ArticlePDF AvailableLiterature Review

Muscle strength, power and adaptions to resistance training in older people

May 2004
European Journal of Applied Physiology 91(4):450-72

DOI:10.1007/s00421-003-0991-3

Source
PubMed

Authors:

Andrea Macaluso

University of Rome Foro Italico

Giuseppe De Vito

University of Padova

Muscle strength and, to a greater extent, power inexorably decline with ageing. Quantitative loss of muscle mass, referred to as "sarcopenia", is the most important factor underlying this phenomenon. However, qualitative changes of muscle fibres and tendons, such as selective atrophy of fast-twitch fibres and reduced tendon stiffness, and neural changes, such as lower activation of the agonist muscles and higher coactivation of the antagonist muscles, also account for the age-related decline in muscle function. The selective atrophy of fast-twitch fibres has been ascribed to the progressive loss of motoneurons in the spinal cord with initial denervation of fast-twitch fibres, which is often accompanied by reinnervation of these fibres by axonal sprouting from adjacent slow-twitch motor units (MUs). In addition, single fibres of older muscles containing myosin heavy chains of both type I and II show lower tension and shortening velocity with respect to the fibres of young muscles. Changes in central activation capacity are still controversial. At the peripheral level, the rate of decline in parameters of the surface-electromyogram power spectrum and in the action-potential conduction velocity has been shown to be lower in older muscle. Therefore, the older muscle seems to be more resistant to isometric fatigue (fatigue-paradox), which can be ascribed to the selective atrophy of fast-twitch fibres, slowing in the contractile properties and lower MU firing rates. Finally, specific training programmes can dramatically improve the muscle strength, power and functional abilities of older individuals, which will be examined in the second part of this review.

Effect of resistance training on muscle strength and size of the quadriceps muscle in older individuals. (CSA Cross-sectional area,CT computerized tomography, F female,

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ORIGINAL ARTICLE

Andrea Macaluso Æ Giuseppe De Vito

Muscle strength, power and adaptations to resistance training

in older people

Accepted: 4 September 2003 / Published online: 25 November 2003

Ó Springer-Verlag 2003

Abstract Muscle strength and, to a greater extent,

power inexorably decline with ageing. Quantitative loss

of muscle mass, referred to as ‘‘sarcopenia’’, is the most

important factor underlying this phenomenon.

However, qualitative changes of muscle ﬁbres and

tendons, such as selective atrophy of fast-twitch ﬁbres

and reduced tendon stiﬀness, and neural changes, such

as lower activation of the agonist muscles and higher

coactivation of the antagonist muscles, also account for

the age-related decline in muscle function. The selective

atrophy of fast-twitch ﬁbres has been ascribed to the

progressive loss of motoneurons in the spinal cord with

initial denervation of fast-twitch ﬁbres, which is often

accompanied by reinnervation of these ﬁbres by axonal

sprouting from adjacent slow-twitch motor units

(MUs). In addition, single ﬁbres of older muscles

containing myosin heavy chains of both type I and II

show lower tension and shortening velocity with respect

to the ﬁbres of young muscles. Changes in central

activation capacity are still controversial. At the

peripheral level, the rate of decline in parameters of

the surface-electromyogram power spectrum and in the

action-potential conduction velocity has been shown to

be lower in older muscle. Therefore, the older muscle

seems to be more resistant to isometric fatigue (fatigue-

paradox), which can be ascribed to the selective atro-

phy of fast-twitch ﬁbres, slowing in the contractile

properties and lower MU ﬁring rates. Finally, speciﬁ c

training programmes can dram atically improve the

muscle strengt h, power and functional abilities of older

individuals, which will be examined in the second part

of this review.

Keywords Ageing Æ Muscle power Æ Muscle strength Æ

Resistance trainin g

Introduction

Human muscle strength, which can be deﬁned as the

maximum force generation capacity of an individual,

reaches its peak between the second and third decades,

shows a slow or imperceptible decrease until about

50 years of age and then begins to decline thereafter at

the rate of approximately 12% to 15% per decade, with

more rapid losse s above the age of 65 years (Asmussen

and Heeboll-Nielse n 1962; Larsson et al. 1979; Viitasalo

et al. 1985; Vandervoort and McComas 198 6; Borges

1989; Narici et al. 1991; Metter et al. 1997; Lindle et al.

1997). Since this trend has been measured in studies with

cross-sectional design, it would not be appropriate to use

of words such as ‘‘decrease’’, ‘‘decline’’, ‘‘drop’’, etc.,

which imply that the strength of the older participants in

their younger age was the same as that of the younger

participants. This is a questionable assumption, because

the older people are of a diﬀerent generation, with a

lifetime of physical activity, health and nutrition that is

probably unlike the habits of their young counterparts.

However, as it is common practice in the literature,

words such as ‘‘decrease’’, ‘‘decline’’, ‘‘drop’’, etc., will

be used throughout this review even if inappropriate.

There are few investigations with a longitudinal design

that have determined how strength changes with ageing

and, to the best of the authors knowledge, the same

individuals were followed for no longer than 12 years

(Aniansson et al. 1986; Kallman et al. 1990; Bassey and

Harries 1993; Greig et al. 1993; Winegard et al. 1996;

Rantanen et al. 1997; Frontera et al. 2000a). Most of the

studies indicate that the decline in strength occurs at a

higher rate than reported in cross-sectional investiga-

tions (Aniansson et al. 1986; Kallman et al. 1990; Bassey

and Harries 1993; Winegard et al. 1996; Frontera et al.

2000a), although no change (Greig at al. 1993) or even a

Eur J Appl Physiol (2004) 91: 450–472

DOI 10.1007/s00421-003-0991-3

A. Macaluso (&) Æ G. De Vito

Applied Physiology Department,

Strathclyde Institute for Biomedical Sciences,

University of Strathclyde, 76 Southbrae Drive,

Glasgow, G13 1PP, UK

E-mail: andrea.macaluso@strath.ac.uk

Tel.: +44-141-9503718

Fax: +44-141-9503132

slight increase in strength (Rantanen et al. 1997) have

both been measured. Direct comparisons between

groups of young and older individuals have shown that

the quadriceps muscles of older people aged around

70 years have approximately 60% of the force-genera t-

ing ability of young individuals aged 20–30 years

(Young et al. 1984, 1985; Klitgaard et al. 1990; Ha

kki-

nen and Ha

kkinen 1991; Overend et al. 1992a; Frontera

et al. 2000b), ranging from a minimum of 56% (Klitg-

aard et al. 1990) to a maximum of 76% (Ha

kkinen and

kkinen 1991; Overend et al. 1992a). It is likely that

this variability of strength diﬀerences reﬂects diﬀerent

study populations, levels of physical activity, muscle

groups investigated and testing methodologies. More-

over, Short and Nair (1999) pointed out that the decline

of muscle function in the whole population may be even

greater than that reported, because many older indi-

viduals are excluded from research studies for the pres-

ence of disease. Finally, further evidence of this

inevitable process of decay is given by measurements

performed on master athletes, whose performance

inexorably decline as they age, although 70-year-old

athletes can have levels of strength, power and other

functional capacities equivalent to those of 20-year-old

sedentary individuals (Moore 1975; Meltzer 1994;

Pearson et al. 2002).

The loss of strength with ageing is apparently true for

both men and women (Danneskiold-Samsoe 1984;

Vandervoort and McComas 1986; Borges 1989; Fron-

tera et al. 1991; Lindle et al. 1997). The early study of

Asmussen and Heeboll-Nielsen (1962) reported that

women began to decline at an earlier age than men, but

this observation was not conﬁrmed in further studies.

However, it is evident that women are weaker than men

in absolute strength of various muscle groups in all

stages of life (Danneskiold-Samsoe 1984; Vandervoort

and McComas 1986; Borges 1989; Frontera et al. 1991;

Lindle et al. 1997). For example, Frontera et al. (1991)

reported that women had 60% and 59% the strength of

men in the lower extremities when they were tested at

slow and fast isokinetic speeds, respectively (the term

isokinetic indicates that the movemen t of the displaced

body segment is performed at constant angular speed).

Similarly, Borges (1989) found women in diﬀerent age

groups to have 66% and 53% the isokinetic peak torque

of men in the knee-extension and knee-ﬂexion, respec-

tively. Although males and females ap pear to show

comparable age-related trends when maximal strength is

referred to the unit of muscle cross-sectional area (CSA)

(Vandervoort and McComas 1986; Winegard et al. 1996;

Lindle et al. 1997), in the latter stages of life absolute

values of maximal strength in women can approach the

minimal levels necessary to accomplish daily activities,

thus suggesting that women should be the ﬁrst target

group in intervention and rehabilitation studies (Skelton

et al. 1994).

Regional anatomical diﬀerences in the loss of

strength have also been reported in the literature

(Viitasalo et al. 1985; Frontera et al. 1991; Rantanen

et al. 1997; Lynch et al. 1999). The proximal muscles of

the lower extremities are more aﬀected by strength losses

than those of the upper extremities, which in older

people has been ascribed to a decreasing use of lower

compared with upper limb muscles (Viitasalo et al. 1985;

Frontera et al. 1991; Lynch et al. 1999). This is sup-

ported by the fact that age-related morphologica l

changes are more pronounced in the quadriceps than in

the biceps brachii muscle (Aniansson et al. 1986). By

contrast, longitudinal data of Rantanen et al. (1997)

showed that older men and women slightly increased

their knee extensor strength from 75 to 80 years, as

opposed to a decrease in upper extremity and trunk

strength. The investigators suggested that everyday

activities at low intensity might have provided suﬃcient

stimulus to the knee extensor musculature. This is in

contrast, however, with the evidence that activities of

daily living require little quadriceps strength, as during

normal walking the quadri ceps is activated only brieﬂy

at the beginning and end of the gait cycle (OToole

1997). Moreover, individuals with weak quadriceps tend

to supplement knee extensor movements with other

muscles, such as arm muscles to get up from a chair, or

alter their behaviour to avoid activities such as climbing

stairs.

Muscle strength can be tested using three diﬀerent

modalities: isometric, dynamic or isokinetic (Frontera

and Meredith 1989; Abernet hy et al. 1995). Isometric or

static strength is the maximum force that can be exerted

against an immovable object, which corresponds to the

maximum that can be measured in humans. Dynamic

strength is the heaviest weight that can be lifted, whilst

isokinetic strength is the maximal torque that can be

exerted against a pre-set rate-limiting device. Maximum

strength is measured during isometric contractions, in

which there is no change in muscle length, whilst power

is generated in actions that involve movement and is

calculated as the product of force and speed at which

movement occurs. Fewer studies have been conducted

on muscle power as compared to strength, possibly due

to methodological diﬃculties, as will be discu ssed later

in this review.

Lower levels of strength and, to a greater extent,

power are associated with functional limitations in daily

living activities (Aniansson et al. 1980; Avlund et al.

1994; Skelton et al. 1994; Foldvari et al. 2000). In

addition, muscle weakness is associated with an

increased risk of falls (Tinetti et al. 1988; Campbell et al.

1989; Wolfson et al. 1995), hip fractures (Aniansson

et al. 1984; Langlois et al. 1998) an d adverse physio-

logical changes such as loss of bone mineral density

(Sinaki et al. 1986), which may predispose to osteopo-

rosis.

This review begins with a systematic examination of

the main causes of muscle strength loss in old age. This

is then followed by a review of the few studies speciﬁ-

cally investigating musc le power, with power being more

related to daily functional activities than strength per se.

Finally, the focus is on how these age-related changes in

451

muscle function can be improved through appropriate

training programmes. Some of the studies on strength

training in older people will be critically scrutinized,

followed by the more limited number of investigations

speciﬁcally aimed at improving muscle power and the

few works looking at the eﬀects of training on selected

functional abilities.

Causes of muscle strength loss in old age

Muscle size and morphology

Muscle size is reduced with ageing an d this quantitative

loss of muscle, referred to as ‘‘sarcopenia’’ (Evans 1995),

aﬀects the generation of force. Sarcopenia has been

demonstrated using various techniques, which include

measurements of total body potassium (Allen et al.

1960) or creatini ne excretion (Tzankoﬀ and Norris

1977), histoch emical analysis of muscle ﬁbres, in either

biopsy (for example Larsson et al. 1979) or autopsy

(Lexell et al. 1983, 1988) specimens, and imaging

methods such as ultrasonography (Young et al. 1984,

1985; Ha

kkinen and Ha

kkinen 1991), computerized

tomography (CT) (Rice et al. 1989; Klitgaard et al. 1990;

Overend et al. 1992b) and, more recently, magnetic

resonance imaging (MRI) (Jubrias et al. 1997; Kent-

Braun and Ng 1999; Kent-Braun et al. 2000).

In the early study of Allen et al. (1960) muscle mass

was reduced by 23% in individuals from 20 to 80 years,

as estimated from measurements of total body potas-

sium. Similarly, Tzankoﬀ and Norris (1977), at the end of

the 1970s, reported that the active muscle mass, as

revealed by creatinine excretion, was 35% less in 80-year-

old individuals as opposed to those in their twenties.

Grimby and Saltin (1983) later pointed out that the loss

of muscle mass, which had been reported in the studies of

Allen et al. (1960) an d Tzankoﬀ and Norris (1977), was in

the same order of magnitude as the decline of strength

in leg, back and arm muscles, thus relating the weakness

of older people entirely to muscle wasting.

Starting in 1977 with Tomonaga, many investigators

have carried out a quantitative histochemical analysis of

muscle samples, obtained either by biopsy (for example

Larsson et al. 1979) or autopsy (for example Lexell et al.

1983, 1988), and have attributed the loss of muscle mass

in older people, at the microscopic level, to a reduction

in the number and size of muscle ﬁbres. An extensive

review of these studies is beyond the purposes of this

manuscript and therefore the reader is referred to the

work of Lexell (1995). The loss of muscle tissue in older

people has been attributed to reduced numb ers of both

slow-twitch ﬁbres and fast-twitch ﬁbres, plus a reduction

in the CSA of single ﬁbres, especially of type II. Fast-

twitch ﬁbres are intrinsically stronger than slow-twitch

ﬁbres and therefore muscles with the same area, but

occupied by a relatively smaller area of fast-twitch ﬁbres,

will be able to generate less force (Jones and Round

1990).

With the introduction of modern radiological imag-

ing techniques, it has been possible to estimate muscle

mass more directly. Sarcopenia has been documented

with ultrasound imaging in the works of Young et al.

(1984, 1985), who measured a 25% and 33% small er

quadriceps CSA in older men an d women, respectively,

(age range 70–81 years) than in young individuals (age

range 20–29 years). Also Ha

kkinen and Ha

kkinen

(1991) have shown a 27% smaller mid-thigh CSA of the

quadriceps in older women aged between 66 and

75 years, as compared to the young, aged between 26

and 35 years. In these studies, however, muscle CSA

may have been overestimated, particularly in older

people, due to the presence of fat and connective tissue

within the muscle belly, which cannot be detected and

separated in the ultrasound scan. This problem has been

overcome by the use of CT and MRI (Rice et al. 1989;

Overend et al. 1992b; Kent-Braun et al. 2000), which

enabled investigators not only to outline the muscle

compartment area on each scan, at diﬀerent levels of

section, but also to estimate the muscle contractile

component separately from the intramuscular non-

contractile tissues (i.e. connective and fat tissue). The

correlation coeﬃcient between cadaver sections of

human skeletal muscle and corresponding CT or MRI

scans approached unity, with the relative diﬀerence be-

tween cadaver and imaging measurements being 1.3%

for both techniques (Mitsipoulos et al. 1998). The use of

both these advanced imaging techniques has provided

further evidence of the wasting of contractile muscle

occurring in older people with a substantial increase of

fat and connective tissue, both within the muscle and in

the overall body (Rice et al. 1989; Overend et al. 1992b;

Jubrias et al. 1997; Kent-Braun and Ng 1999; Janssen

et al. 2000; Kent-Braun et al. 2000; Macaluso et al.

2002). In particular, the recent data of Janssen et al.

(2000), which present reference values for whole-body

skeletal mass of 468 individuals from 18 to 88 years,

show a decline after the end of the ﬁfth decade of 1.9 and

1.1 kg/decade in men and women, respectively, with a

preferential decrease in the lower body.

Comparing the time course of the decline in muscle

mass with that of the decline in force indicates that the

decline in force with ageing is generally greater and

starts sooner than that of muscle bulk (Bruce et al.

1997). Similarly, Kallman et al. (1990) have shown that

there is no relationship between how quickly subjects

lose muscle mass, as indicated by the slope of creatinine

excretion versus age, and how fast they lose grip

strength, as indicated by the slope of grip strength versus

age. This suggests that there may be other causes con-

tributing to the decline in strength, other than muscle

wasting, which are speciﬁc to the remai ning muscle. In

further support of this observation, some investigators

have shown that muscle strength of the knee extensors is

highly correlated with muscle size, but this relationship

is higher in young than older populations (Ha

kkinen

and Ha

kkinen 1991; Overend et al. 1992a). In most of

the research studies, however, it is the ratio between

452

muscle strength and CSA, referred to as speciﬁ c

strength, which has been compared between young and

older people, or examined across the life span, in order

to determine whether strength losses could be attributed

entirely to reduced muscle mass or whether other factors

had to be taken into account (Young et al. 1984, 1985;

Klitgaard et al. 1990; Ha

kkinen and Ha

kkinen 1991;

Phillips et al. 1993; Jubrias et al. 1997; Kent-Braun and

Ng 1999; Frontera et al. 2000b). The results of these

studies are still controversial. You ng et al. (1985 ) and

Klitgaard et al. (1990) have reported that the isometric

speciﬁc strength of quadriceps muscle was 19% and

27% smaller, respectively, in young than older sedentary

men. Similarly, other studies have observed a drop of

isometric (Phillips et al. 1993) and isokinetic (Jubrias

et al. 1997) speciﬁc strength of adductor pollicis and

quadriceps muscles, respectively, in older subjects of

both genders. Conversely, Kent-Braun and Ng (1999)

and Frontera et al. (2000b) have reported no age-related

diﬀerences in isometric strength of the ankle dorsiﬂexors

and isokinetic strength of the knee extensors, respec-

tively, after adjusting for CSA. Young et al. (1984) and

kkinen and Ha

kkinen (1991) have also indicated that

the quadriceps weakness of healthy women in their

seventies can be adequately explained by the similarly

smaller size of the muscle group. All of these investiga-

tors (Young et al. 1984, 1985; Klitgaard et al. 1990;

kkinen and Ha

kkinen 1991; Phillips et al. 1993;

Jubrias et al. 1997; Kent-Braun and Ng 1999; Frontera

et al. 2000b) have used the area of muscle cross-section

at right angles to the long axis of the limb, referred to as

anatomical CSA, to interpret data on muscle strength

relative to muscle size in ageing muscle. However, in

pennate muscles, such as the quadriceps, ﬁbres run ob-

liquely to the force-generating axis and insert into the

tendon with an angle, referred to as ‘‘angle of penna-

tion’’ (Narici et al. 1992). Therefore, the anatomical

CSA cuts a limited number of ﬁbres, whilst it is the sum

of the cross-sectional areas of all the muscle ﬁbres within

the muscle which should be used. This has been referred

to as physiological CSA (PSCA), which can be calcu-

lated by measuring, with a combined use of MRI and

ultrasonography, three parameters: muscle volume, ﬁbre

length and pennation angle. It has been recently found

that pennation angle and ﬁbre length in the human

gastrocnemius were 13% and 8% less in older than

young individuals, respectively (Narici et al. 1999).

Therefore, PCSA is expected to decrease with ageing at a

diﬀerent rate than anatomical CSA, which may lea d to a

misinterpretation of the ratio between force and CSA

(Narici 1999). Recently, Miyatani et al. (2001) have

suggested that determination of muscle volume is a way

of approximating its physiological CSA. Moreover,

because the muscle volume can be expressed as a pro-

duct of physiological CSA and muscle ﬁbre length,

torque relative to muscle volume may theoretically be

considered as an index with the same dimension as that

of muscle force relative to physiological CSA, i.e. spe-

ciﬁc tension (kNÆm

) (Miyatani et al. 2001). In light of

these considerations, Macaluso et al. (2002) have

expressed the speciﬁc tension of the knee extensors and

ﬂexors as the ratio between maximum isometric torque

and contractile muscle volume, distinguished from

intramuscular non-contractile tissue, and compared it

between young and older women. Women in their

seventies showed a lower speciﬁc tension than women in

their twenties, thus suggesting that the lower level of

muscle strength in older women is not completely

explained by their smaller contractile muscle mass, but

also by other factors, which will be presented in the

following paragraphs of this review.

Muscle excitability and contractility

Narici (1999), in his comprehensive review of studies on

the changes of muscle contractile properties with ageing,

has attributed the loss of strength in older people not

only to reduced muscle mass but, more exhaustively, to

reduced excitable muscle mass. Therefore, it is suggested

to take into account only the amo unt of muscle that is

functionally active, as indicated by the term ‘‘excitable’’.

This is strictly dependent, in turn, on the integrity of

both the muscle ﬁbres and the nerve cells that control

them, namely the motoneurons. Neural and muscular

systems cannot therefore be separated and it is appro-

priate to consider muscle ﬁbres and motoneurons as a

whole. A single motoneuron and its family of innervated

muscle ﬁbres have been deﬁned by Sherrington (1929) as

the motor unit (MU). Fast-twitch MUs are composed by

relatively large motoneurons with fast conduction

velocities, which generally innervate between 300 and

500 muscle ﬁbres (McArdle et al. 1996). On the contrary,

slow-twitch MUs are composed of smaller motoneurons

with slow conduction velocities, which innervate a

smaller number of ﬁbres. There are at least nine elec-

trophysiological techniques of MU estimation in hu-

mans, most of which involve applying electric shocks of

varying intensity to a peripheral nerve and measuring

the evoked responses in the muscle (for review see

McComas 199 8). The number of MUs is obtained by

comparing an average parameter of the single MU,

usually its action potentials, with the corresponding

parameter of the whole muscle. The relative size of MUs

is determined by comparing their mechanical responses

to single or maximal repetitive stimulation, which are

referred to as twitch or tetanic contractions, respectively.

MUs have been shown to be reduced with ageing in both

number and size, thus aﬀecting the capacity of skeletal

muscles to produce force (Brown et al. 1988; Doherty

and Brown 1993; Doherty et al. 1993). This is in agree-

ment with previous evidence of a reduced number of

limb motoneurons, in the human lumbosacral cord, by

approximately 25% from the second to the tenth decade

(Tomlinson and Irving 1977). Consistent wi th this

observation is the reduction in the number and diameter

of motor axons in the ventral roots (Kawamura et al.

1977a, 1977b), which is accompanied by slower axonal

453

conduction velocity (Metter et al. 1998; Wang et al.

1999). In animal studies, the selective atrophy of fast-

twitch ﬁbres, which has been reported earlier in this re-

view, has been ascribed to the progressiv e loss of

motoneurons in the spinal cord with initial denervation

of fast-twitch ﬁbres and reinnervation of these ﬁbres by

axonal sprouting from adjacent slow-twitch MUs

(Brooks and Faulkner 1994). This phenomenon of

remodelling is supported, at the microscopic level of

muscle analysis in humans, by morphological changes

similar to those occurring in motoneuron diseases and

chronic neuropathies, which include the presence of

larger groups of muscle ﬁbres of the same histochemical

type, referred to as ﬁbre type grouping, small and dark

ﬁbres with a peculiar geometrical shape, referred to as

angulated ﬁbres, and group atrophy (Jennek ens et al.

1971; Lexell and Downham 1991). Recent results of in-

creased coexpression of myosin heavy chain (MHC)

isoforms in the same ﬁbre as measured with electro-

phoretic techniques, which will be presented later in this

review, are further evidence of this process of ongoing

denervation and reinnervation (Andersen et al. 1999).

Results of Galea (1996) suggest that the reduction of

excitable muscle mass in the upper limbs is of neuro-

pathic origin, i.e. denervation of MUs due to loss of

peripheral motoneurons, for distal muscles, but of

myopathic origin, i.e. atrophy of muscle ﬁbres, in

proximal muscles. The neuromuscular excitability has

been evaluated by measuring the muscles electrical re-

sponse evoked by the electrical stimulation of the motor

nerve. An M-wave is the result of the direct depolar-

isation of motoneurons that directly innervate muscle

ﬁbres, with its area and amplitude being a measure of

their excitability. The number of MUs has been esti-

mated by comparing the average area of a sample of

MU action potentials, which are obtained by incre-

mental stimulation of the peripheral nerve, and the area

of the maximum M-wave, which is the compound action

potential of a muscle. The authors studied the number of

MUs and the maximum M-wave of both the thenar

muscle, a distal muscle, and the biceps brachii, a prox-

imal muscle, in individual s in their eighties as com pared

to those in their twenties. In the thenar muscle, older

individuals had maximum M-wave area and amplitude

values that were 22% and 33% lower, respectively, than

the young subjects values, with a 50% lower number of

MUs, thus revealing a loss of MUs with the presence of

collateral reinnervation. In contrast, in the biceps bra-

chii, the area and amplitude of maximum M-waves were

50% and 30% less in the older than in the young sub-

jects, respectively, with no signiﬁcant diﬀerences in the

number of MUs, thus indicating the presence of less

excitable muscle mass entirely due to ﬁbre atrophy.

The older muscle is not only atrophied, but is also

slower (Vandervoort and McComas 1986; Narici et al.

1991; Roos et al. 1999), tetanizes at lower fusion fre-

quencies (Narici et al. 1991; Kamen et al. 1995; Connelly

et al. 1999; Roos et al. 1999), and is more resistant to

isometric fatigue than the young muscle (Narici et al.

1991; Bilodeau et al. 2001a). Slowing in the contract ile

properties has been demonstrated in various muscles of

the lower limbs (Vandervoort and McComas 1986;

Roos et al. 1999), and in the adductor pollicis muscle

(Narici et al. 1991), by measuring the duration of

twitch contraction. Tibialis anterior, tibialis posterior

(Vandervoort and McComas 1986) and vastus medialis

(Roos et al. 1999) are slower in individuals over 73 than

in young people in their twenties, as demonstrated by the

longer time taken not only to reach the peak of tension

but also to relax after the twitch. Similarly, the maximum

relaxation rate has been shown to decline from 20 to

91 years of age in the adductor pollicis muscle (Narici

et al. 1991). The reason for this slower relaxation rate (or

longer twitch contraction duration) is probably related to

the selective atrophy of type II ﬁbres. The muscle relaxes

at a lower rate as a result of the predominance of type I

ﬁbres, which are slo wer. At the microscopic level of

muscle analysis, the slower relaxation rate can be

ascribed to a reduction in sarcoplasmic reticulum activity

(Klitgaard et al. 1989; Delbono et al. 1997; Hunter et al.

1999) and actin sliding speed on myosin (Ho

k et al.

2001). As a result of slowing of relaxation and probably

of type II ﬁbre atrophy, it has been shown that muscles of

older individuals demonstrate tetanic fusion, i.e. a max-

imum maintained contraction in response to repetitive

stimulation, at lower frequencies of stimulation than is

required for young adults, thus enabling the individual

MUs to use lower ﬁring frequencies to achieve a full

contraction (Narici et al. 1991; Kamen et al. 1995;

Connelly et al. 1999; Roos et al. 1999). Slowing of

relaxation and type II atrophy may also explain why

older people demonstrated more resistance to isometric

fatigue that young individuals (Narici et al. 1991), a

process which is referred to as the ‘‘fatigue-paradox’’

(Narici 1999).

Recently, Scaglioni et al. (2002) have shown that the

excitability in the spinal reﬂex pathway, expressed as the

ratio between the maximum H-reﬂex and the maximum

M-wave, is functionally impaired in older male individ-

uals. Scaglioni et al. (2002) concluded that the impaired

functionality of the reﬂex pathway with ageing, in

addition to the lack of changes following resistance

training as will be mentioned later in this review, sug-

gests that the H

max

ratio may be related to

degenerative phenomena rather than physical decondi-

tioning. At higher levels of the central nervous system,

there are still conﬂicting results on the possibility of a

reduction in the descending drive from supraspinal

centres to the motoneurons during maximum voluntary

contractions in older individuals, which would be a

further explanation for the loss of muscle streng th with

ageing (Phillips et al. 1992; De Serres and Enoka 1998;

Connelly et al. 1999; Harridge et al. 1999b; Kent-Braun

and Ng 1999; Roos et al. 1999; Yue et al. 1999; Bilodeau

et al. 2001a; Jakobi and Rice 2002; Scaglioni et al. 2002).

Some experimenters showed by a twitch interpolation

technique, a method based on delivering one or a brief

series of electrical stimuli to the motor axons of muscles

454

during maximum voluntary contraction, that older

adults were not able to maximally activate a muscle or

muscle group (Harridge et al. 1999b; Yue et al. 1999;

Bilodeau et al. 2001a). Other investigators, however,

demonstrated that a superimposed stimulus, in the form

of a single twitch or a short tetanus, added little or

nothing to the volitional force of older people (Phillips

et al. 1992; De Serres and Enoka 1998; Connelly et al.

1999; Kent-Braun and Ng 1999; Roos et al. 1999; Jakobi

and Rice 2002; Scaglioni et al. 2002). It must be noted

that central failure in activation was measured in very

old individuals by Harridge et al. (1999b) and that not

enough practice may have been given to the subjects in

measuring maximum voluntary contraction by Yue et al.

(1999) and Bilodeau et al. (2001a). The possibility of

underestimating the ‘‘real’’ maximum should therefore

be discounted if at least one session of familiarization is

practised (Jakobi and Rice 2002) and if the subjects are

not too old. Narici (2001) pointed out that activation

capacity could be muscle speciﬁc, as it appears to be

preserved in distal muscles of both upper and lower

extremities (Vandervoort and McComas 1986; Phillips

et al. 1992), but this issue deserves further investigation

as conclusive results on the activation capacity of a

muscle may largely depend on the stimulation technique

that is adopted. In fact, Behm et al. (2001) have recently

indicated that tetanic stimulation superimposed upon

single maximal or multiple contractions seems to pro-

vide the most valid measure of muscle inactivation.

Finally, the authors of this review suggest that voluntary

strength testing should be preferred, since electrical

stimulation determines a full synchronization of MUs

(Solomonow et al. 1994), which is unlikely to occur in

real life.

Surface electromyography is a non-invasive method

that has been used often to monitor changes due to

ageing in the overall neural activation of both agon ist

and antagonist muscles which, in turn, aﬀects the gen-

eration of force (Merletti et al. 199 2; Esposito et al.

1996; Ha

kkinen et al. 1998a; Izquierdo et al. 1999;

Merletti et al. 2002). Inter-subject com parisons of

sEMG parameters as indicators of muscle function are

limited by the presence of skin, subcutaneous and fat

layers between the muscle and the recording electrodes,

which have a ﬁltering eﬀect on the signal (De Luca

1997). Therefore, although it is evident from the data

reported in various studies (Moritani and de Vries 1980;

kkinen and Ha

kkinen 1995; Ha

kkinen et al. 1998c)

that the absolute values of sEMG amp litude were lower

in groups of older individuals as compared to young or

middle-aged subjects, most of the investigators did not

remark on the physiological meaning of this diﬀerence,

with the exception of Esposito et al. (1996) and Merletti

et al. (2002). The authors speculated that the lower RMS

in the biceps brachii of individuals in their seventies, as

compared to those in their twenties, may be due not only

to the diﬀerent thickness or conductivity of the layers

between the muscle and the recording electrodes, but

also to the diﬀerent MU ﬁring rates betw een the two

groups. This is supported by the observation of a de-

crease in the maximal MU ﬁri ng rate with ageing in

studies where the intramuscular EMG signal was re-

corded (Kamen et al. 1995; Connelly et al. 1999; Erim

et al. 1999). The lower sEMG amplitude in olde r people

has also been ascribed to the reduced number of MUs

(Esposito et al. 1996), in agreement with the observation

made by others with electrophysiolog ical techniques of

MU estimation (Brown et al. 1988; Doherty and Brown

1993; Doherty et al. 1993), which are reported at the

beginning of this paragraph . In agreement with Esposito

et al. (1996) and Merletti et al. (2002), Macaluso et al.

(2002) have attributed the smaller RMS in older indi-

vidual to either a smaller number of recruited MUs or a

decreased ﬁring rate of the individual MUs. As an

additional factor to explain the smaller RMS, the au-

thors have suggested the possibility of a decreased MU

synchronization. However, the relative roles played by

MU recruitment, MU ﬁring rate and synchronization of

the individual MUs cannot be distinguished with surface

EMG. Some authors (Ha

kkinen et al. 1998a; Izquierdo

et al. 1999) , alternatively, used the sEMG to measure the

coactivation of antagonist muscles, which is referre d to

the maximum EMG activity of the same muscle group

during the agonist action. In this measure the inﬂuence

of subcutaneous and skin layers can be discounted, al-

though it may increase the danger of cross-talk from

nearby muscles (Solomonow et al. 1994), thus contam-

inating the signal and misleading its interpretation.

Coactivation, also referred to as cocontraction, may

serve to prot ect and stabilize the joint during forceful

contractions (Baratta et al. 1988). The higher levels of

antagonist coactivation, which have been observed in

70-year-old men and women as opposed to 40- and 20-

year-old individuals (Ha

kkinen et al. 1998a; Izquierdo

et al. 1999; Macaluso et al. 2002), could be an additional

explanation for the age-related decline in force produc-

tion. In other words, the net force exerted about a joint

during a given action, e.g. knee extension, would be

reduced in older people due to the greater simultaneous

activation of the musc les exerting a torque in the

direction which is opposite to that of the movement (i.e.

hamstrings).

Comparisons between young and older indivi duals in

the frequency-domain parameters of sEMG have been

performed during sustained isometric contractions of

the biceps brachii (Bilodeau et al. 2001b; Merletti et al.

2002) and tibialis anterior muscles (Merletti et al. 1992).

A decline in median frequency (MDF), mean frequency

(MNF) or other parameters of the power spectrum,

which can be attributed to muscle fatigue, has been

observed in both young and older individuals, with the

rate of decrease of spectral parameters being lower in

the older subjects. This observation further supports the

‘‘fatigue-paradox’’ reported in the previous pages

(Narici et al. 1999), which can be ascribed to selective

atrophy of type II ﬁbres, slowing in the contractile

properties, and lower MU ﬁring rates of the older

muscle. By contrast, Hara et al. (1998) and Bilodeau

455

et al. (2001a) did not ﬁnd signiﬁcant diﬀerences between

young and older subjects in the decline of spectral

parameters of the abductor digiti minimum and elbow

ﬂexors, respectively, during sustained contractions. The

interpretation of these results is unclear. Hara et al.

(1998) have explained the occurrence of a similar decline

in spectral parameters of young and older individuals

with ischemia, which is due to a stronger muscle con-

traction in the young and to a smaller capillary bed in

the older subjects. This would result in a similar decrease

in the two groups of the pH of interstitial ﬂuids, as blood

ﬂow determines the rate of metabolites removal, thus

aﬀecting the decline in spectral parameters. Merletti

et al. (2002) attributed the results of Bilodeau et al.

(2001a) to the lack of speciﬁc criteria in selecting the best

electrode location.

Tendinous factors

Human tendons are responsible for the transmission of

force from muscles to bones and in vitro studies have

shown that ageing is associated with a decrease in their

stiﬀness (Vogel 1991). Only in recent times have tendon

mechanical properties been studied in vivo using ultra-

sonography (Maganaris 2001; Kubo et al. 2003; Reeves

et al. 2003a, 2003b). A lower gastrocnemius tendon

stiﬀness, reported as the ratio between force and tendon

elongation, was measured in a group of six older indi-

viduals in their seventies with respect to a control group

of six healthy young adults in their twenties (Maganaris

2001). The older tendons were more compliant in the

higher regions of the maximal tendon force, thus sug-

gesting that older individuals may operate closer to

muscle optimal length as a compensatory mechanism for

the reduction in force (Maganaris 2001; Reeves et al.

2003b). A functional consequence of the reduced tendon

stiﬀness in older people is likely to be a decrease in the

rate of force development, aﬀecting for example the time

necessary to decelerate body mass, a factor related to the

prevention of falls (Kubo et al. 2003; Reeves et al.

2003b). Increased tendon compliance would also in-

crease the likelihood of strain injury (Reeves et al.

2003a). The mechanisms contributing to reduced tendon

stiﬀness are unclear, with decreased diameter and

packing density of collagen ﬁbrils and changes in col-

lagen crimp structure being likely factors (Reeves et al.

2003b).

Hormonal factors

A decline in the levels of many hormones has been

measured in older people, particularly growth hormone

(GH) (Rudman et al. 1990; Welle et al. 1996a), insulin-

like grow factor 1 (IGF-1) (Butterﬁeld et al. 1997), tes-

tosterone (Tenover 1992; Urban et al. 1995; Baumgart-

ner et al. 1999) and oestrogen (Phillips et al. 1993; Taaﬀe

et al. 1995; Bassey et al. 1996; Skelton et al. 1999;

Onambele et al. 2001), but how these changes are related

to strength and power is still largely unclear (Short and

Nair 1999).

The action of GH, which is known to promote

growth, protein synthesis and fat mobilization, is med-

iated by IGF-1 (Rooyackers and Nair 1997). The hor-

mone deﬁciency, which has been measured in older

individuals (Rudman et al. 1990), determines a reduction

in muscle mass and an increase in fat mass. The

administration of recombinant human GF to healthy

older adults for 1 month increased circulating IG F-1

and led to a 50% increase in mixed muscle protein

synthesis, as measured using biopsy samples (Butterﬁeld

et al. 1997). However, 3 months of treatment with

slightly higher doses did not produce any signiﬁcant

change in protein synthesis of another group of older

subjects, although muscle mass and strength increased

(Welle et al. 1996a). Yarasheski at al. (1992, 1995) have

suggested that, as adding GH to a 12- or 16-week

resistance training programme led to an increase in fat-

free mass and whole body protein synthesis but not in

mixed muscle protein synthesis, the hormone enhanced

protein synthesis in non-muscle tissues. Taaﬀe et al.

(1994, 1996) gave evidence that administering GH dur-

ing the ﬁnal 10 weeks of a 24-week resistance training

programme did not produce any further gains in muscle

size and strength. Harridge and Young (1998), in their

notable review on skeletal muscle in older people,

pointed out that a treatment for life with GH may lead

to gains that may be equally achieved with exercise

training, with the latter having no undesirable side-

eﬀects of drug administration, such as ﬂuid retention,

with consequent joint swelling and arthralgias, or

hypertension, atrial ﬁbrillation, gynecomastia, and

hyperglycemia. Undesirable side-eﬀects were also

observed after administration of IGF-I (Butterﬁeld et al.

1997), which was associated wi th an increase in protein

synthesis, but muscle strength and size were not moni-

tored, thus requiring further investigation.

It is known that testosterone has a trophic action on

skeletal muscle, which is mediated by androgen recep-

tors in the myoﬁbrils (Celotti and Negri Cesi 1992).

Iannuzzi-Sucich et al. (2002) recently reported that

bioavailable testosterone is a strong predictor of muscle

mass in men, together with body mass index, strength

and power. As many changes associated with testoster-

one deﬁciency in young men, such as muscle atrophy

and weakness, are similar to the changes associated with

the declined testosterone levels in older men, it has been

suggested that testosterone supplementation may pre-

vent or reverse the eﬀects of ageing (Gruenewald and

Matsumoto 2003). Serum testosterone increased fol-

lowing hormone replacement to a level comparable to

that of young men, with a signiﬁcant gain in muscle

mass after 3 months (Tenover 1992). In the study of

Urban et al. (1995), administering testosterone to hor-

mone-deﬁcient men for 4 weeks was accompanied by an

increase in both strength and mixed protein synthesis.

Kraemer et al. (1999) have shown that older men

456

showed a signiﬁcant increase in testosterone levels in

response to resistance training. Ha

kkinen and Pakarinen

(1994) suggested that the balance between anabolic and

catabolic hormones in ageing men is associated with

muscle atrophy and decreased voluntary force produc-

tion, as the ratio between circulating levels of serum

testosterone and serum cortisol correlated with muscle

CSA and strength. Izquierdo et al. (2001) have shown

that serum total testosterone and free testostero ne cor-

related with the magnitude of the training-related in-

crease in strength and power output in both groups of

middle-aged and older men. Androgens also appear to

be responsible for the reduced release of neurotrans-

mitters and other neuro trophic agents that, in turn, ef-

fect the partial denervation of muscle ﬁbres and the

consequent sarcopenia (Gutmann and Hanzlı

kova

1970,

cited in Narici 1999), as reported earlier in this review. In

a recent cross-sectional study, which involved 121 male

and 170 female volunteers aged 65–97 years (Baum-

gartner et al. 1999), a signiﬁcant correlation between

muscle mass and serum free-testosterone was found in

men, whilst there was no association between oestrogen

levels and strength in women. However, oestrogen levels

in women , which fall at menopause, may play a role in

the loss of force, but conﬂicting results have been found

in postmenopausal women who were recei ving hormone

replacement therapy (HRT) (Phillips et al. 1993; Taaﬀe

et al. 1995; Bassey et al. 1996; Skelton et al. 1999;

Onambele et al. 2001). In the study of Phillips et al.

(1993), 25 postmenopausal women, who had been on

HRT for between 1 and 25 years, showed a 26% higher

speciﬁc strength in the adductor pollicis muscle than

those not receiving HRT. Similarly, another group

of postmenopausal women treated with HRT for

6–12 months increased their strength by 12%, as

opposed to a slight decline in the control group, wi th

both groups being accompanied by no signiﬁcant change

in muscle CSA (Skelton et al. 1999). A follow-up study

after 2–4 years on the same population showed that the

beneﬁt of HRT on the isometric muscle strength of

adductor pollicis was maintained in those women who

continued treatment beyond 1 year, although no further

increase in muscle strength was found (Onambele et al.

2001). However, other studies suggest that oestrogen

status does not have any eﬀect on the maximal

muscle strength of older women (Taaﬀe et al. 1995;

Bassey et al. 1996).

Levels of physical activity

It has been shown that the amount of physical activity

decreases with ageing but it is unclear whether this is a

cause or an eﬀect of the age-related loss of muscle

function (Harridge and Young 199 8). It seems that the

decrease in strength with ageing cannot be explained only

on the basis of a decreased level of physical activity, since

also highly competitive veteran sportsmen inexorably

decline (Meltzer 1994; Pearson et al. 2002). In addition,

muscle disuse results in a reduction in muscle ﬁbre size

(Ferretti et al. 1997) and not, like ageing, in muscle ﬁbre

number (Larsson et al. 1979; Lexell et al. 1988). As a

limitation of current studies, habitual physical activity

levels have often been reported in descriptive rather than

quantitative terms, making them diﬃcult to interpret

(OToole 1997). For example, individuals were classiﬁed

as inactive if they participated in normal activities of

daily living or sedentary leisure-time activities and active

if they participated in moderated physical activity more

than once a week or had physically demanding jobs

(Borges 1989). In another study (Rantanen et al. 1997),

the active category was made by individuals reporting at

least 1 or 2 h a week of moderate activity, but include

also individuals involved in strenuous activities several

times a week. Interestingly, OToole (1997 ) pointed out in

her editorial that it is intui tive to think that higher

activity levels should preserve strength better, but further

studies are required to see whether habitual low to

moderate intensity activity for just a few hours a week

can provide an adequate stimulus to maintain muscle

strength and function. The author reported the results of

Rantanen et al. (1997), who observed similar rates of

changes in strength over a period of 5 years in individuals

classiﬁed as active, who maintained their level of physical

activity, and those classiﬁed as sedentary and remained

sedentary. However, individuals who were sedentary

initially and became more active were able to slow down

their decline in strength, even though their level of

strength remained lower than that of active individuals.

Those who were active initially but for some reason

became sedentary, without changes in their health status,

incurred in the highest rate of strength decline.

Muscle power in older people

Muscle power, i.e. the rate of performing mechanical

work, is also signiﬁcantly reduced with advancing age

(see for example Bassey et al. 1992; Ferretti et al. 1994;

De Vito et al. 1998). In this review, muscle power will be

considered as that generated in a single explosive

movement, lasting a fraction of a second, where muscle

metabolism does not limit the performance. This is dif-

ferent from ‘‘sustained power’’, which describes the

ability to maintain a submaximal level of power output

in activit ies of longer duration such as cycling or running

(Sargeant 1994). Skelton et al. (1994) have reported that,

between the ages of 65 and 89 years, its decline occurs at

an even higher rate than isometric strength (3–4% per

annum as compared to 1–2% of the value for a 77-year-

old), thus revealing that power is more vulnerable than

strength to the ageing process. This loss of power has

been demonstrated to have severe functional conse-

quences (Bassey et al. 1992; Krebs et al. 1998; Suzuki

et al. 2001). Bassey et al. (1992) indicated that very old

adults who require the use of aids to perform functional

tasks, such as stair climbing, raising from a chair and

walking, had 42–54% less leg exten sor power than those

457

who could complete these tasks without assistance. Su-

zuki et al. (2001) have recently demonstrated a positive

association between muscle power of the ankle ﬂexors

and func tional limitations in community-dwelling older

women. Among a group of sedentary, community-

dwelling older women, aged between 70 and 95 years,

leg power was the strongest predi ctor of functional sta-

tus, as compared to other physiological parameters

(Foldvari et al. 2000). Rantanen and Avela (1997) have

suggested that diminished leg extensor power may be a

key predictor for mobility problems. Improved knowl-

edge on the mechanisms of power decline is therefore

crucial with regard to the development of eﬀective pre-

vention and treatme nt programmes for restoring

mobility and independence in older people.

As reported earlier in this review, power is the

product of force and speed at which mov ement occurs.

The amount of force produced during a muscle con-

traction varies with the velocity of shortening, as ﬁrst

demonstrated by Fenn and Marsh in 1935 and then

further investigated by Hill in 1938, who described a

‘‘characteristic equation’’ for the speed of shortening

under a load in isolated animal muscle. The nature of

the force/velocity relationship dictates that power will

have its own distinct relationships with force and

velocity of movement. Thus, there is an optimum force

and an optimum velocity at which maximum power is

developed. In isolated animal muscle, these correspond

to about 30% of P

and 25–30% of V

max

, respectively

strand and Rodahl 1986) . However, measurements of

force/velocity and power/velocity relationships in hu-

mans in vivo represent the resultant of relatively com-

plex situations, as human muscles are attached to bones

via tendons that cross over one or two articular joints in

order to produce a moment. Not only must the con-

tractile elements of muscle cells be considered, but also

factors such as neural inﬂuences, muscle architecture

and intercellular connective tissue. The optimal values of

force and speed for maximum power production in vivo

may therefore be diﬀerent from those recorded in iso-

lated muscles or skinned single ﬁbre preparations.

In the literature, investigations looking at muscle

power in older people are not as numerous as those

carried out on muscle strength. According to Earles

et al. (2000), this is because muscle power in single

explosive movements is much more diﬃcult to measur e

than muscle strength. Isokinetic dynamometers allow

power to be assessed in single muscle groups from the

torque measur ed whilst the limb rotates at constant

angular velocity, but do not reﬂect everyday activity

where older subjects have to work against resistance and

overcome speed (Harridge et al. 1999a). Moreover, the

maximum speed of isokinetic devices (about 300°/s) is

too low with respect to the maximum that can be

achieved during ‘‘unloaded’’ movements of human limbs

(estimated at 832°/s, as reported by Perrine 1986), which

limits the measure of power above these value s of speed.

Explosive power has been traditionally measured in

older people using a simple whole body movement, that

is a vertical jump on a force platform (Grassi et al. 1991;

Ferretti et al. 1994; De Vito et al. 1998). Force platforms

essentially operate like a scale for measuring weight.

Velocity of movement of the individuals centre of

gravity is calculated by integration of the acceleration,

which in turn is calculated by the vertical component of

the ground reaction force (VGRF) according to the

Netwons Second Law of Motion, and power is the

product of force (obtained by subtracting body weight

from VGRF) and velocity of movement (Davies and

Rennie 1968). This method presents tw o limitations: (1)

it may not be safe in very-old frail individuals; (2) older

people have to lift their body weight, which represents a

high percentage of their maximum strength, and there-

fore may be forced to work in a less favourable portion

of the force/velocity curve, thus performing the move-

ment at slower speed that is away from the optimal

speed for maximum power production. The ﬁrst prob-

lem has been overcome by Bassey and Short (1990), who

developed a piece of apparatus that measures the aver-

age power generated by the lower limb muscles during a

single leg extensor thrust agains t a pedal which, in turn,

accelerates a heavy ﬂywheel of a known inertia. This,

however, does not solve the second issue, in that both

young and older individuals have to overcome the same

ﬁxed inertia, with weak-old individuals being forced to

use a relatively greater proportion of their maximum

force-generating ability. Pearson et al. (2001) have

recently introduced a variable inertial testing system

mounted in the apparatus designed by Bassey and Short

(1990), which has the potential to overcome the limita-

tions due to the use of a single inertia. A safe device for

testing older individuals coul d also be the sledge-

ergometer designed by Antonutto et al. (1999), on which

individuals sitting on a carriage-seat push with both feet

on two force platfor ms. In this device diﬀerent inert ial

loads can be obtained by positioning the sledge at dif-

ferent angles with respect to the horizontal (Zamparo

et al. 2000). Power can also be tested at diﬀerent inertial

loads by using instrumented weight-stack machines,

where force is measured by a transducer or derived by

the mass of the stack, and velocity by an electrogoni-

ometer at the individuals joint (Harman 1995). How-

ever, the masses of plates in weight-stack machines have

been found both to be variable and to diﬀer from

labelled values, thus complicating the measurement

technique. All things considered, dynamic force testing,

which is performed by measuring the maximum weight

that can be lifted, is also a test of power, as the indi-

vidual is exerting a given force at a given speed that,

however, is not measured, thus not enabling the calcu-

lation of power. Thomas et al. (1996) have developed a

pneumatic system mounted on a double leg-press ma-

chine that also allows one to use the optimal resistance

for maximum power, expressed as a percentage of

maximum dynamic strength. To the authors knowledge,

there is only one study in which explosive power has

been compared between middle-aged and older men

after optimizing the load (Izquierdo et al. 1999), and

458

another investigation (Macaluso and De Vito 2003),

which has considered this issue in women. In both

studies, the maximum power of the lower limbs, which

was lower in the older individuals, was measured by

pushing a load corresponding to about 60% of maxi-

mum force. Onl y Macaluso and De Vito (2003), how-

ever, have focused their attention on the two

determinants of power output, i.e. optimal force and

optimal speed, and reported that the inferior ability to

generate power was due to a lesser ability to develop

both force and speed. Moreover, as optimal speed was

measured when optimal force was the same percentage

of maximum voluntary contraction in the two groups

(60%), thus standardizing for diﬀerences in optimal

force, the authors concluded that the older women

generated less power because they were slower than the

young even when using the same percentage of their

maximum strength. This ﬁnding is in contrast with the

recent results of the same authors, carried out in col-

laboration with some other investigators (Pearson et al.

2002), who compared a population of weightlifter ath-

letes with untrained healthy individuals by using a var-

iable inertial testing system (Pearson et al. 2001)

mounted in apparatus designed for measuring explosive

power in older people (Bassey and Short 1990). In their

study, weaker individuals achieved max imum power

output at lower inertial loads than stronger individuals,

but the speed component was found to be not signiﬁ-

cantly diﬀerent between the two groups. The discrep-

ancy between the results of these two studies could be

ascribed to the diﬀ erent study population and testing

methodology and deserves further investigation.

As power is the product of force and velocity, any-

thing that will aﬀect force production or speed of

shortening of a muscle will also aﬀect its power output.

Therefore, all of the factors that have been reviewed in

the previous paragraph to explain the loss of muscle

strength in older age can be transferred to power. In

addition, all of the factors that may aﬀect speed of

shortening must be taken into account. The selective

atrophy of type II ﬁbres with advancing age may partly

explain power losses, because the power output of type

II muscle ﬁbres is four times that of type I ﬁbres, as

reported in animal studies (Faulkner et al. 1986). The

speed at which a muscle ﬁbre shortens is determined by

the expression of the diﬀerent isoforms of the MHC, as

measured with a technique based on identiﬁcation of

MHC as molecular marker with gel electrophoresis

(Harridge et al. 1996). Type I, IIa and IIb ﬁbres of the

traditional classiﬁcation, based on ATPase sensitivity to

pH, express mainly MHC-I, MHC-IIA and MHC-IIX

isoforms, respectively. Fast ﬁbres have the potential to

generate higher power output, with greater forces and

higher speed of shortening than slow ﬁbres. The signif-

icant reduction in the size of type II ﬁbres, as described

earlier in this review (Lexell et al. 1988), would result in

a decrease in the proportion of the muscle that is

occupied by fast contracting MHC isoforms (Harridge

and Young 199 8). Moreover, it has been shown, by

using electrophoretic techniques, that ﬁbres in older

muscle co-express more than one MHC isoform (hybrid

ﬁbres) to a greater extent than ﬁbres in young muscle

(Klitgaard et al. 1990; Andersen et al. 199 9). This in-

creased presence of hybrid ﬁbres, which had not been

clearly identiﬁed by the earlier technique based on the

ATPase reaction, may indicate a shift towards a slower

older muscle (Harridge and Young 1998) and, as re-

ported earlier in this review, is further evidence of the

process of ongoing denerv ation and reinnervation of the

ageing muscle (Vandervoort 2002). The shift towards a

slower muscle with ageing has been conﬁrmed by studies

on the contractile mechanics of single ﬁbres, in which it

was shown that both MHC-I and MHC-IIA single ﬁbres

have lower speciﬁc tension and maximum shortening

velocity if they originate from the muscles of an older

person as opposed to a young person (Larsson et al.

1997; Frontera et al. 2000b). Moreover, Ho

k et al.

(2001) recently reported an age-related slowing in the

actin sliding speed on myosin by studying an in vitro

motility assay, in which myosin is extracted and immo-

bilized from a 2- to 4-mm single muscle ﬁbre segment, in

order to focus on actomyosin interactions without

interference from cytoskeletal or regulatory proteins. As

previously pointed out, measurements taken from skin-

ned single ﬁbre preparations or in vitro motility assays

have the advantage that they allow the researcher to

study directly the contractile elements of muscle cells,

thus ruling out the confounding eﬀects of factors such as

neural inﬂuences, muscle architecture, and intercellular

connective tissue (Frontera et al. 2000b). However, these

factors should be taken into account when examining

the speed of shortening of the whole body segments in

vivo and the exact interpretation of these phenomena is

still unknown. Metter et al. (1997) speculated that nor-

mal ageing changes in the basal ganglia, consisting of a

continuing loss of dopaminergic neurons in the sub-

stantia nigra (Morgan et al. 1994), could contribute to

the observed slowing in speed, coord ination and power,

together with peripheral changes such as slowing in

nerve conduction velocities (Norris et al. 1953). Another

potential cause of lower power production could be an

age-related decrease in tendon stiﬀness, which has been

brieﬂy reviewed in a previous paragraph.

The effect of resistance training on muscle strength,

power and selected functional abilities in older

individuals

Strength

At the end of the 1980s, Frontera et al. (1988) reported

that a heavy-resistance training programme led to an

increase in strength of the quadriceps muscles of older

men aged between 60 and 72 years, which was accom-

panied by an increase in muscle ﬁbre size. Since then, a

growing number of studies each year continue to docu-

ment the beneﬁts of resistance training in older people,

459

even in individuals over 90 yea rs of age. Resistance-

training programmes are based on the application of the

overload principle, which states that muscles worked

close to their force-generating capacity will increase in

strength (McArdle et al. 1996). The term exercise session

refers to the block of time devoted to the training. The

number of training sessions completed per week is

termed frequency. The basic unit of a resistance training

session is the repetition which, for a given training

movement, is the completion of a whole cycle from the

starting position, through the end of the movement and

back to the start. When a series of repetitions is com-

pleted this is termed a set. Volume is typically calculated

as the product of repetitions and sets (Fry and Newton

2002). Intensity refers to the relative load or resistance

that the muscle is exercising against, usually expressed as

a percentage of the maximum weight that could be lifted

once, i.e. 1 repetition maximum (1-RM). In the older

population, these programmes have a similar structure

to those undertaken by younger people, for example

they can have a duration of 12 weeks, with a frequency

of 3 times per week, where subjects perform 3–4 sets of 8

repetitions at an intensity equal to 80% of 1-RM. To

elicit gains in muscle strength, the loads must be pro-

gressively increased so that the relative intensity remains

high enough to provide an adequate overload through-

out the whole duration of the training programme. The

results of some of the stud ies that have examined the

eﬀects of heavy-resistance training on muscle strength

and size on the most frequently studied muscle, the

quadriceps group, are summarized in Table 1.

One of the main problems of between-study com-

parisons is the variation in the population of subjects.

Studies have been performed on males, females and

mixed populations. The mean age of the participants has

also varied greatly, with older people ranging from those

in their sixties (Ha

kkinen et al. 1998b) to those in their

nineties (Fiatarone et al. 1990; Harridge et al. 1999b).

The presence of med ical conditions and the baseline

ﬁtness of participants are among other variables that

have to be taken into account (Greig et al. 1994).

Additional factors that make the comparison between

studies problematic are the duration and frequency of

the training programme, the number of sets and repeti-

tions of each session, and ﬁnally the intensity at which

these repetitions are performed. Resistance training

programmes in older people varied considerably in

length from just 4 weeks (Sherrington and Lord 1997) to

84 weeks (McCartney et al. 1996). On average, however,

most resistance training studies lasted 8–12 weeks

(Frontera et al. 1988; Fiatarone et al. 1990; Charette

et al. 1991; Gr imby et al. 1992; Lexell et al. 1995;

kkinen et al. 1998b; Harridge et al. 1999b; Hunter

et al. 1999; Tracy et al. 1999; Hortoba

gyi et al. 2001).

Most studies involved subjects performing three training

sessions per week, but the number could be as great as

seven per week (Sherrington and Lord 1997). Recently,

Taaﬀe et al. (1999) have demonstrated that strength

gains can be obtained even with a training frequency of

once per week. The number of sets was three in most of

the studies, but could vary up to six (Charette et al. 1991;

kkinen et al. 1998b; Ha

kkinen et al. 2001). The

number of repetitions also changed from study to study,

with the most common number of repetitions being

eight, although it ranged between 3 (Ha

kkinen et al.

1998b, 1998c) and 18 (Ha

kkinen et al. 2001). The

intensity of heavy-resi stance training was around 80% in

most studies and there have also been few studies com-

paring low- versus high-intensity resistance training

(Pruitt et al. 1995; Hortoba

gyi et al. 2001; Fielding et al.

2002). Strength changes have been quite variab le across

studies, which is likely a reﬂection of the key design

factors that ha ve just been reviewed. Moreover, vari-

ability in the results depends on the modalities used to

test strength (isometric vs. 1-RM), which are deﬁned at

the beginning of this review. It is important to remark

that even very old people can beneﬁt from progressive

resistance training (Fiatarone et al. 1990; Harridge et al.

1999b). Lexell (2000) pointed out that the lower the

initial levels of strength, such as those in individuals in

their nineties, the higher the magnitude of the percentage

increase with res pect to the baseline.

Various factors can contribute to strengt h gains fol-

lowing heavy-resistance training in both young and

older subjects. These include, in the ﬁrst phase of

training (about 1–2 weeks), a rapid improvement in the

ability to perform a training exercise, such as lifting

weights, which is mainly the result of a learning eﬀect.

The learning eﬀect, which is mediated by changes in

motor skill coordination and level of motivation, can be

substantial especially when the test adopted to evaluate

muscle strength req uires high levels of skill (Jones and

Round 1990). In the second phase, which lasts

3–4 weeks, muscle strength gains are obtained without a

matching increase in size of the trained muscles. The

improvement in this phase has been mainly attributed to

neural adaptations (Moritani and de Vries 1980; Sale

1988; Ha

kkinen et al. 2001). The term neural adapta-

tions includes many elements such as an increased acti-

vation of prime mover muscles (number of recruited

MUs or ﬁring rate and synchronization of the individual

MUs), a better coordination of synergistic and antago-

nist muscles, and an increased neural drive from the

highest levels of the central nervous system (Sale 1988).

There are, however, few studies investigating changes in

neural properties following strength training in aged

humans (Rice 2000). Some investigations have shown

signiﬁcant improvements in agonist sEMG with con-

current reductions in the activity of antagonist muscles

(Ha

kkinen et al. 1998b, 1998c). An increase in M-wave

potentials following strength training has been measured

in older individuals by Hicks et al. (1992), thus sug-

gesting a training-induced increase of muscle membrane

excitability. However, Scaglioni et al. (2002) have

recently shown that the modulation of neuromuscular

excitability, expressed as the ratio between the maximum

Hoﬀman reﬂex and the maximum M-wave, did not

change in older male adults following 16 weeks of

460

Table 1 Eﬀect of resistance training on muscle strength and size of the quadriceps muscle in older individuals. (CSA Cross-sectional area,CT computerized tomography, F female,

GWBE general weight bearing exercises, Isok isokinetic, KE knee extension, LP leg press, M male, MRI magnetic resonance imaging, MVC maximal voluntary contraction, NS non-

signiﬁcant, 1RM one repetition of maximum weight that could be lifted, 5RM ﬁve repetitions of maximum weight that could be lifted, Ultr ultrasonography, vol volume)

Authors Subjects Training programme % Change

Age Gender N Exercise

movement

Duration

(weeks)

Sessions

per week

Sets Repetitions % of 1RM Strength CSA Fibre size increase

1RM MVC Type I Type II

Frontera et al. 1988 60–72 M 12 KE 12 3 3 8 80 107 17 CT: 9 34 28

Fiatarone et al. 1990 86–96 M/F 10 KE 8 3 3 8 80 174 – CT: 11 – –

Charette et al. 1991 64–86 F 13 LP, KE 12 3 6 6 75 28–93 – – 7 NS 20

Grimby et al. 1992 78–84 M 9 KE 8–12 3 3 8 Isok 10 at 30°s

CT: 3 8 NS 5 NS

Pyka et al. 1994 61–78 M/F 25 LP, KE 52 3 3 8 75 53–95 – – 59* 67*

Lexell et al. 1995 70–77 M/F 23 KE 11 3 3 6 85 163 – - -4 NS -8 NS

McCartney et al. 1996 60–80 M/F 113 LP 84 2 3 12 80 32 – CT: 9 – –

Sherrington and Lord 1997 64–94 M/F 21 GWBE 4 7 – – – – 22 – – –

kkinen et al. 1998b 61 M 10 KE 10 3 3–6 3–15RM – – 17 MRI: 9 23 27

kkinen et al. 1998c 70 M/F 20 KE 26 2 3–6 3–15 50–80 26 – Ultr: 6 (F) – –

Harridge et al. 1999b 85–97 M/F 11 KE 12 3 3 8 80 134 37 MRI: 10 – –

Taaﬀe et al. 1999 65–79 M/F 46 LP, KE 24 1 3 8 80 23–71 – – – –

Tracy et al. 1999 65–75 M//F 23 KE 9 3 3 5–10RM – 28 – MRI(vol): 12 –

Hunter et al. 1999 64–79 M/F 11 KE 12 3 3 8RM – 39 – – )4NS )2NS

Hortoba

gyi et al. 2001 66–83 M/F 27 LP 10 3 5 4–12 40–80 35 26 – – –

kkinen et al. 2001 71 M/F 21 LP 26 2 3–6 10–18 70–80 26 26 – 32 (F) 32 (F)

After 30 weeks

461

resistance training of plantar ﬂexor muscles. A 30%

increase in maximal MU ﬁring rate has been measured

in the tibialis anterior muscle of six older individuals in

their seventies following 2 weeks of train ing (Patten and

Kamen 2000). However, the same authors (Patten et al.

2001) reported a bimodal response of maximal MU

discharge rate of the adductor digiti minimi to 6 weeks

of isometric training – an initial increase followed by a

return towards baseline – which is surprising and diﬃ-

cult to explain. The third phase of adaptation to strength

training (>6 weeks) is characterized by an increase in

both the size and strength of the exercised muscles.

Muscle size has been measured before and after training

using various non-invasive techniques, such as ultra so-

nography (Ha

kkinen et al. 1998c), CT (Frontera et al.

1988; Fiatarone et al. 1990; Grimby et al. 1992; Mc-

Cartney et al. 1996) or MRI (Ha

kkinen et al. 1998b;

Harridge et al. 1999b). Notable is the recent study of

Tracy et al. (1999), who measured a 12% increase in

quadriceps muscle volume, by MRI, following 9 weeks

of resistance training in both groups of 65- to 75-year-

old men and women. Both type I and type II ﬁbres retain

their capacity for hypertrophy in response to resistance

training (Frontera et al. 1988; Brown et al. 1990; Cha-

rette et al. 1991; Pyka et al. 1994; Ha

kkinen et al. 1998b,

2001), although some studies demonstrated little or no

change (Grimby et al. 1992; Lexell et al. 1995; Hunter

et al. 1999). Ha

kkinen et al. (1998b) have reported a type

MHC II subtype transformation going from type MHC

IIb to IIab to IIa in older men, similar to previous

training studies in young individuals (Adams et al. 1993;

Harridge et al. 1998). In a further study (Sharman et al.

2001), the same result was also found in a group of 65-

year-old women following 24 weeks of heavy resistance

training. Williamson et al. (2000) have measured a sig-

niﬁcant increase in the expression of MHC I as a result

of 12 weeks of low-intensity resistance training, thus

indicating that higher-threshold MUs may not have

been recruited during the programme. Recent investi-

gations on single ﬁbres indicated that 12 weeks of pro-

gressive resistance training increased muscle cell size,

strength and peak power in both older men (Trappe

et al. 2000) and women (Trappe et al. 2001). Interest-

ingly, in contrast with the older men, no change in ﬁbre

unloaded shortening velocity or peak power normalized

to cell size was observed in older women, thus suggesting

that men and women respond diﬀerently, at the cell le-

vel, to the same resistance-training stimulus. The

mechanisms of this phenomenon are still unknown.

Some studies (Yarasheski et al. 1995; Welle et al. 1999)

have also given evidence that resistance training leads to

an increase in protein synthesis, accompanied by sig-

niﬁcant improvements in muscle strength. Less is known

about the long-term eﬀects of resistance training in older

people, as most of the available studies did not continue

after 12–24 weeks. Following a 1-year exercise trial

involving 25 individuals aged between 61 and 78, Pyka

et al. (1994) measured increases in strength ranging from

30 to 97% over the ﬁrst 3 months, which then

maintained a plateau in the remaining months of the

experiment. Similarly, Morganti et al. (1995) showed that

a 1-year training programme with a frequency of twice a

week resulted in strength gains of various muscle groups,

ranging from 4% to 74% in a population of 39 healthy

postmenopausal women, with the greatest gains seen in

the ﬁrst 3 months of training. McCartney et al. (1996)

reported instead progressive strength gains and moderate

muscle hypertrophy in an older group that continued to

participate in a resistance-training programme for

2 years. Rapid detraining will result if programmes are

interrupted, but the initial gains can be maintained with a

reduced exercise frequency of even once per week (Lexell

et al. 1995; Taaﬀe et al. 1999; Trappe et al. 2002).

Although several investigations have shown that the

capacity to impr ove muscle strength is not impaired with

increasing age (see the studies of Table 1), few investi-

gations have made a direct comparison of the magnitude

of the responses in older and young individuals to the

same training programme. Jozsi et al. (1999) have shown

that 60-ye ar-old individuals can improve strength and

power in response to 12 weeks of resistance training,

with the same magnitude as that of individuals in their

twenties. Welle et al. (1996b) found that the increase in

speciﬁc tension following 3 months of resistance training

in young (22–31 years) and older (62–72 years) individ-

uals was similar for elbow ﬂexion (about 20%) and knee

extension (about 35%), but was more than double in the

older subjects for the knee ﬂexors. Larsson (1982), on

the contrary, had previously shown that the average

increase in isokinetic strength of the knee extensors

following 15 weeks of training at low resistance and high

repetition tended to be higher in a group of 56- to 65-

year-old males (7.5%) as compared to the 20- 39-year-

old group (2.9%). In contrast, in the study of Macaluso

et al. (2000), after 6 weeks of isometric training the force

gain of the bicep s brachii muscle followed the same

temporal pattern in the two groups of young and older

women, in that after 4 weeks the maximum voluntary

contraction reached a plateau, but the absolute force

increments were diﬀerent, 22.4% in the young and

13.4% in the older women.

Power

In recent times, greater attention has been focused on

the need to design exercise strategies in order to increase

muscle power (Earles et al. 2000; Evans 2000; Fielding

et al. 2002). ‘‘The preservation of muscle power into late

life can greatly decrease the risk of disability and en-

hance functional independence’’ (Evans 2000). As

pointed out by Earles et al. (2000), it is important to

determine whether high-velocity training is comparable

or superior to low-velocity high-resistance programmes

in order to improve function and quality of life in older

individuals. Table 2 summarizes the results of the

training studies in which, to the authors knowledge,

explosive power has been measured. The subjects age

462

and gender, training mode and testing methodology are

included.

To date there is only one published study, to the

authors knowledge, which was speciﬁcally designed to

increase power in older people (Fielding et al. 2002).

Older women in their seventies with mild functional

limitations were randomized into one of two groups:

high-velocity (HI) and low-velocity (LO), in which

absolute training force and total work performed were

similar between groups, but power output was signiﬁ-

cantly higher in HI, since individuals were asked to

perform each repetition as fast as possible. HI and LO

have improved leg-press power output by 97% and

45%, respectively, over 16 weeks of training. This

training programme was designed starting from the

assumption that muscle-strengthening exercises may not

always produce an optimum increase in power. The

authors pointed out that traditional low-velocity resis-

tance training resulted in a small but signiﬁcant increase

in muscle power ranging from 18% to 25% (Fiatarone

et al. 1994; Skelton et al. 1995; Jozsi et al. 1999) despite

much larger increases in muscle strength. Fiatarone et al.

(1994) showed that in frail very old nursing-home resi-

dents progressive resistance exercise over a 10-week

period produced 113% and 28% increases in muscle

strength (kne e-extension 1-RM) and power output,

respectively, with power being assess ed during a stair-

climbing test (Bassey et al. 1992). Similarly, in 20 heal-

thy, independent, very old women, Skelton et al. (1995)

observed a 27% increase in knee-extension isometric

strength and an 18% increase in leg-extension power

standardized for body weight, which was measured

using the Nottingham Power Rig (Bassey and Short

1990). Jozsi et al. (1999) found that 12 weeks of

progressive resistance-training in men and women in

their sixties resulted in an increase in the strength of knee

extension and arm pull movements of 30% and 18%,

respectively, versus an increase of 26% and 10% in

power, with power being measured with a pneumatic

resistance machine (Thomas et al. 1996). Also Frontera

et al. (1988), in their notable study that has been cited in

the previous paragraph on the eﬀects of resistance

training on strength, examined the eﬀect of resistance

training on quadriceps power, wi th power being mea-

sured with an isokinetic dynamometer, but did not see

any signiﬁcant change. It can be speculated that the ef-

fect of training on muscle power may have been seen if

power had been measured with the same dynamometer

used to carry out the training programme. Even a low-

intensity general conditioning programme has been

successful in obtaining a 24% increase in peak pow er

during a vertical jump on a force platform in 20 healthy

older women in their sixties (De Vito et al. 1999), but

higher improvements may ha ve been observed if the

programme had been more speciﬁc. Earles et al. (2000)

have therefore pointed out the importance of performing

movements at high velocity during resistance training in

order to increase power output. Forty-three volunteers

over the age of 70 years were randomized into one of

two groups: the power group, in which individuals

participated in high-velocity resistance exercise 3 times a

week, and the walking group, in which individuals per-

formed moderate intensity exercise 30 min daily, 6 days

per week. Leg-press power and maximal knee extensor

strength substantially increased in the ﬁrst group, but

not in the second. Similarly, Izquierdo et al. (2001) re-

ported that in middle-aged and older men a prolonged

total strength training programme, which included high

Table 2 Eﬀect of resistance training on muscle power of various

muscle groups in older individuals. (AP arm pull, BP bench press,

F female, HE hip extension, HI high velocity, HS half-squat, KE knee

extension, LO low velocity, LP leg press, M male, NS non-signiﬁcant,

PRM pneumatic resistance machine, 1RM one repetition of maxi-

mum weight that could be lifted, SC stair climbing, VJ vertical jump)

Authors Subjects Training programme Testing

movement

Power gain Measurement

apparatus

Age Gender N Exercise

movement

Duration

(weeks)

Sessions

per week

Sets Repe-

titions

%of

1RM

Frontera

et al. 1988

60–72 M 12 KE 12 3 3 8 80 KE None Isokinetic

dynamometer

Fiatarone

et al. 1994

72–98 M/F 100 KE, HE 10 3 3 8 80 SC 28% Stair-climbing

Skelton

et al. 1995

76–93 M/F 20 Elastic tubing

or rice bags

12 3 3 4–8 – LP 18% (NS) Nottingham Rig

De Vito

et al. 1999

60–70 F 11 Low-intensity

general

conditioning

12 3 – – – VJ 24% Force platform

Jozsi

et al. 1999

56–66 M/F 17 KE, AP 12 2 3 8–12 80 KE, AP 10–26% PRM

Izquierdo

et al. 2001

64 (2) M 11 KE, HS, BP 16 2 3–4 8–15 50–80 KE, HS,

21–37% Instrumented

weight-stack

machines

Earles

et al. 2000

77 (5) M/F 18 LP 12 3 3 10 50–70 LP 22% PRM

Fielding

et al. 2002

73 (1) F 30 LP 16 3 3 8–10 70 LP HI: 97%

LO: 45%

PRM

463

velocity movements, led to gains in maximal strength

and power of the upper and lower extremity muscles,

with the improvements being limited in magnitude pos-

sibly due to neuromuscular or age-related endocrine

impairments.

As reported in the paragraph entitled ‘‘muscle power

in older people’’, there is an optimum value of force and

velocity at which maximum power is generated. De Vito

et al. (1999) showed that the training-induced increase in

peak power output, which was measured by performing

a vertical jump on a force platform, was due to an in-

crease in both optimal force (18%) and optimal velocity

(13%) at which maximum power was measured. How-

ever, as argued earlier in this review, explosive power

output on a force platform was assessed using a ﬁxed

inertia, which is the subjects body weight and corre-

sponds to a high percentage of their maximum, thus not

representing the optimal value of force for maximu m

power production. Other authors have overcome this

problem by measuring power output using diﬀerent

loads before and after training (Jozsi et al. 1999; Earles

et al. 2000; Izquierdo et al. 2001; Fielding et al. 2002),

but do not seem to have focused much of their at tention

on whether the training-induced changes in maximum

power were due to an increase of optimal force, optimal

velocity or both. Most of the investigators (Jozsi et al.

1999; Izquierdo et al. 2001; Fielding et al. 2002) showed

how power output varied as a function of diﬀerent

loads, expressed as a percentage of 1-RM, thereby

identifying the load at which maximum power was

measured, but they did not discuss the role of velocity of

movement in maximum power generation. In the work

of Earles et al. (2000), both power output and velocity of

movement were plotted in two diﬀerent graphs against

the resistance used, expressed as a percentage of body

mass. However, also these authors did not discuss the

relative role of both optimal force and velocity, but

simply commented that minimal improvements in power

were measured at low resistance, whilst large improve-

ments occurred at higher resistance, in agreement with

the principle of speciﬁcity of training (McCaﬀerty and

Horvath 1977). From a closer analysis of the graphical

results of Earles et al. (2000), it is clear that maximum

power was measured before and after training at 30%

and 50% of body mass, respectively, with the optimal

speed at maximum power being decreased after train ing,

although this was not remarked upon by the authors.

Improvements in power can therefore be interpreted

with the fact that individuals were able to push higher

loads despite a decrease in optimal speed of movement.

Possible mechanisms underlying the improvement in

peak power may include speciﬁc increa ses in the CSA of

type II musc le ﬁbres and increases in speciﬁc force and

shortening velocity of single muscle ﬁbres, as speculated

by Fielding et al. (2002). Notably, Van Cutsem et al.

(1998) have shown that changes in single MU behaviour

contribute to the increase in contraction speed after

dynamic training in humans, which could justify train-

ing-induced changes in power. These changes in MU

behaviour include earlier MU activation, extra doublets,

i.e. brief (2–5 ms) MU interspike intervals, and en-

hanced maximal ﬁring rate. Also Izquierdo et al. (2001)

have speculated that power increases could be due to

training-induced changes in the neural component,

which they vaguely refer to as ‘‘voluntary or reﬂex/in-

duced rapid neural activation of MUs’’. Indeed, the

various factors included under the term ‘‘neural adap-

tations’’ that have been presented in the previous para-

graph to explain gains in strength following heavy-

resistance training, i.e. increased activation of the prime

mover muscles, better coordination of synergistic and

antagonist muscles and increased neural drive from the

highest levels of the central nervous system, can also

explain improvements in power. However, studying

these mechanisms during dynamic contractions by

sEMG or other electrophysiological techniques could be

problematic, as various mechanical, physiological, ana-

tomical and electrical modiﬁcations occur throughout

the contraction that aﬀect, in substantial ways, the

relationship between signal amplitude and muscle force

(De Luca 1997). Also the slowing in nerve conduction

velocities due to ageing (Norris et al. 1953) may be a

mechanism to be recovered following power training.

However, Scaglioni et al. (2002) have recently shown

that the conduction velocity of the posterior tibial nerve

did not change in older male adults foll owing 16 weeks

of resistance training of plantar ﬂexor muscles, thus

suggesting that decreased nerve conduction velocity may

be due to degenerative phenomena rather than disuse.

Moreover, if decreased tendon stiﬀness, as recently

shown in vivo on human individuals by Maganaris

(2001), is another potential cause for decreased power

with ageing, an appropriate training intervention may

also aﬀect this mechanism. Re cent results of Reeves

et al. (2003a, 2003b) showed that the pa tella-tendon

stiﬀness increased by 65% in 74-year-old men following

14 weeks of resistance training, which was accompanied

by a 14–23% increa se in dynamic strength during leg

extension and leg press. In contrast, in a population of

older women, it has been shown that 6 months of low-

load resistance training produced no signiﬁcant changes

in stiﬀness of the tendon-aponeurosis structures of the

vastus lateralis muscle, but an increase in elasticity

(Kubo et al. 2003).

Selected functional abilities

Functional ability can been described as an individuals

competence in performing everyday physical tasks, like

rising from a chair, climbing stairs or lifting shopping

bags (Harridge et al. 1999a). Although, as reported in

the previous paragraph on muscle power, it has been

shown that muscle strength and pow er correlate with the

ability to perform functional tasks (Bassey et al. 1992;

Skelton et al. 1994; Rantanen and Avela 1997; Fold vari

et al. 2000; Suzuki et al. 2001), this is an area of research

that remains unclear. Some authors (Skelton et al. 1994;

464

Buchner et al. 1996; and Levy et al. 1994, cited in Har-

ridge et al. 1999a) have attempted to identify functional

‘‘threshold’’ values of muscle performance below which

older people lose their ability to perform basic daily

tasks. Skelton et al. (1994), who measured the greatest

height of step that could be mo unted without using the

hands in healthy men and women aged from 65 to

89 years, found a signiﬁcant correlation between step

height and lower limb extensor power, but failed to

identify a universally applicable threshold value, even

after adjusting for limb length. In contrast, Levy et al.

(1994, cited in Harridge et al. 1999a) reported that uni-

lateral power/weight ratios of 1.5 and 2.5 W/kg could be

considered as threshold values for mounting a 30- and

50-cm step, respectivel y. Buchner et al. (1996), who used

a slightly diﬀerent model based on an inverse transfor-

mation of strength, found a signiﬁcant correlation be-

tween lower limb strength and self-chosen walking

speed. However, they also concl uded that a ‘‘universal

threshold’’ might not exist due to a compensation for

deﬁciencies in strength by using reserve capac ity in other

determinants of walking speed. Regardless of the diﬃ-

culties in identifying a precise value, this ‘‘threshold’’

concept makes it easy to recognize that small changes in

physiological capability can have large eﬀects on the

functional ability of a frail person, but little or no eﬀect

in a more robust person.

There are few studies that have investigated the ef-

fects of training protocols on selected functional abilities

in older people (Table 3). It should be noted that in most

of these studies (Judge et al. 1993; Fiatarone et al. 1994;

Hunter et al. 1995; Skelton et al. 1995; Skelton and

McLaughlin 1996; Rooks et al. 1997; Sherrington and

Lord 1997) the age range of participants is widespread,

with individuals in their nineties and in their sixties or

seventies being included in the same group. As these

investigations, with the exception of Fiatarone et al.

(1994), were carried out on a relatively small number of

individuals, the interpretation of results might have been

limited by pulling together subjects within a wide age

range, since in the very old individuals small changes in

physiological capability are likely to have larger eﬀects

on functional ability than in relatively younger subjects.

Health and functional status also vary considerably be-

tween studies, with the participants ranging from frail-

institutionalized (Fiatarone et al. 1990, 1994) to rela-

tively independent (Judge et al. 1993; Skelton and

McLaughlin 1996) or healthy active (Earles et al. 2000;

kkinen et al. 2000), thus limiting inter-study com-

parisons. Regardless of these limitations, the relation-

ship between resistance training and the ability to

perform functional tasks remains unclear. As reported in

Table 3, the eﬀects of resistance training on selected

functional abilities vary from no change (Earles et al.

2000) to a 48% gain (Fiatarone et al. 1990), with most of

the studies reporting improvements in only some of the

functional abilities investigated (Skelton et al. 1995;

Rooks et al. 1997; Sherrington and Lord 1997; Taaﬀe

et al. 1999). Skelton and McLaughlin (1996) have

concluded that improvements in functional abilities may

be a carried-over eﬀect of strength and power training,

but are more likely to occur if the functional task is also

practised. More recently, Earles et al. (2000) have also

pointed out that power training per se may not neces-

sarily improve functional task performance. Following a

12-week high-velocity training programme, 34 individ-

uals of both genders in their late seventies increased their

muscle strength and peak powe r of lower limb muscles

by 22%, but did not improve functional task perfor-

mance. Rooks et al. (1997), on the contrary, noted a

20% improvement in stair clim b time after 10 months of

moderate-resistance high-velocity training, but similar

changes were also observed in a group of individuals

performing a walk ing programme.

Three selected functional abilities appear to be the

most frequently used in assessing the eﬀects of inter-

vention programmes: chair raising, stair climbing and

maximum or self-sel ected walking speed. Chair raising is

usually assessed as the time taken by older individuals to

rise from a standard chair (seat height 43 cm) with their

arms folded (Fiatarone et al. 1990; Skelton et al. 1995;

Skelton and McLaughlin 1996; Taaﬀe et al. 1999; Earles

et al. 2000). Stair climbing normally consists of

ascending and often descending a staircase without

stopping at a comfortable pace, with or without using

the handrail (Fiatarone et al. 1994; Skelton et al. 1995;

Skelton and McLaughlin 1996; Rooks et al. 1997). The

time to complete the task is timed. Maximum walking

speed is measured by asking the subjects to walk as fast

as possible over a distance, which is usually around 6 m,

and recording the time taken to cover that distance

(Hunter et al. 1995; Skelton and McLaughlin 1996).

Other authors have measured the so-called usual or

habitual velocity, by instructing the subjects to walk as

they normally do (Fiatarone et al. 1990, 1994; Judge

et al. 1993; Skelton et al. 1995; Sherrington and Lord

1997). In some cases, subjects were asked to walk placing

the heel of one foot directly in front of the toe of the

other with the shoe touching, which is referred to as

tandem walk (Fiatarone et al. 1990), or walking back-

ward using the same pattern (Taaﬀe et al. 1999). It is

reasonable to expect that the choi ce of discriminatory

tests of functional abilities in a population of older

individuals who are healthy and active could be more

problematic than in a group of frail indivi duals.

Macaluso et al. (2003) have been the ﬁrst authors to

adopt cycling as a novel approach to perform three

diﬀerent regimes of resistance training (one performed at

a light intensity with a high speed of movement, another

performed at a heavy intensity with a slower speed of

movement, a third based on a combination of both) in a

population of healthy older women aged 65 to 74 years.

All of the participants improved their maximum walking

speed and other functional abilities (box-stepping and

vertical jumping), with a parallel increase in muscle

strength and power, regardless of the training regime

adopted (Macaluso et al. 2003). These are the novel

ﬁndings which deserve to be remarked upon: (1) even ﬁt

465

Table 3 Eﬀect of resistance training on selected functional abilities in older individuals. (AD ankle dorsiﬂexion, BL bag lifting, BP bench press, BS box stepping, BT time of getting in

and out of a bath, CB carrying box, CR chair raise time, EE elbow extension, EF elbow ﬂexion, F female, FR functional reach, FRS ﬂoor rising, GWBE general weight bearing exercises,

HA hip abduction, HE hip extension, HF hip ﬂexion, HR heart rate during stair climbing or corridor walk, iEMG integrated electromyogram, KE knee extension, KF knee ﬂexion, KR

kneel rise time, LP leg press, M male, MWV maximum walking velocity, PCE postural control exercises, PF ankle plantarﬂexion, 1RM one repetition of maximum weight that could be

lifted, S supervised, SC self-paced stair climbing speed, sp self-paced, SR self-paced step rate, SU step-up, SUT sit-up for the trunk ﬂexors, TUG timed up and go, U unsupervised, WR

self-paced walking rate)

Authors Subjects Training programme Functional ability

investigated

Gain %

Age Gender N Health and

functional

status

Practice of

functional

task

Exercise

movement

Duration

(weeks)

Sessions

Per wk

Sets Reps % of

1RM

Fiatarone

et al. 1990

86–96 M/F 10 Frail institutionalized NO KE 8 3 3 8 80 CR, MWV, tandem

gait speed

Judge

et al. 1993

71–97 M/F 31 Relatively healthy NO KE, HA, AD, HE,

KF, PCE

12 3 3 8–12 75–80 MWV, usual gait

velocity

8°

Fiatarone

et al. 1994

72–98 M/F 100 Frail institutionalized NO KE, HE 10 3 3 8 80 Habitual gait

velocity

Hunter

et al. 1995

60–77 F 14 Healthy independent NO LP, BP, EF, EE,

KE, HA

16 3 2 12RM – MWV, iEMG during

CR, CB

18 §

Skelton

et al. 1995

76–93 F 20 Healthy independent NO Elastic tubing,

rice bags

12 3 3 4–8 – FR, SC, WR, SR, HR,

BL, CR, FRS, KR, BS

Only KR,

Skelton and

McLaughlin

1996

74–89 F 18 Relatively independent YES Elastic tubing,

tin cans, sponge

balls

8 1S-2U 1–3 4–8 – FR, CR, TUG, MWV,

SR, FRS, SC, BT

11–22 in

only ﬁve

Sherrington

and Lord 1997

64–94 M/F 21 Indep. after hip fracture NO GWBE 4 7 – – – FR, habitual walking

velocity

n.r.#

Rooks

et al. 1997

65–95 M/F 37 Independent community

dwelling

YES* HE, KE, AD, PF,

43 3 3 8–15 sp SC time; pen pick-up

task

20; -24

Taaﬀe

et al. 1999

65–79 M/F 46 Healthy NO LP, KE 24 1 3 8 80 CR, backward tandem

walk

kkinen

et al. 2000

62–77 M/F 10 Healthy active NO LP, KE, BP, KF,

EF, SU

24 2 3–5 8–12 50–80 MWV 13

Earles

et al. 2000

77±5 M/F 18 Highly functioning YES

LP, HF, SU, CR,

12 3 3 10 50–70 CR; 8-foot walk;

6-min walk

None

Tandem gait speed only;

usual only;

decreased iEMG in CR and CB by36% and 40%, respectively;

SC only;

CR only; n.r.# habitual walking velocity only; percentage increase not reported

466

older women still have a margin for improvement in

functional abilities; (2) the training-induced increase in

power output was due to a combined increase in both of

the two determinants of power, optimal force and

optimal velocity; (3) the lack of diﬀerence between the

three training regimes seems to suggest that it is not the

intensity nor the speed of movement, but the level of

mechanical work, with this being similar in the three

groups, which represents the training stimulus.

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