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Central and Peripheral Fatigue in Sustained Maximum Voluntary Contractions of Human Quadriceps Muscle



1. The fatigue of force that occurs during the first 60 s of a maximum voluntary contraction of the human quadriceps has been examined by comparing the voluntary force with that obtained by brief tetanic stimulation at 50 Hz in nine healthy subjects. In three subjects the voluntary force declined in parallel with the tetanic force whereas in the remainder it fell more rapidly, suggesting that central fatigue was present. 2. For those subjects who showed little or no central fatigue, surface electromyograph (EMG) activity remained approximately constant while the force declined by about 60%. In the others, EMG activity and force declined in parallel but when an extra effort was made the subjects could briefly increase their force and this was accompanied by a proportionately greater increase in EMG activity (generally up to the original value). 3. It is concluded that in sustained maximum voluntary contractions of the quadriceps (a) central fatigue may account for an appreciable proportion of the force loss, (b) surface EMG recordings provide no evidence that neuromuscular junction failure is the limiting factor determining the loss of force in this muscle.
Clinical Science and Molecular Medicine
(1978) 54,609-614
Central and peripheral fatigue in sustained maximum
voluntary contractions of human quadriceps muscle
Lewis Muscle Research
Postgraduate Medical
(Received 9
accepted 4 January
The fatigue of force that occurs during the
first 60 s of a maximum voluntary contraction of
the human quadriceps has been examined by com-
paring the voluntary force with that obtained by
brief tetanic stimulation at SO Hz in nine healthy
subjects. In three subjects the voluntary force
declined in parallel with the tetanic force whereas in
the remainder it fell more rapidly, suggesting that
central fatigue was present.
For those subjects who showed little or no
central fatigue, surface electromyograph (EMG)
activity remained approximately constant while the
force declined by about 60%. In the others, EMG
activity and force declined in parallel but when an
extra effort was made the subjects could briefly
increase their force and this was accompanied by a
proportionately greater increase in EMG activity
(generally up to the original value).
It is concluded that in sustained maximum
voluntary contractions of the quadriceps (a) central
fatigue may account for an appreciable proportion
of the force loss, (b) surface EMG recordings pro-
vide no evidence that neuromuscular junction
failure is the limiting factor determining the loss of
force in this muscle.
•Present address: Quinnipiac College, Hamden, Conn.
f Present address: Department of Human Metabolism, Uni-
versity College Hospital Medical School, University Street,
London WC1E6JJ.
Correspondence: Professor R. H. T. Edwards, Department of
Human Metabolism, University College Hospital Medical
School, University Street, London WC1E 6JJ.
Key words: fatigue, muscle, quadriceps, voluntary
Abbreviations: EMG, electromyograph; MVC,
maximum voluntary contraction.
Force fatigue during maximum voluntary muscular
contractions (MVC) may occur from failure either
at the neuromuscular junction or at sites proximal
or distal to this. Failure proximal to the junction
may be the result of a decreased voluntary effort,
change in motor neuron excitability (direct or due
to inhibition of afferents from the muscle) or to pre-
synaptic block. Which, if any, of these may limit
the extent of voluntary contractions has been the
subject of controversy in the last 50 years (Reid,
Merton, 1954). A failure of transmission at
the neuromuscular junction may arise from either a
loss of excitability of the postjunctional membrane
or from depletion of acetylcholine stores. Distal
failure may arise if action potentials fail to
propagate, if coupling between the action potential
and the release of
fails within the
or if
the contractile elements fail to function correctly.
We have examined the ability of normal subjects to
maintain maximum voluntary isometric con-
tractions of the quadriceps for periods up to 60 s.
Force and surface electromyograph (EMG) ac-
tivity have been measured and the response of part
of the muscle to maximal electrical stimulation at
different times during the MVC and has been used
Bigtand-Ritchie et al.
as an index of muscle contractility that is indepen-
dent of central drive.
This work follows our previous studies of
quadriceps muscle function (Edwards, Young,
Hosking & Jones, 1977) and was designed to help
answer the question 'Does the fatigue (i.e. failure to
generate the required force) arise because the
muscle machinery is failing, or because the subject
is not willing to go on driving it with the same
motivation as at the start?'.
We studied nine healthy adult laboratory person-
nel (one female; ages 25-50 years) who were
familiar with the procedures and gave their
informed consent as required by the local Research
Ethics Committee. Both legs were studied and no
consistent differences in the ability to maintain
force were detected.
Measurements of force
The blood supply to leg muscles is occluded
during isometric contractions of more than about
of the maximal voluntary force (Barcroft &
Millen, 1939; Edwards, Hill & McDonnell, 1972),
but since many of the present experiments involved
interruption of the contraction for brief periods of
stimulation, a cuff around the upper thigh was
inflated to 200 mmHg throughout to prevent any
return of blood supply. The force produced by iso-
metric contractions of the quadriceps muscle was
measured with a strain gauge attached to the ankle
while the subject was seated in an adjustable chair
(Edwards et al., 1977) and recorded on a u.v.
Surface EMG was recorded with two adhesive
cup electrodes filled with electrode paste, one
situated on the skin over the lower third of the
vastus lateralis muscle, the other over the tendon
on the lateral aspect of the knee. An earth electrode
was placed on the lower leg. The EMG activity was
amplified, rectified, smoothed with a time constant
of 0-2 s and displayed on a u.v. oscillograph.
Electrical stimulation
Stimulation of the quadriceps, either per-
cutaneously with surface pad electrodes or by
localized stimulation of the femoral nerve, was
carried out as previously described (Edwards, Hill
& Jones, 1975; Edwards et al., 1977) with a short
stimulus pulse duration (50 ßs) so as to limit
excitation to nerve branches in the muscle. Direct
excitation of human muscle fibres required longer
pulses (100-500 μβ) when studied in vitro (Moulds,
Young, Jones & Edwards, 1977).
To maintain a maximal isometric contraction
(MVC) of the quadriceps for 60 s takes a con-
siderable effort. During the first 30 s discomfort is
mild but between 30 and 45 s pain in the thigh
becomes increasingly severe. There is a pro-
gressive change in the perceived sensations from
the leg with the result that a subject, without visual
feedback, is uncertain of the force exerted towards
the end of the contraction. Pain in the thigh is not
due to ischaemia alone since it largely disappears
as soon as the contraction ceases, even when the
cuff around the thigh remains inflated. To reduce
variation due to pain the results presented here are
all from subjects experienced in the experimental
Time course of a sustained MVC
Subjects were first asked to make a series of brief
maximal contractions, the highest of which was
taken as 100% MVC. Aided by visual feedback
subjects were then asked to maintain a contraction
as close as possible to this maximum for 60 s.
During this period force fell to about 30% of the
initial level, but the precise time course varied be-
tween individuals. The consistent performances of
the two subjects with the greatest difference in the
time course are shown in Fig. 1, the records having
been made on several occasions over a 2 months
Central and peripheral fatigue
In one subject the maintained maximum volun-
tary force was compared with the force obtained by
supramaximally stimulating the muscle via the
femoral nerve. In the unfatigued muscle the MVC
was matched by stimulating at 50 Hz, at which
frequency a fully fused tetanus of the quadriceps
develops (Edwards et al., 1977). During each
tetanus the stimulus voltage was increased to con-
firm that nerve stimulation was supramaximal. The
subject then held a MVC which was interrupted
every 15 s for a brief period of stimulation at 50 Hz
(Fig. 2a).
Human muscle fatigue 611
The voluntary and stimulated contractions fell to
a similar extent during the first 30 s. Thus fatigue
during this period could only be due to a failure at
or distal to the neuromuscular junction. After 45 s
the voluntary force was less than the force
produced by the test tetani, indicating that in this
later period a loss of central neural drive con-
tributed to the force fatigue.
Femoral nerve stimulation is painful and carries
a risk of dislocating the patella, whereas per-
cutaneous stimulation of about 50% of the quad-
riceps with large pad electrodes is quite acceptable
(Edwards et al., 1977). Heat measurements during
stimulated and maximal voluntary contractions
indicate that percutaneous stimulation can maxi-
mally activate a portion of the muscle (Edwards,
The experiment was repeated with the same
subject but, instead with percutaneous stimulation
of the quadriceps at 50 Hz, activating 48% of the
muscle (Fig.
initial value).
During the first part of
MVC voluntary and
stimulated force fell to a similar degree. Thereafter
the voluntary force fell more than the tetanic force
so that the stimulated contraction became 80% of
the voluntary force after 60 s (Fig.
The relative
changes in the voluntary and stimulated forces
were very similar to those seen in the same subject
with femoral nerve stimulation (Fig. 2a). This
indicates that percutaneous stimulation of
of the muscle can be used to determine function
independently of central drive.
Possible fatigue of the central neural drive during
60 s MVC was assessed, with percutaneous
stimulation, in all subjects. Central fatigue was
considered to be present when the MVC force
declined more rapidly than did the tetanic force
produced by the brief stimulated contractions.
nine subjects studied, four showed little or
no central fatigue. In five it amounted to between
24 36
Time (s)
1. Time course of uninterrupted maximum voluntary con-
tractions (MVC) of the quadriceps. Subjects were asked to hold
a MVC for 1 min. Points are the mean1
of four separate
contractions for subject D.O. (#), and six contractions for
subject G.H. (O). Force is expressed as a percentage of the
initial value.
Time (s) 60
2. Comparison of the force obtained by electrical stimulation with the voluntary force during a sustained
maximum voluntary contraction (MVC) of the quadriceps, (o) Femoral nerve stimulation: stimulus marker (shaded
area) gives a record of the stimulation voltage and duration.
Percutaneous stimulation. Duration of stimulation (at
constant voltage) is indicated by the shaded areas. Force (newtons) and time scales apply to both
Values given
just below the stimulus markers are the tetanic force as a percentage of the voluntary force measured immediately
before the tetani. Results in (a) and (6) are from the same subject.
Bigland-Ritchie et al.
Time (s) 45
g 300
50 W5
20 30
Time (s) 50
3. Force and smoothed, rectified EMG (SREMG) during sustained maximum voluntary contraction (MVC) of
quadriceps in (a) a subject (D.O.) with no central fatigue and (4) in subject D.J. with central fatigue; 'extra efforts'
were made at times shown by the arrows. Force scale is in newtons. The smoothed, rectified EMG is given as a per-
centage of the value at the start of
10 and 30% of the force loss after 60 s of sustained,
uninterrupted contraction. With practice, the de-
gree of central fatigue generally became less though
these subjects were not able completely to over-
come it.
Surface EMG activity during sustained MVC
The smoothed, rectified EMG activity and force
were measured at intervals throughout the con-
traction in seven of the subjects (Fig. 3). To show
how these vary the ratio of the smoothed, rectified
EMG divided by the force (here called the E/T
ratio) as first used by Stephens & Taylor (1972) is
shown in Fig. 4. The subjects could be divided into
two groups. In group 1, where the smoothed,
rectified EMG declined roughly in proportion to
the fall in force (resulting in relatively constant
ratios), all the four subjects showed central
fatigue when previously tested, as described above.
In group 2 the smoothed, rectified EMG increased
during the first 10-15 s and then remained high
while the force fell to between 20 and 35% of the
5 ·
Group 2
Group 1
4. Relationship between smoothed, rectified EMG
(SREMG) and force during prolonged maximum voluntary con-
traction (MVC) in seven subjects. The SREMG is divided by
force (E/T ratio, ordinate) and the value at the start of
traction expressed as 1·0. Group 2 subjects exhibited no central
fatigue but were in brief extra efforts able to achieve E/T ratios
close to those found in group 2 subjects.
fresh MVC. This gave E/T ratios three to five times
the control value at the end of the contraction. The
three subjects in this group had previously shown
no central fatigue.
muscle fatigue 613
Subjects were asked to maintain a MVC and, at
15 s intervals, to make a brief extra effort. Subject
typical of those exhibiting little or no central
fatigue, showed the initial rise in smoothed,
rectified EMG to a value which was then main-
tained until near the end of the contraction (Fig.
When asked, he could produce virtually no
extra force nor was there any increase in the
smoothed, rectified EMG. The second subject
in whom approximately 10% central fatigue
had been demonstrated, showed the same initial
rise in smoothed, rectified EMG but after about 10
s the smoothed, rectified EMG and force fell at the
same rate (Fig. 3b). When this subject made an
extra effort both smoothed, rectified EMG and
force increased momentarily but with a greater pro-
portional increase in the electrical activity. If the
ElT ratio was expressed as 1-0 at the start of the
contraction, it became
just before the
effort, increasing to 1-7 during this brief effort.
After 30 s the value before was again 1-4,
increasing to 2-0 during the brief effort and for the
last effort the values were 1-7 and 2-9 respectively.
Although the 'before' values are similar to those
seen for the group 1 subjects shown in Fig. 4,
values during the brief extra efforts approached
those maintained by group 2 subjects. All the
subjects showing a fall in smoothed, rectified EMG
during the contraction could increase the level pro-
portionately more than the increase in force during
the extra efforts, thus briefly increasing the ElT
Our studies show that changes occurring proximal
to the neuromuscular junction (i.e. central fatigue)
can consistently account for up to 30% of
force loss even in apparently well-motivated sub-
jects during sustained contractions of the quad-
riceps. All subjects who showed evidence of central
fatigue could overcome this in brief extra efforts.
The results demonstrate that fatigue cannot be all
attributed to factors at, or distal to, the neuro-
muscular junction, without first ascertaining, par-
ticularly when studies are done on naive subjects,
that central components are not also present.
In experiments on the first dorsal interosseous
muscle Stephens & Taylor (1972) found that the
smoothed, rectified EMG and force declined in
parallel, resulting in a constant ElT ratio
(smoothed, rectified EMG divided by force) during
the first 60 s of the contraction. They interpreted
this to show that fatigue was mainly due to failure
of neuromuscular transmission, having excluded
central fatigue because the evoked synchronous
muscle action potential was also shown to de-
number of
subjects showed a
constant ElT ratio with contractions of the
quadriceps, we believe that for these subjects this
constancy could be accounted for by central
fatigue. For subjects without central fatigue the
smoothed, rectified EMG, after an initial rise,
remained fairly constant at a time when the force
was falling, so that the ElT ratio increased
throughout the contraction. Those subjects with
evidence of central fatigue could briefly increase
their ElT ratios to about the same level by making
brief extra efforts. Our findings for the quadriceps
therefore differ in this respect from those of
Stephens & Taylor (1972) for the first dorsal
The smoothed, rectified EMG signal recorded
during a contraction depends on the number, size
and distribution of active units in the muscle, the
size of the individual fibre action potentials, the
firing frequency and the degree of synchronization
of motor unit activity. The influence of
on the
surface recorded signal is not known, and we are
therefore reluctant to speculate about the site of
failure on the basis of smoothed, rectified EMG
It was noticed that towards the end of a
sustained MVC subjects with little central fatigue
appeared actually to improve their voluntary per-
formance as compared with their response to
stimulation at 50 Hz. However, this effort seems
likely to arise from a diminished response to
stimulation at 50 Hz rather than an improvement
in voluntary performance. Parallel studies on the
adductor pollicis (B. Bigland-Ritchie, R. H. T.
Edwards & D. A. Jones, unpublished work) have
shown that fatigued muscle responds to stimulation
at 20 Hz better than at 50 Hz. As a practical con-
sideration, if muscle contractility is to be tested
during the course of a prolonged MVC, a 50 Hz
tetanus is appropriate at the start of the con-
traction but after about 30-45 s stimulation at a
lower frequency (20 Hz) is a better match to the
force of the voluntary contraction, so giving more
reliable information. This suggests that in order to
maintain optimum force during a fatiguing con-
traction the
frequency of motor neurons must
decline, as has been observed (Marsden, Meadows
& Merton, 1971). The smoothed, rectified EMG
might therefore be expected to fall somewhat even
without any neuromuscular block.
In any investigation involving prolonged MVC
Bigland-Ritchie et al.
of the quadriceps the possibility
central fatigue
taken into account. We suggest that this
force with tetani
50 Hz during the first 30
s of
the contraction,
and at a
lower frequency there-
would appear that central neural drive pro-
gressively falls during sustained MVC
overcome this tendency
This work was supported
the Wellcome Trust
Muscular Dystrophy Group
Britain and also
part by grant NS 09960 from
the U.S.P.H.S. to B.B.-R.
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... Bien que partiellement remis en question au cours des dernières années, les causes de la fatigue p. 105 ont été longtemps divisées en deux groupes : les causes centrales désignant les modifications ayant lieu en amont de la jonction neuromusculaire et les causes périphériques qui désignent les modifications en aval de la jonction neuromusculaire (Amann et al., 2013;Babault et al., 2006;Bigland-Ritchie et al., 1978;Gandevia, 2001;Lambert et al., 2005). Les causes centrales regroupent généralement les modifications de la commande nerveuse qui vont avoir pour résultat une diminution de l'influx nerveux descendant en direction du muscle. ...
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La drépanocytose est une maladie caractérisée par la production d’une hémoglobine mutée, l’hémoglobine S, dont les formes les plus répandues sont la forme homozygote S/S et double hétérozygote composite S/C. L’hémoglobine S possède la capacité de polymériser au sein du globule rouge en condition désoxygénée modifiant sa morphologie provoquant ainsi des complications cliniques multi-organiques. Ces patients présentent une capacité à l’exercice réduite dont les causes sont des altérations cardio-vasculaires cependant l’implication du muscle squelettique n’est pas encore totalement caractérisée dans la drépanocytose. En effet, le muscle squelettique drépanocytaire présente une amyotrophie, un remodelage profond de la microcirculation et une diminution de sa capacité à produire de la force cependant la présence d’une dysfonction musculaire à l’exercice n’a pas encore été décrite. Plusieurs études suggèrent que le stress oxydatif pourrait être élevé dans le muscle drépanocytaire expliquant ainsi en partie les altérations observées. Chez les sujets sains, l’exercice chronique aérobie est connu pour améliorer la capacité antioxydante musculaire notamment par l’augmentation de l’activité des enzymes antioxydantes. Cependant, les effets de ce type d’exercice sur le stress oxydatif n’ont pas encore été caractérisés dans la drépanocytose. Ainsi, les objectifs de cette thèse sont de 1) mettre en évidence une dysfonction musculaire chez des patients drépanocytaires S/S et S/C et de 2) déterminer les modifications induites par un exercice chronique aérobie sur le stress oxydatif au sein du muscle squelettique dans un modèle murin drépanocytaire. Lors d’une première étude, des sujets sains (A/A) et patients drépanocytaires S/S et S/C ont effectué un exercice mono-articulaire sous-maximal du quadriceps à une intensité correspondant à 25% de leur force maximale isométrique (Fmax) suivi d’un test de marche de 6 minutes (T6M). Fmax a été mesurée avant et après cet exercice mono-articulaire, et après le T6M et la perte de Fmax a été utilisée pour quantifier la fatigabilité musculaire. Au cours du protocole, les activités électromyographiques du Vastus Lateralis (VL) et du Vastus Medialis ainsi que l’oxygénation musculaire par spectroscopie proche infrarouge du VL ont été mesurés au cours de l’exercice. Le principal résultat de cette étude est que les patients drépanocytaires S/S et S/C présentent une fatigabilité musculaire accrue comparés aux sujets A/A. Cette fatigabilité accrue semble être provoquée par des altérations musculaires plutôt qu’une modification de l’oxygénation musculaire. De plus, il semblerait que les mécanismes d’apparition de la fatigue musculaire soient différents entre les S/S et S/C comme le suggère les modifications des paramètres électromyographiques. Ainsi, le muscle des patients S/C semble moins altéré que celui des patients S/S. Dans une seconde étude, des souris Townes A/A et S/S ont suivi un exercice chronique aérobie d’intensité modérée de 8 semaines puis la mesure de marqueurs permettant de caractériser la balance pro/anti-oxydant a été effectuée dans le gastrocnemius, le plantaris et le soleus. A l’issue de ces 8 semaines, nous avons observé des modifications uniquement dans le gastrocnemius des souris S/S entraînées. En effet, l’activité de la NADPH oxydase ainsi que de la superoxyde dismutase et de la catalase étaient plus élevées chez les souris S/S entraînées. Ces résultats suggèrent une altération de la balance pro/antioxydante en réponse à un exercice chronique aérobie d’intensité modérée au sein du muscle squelettique drépanocytaire. Enfin, contrairement aux souris S/S entraînées, nous n’observons pas de modifications de la balance pro/antioxydante dans les souris S/S sédentaires indiquant que le stress oxydatif ne serait pas augmenté dans le muscle drépanocytaire.
... It stems from active skeletal muscles that are involved in the peripheral processes as well as supraspinal mechanisms within the brain, thus comprising physiological and psychological aspects (Gandevia, 2001;Berchicci et al., 2013). The fatigue processes at or distal to the neuromuscular junction is referred to as peripheral fatigue and those attributed to the central nervous system affecting the neural drive to the muscle as central fatigue (Bigland-Ritchie et al., 1978). ...
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Repeatedly performing a submaximal motor task for a prolonged period of time leads to muscle fatigue comprising a central and peripheral component, which demands a gradually increasing effort. However, the brain contribution to the enhancement of effort to cope with progressing fatigue lacks a complete understanding. The intermittent motor tasks (IMTs) closely resemble many activities of daily living (ADL), thus remaining physiologically relevant to study fatigue. The scope of this study is therefore to investigate the EEG-based brain activation patterns in healthy subjects performing IMT until self-perceived exhaustion. Fourteen participants (median age 51.5 years; age range 26−72 years; 6 males) repeated elbow flexion contractions at 40% maximum voluntary contraction by following visual cues displayed on an oscilloscope screen until subjective exhaustion. Each contraction lasted ≈5 s with a 2-s rest between trials. The force, EEG, and surface EMG (from elbow joint muscles) data were simultaneously collected. After preprocessing, we selected a subset of trials at the beginning, middle, and end of the study session representing brain activities germane to mild, moderate, and severe fatigue conditions, respectively, to compare and contrast the changes in the EEG time-frequency (TF) characteristics across the conditions. The outcome of channel- and source-level TF analyses reveals that the theta, alpha, and beta power spectral densities vary in proportion to fatigue levels in cortical motor areas. We observed a statistically significant change in the band-specific spectral power in relation to the graded fatigue from both the steady- and post-contraction EEG data. The findings would enhance our understanding on the etiology and physiology of voluntary motor-action-related fatigue and provide pointers to counteract the perception of muscle weakness and lack of motor endurance associated with ADL. The study outcome would help rationalize why certain patients experience exacerbated fatigue while carrying out mundane tasks, evaluate how clinical conditions such as neurological disorders and cancer treatment alter neural mechanisms underlying fatigue in future studies, and develop therapeutic strategies for restoring the patients' ability to participate in ADL by mitigating the central and muscle fatigue.
... Yorgunluk, bir kasın verilen bir aktiviteyi gerçekleştirmek için istenen ya da beklenen kuvveti devam ettirmede başarısızlığı olarak tanımlanır (37). Fizyolojik olarak maksimal güç üretme kapasitesindeki düşüşün zaman ile ilişkili bir olgusudur (38). ...
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Background: All of the amazing advances of the present world are the result of human learning. Durability and survival of human being in the world is possible by creating and discovering new ways of teaching and learning. Objective: The purpose of this study was to investigate the transitional effect of fatigue due to a period of physical and mental activity on the level of learning a motor skill. Method: Participants in this research were all of the students of a non-profit female school; among which 36 students, aged between 15-18 years, were selected in simple random method and divided into 3 groups of mental activity, a physical activity and a control group-each group consisting of 12 individuals. The present study was semi-experimental and of repeated measurement type and included three experimental groups. The exercise protocol was considered for 4 weeks, 3 sessions per week. For each session, 5 close and 20 attempts were considered. AAHPERD standard test was used in acquisition, retention and transfer tests. Statistical method of variance analysis with repeated measurements was used to analyze the data. Results: The results showed that the physical activity group had a significant difference compared to the control group, while the mental activity group did not show significant difference. There was also a significant difference between physical activity and mental activity in motor skill variable. Conclusion: The overall conclusion that can be drawn is that, learning motor skill is not equal in all individuals and could be affected by all negative factors.
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1. A new method of studying isolated human skeletal muscle has been evaluated. This involves the incubation and electrical stimulation of strips of muscle, obtained at surgical biopsy, that are tied at the cut ends of the fibre bundles. 2. Morphological examination showed that the fibres were sealed off at the cut ends. Damage appeared to be restricted to the areas immediately adjacent to the ties. 3. Contractile properties were well maintained for several hours and measurements of tissue metabolites showed that muscle contents of the high-energy phosphate compounds were well preserved. 4. The isolated preparations were found to have the same contractile properties as human quadriceps femoris studied in vivo by the methods described in the preceding paper. 5. Correlation was found between the relaxation speed of the isolated preparations and their fibre-type composition histochemically determined. 6. It is concluded that this technique is a valid addition to the present methods of studying the physiology and pharmacology of human skeletal muscle.
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1. The force produced by isometric contractions of the quadriceps muscle have been studied during maximal voluntary contractions and when a substantial part of the muscle was electrically stimulated via surface electrodes. 2. In normal children and adults, the force of a maximal voluntary contraction of the quadriceps was proportional to body weight. 3. The function of the quadriceps has been described in terms of the force/frequency curve, speed of relaxation and the rate of loss of force during 18 s stimulation at 30 Hz and 100 Hz. 4. The functional characteristics of adductor pollicis when stimulated via the ulnar nerve were essentially similar to those of the quadriceps. 5. Studies of the function of these two muscles are complementary since quadriceps femoris is amenable to needle biopsy investigations of its structure and chemistry whereas adductor pollicis is more suitable for electrophysiological studies.
Muscle fatigue is a common symptom but there are no universally accepted methods for quantitating the function of voluntary muscle. This paper describes three main methods of assessment: simple clinical tests of muscle function; thermal probe measurements of metabolic heat production during muscular contraction; needle biopsy studies of muscle structure and chemistry. These methods, though at a relatively early stage of development, have given promising results which suggest that they could be useful in assessing possible new forms of treatment in patients with neuromuscular disorders.
1. Development of a new thermal probe and use in conjunction with chemical analysis of needle biopsy samples, has made possible a thermodynamic study of the energetics of muscular contraction in the human quadriceps. 2. The observed rate of muscle temperature rise was proportional to the force of the contraction. During maximal contractions the rate of heat production was 54 +/-8-5 W/kg wet muscle (mean +/- s.d.). 3. The observed rates of muscle temperature rise agreed well with the rates calculated from the measured metabolite changes when standard values for the enthalpy changes of the reactions involved were used. 4. During prolonged stimulation of the quadriceps at 15/sec via the femoral nerve, the rate of heat production per unit force fell to nearly half the initial value. It is estimated that this represented a two- to fourfold increased in economy of ATP turn-over required to maintain a given force. 5. Relaxation becomes progressively slower during prolonged contractions and it is suggested that the slowing of relaxation and the increased economy of force maintenance may both be due to an increased cross-bridge cycle time in the fatigued muscle.
1. The mechanism of fatigue has been studied in maintained maximal voluntary contractions of the first dorsal interosseous muscle of the hand.2. Fatigue occurs in two phases. In the first, lasting 1 min, force falls to about 50%. The smoothed rectified e.m.g. (s.r.e.) falls with the same time course and the normal linear relation between s.r.e. and force of unfatigued muscle is preserved.3. In the second phase, force falls relatively faster than s.r.e.4. Arterial occlusion does not affect the first phase, but in the second phase causes force to fall to zero, whereas without occlusion it tends to stabilize at about 25%.5. The size of the synchronous muscle action potential evoked by ulnar nerve stimulation falls to about 65% of normal, most of this fall occurring in the first phase.6. During recovery after prolonged fatigue, the relation between force and s.r.e. is changed for weak voluntary contractions much more than for strong ones, such that force is less for a given s.r.e. than normal.7. These results are interpreted as evidence that, in a maximal voluntary contraction, neuromuscular junction fatigue is most important at first, but later, contractile element fatigue increases, particularly when the blood supply is obstructed.8. Neuromuscular junction fatigue is believed to be most marked in high threshold motor units, while contractile element fatigue more especially affects low threshold units.