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Sports Med 2004; 34 (7): 443-449
R
EVIEW
A
RTICLE
0112-1642/04/0007-0443/$31.00/0
2004 Adis Data Information BV. All rights reserved.
Stretching and Injury Prevention
An Obscure Relationship
Erik Witvrouw,1 Nele Mahieu,1 Lieven Danneels1 and Peter McNair2
1 Department of Rehabilitation Sciences and Physical Therapy, Faculty of Medicine and Health
Sciences, Ghent University, Ghent, Belgium
2 School of Physiotherapy, Physical Rehabilitation Research Centre, Auckland University of
Technology, Auckland, New Zealand
Contents
Abstract ....................................................................................443
1. Working Mechanism of the Musculotendinous Unit during Movement .........................444
2. How Can Stretching Reduce the Risk of Injuries? ............................................445
3. Stretching and Injuries in Sports with High Stretch-Shortening Cycle (SSC) Movements ...........446
4. Stretching and Injuries in Sports with No or Low SSC Movements ..............................447
5. Conclusions .............................................................................448
It is generally accepted that increasing the flexibility of a muscle-tendon unit
Abstract promotes better performances and decreases the number of injuries. Stretching
exercises are regularly included in warm-up and cooling-down exercises; howev-
er, contradictory findings have been reported in the literature. Several authors
have suggested that stretching has a beneficial effect on injury prevention. In
contrast, clinical evidence suggesting that stretching before exercise does not
prevent injuries has also been reported. Apparently, no scientifically based
prescription for stretching exercises exists and no conclusive statements can be
made about the relationship of stretching and athletic injuries. Stretching recom-
mendations are clouded by misconceptions and conflicting research reports. We
believe that part of these contradictions can be explained by considering the type
of sports activity in which an individual is participating. Sports involving bounc-
ing and jumping activities with a high intensity of stretch-shortening cycles
(SSCs) [e.g. soccer and football] require a muscle-tendon unit that is compliant
enough to store and release the high amount of elastic energy that benefits
performance in such sports. If the participants of these sports have an insufficient
compliant muscle-tendon unit, the demands in energy absorption and release may
rapidly exceed the capacity of the muscle-tendon unit. This may lead to an
increased risk for injury of this structure. Consequently, the rationale for injury
prevention in these sports is to increase the compliance of the muscle-tendon unit.
Recent studies have shown that stretching programmes can significantly
influence the viscosity of the tendon and make it significantly more compliant,
and when a sport demands SSCs of high intensity, stretching may be important for
injury prevention. This conjecture is in agreement with the available scientific
clinical evidence from these types of sports activities. In contrast, when the type of
444 Witvrouw et al.
sports activity contains low-intensity, or limited SSCs (e.g. jogging, cycling and
swimming) there is no need for a very compliant muscle-tendon unit since most of
its power generation is a consequence of active (contractile) muscle work that
needs to be directly transferred (by the tendon) to the articular system to generate
motion. Therefore, stretching (and thus making the tendon more compliant) may
not be advantageous. This conjecture is supported by the literature, where strong
evidence exists that stretching has no beneficial effect on injury prevention in
these sports. If this point of view is used when examining research findings
concerning stretching and injuries, the reasons for the contrasting findings in the
literature are in many instances resolved.
Traditionally, it is generally accepted that such as in cycling, jogging and swimming. In the
stretching promotes better performances and de- former role, an eccentric muscle action is immedi-
creases the number of injuries.[1-6] Consequently, ately followed by a concentric action. It is well
stretching exercises are regularly included in warm- known that if an activated muscle is stretched before
-up and cooling-down exercises. However, today shortening, its performance is enhanced during the
the scientific evidence concerning the preventive concentric phase. Consequently, jumping, hopping
effect of stretching on injuries seems unclear. In the and leaping movements are improved by making a
literature, prospective studies are lacking and con- counter-movement. Many previous studies have in-
tradictory findings have been reported concerning dicated that this phenomenon is the result of strain
the relationship between stretching and injury pre- energy stored in the tendon structures.[9-18] Muscle-
vention. The purpose of this article is to review the tendon units can store mechanical work as elastic
pertinent literature and to advance a new theory to energy during eccentric contractions. The storage
explain the relationship between stretching and inju- and subsequent release of elastic energy during
ry prevention. stretch-shortening cycles (SSCs) have generally
been considered as an ‘energy-saving’ mechanism.
Before looking at the available literature on
However, the effect of the re-utilisation of elastic
stretching and athletic injuries, it seems essential to
energy on the efficiency of movement has been
examine how the muscle-tendon unit works during
recently debated.[19]
movements and how stretching would be able to
reduce the risk of athletic injuries. We believe that When a muscle has a less compliant muscle-
the contrasting results in the literature concerning tendon unit, more work is directly converted into
the relationship between stretching and injuries can external work. Activities like cycling, flying, skat-
be explained by taking into account the type of ing and swimming use predominantly positive
sports activity in which the individual participates. work-loops and little opportunity exists for absorb-
ing amounts of energy during the task or skill.[8,20,21]
A more compliant muscle-tendon unit allows for the
1. Working Mechanism of the effective storage and release of series elastic energy,
Musculotendinous Unit but seems to be less suited for a task with a predomi-
during Movement nantly positive work-loop. Wilson et al.[22] conclud-
Muscle-tendon systems may generate forces in ed that musculotendinous stiffness was significantly
two distinctly different ways: (i) as an elastic-like related to isometric and concentric performance but
spring in stretch-shortening motion[7] that occurs, not to eccentric performance. In their study, they
for example, during jumping-type activities; and (ii) found that the stiffer subjects performed significant-
as converters of metabolic energy into mechanical ly better than the more compliant subjects on both
work in predominantly concentric contractions,[8] the isometric tests and on the majority of the concen-
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (7)
Stretching and Injury Prevention 445
tric tests, since the stiff muscles immediately trans- tion and hence compliance can be modified to suit
fer force to the muscle-bone junction. In contrast, different tasks. As such, Bach et al.,[25] and more
compliant muscles generated less power due to the recently Wilson et al.,[26] have noted that when the
delayed transfer of energy through the musculo- mechanical properties of the unit are optimised then
tendinous unit. Wilson et al.[23] observed in another maximal performance (e.g. for rapid force produc-
study that increasing the compliance of the mus- tion or economy) is obtained.
culotendinous unit through stretching, increased the
contribution of elastic strain energy to movement, 2. How Can Stretching Reduce the Risk
facilitating performance in an SSC movement. of Injuries?
Therefore, it seems that different types of sports
need different levels of musculo-tendinous compli- Before looking at the available literature where
ance. Many physical pursuits such as cycling, swim- the relationship between stretching and injury pre-
ming, skating, wrestling and boxing involve the vention is examined, we need to understand how
rapid development of force in an isometric or con- stretching would be able to reduce the risk of athletic
centric muscular contraction, and it would appear injuries. Firstly, consideration of the compliance of
that such performances could be enhanced through the muscle-tendon unit is essential. To fully under-
an increase in musculotendinous stiffness. The stif- stand the effect of compliance, we need to appreci-
fer the muscle-tendon unit, the faster the force is ate differences between the active contractile (mus-
transferred to the bones, and the resulting movement cle) component and the passive (tendon tissue) com-
of the joint is quicker. Therefore, looking only at ponent of the muscle-tendon unit. According to
performance, it might be possible that in these sport Safran et al.,[27] the ability of a muscle to absorb
activities there is no need for a highly elastic mus- energy is dependent on both components. In a com-
cle-tendon that acts like a spring. The aim of sports pliant system when the contractile elements are ac-
with a high amount of positive work-loops is to tive to a high level, more energy can be absorbed by
convert metabolic energy as fast as possible into the tendon tissue, thereby reducing trauma to muscle
mechanical work. fibres. However, in case of a low compliance of the
Conversely, in sports with a high-intensity SSC, tendon, forces will be transferred to the contractile
a more compliant muscle-tendon unit may be re- apparatus with little energy absorption in the tendon.
quired for the storage and release of elastic energy. This provides a mechanism to explain the noted
A muscle-tendon unit involved in such SSCs, needs association between reduced flexibility and occur-
a high storage capacity for potential energy and rence of muscle injury during SSC motion. Evi-
must, therefore, be sufficiently compliant. For en- dence for this conjecture is found in the in vivo work
hanced performance, it seems that for these kinds of by McHugh et al.,[28] who found increased evidence
sports there is a great need for a more compliant of muscle damage following eccentric exercise in
muscle-tendon unit. subjects with greater passive stiffness. In addition,
It should be considered that there may be an ideal this is consistent with other research,[29] which
level of compliance for a musculo-tendinous unit showed that in the outer ranges of movement, as
during a task. This level can be influenced by struc- tendon stiffness increases, greater passive forces are
tural characteristics of the unit. For instance, generated within the muscle. In people with stiff
Shadwick[24] has shown that compared with mature tendons, even greater passive muscle forces would
tendons, those of younger animals have a lower be expected to develop during SSC, which would
capacity to store and release strain energy as a result therefore increase the risk of muscle injury. In con-
of higher compliance levels and greater amounts of trast, a more compliant tendon, with greater energy-
hysteresis. The level of overall muscle compliance absorbing capabilities, would therefore seem to re-
can also be influenced by contractile element activa- duce the risk of muscle injury during SSC motion.
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (7)
446 Witvrouw et al.
On the basis of these findings, the rationale for Nevertheless, transient or chronic increase in ten-
don compliance as an acute or chronic adaptation of
stretching as part of an injury prevention programme
stretching will theoretically lead to a higher ability
is to increase the compliance of the tendon unit, and of the tendon to absorb energy. In the case of a high-
consequently more energy can be absorbed for a intensity SSC movement (when a large amount of
given SSC performance.[30] Can stretching influence energy needs to be absorbed), the greater energy
the compliance of the tendon structure? Recently, absorbing capacity of the stretched tendon will theo-
Kubo et al.[31] investigated this question and looked retically lead to a lower injury risk in the tendon and
for the acute and long-term effects of stretching on the muscle structures: since (i) the tendon is able to
human tendons in vivo. They showed, using ultraso- absorb more energy, the high stresses on the tendon
nography, that it was possible to quantify the visco- (typically coming from the high SSC movements)
elastic properties of human tendon in vivo. Their will less likely reach the maximal energy-absorbing
results on seven healthy men showed that immedi- capacity of the tendon, and thus will less likely lead
ately after the execution of static stretching exer- to injury to the tendon; and (ii) since the stretched
cises the tendon stiffness was transiently de- tendon is able to absorb more energy, less energy is
creased.[31] In a more recent study,[32] the same au- transferred to the contractile apparatus, therefore
thors investigated whether resistance and stretching reducing the risk of injury within this component of
training programmes altered the viscoelastic proper- the muscle.
ties of human tendon structures. In that study on Does this theoretical background for stretching in
eight healthy males they showed that an 8-week injury prevention in sports with a high SSC compo-
stretching programme (two stretching sessions dai- nent, as described in the paragraph above, stand-up
ly, 7 days per week) made the tendon structures when examined with the available literature in
significantly more compliant.[32] Their findings are sports with high SSC movements?
in agreement with previous animal studies that re-
ported an increase in tendon compliance as a result 3. Stretching and Injuries in Sports with
of a stretching regime.[33-35] Kubo et al.[32] speculat- High Stretch-Shortening Cycle
ed that stretching may be an effective means to (SSC) Movements
increase the elastic energy to be utilised during Ekstrand et al.[3] found that a group of elite soccer
exercise involving a SSC, by reducing the viscosity teams randomised to a routine of warm-up and
of tendon structures. stretching before exercise, leg guards, special shoes,
Concerning the relationship between stretching ankle taping, controlled rehabilitation, education
and injury prevention, stretching and the subsequent and close supervision had 75% fewer injuries than
decrease in tendon stiffness may lessen the imposed the control group, which received no intervention.
load across the muscle-tendon unit during SSC They concluded that the proposed prophylactic pro-
movements.[31] The mechanism by which the de- gramme, including close supervision and correction
crease in stiffness occurs immediately after stretch- by doctors and physiotherapists, significantly
ing and on the long term cannot be determined from reduces soccer injuries. The same authors[2] hy-
the available research. However, McNair et al.[36] pothesised that a redesign of the warm-up with more
states that immediately after stretching, the mecha- emphasis on stretching and the addition of cooling-
nism could involve the movement of the mobile down exercises reduces injuries. According to Bix-
components/elements within the tissues. That is, ler and Jones[1] high-school football injuries are very
liquid and polysaccharides may be redistributed frequent each year in the US. In a randomised inter-
within the collagen matrixes.[36] After a periodic vention study, they investigated whether completing
stretching programme, the changes are more likely a warm-up and stretching routine after halftime re-
to involve structural changes to collagen. duced the incidence of third-quarter injuries. The
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (7)
Stretching and Injury Prevention 447
results of their study showed a reduction in injuries crease the risk of injuries by instituting a stretching
with the warm-up and stretching exercises. programme. Furthermore, stretching in these ath-
letes will probably not lead to an increase of inju-
Witvrouw et al.[5] determined the intrinsic risk
factors for the development of patellar tendinopathy ries. Subsequently, why shouldn’t they stretch if it
in an athletic population. Before the study, 138 male doesn’t harm? The answer is related to performance.
and female physical education students had been If these athletes stretch a lot and make their tendons
evaluated for anthropometric variables, leg align- more compliant, they may be less adapted for their
ment characteristics, muscle tightness and strength sports activities and consequently be less efficient
parameters. The study revealed that the only signifi- during movement. In some sporting activities, stiff
cant determining factor was muscular flexibility, tendons are advantageous for performing brisk, ac-
with the patellar tendinosis group being less compli- curate movements because they allow rapid tension
ant in quadriceps and hamstring muscle-tendon unit. changes and hence faster joint motion responses,
Lower flexibility of the quadriceps and hamstring and perhaps provide more sensitive feedback to the
muscles may contribute to the development of patel- central nervous system concerning muscle length
lar tendinosis in an athletic population. Therefore, and tension.[11,12,37] Looking at the literature con-
the authors concluded that a stiff quadriceps and cerning the effect of stretching on low SSC sports
hamstring muscle-tendon unit was a risk factor for there seems to be some evidence for the above-
the development of patellar tendinopathy. The same stated concept.
authors published a similar prospective study of 146 In 1993, van Mechelen et al.[38] studied the effect
professional soccer players.[6] Players with a ham- of a health-education intervention on jogging inju-
string or quadriceps lesion were found to have a ries. The intervention consisted of information/edu-
statistically lower compliance of these muscle-ten- cation and the subsequent performance of
don units prior to their injury compared with non- standardised warm-up, cooling-down and stretching
injured soccer players. On the basis of these find- exercises. Male recreational joggers (n = 421) were
ings, they suggested that stretching might play an randomly split into an intervention and a control
important role in the prevention of this condition. group. During the 16-week study, both groups kept a
daily diary of their jogging distance and time, and
4. Stretching and Injuries in Sports with reported all injuries. The results of this study did not
No or Low SSC Movements identify any evidence of a reduction in soft tissue
injuries in the intervention group. The authors con-
If one participates in a sport with a low or no cluded that the intervention was not effective in
frequency of SSC movements (e.g. cycling, swim- reducing the number of jogging injuries. Recently,
ming), or a sport with a high frequency of SSC Yeung and Yeung[39] assessed in their review the
movements but always at a low percentage of the available evidence for preventive strategies for low-
maximum (e.g. jogging), these movements utilise er limb soft tissue injuries caused by jogging. Their
little of the energy-absorbing capacity of the mus- review identified five eligible trials (1944 partici-
cle-tendon unit. For optimal performance in such pants in intervention groups, 3159 controls) that
activity, the tendons do not need to function as good examined the effect of a stretching regimen on lower
energy-absorbing structures. Since the maximal en- limb injuries caused by jogging.[4,38,40-42] Two stud-
ergy-absorbing capacity of these unstretched (stiff) ies evaluated the effect of stretching outside the
tendons is less likely to be exceeded during these
training sessions.[4,40] The remaining three studies
sporting activities, the risk of tendon or muscle
examined the effectiveness of stretching immediate-
damage will be relatively low. A stiff tendon will
ly before training.[38,41,42] Their exploratory analysis
theoretically be sufficient to deal with the loads
of these five studies showed that in only one study[4]
imposed on the musculo-tendinous structures during
a significant effect of stretching on the incidence of
these sports, and hence one cannot expect to de-
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (7)
448 Witvrouw et al.
injuries could be found. The authors concluded on energy, which may lead to tendon and/or muscle
the basis of these findings that insufficient evidence damage. When the sports activity contains no, or
exists to suggest that stretching is effective in only low SSC movements (cycling, jogging), all or
preventing lower limb injuries in joggers.[39] most of the work is directly converted to external
work. In these cases, there is no need for a compliant
Looking in the literature concerning swimming tendon since the amount of energy absorption re-
and cycling, no prospective studies could be found mains low. Hence, additional stretching exercises to
examining the effect of stretching on the incidence improve the compliance of the tendon may have no
of injuries. However, looking at the injury incidence beneficial effect on injury prevention.
in these sports,[43,44] the rather low incidence of
musculo-tendinous injuries is interesting and sup- It must be acknowledged that the aetiology of
ports our model. Looking at the regular training injuries can be multifactorial. Taking out only one
schedule of professional cyclists in Europe, it is aspect (e.g. stretching) and examining its effect on
surprising to see how little stretching is performed in the incidence of injuries is a rather narrow outlook
most teams. In contrast, swimmers tend to devote on this problem. For example, fatigue is widely
considerable time to stretching. However, recent believed to be predisposing factor in muscle inju-
literature[45] advises to minimise stretching, particu- ry.[38] In addition, other problems remain. Even
larly at the shoulders where hyper-mobility is often within the same sport, the demands on different
apparent. players (position on the field) may be different.
However, we believe that far greater attention
should be given to an examination of the type of
5. Conclusions
activity in which the athlete participates when one
In summary, stretching is perhaps the most com- considers the merits of stretching to reduce injury.
mon routine advocated by sports coaches and sports-
medicine professionals. However, in the literature, Acknowledgements
conflicting data have been reported concerning the
The authors have provided no information on sources of
relationship between flexibility and athletic injury. funding or on conflicts of interest directly relevant to the
Stretching recommendations are clouded by mis- content of this review.
conceptions and conflicting research reports. The
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