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Late swing or early stance? A narrative review of hamstring injury mechanisms during high‐speed running


Abstract and Figures

Hamstring injuries are highly prevalent in many running‐based sports, and predominantly affect the long head of biceps femoris. Re‐injury rates are also high and together lead to considerable time lost from sport. However, the mechanisms for hamstring injury during high‐speed running are still not fully understood. Therefore, the aim of this review was to summarise the current literature describing hamstring musculotendon mechanics and electromyography activity during high‐speed running, and how they may relate to injury risk. The large eccentric contraction, characterised by peak musculotendon strain and negative work during late swing phase is widely suggested to be potentially injurious. However, it is also argued that high hamstring loads resulting from large joint torques and ground reaction forces during early stance may cause injury. While direct evidence is still lacking, the majority of the literature suggests that the most likely timing of injury is the late swing phase. Future research should aim to prospectively examine the relationship between hamstring musculotendon dynamics and hamstring injury. This article is protected by copyright. All rights reserved.
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MISS CLAIRE KENNEALLY-DABROWSKI (Orcid ID : 0000-0001-5433-5082)
DR WAYNE SPRATFORD (Orcid ID : 0000-0002-6207-8829)
Article type : Review Article
Late swing or early stance? A narrative review of hamstring injury mechanisms during high-
speed running
Running head: Hamstring injury mechanisms during running
Authors: Claire J. B. Kenneally-Dabrowski1,2, Nicholas A.T. Brown2,3, Adrian K.M. Lai4, Diana
Perriman1,5,6, Wayne Spratford3,7, Benjamin G. Serpell3,8
1 ANU Medical School, Australian National University, Canberra, ACT, Australia
2 Australian Institute of Sport, Canberra, ACT, Australia
3 University of Canberra Research Institute for Sport and Exercise, Faculty of Health, University of
Canberra, Canberra, ACT, Australia
4 Department of Biomedical Physiology and Kinesiology, Simon Fraser University, Burnaby, BC,
5 Trauma and Orthopaedic Research Unit, Canberra Hospital, Canberra, ACT, Australia
6 Discipline of Physiotherapy, Faculty of Health, University of Canberra, Canberra, ACT, Australia
7 Discipline of Sport and Exercise Science, Faculty of Health, University of Canberra, Canberra,
ACT, Australia
8 Brumbies Rugby, Canberra, ACT, Australia
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Corresponding author: Claire Kenneally-Dabrowski, Australian Institute of Sport, Movement
Science Department, Leverrier St, Bruce, ACT, Australia. Tel: +614 3333 1214, Fax: +612 6214
1593, E-mail:
Hamstring injuries are highly prevalent in many running-based sports, and predominantly affect the
long head of biceps femoris. Re-injury rates are also high and together lead to considerable time lost
from sport. However, the mechanisms for hamstring injury during high-speed running are still not
fully understood. Therefore, the aim of this review was to summarise the current literature describing
hamstring musculotendon mechanics and electromyography activity during high-speed running, and
how they may relate to injury risk. The large eccentric contraction, characterised by peak
musculotendon strain and negative work during late swing phase is widely suggested to be potentially
injurious. However, it is also argued that high hamstring loads resulting from large joint torques and
ground reaction forces during early stance may cause injury. While direct evidence is still lacking, the
majority of the literature suggests that the most likely timing of injury is the late swing phase. Future
research should aim to prospectively examine the relationship between hamstring musculotendon
dynamics and hamstring injury.
Athletic injuries, Musculotendon function, Literature review, Biceps Femoris long head,
Musculoskeletal modelling
Hamstring injuries are one of the most common and debilitating injuries in many running-based
sports, including the football codes and track sprinting.1-3 High rates of initial injury coupled with
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frequent recurrence1-4 lead to significant periods of convalescence.2, 4 The resulting player
unavailability in team sports negatively impacts team performance,5 while individual performance
may also remain diminished upon return to play.6
It is well established that hamstring injuries most commonly occur during high-speed running.2, 7, 8
While the hamstring complex is comprised of four muscles (biceps femoris long head, biceps femoris
short head, semimembranosus and semitendinosus), injuries primarily affect the biceps femoris long
head muscle (BFlh) at the proximal musculotendinous junction.9, 10 In professional soccer, 57-72% of
all hamstring injuries occurred during high-speed running, and in nearly all of these injuries (up to
94%) the primary injury site was the BFlh.2, 11, 12 Therefore, the focus of the majority of research, and
by extension, this review, is the BFlh musculotendon complex. The BFlh is positioned laterally within
the hamstring complex and is bi-articular, crossing both the hip and knee joints and therefore
contributing to hip extension and knee flexion.13
Several studies have attempted to determine the mechanisms of BFlh injury during running through
anatomical studies,13-16 or kinematic and kinetic analyses of running.17-19 However, although it is the
subject of significant speculation, the exact mechanisms for hamstring injury during high-speed
running have still not yet been definitively established. It has been suggested that anatomical risk
factors for BFlh injury may include the morphology of the aponeuroses15, 16 as well as it’s synergistic
relationship and common proximal tendon with semitendinosus.13 However, it is potentially the
function of the hamstring musculotendon complex during high-speed running that may provide the
most insight into injury mechanisms. Advances in musculoskeletal modelling techniques over the past
decade have facilitated several investigations into hamstring musculotendon mechanics during high-
speed running. These modelling studies have indicated that the mechanism of injury may relate to the
timing of peaks in musculotendon variables such as length and force.17, 19-24 These peaks occur in
distinct phases of the gait cycle in response to the task demands. Two distinct arguments have been
proposed for the timing of injury, and pose the question: Do hamstring injuries occur during late
swing or early stance?
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Therefore, the objective of this review is to provide an overview of:
1. Musculotendon mechanics and muscle excitation of the BFlh during high-speed running.
2. How these musculotendon mechanics and muscle excitation may relate to the potential for
injury during the late swing or early stance phases of high-speed running.
Literature search
The databases Scopus, Web of Science and PubMed were searched using combinations of the
following terms, hamstring, inj*, sprint*, run*, EMG, activ*, kinematic*, kinetic*. The reference lists
of articles retrieved were also manually searched for any relevant articles that were not identified
Hip and knee kinematics during high-speed running
In this paper, the gait cycle will be discussed starting at toe off, as this approach is more conducive to
the discussion of hamstring function at the late swing and early stance phases where the interest in
hamstring injury risk is greatest. Hip and knee angles are discussed as relative angles between the
trunk and thigh, and thigh and shank, respectively. Further, all kinematic and kinetic data presented in
the following two sections of this review refer to overground sprinting data only.
As depicted in Figure 1, each stride commences with early swing phase. Early swing phase begins at
toe-off after which the hip transitions from maximum extension (approximately 195 degrees) into
flexion, and the knee flexes.25, 26 At the transition to mid swing, the knee reaches peak flexion
(approximately 40 degrees) and the hip continues to flex,25, 26 before beginning to extend rapidly,
reaching a peak extension velocity of 1195 degrees/second.25 Late swing commences as the hip
reaches peak flexion (approximately 100 degrees) and then starts to extend in preparation for foot
strike.25, 26 The knee extends until the shank is decelerated, at which point the knee starts to flex just
prior to foot strike. Early stance begins at foot strike, where the hip extends while the knee flexes. Mid
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stance defines the transition from knee flexion to extension, and the hip continues to extend.
Following this point, late stance commences and the knee continues to extend until just prior to toe
off, where it starts to flex. The hip continues to extend throughout late stance, nearing peak hip
extension at toe off.25, 27
Hip and knee kinetics during high-speed running
Throughout the first half of swing phase, the hip produces a large flexion moment, reported to peak at
4.3 N·m·kg, while the knee produces an extension moment of smaller magnitude (1.0 N·m·kg).17, 28 In
the second half of swing, the hip produces a large extension moment (4.2 N·m·kg) while the knee
displays a smaller flexion moment (1.8 N·m·kg).17, 28 During swing, net joint torques are primarily a
result of muscle torques (generated from muscle contractions) and motion-dependent torque (arising
from the mechanical interaction of segments eg. the angular acceleration of the shank).29, 30
In contrast, during stance, the net joint torques primarily result from muscle torques and external
forces (resulting from ground reaction forces).29, 30 During the beginning of the stance phase, the hip
exhibits extensor dominance, reaching peak extension torque at approximately 4.1 N·m·kg, before
changing to a flexion moment towards the latter half of stance.17, 31, 32 The reported knee moments
during stance vary much more between studies, which may be attributed to different filtering
techniques utilised.33, 34 Schache et al.17, 28 described an extension moment for the first half of stance,
with a peak torque of 3.6 N·m·kg, before a flexion moment is produced towards late stance. However,
other studies have reported a much more variable knee moment, sometimes switching several times
from extension to flexion dominance throughout stance, although some researchers have argued that
this may be an artefact of data processing.31
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Muscle excitation of the BFlh during high-speed running
Surface electromyography (EMG) has been used to define EMG amplitude (ie. muscle excitation)35 of
the lateral hamstrings (BFlh and biceps femoris short head) during both overground36-38 and
treadmill19, 39 high-speed running. Muscle excitation is evident throughout the whole gait cycle;
however, muscle excitation is low through the early and middle phases of swing.19, 36, 37 Two large
peaks in excitation are observed during late swing and early stance.19, 36, 37, 39 The exact magnitudes of
these peaks have not been clearly reported in the literature; however, when normalised to maximum
excitation within a stride, hamstring EMG has been reported as being of similar magnitude during late
swing and early stance.37, 39, 40 When normalised to excitation during a maximum voluntary isometric
contraction (MVIC), BFlh excitation during late swing and early stance is reported to exceed 100%
MVIC.38 It is not unusual to observe excitation levels in excess of 100% MVIC when using this
normalisation method during high-velocity tasks such as sprinting.41 In comparison to the rest of the
gait cycle, the magnitude of excitation in late swing has been reported as two to three times greater
than during early swing and late stance,36 and the excitation during early stance is significantly greater
than during late stance.37 In addition, studies of strength training exercises have demonstrated
differences in proximal to distal regional excitation of the hamstring muscles during exercises such as
the Nordic hamstring exercise, stiff-leg deadlift, bent-knee bridge, prone leg curl and sliding leg
curl.42-45 While differences in regional muscle excitation have not yet been investigated during high-
speed running, it is likely that this could provide further insight into muscle function and potential
injury mechanisms, as injuries often primarily affect one region (the proximal musculotendinous
Musculotendon mechanics of the BFlh during high-speed running
Musculoskeletal modelling studies have provided valuable insight into the mechanics of the BFlh
musculotendon complex during high-speed running. Predictions of functional variables such as
musculotendon length and force changes throughout the gait cycle provide information that is unable
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to be derived directly from human subjects. While there are still many limitations associated with
musculoskeletal modelling such as anatomical assumptions and approximations, and varying levels of
model complexity and subject-specificity, the limitations are reasonable if findings are interpreted in
context of these limitations.47, 48 Therefore, these studies are imperative to furthering understanding of
BFlh musculotendon function. However, to date, there have been limited musculoskeletal modelling
studies of overground running,17, 36 as most have collected data on a treadmill.18, 19, 49, 50 Data from
treadmill running has limited applicability since it induces kinematic and kinetic adaptations in an
athlete’s sprinting mechanics.51 Furthermore, while some have used recreational or sub-elite athletic
populations,17, 18, 36 there is a distinct lack of literature pertaining to athletic populations such as elite
level sprinters and football players, for whom hamstring injury mechanics are most relevant. A
summary of reported peak musculotendon stretch and force for running can be found in Table 1.
Musculotendon length changes
The hamstring muscles undergo a stretch-shortening cycle throughout high-speed running.49, 52 In the
first part of swing phase the BFlh shortens,17, 19, 49 as the knee flexes and the hip moves from extension
into flexion.18, 49 The hip continues to flex until terminal swing, while the knee starts to extend
throughout the second half of swing,18, 19, 49 causing the BFlh to rapidly lengthen.17-19, 49 Several studies
have reported peak lengthening velocity at the transition from knee flexion to extension,17, 18 and a
second, smaller peak in lengthening velocity has been described closer to terminal swing.17 During
terminal swing the BFlh reaches peak length and strain at approximately 112% of upright standing
muscle length.17-19, 22, 52 Just prior to foot strike, the BFlh starts to shorten as the hip extends and the
knee flexes in preparation for foot strike.17-19, 49 The hip continues to extend throughout stance, and the
knee flexes for the first half, before starting to extend.18, 49 Most studies suggest that the BFlh shortens
throughout stance.17, 19 However, two studies36, 53 have reported an eccentric (lengthening) contraction
of the hamstrings during late stance. The extension of the hip places the hamstrings in a shortened
position, and while the knee does extend after mid-stance, the hamstring sagittal plane moment arms
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at the hip are greater than at the knee.18, 54 Therefore, it is argued that for knee extension to cause
lengthening of the hamstring muscles, the velocity of knee extension would have to substantially
exceed the velocity of hip extension.19 Although it seems likely that in most cases, the BFlh will
shorten throughout late stance, it may be possible that the length change is dependent on the degree of
hip and knee extension exhibited by an individual athlete.
While this information provides valuable insight into BFlh function during running, a limitation of the
majority of this work is that it considers only the length changes of the musculotendinous unit as a
whole. Relative lengthening and velocity contributions from the muscle and tendon components have
not been discussed in the majority of current research. Recently, Hooren and Bosch55, 56 have
challenged the view that changes in the whole musculotendinous unit may reflect muscle fascicle
behaviour. They propose that the muscle fascicles of the BFlh may not act eccentrically in the late
swing phase of high-speed running. Rather, that the tendinous element facilitates lengthening and the
fascicles remain closer to isometric. However, it should be noted that this proposal is based on
evidence from other lower limb muscles57, 58 and animal studies,59 and at this time there is no direct
evidence for this theory in the human hamstring muscles during running. Quasi-isometric fascicle
behaviour has been previously observed during high-speed running at the ankle plantar-flexor
complex and also in the vastus lateralis.57, 58 While we cannot assume that this fascicle behaviour
applies also to the hamstrings during running, it may suggest that this phenomena is plausible.
Further, both Chumanov et al.50 and Thelen et al.49 have differentiated BFlh muscle and tendon
contributions to lengthening using musculoskeletal modelling, and predicted small discrepancies in
lengthening patterns between the two elements. In fact, Thelen et al.49 noted that the stretch of the
muscle component slows once the muscle is excited at late swing, and that tendon stretch is primarily
responsible for negative work done during terminal swing. Hence, these modelling studies suggest
that length and velocity changes in the muscle fibres of the BFlh could be decoupled from the whole
musculotendon complex, although more focussed research is needed to clarify this relationship.
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Musculotendon force
The BFlh produces very little force throughout the first half of swing phase, after which a large peak
in musculotendinous force occurs. During late swing, the reported peak musculotendinous force is
between 10.5- 26.4 N·kg-1.17, 50, 52 This typically decreases prior to foot contact before a second
smaller peak is observed in early stance (3- 11.6 N·kg-1).17, 52 However, it has also been proposed that
the BFlh musculotendon force during early stance may have been underestimated due to over-
filtering, causing erroneously low hip and knee torques.29, 34 It is argued that in early stance, ground
reaction forces cause a large extension torque at the knee and a flexion torque at the hip.
Consequently, the hamstrings would be under very high load, in order to produce large flexion torque
at the knee, and extension torque at the hip, to counter the large passive forces.29, 34
The potential for injury during late swing phase
The occurrence of peak musculotendon force, high muscle excitation, negative work and peak
musculotendon length during terminal swing, have all been suggested to contribute to the increased
risk of injury in the late swing phase of high-speed running.17, 19, 20, 36 Chumanov and colleagues19
observed that peak stretch and negative work occurred exclusively during the swing phase of high-
speed treadmill running. Large loads were evident during both late swing and early stance; however,
peak musculotendon force of the BFlh increased significantly with increasing running velocity only in
the late swing phase. When combined with increasing negative work, the authors concluded that the
large inertial loads placed on the hamstrings during the late swing phase suggest an increased
likelihood of injury during late swing, rather than early stance.
Researchers proposing an isometric function of the hamstring fascicles at late swing also suggest that
this large force at late swing may be important in hamstring injury occurrence.55, 56 The authors
propose that forces at late swing may become too high for fascicles to remain isometric, and therefore
cause an eccentric contraction and concomitant vulnerability to injury.55, 56 However, this mechanism
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is speculative, as there is not currently any data to support this theory in the hamstring muscles during
The extensive strain experienced by the BFlh in late swing is also commonly suggested as a
contributor to, or even the primary cause for injury.17, 19, 21, 60 Schache and colleagues17 suggested that
it is the greater strain experienced by the BFlh compared to the other hamstring muscles, that makes it
more susceptible to injury. The strain experienced by the BFlh was 2.2% and 3.3% greater than for
semimembranosus and semitendinosus, respectively, while peak musculotendon force and negative
work were comparatively less for the BFlh. The relatively longer hip extension moment arm of the
BFlh compared to other hamstring muscles causes this greater lengthening and strain when the hip is
flexed in the late swing phase.18 Therefore, the peak strain experienced by BFlh during terminal swing
seems to be the parameter that distinguishes it from the other hamstring muscles, and therefore may
be the most relevant parameter for understanding why the BFlh in particularly is vulnerable to injury
during high-speed running.
The relationship between the level of muscle excitation and injury risk has been explored in animal
studies. Yu et al.36 suggested that the high levels of muscle excitation observed during late swing
exacerbate the susceptibility to strain injury. This proposal was based upon a study of the mouse
extensor digitorum longus muscle, which demonstrated that when a muscle is maximally activated,
the amount of strain needed to cause a muscle strain injury is decreased.61 This indicates that the high
levels of both excitation and muscle strain during late swing are both conditions which are likely to
increase the risk of hamstring strain injury.
Other researchers have drawn on evidence from animal models to support the argument that the
conditions during late swing are conducive to injury.61, 62 Early research on the rabbit extensor
digitorum longus muscle during active and passive lengthening first suggested that strain injury was
the result of reaching a critical strain, rather than exceeding a critical force.63 Notably, in this study,
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failure always occurred at the musculotendinous junction, which suggests that these experiments
successfully mimicked real injury occurrence. In a study of the rabbit tibialis anterior, Lieber and
Friden62 measured the contractile properties of the muscle (as an indicator of muscle damage) after
eccentric contractions at various levels of force and strain. The results suggested that muscle damage
is not a function of muscle force, but rather the amount of muscle strain is the best indicator of injury.
Moreover, the study of the mouse extensor digitorum longus muscle also demonstrated that the
amount of negative work done by the muscle was the best predictor of injury magnitude, as indicated
by force deficits following active or passive stretching of the muscle.61 These principles support the
proposition that the mechanics of the BFlh during late swing are likely to be the cause of BFlh injury
during high-speed running.
Two incidences where injury to the BFlh has occurred during studies of high-speed running further
support late swing as the likely phase for injury occurrence.21, 23, 24 While the first kinematic deviations
to indicate injury occurred during stance, once neuromuscular latencies were accounted for, both
studies concluded that the late swing phase was the most likely time of injury occurrence.21, 23 In the
running trials prior to the injury occurrence, the BFlh of the subsequently injured leg demonstrated
greater peak length and force in late swing phase, compared to the contralateral muscle, as well as a
greater vertical ground reaction force and loading rate for the subsequently injured leg.23, 24 Following
the injury, the BFlh exhibited intolerance to eccentric contraction, performed less negative work and
had a reduced peak lengthening velocity.24 Despite having kinematic and kinetic data during an injury
occurrence, these studies have not provided conclusive evidence that the injury occurred during late
swing. While the information provided is no doubt unique and valuable, the arguments for the timing
of injury are still speculative, due to the uncertainty of predicting neuromuscular latency.23 This
highlights the complexity of predicting injury timing, and therefore, further work is needed to
conclusively determine the timing of injury.
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Finally, only one to date study has prospectively examined the relationship between running
mechanics and subsequent hamstring injury. Schuermans et al.64 determined that subsequently injured
soccer athletes displayed kinematic differences during high-speed running that were only observed
during the swing phase. Subsequently injured athletes displayed greater anterior pelvic tilt during mid
swing and greater lateral flexion of the thorax during the late swing phase. No kinematic differences
were found between subsequently injured and uninjured athletes during the stance phase. The same
authors also found differences in muscle excitation between subsequently injured and uninjured
soccer athletes, that were again constrained to the swing phase.65 During mid-swing, injured athletes
showed significantly lower muscle excitation of the trunk, and lower gluteal excitation during late
swing. Again, no differences were observed between injured and non-injured athletes during stance.
These findings support the contention that the late swing phase is the high-risk period for hamstring
injury occurrence during high-speed running.
The potential for injury during early stance
Fewer researchers have suggested that BFlh injury risk is greater during the early stance phase of
high-speed running.32, 66 Early research proposed that the greatest knee flexion and hip extension
moments occur during early stance phase, leading to very high loads on the hamstring muscles and
increased injury risk.32 One of the strongest voices in support of increased risk of injury during stance
phase rather than swing has been Orchard.66 He has reasoned that strong opposing forces resulting
from ground reaction forces during stance are likely to increase the risk of hamstring injury.
Orchard 66 also observed that muscle strains do not typically occur during open chain activities. For
example, while divers experience great stretch and high angular velocities when performing a pike,
they do not typically succumb to injury. This observation was used to reinforce the point that injuries
are more likely to occur when forces are high, rather than when the muscle experiences great strain.
However, this theory lacks experimental evidence to support this anecdotal observation.
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Some researchers suggest that injury risk is increased during both phases of high-speed running. A
recent opinion paper by Liu and colleagues34 supported Orchards arguments, stating that the
hamstrings would be under high load in early stance, due to ground reaction forces causing large joint
moments. They argued that the lower estimates of muscle forces during early stance reported in
musculoskeletal modelling studies is due to over-filtering of data, and that the hamstrings would be
vulnerable to injury in early stance due to this force peak. However, Liu and colleagues suggest that
both early stance and late swing are potentially hazardous times for injury during high-speed running.
They characterised these two phases as one “swing-stance transition” phase and advocated for a more
general principle, whereby the BFlh is more prone to injury when the muscle has to counter large
passive forces. In late swing, the muscle torque is high, to counteract a large motion dependent torque
created by the angular acceleration of the shank. Therefore, they propose that the inertial loads at late
swing, and external ground reaction force loads in early stance, both have the potential to cause injury
to the BFlh during high-speed running.
Future directions
This review has identified several gaps in the current knowledge of hamstring mechanics during
sprinting and how they may relate to the timing of hamstring injury. Firstly, there is a paucity of
studies examining athletic populations such as football players and sprinters, especially at the elite
level. Secondly, there may be further insights gained from exploring regional excitation of the
hamstrings during high-speed running. Third, there is also a need to examine relative lengthening and
velocity contributions of both the muscle and tendon when performing musculoskeletal modelling
analyses. This is important for distinguishing muscle and tendon contributions to peak strain and force
during the late swing phase, and will assist in refining injury prevention strategies. For example, there
is growing support for the importance of injury prevention exercises that result in lengthening of
muscle fascicles.67-69 Understanding the relative contribution of muscle fascicles to overall strain of
the musculotendon complex would serve to validate and inform this training principle. Finally, there
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is a distinct lack of prospective studies examining the relationship between running mechanics and
hamstring injury occurrence. Schuermans et al.64 provided the first ever prospective link between
running mechanics and hamstring injury, using a strong study design which should be replicated in
other cohorts such as sprinters and other football codes. Further, this work could be improved in
future investigations by also examining kinetic variables, such as joint moments and powers. Indeed,
it would be a natural progression to also conduct prospective musculoskeletal modelling studies
examining the relationship between hamstring musculotendon dynamics and hamstring injury. This
would allow the identification of musculotendon dynamics, and the gait cycle phase, that are
associated with hamstring injury.
The timing of hamstring injury during high-speed running has been a topic of debate for many years.
While we still lack direct evidence, the majority of the literature suggests that the most likely timing
of injury is the late swing phase. Future research should focus on prospectively examining the
relationship between running mechanics, hamstring musculotendon function and hamstring injury in
athletic populations.
This research is supported by an Australian Government Research Training Program (RTP)
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Accepted Article
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Accepted Article
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Accepted Article
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Table 1 Summary of literature describing BFlh musculotendon stretch and force during high-speed running
Data is presented as the mean (SD) where possible. Reported timings of peaks within gait cycle (% of gait cycle) reflect stance followed by swing, where the
transition from stance to swing occurred at 16.1 26.8% (range). MT = musculotendon, M = male, F = female. a Normalised to length during upright posture/
optimum length or change in length (mm) from upright posture, b Estimated from a graph. c As described by authors, peak occurred within reported range.
Study details
BFlh MT stretch
BFlh MT force
Authors (year)
speed (m/s)
Peak MT stretcha
Peak MT force
during swing
Peak MT force
during stance
Timing of peak
(% gait cycle)
Timing of peak
(% gait cycle)
Timing of peak
(% gait cycle)
Thelen et al.
(2005) 49
1 adult male
~60 mm b
~90 %
17.6 N·kg-1
~85 % b
Nagano et al.
(2014) 52
8 male track and field
athletes (sub-elite)
9.5 (0.2)
1.2 (0.0)
82.8 (1.9) %
10.5 (0.7) N·kg-1
80.3 (3.0) %
~3 N·kg-1 b
~5 % b
Schache et al.
(2012) 17
7 experienced sprinters
(5 M, 2 F)
8.9 (0.7)
1.1 (0.0)
86.4 (1.71) %
26.4 (5.2) N·kg-1
82.9 (1.0) %
4.6 (1.0) N·kg-1
0.0 (1.3) %
Chumanov et al.
(2011) 19
12 adults
(9 M, 3 F)
8.0 (M),
7.1 (F)
1.1 (0.0)
~90 %
13.2 (1.5) N·kg-1
85-95 % c
11.6 (1.9) N·kg-1
~10% b
Thelen et al.
(2005) 18
14 athletes
(9 M, 5 F)
9.4 (0.6; M),
8.1 (0.8; F)
1.1 (0.0)
89.6 (3.7) %
Chumanov et al.
(2007) 50
19 athletes
(14 M, 5 F)
9.1 (0.6; M),
8.2 (0.8; F)
51.2 (4.4) mm
~90 %
21.4 (5.4) N·kg-1
85-95 % c
Wan et al. (2017)
20 college students
(10 M, 10 F)
8.0 (0.49; M),
6.9 (0.43; F)
1.1 (0.1)
~90 % b
Accepted Article
This article is protected by copyright. All rights reserved.
Figure Legends
Figure 1 The sprinting gait cycle. a) Early swing b) Mid swing c) Late swing d) Early stance/ foot
strike e) Mid stance f) Late stance/ toe off 25,26
... The late-swing phase of running was pointed out as the most vulnerable time of the hamstring muscles [52][53][54]. At the late-swing phase of running, hamstrings eccentrically contract to decelerate the tibia and to control the antagonist quadriceps femoris muscles' concentric force [55]. ...
... The HSIs most commonly occur when the muscle fascicles cannot resist an excessive elongation during the late-swing phase of running [57]. Therefore, shorter BFlh fascicles [40] and insufficient eccentric hamstring contractions were considered risk factors for HSIs [40,52,53]. ...
... Additionally, it has previously been pointed out that hamstrings undergo elongations with eccentric contraction during the late-swing phase of running [56]. During this time, an excessive antagonist force higher than the eccentric force of the hamstrings elongates the hamstrings and can lead to damage and strains in BFlh fascicles [40,52,53]. Accordingly, a shorter BFlh fascicle length was defined as a risk factor for HSIs because of a possible lesser ability to be stretched and possible greater damage due to lesser sarcomeres in the series of the BFlh fascicles than longer fascicles during the eccentric muscle activation of the hamstrings [40]. ...
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Introduction Football matches show higher hamstring strain injuries (HSIs) than football training. The occurrence of HSIs increases in the last fifteen minutes of both halves of football matches and shows an incremental trend towards the end of the ninety minutes. Objectives This study aimed to examine football-specific fatigue-induced alterations in risk factors of the HSIs, including biceps femoris long head fascicle length via ultrasonography (BFlh FL), single-leg hop distance, hamstrings’ maximal eccentric strength, and single-leg hamstring bridge test (SLHB) performance. Methodology During ninety minutes of the TSAFT ⁹⁰ football simulation, the BFlh FL and single-leg hop distance were measured three times (before, at half-time and after 90 minutes of simulated match-play), and maximal hamstrings eccentric strength and SLHB test scores were recorded twice (before and after simulated match-play) for both legs in physically active participants (n = 15). Results Maximal eccentric hamstrings’ strength (dominant leg (D): p < 0.001, Hedges’ (adjusted) g effect size = -0.969; non-dominant leg (ND): p < 0.001, g = -0.929) and the SLHB performance (D: p < 0.001, g = -1.249; ND: p < 0.001, g = -1.108) showed large decrements immediately after the TSAFT ⁹⁰ intervention. There were no significant alterations in the BFlh FL, and the single-leg hop distance. Conclusions Maximal eccentric strength and the SLHB performance of hamstrings are reduced after 90 minutes of simulated football match-play. Practitioners may consider focusing on improving eccentric strength and the SLHB performance. Future studies should examine alterations in the BFlh fascicles’ dynamic lengthening and shortening ability during a football match.
... It has been reported that 57-72% of all hamstring injuries occur during sprinting. Nearly 94% of these injuries appear in the biceps femoris long head (BFLH) [12][13][14][15][16]. For example, in 17 subjects with single injuries or isolated re-injuries, 12 of these were injured on the proximal part [17]. ...
... Researchers explored the underlying mechanism of hamstring injury through anatomical, kinematic, and kinetic analyses of sprinting [14,[18][19][20]. The injury's timing, location and tissue involved are the discussions' foci due to the discordance of muscle-tendon length and force changes in leg swing progression throughout the gait cycle [15,[21][22][23][24]. ...
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Hamstring injury has been considered one of the most common exercise-induced injuries in sports. Hamstring injuries mostly occur proximal to the biceps femoris. However, the reasons and mechanisms remain unclear. To summarize hamstring morphological structure features and what the relationship is between their structure and risk of injury from the current literature, this review discussed the possible injury mechanism of hamstrings, from the morphological and connected pattern diversity, the mechanical properties, and the stress–strain performance, to probable changes in action control. Morphological and connected pattern diversity of hamstrings components show heterogeneous loads under muscle tension. Connections of gradient compliance between different tissues may lead to materials’ susceptibility to detachments near the tendon–bone junction sites under heterogeneous load conditions. The hamstrings muscle’s motor function insufficiency also brings the risk of injury when it performs multi-functional movements during exercise due to the span of multiple joints’ anatomical characteristics. These structural features may be the primary reason why most damage occurs near these sites. The role of these biomechanical characteristics should be appreciated by exercise specialists to effectively prevent hamstring injuries. Future work in this research should be aimed at exploring the most effective prevention programs based on the material structure and motor control to enhance the properties of hamstring muscle materials to minimize the risk of injury.
... Previous work has shown that pre-activation of the hamstrings and plantar flexors increase with running velocity and may prevent unnecessary breaking forces during the contact phase of the sprint (34). While, both muscle groups are prone to injury, hamstring injuries have been a common cause for concern as the prevalence is high in both field based sports and track and field events (35). Recently, it was shown that a proximal neuromuscular control strategy may be associated with decreased occurrence of hamstring injuries in a large group of amateur soccer players (36). ...
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Sprinting is multifactorial and dependent on a variety of kinematic, kinetic, and neuromuscular features. A key objective in sprinting is covering a set amount of distance in the shortest amount of time. To achieve this, sprinters are required to coordinate their entire body to achieve a fast sprint velocity. This suggests that a whole-body kinematic and neuromuscular coordinative strategy exists which is associated with improved sprint performance. The purpose of this study was to leverage inertial measurement units (IMUs) and wireless surface electromyography (sEMG) to find coordinative strategies associated with peak over-ground sprint velocity using machine learning. We recruited 40 healthy university age sprint-based athletes from a variety of athletic backgrounds. IMU and sEMG data were used as inputs into a principal components analysis (PCA) to observe major modes of variation (i.e., PC scores). PC scores were then used as inputs into a stepwise multivariate linear regression model to derive associations of each mode of variation with peak sprint velocity. Both the kinematic (R2 = 0.795) and sEMG data (R2 = 0.586) produced significant multivariate linear regression models. The PCs that were selected as inputs into the multivariate linear regression model were reconstructed using multi-component reconstruction to produce a representation of the whole-body movement pattern and changes in the sEMG waveform associated with faster sprint velocities. The findings of this work suggest that distinct features are associated with faster sprint velocity. These include the timing of the contralateral arm and leg swing, stance leg kinematics, dynamic trunk extension at toe-off, asymmetry between the right and left swing side leg and a phase shift feature of the posterior chain musculature. These results demonstrate the utility of data-driven frameworks in identifying different coordinative features that are associated with a movement outcome. Using our framework, coaches and biomechanists can make decisions based on objective movement information, which can ultimately improve an athlete's performance.
... One HSI mechanism occurs when the hamstrings are lengthening while producing force (eccentric) to slow the limb during the swing phase of high-speed running (Chumanov et al., 2011;Heiderscheit et al., 2005;Hickey et al., 2022;Kenneally-Dabrowski et al., 2019). For this reason, eccentric strength assessments are frequently used as targets for predicting HSI injury risk and determining an athlete's readiness for return to sport (RTS) (Bourne et al., 2015;Opar et al., 2015;Timmins et al., 2016;Wille et al., 2022). ...
Objectives The primary aim of this study was to describe eccentric hamstring strength magnitude and asymmetry at the time of return-to-sport (RTS) after an index hamstring strain injury (HSI) and determine if there were differences in strength asymmetry at RTS between those who did and did not go on to re-injure within 1-month and within 3-months of RTS. Design Cross-sectional study. Setting Laboratory-based. Participants Sixty National Collegiate Athletic Association Division I athletes with index HSI. Main Outcome Measures: Maximum hamstring eccentric strength for each limb, total maximum strength summed across limbs, and between-limb asymmetry at the time of RTS following the index HSI, assessed using the NordBord Hamstring Testing System. Results Of the 60 index HSIs, 8 (13%), and 11 (18%) re-injuries occurred within 1 and 3-months of RTS, respectively. There were no differences between those who did and did not re-injure in maximum eccentric force of either limb (p-values≥0.52), total force from both limbs (p-values≥0.47), and between limb force asymmetry (p-values≥0.91), regardless if re-injury occurred within 1 or 3-months after RTS. Conclusions Eccentric hamstring strength and asymmetry measured at the time of RTS did not differ between those who did and did not re-injure within 3-months of RTS.
... Future work should compare these task-specific effects on MTU mechanics during various movements while rectify the aforementioned shortcoming of prior studies. Controversy exists regarding whether hamstring muscle strain injury occurs during swing or early stance (Kenneally-Dabrowski et al., 2019). Interestingly, all four hamstring muscles changed from eccentric to concentric contraction at the point of transition from swing to stance (i.e., foot-ground contact), as indicated by shortening fibre velocities in combination with substantial activations. ...
... In contrast, Pinniger et al. (2000) [43] found that, when fatigued, athletes adopted a movement pattern that would be more protective (i.e., associated with less hamstring strain). Specifically, pilot studies showed that modifications in trunk, hip, or knee angle led to changes in hamstring length that were prospectively associated with hamstring injury risk [45][46][47][48][49]. ...
Full-text available
The aim of this study was to analyse the influence of fatigue on sprint biomechanics. Fifty-one football players performed twelve maximal 30 m sprints with 20 s recovery between each sprint. Sprint kinetics were computed from running speed data and a high-frequency camera (240 Hz) was used to study kinematic data. A cluster analysis (K-mean clustering) was conducted to classify individual kinematic adaptations. A large decrease in maximal power output and less efficiency in horizontally orienting the ground reaction force were observed in fatigued participants. In addition, individual changes in kinematic components were observed, and, according to the cluster analysis, five clusters were identified. Changes in trunk, knee, and hip angles led to an overall theoretical increase in hamstring strain for some players (Cluster 5, 20/51) but to an overall decrease for some others (Cluster 1, 11/51). This study showed that the repeated sprint ability (RSA) protocol had an impact on both kinetics and kinematics. Moreover, fatigue affected the kinematics in a different way for each player, and these individual changes were associated with either higher or lower hamstring length and thus strain.
... Planning high-speed and sprint running training receives particular attention among soccer coaches and practitioners as optimal exposure strategies may also have a preventive role against injuries for which inadequate training dose is considered as a modifiable risk factor [235]. Unaccustomed volumes and spikes in sprint and near-to-maximal speed distances during competitive match-play have been reported to have harmful association with muscle injury occurrence [255,256]; therefore, exposing soccer players to progressive and optimal sprint running doses may provide a preventive effect, especially for non-contact hamstrings injuries [257,258]. This likelihood of muscle injuries is reasonably increased among non-starting players owing to the lack of match-induced high-speed and sprint running exposure, especially if these are not adequately compensated for during the training micro-cycle. ...
Full-text available
Background Sided games (i.e., small sided, medium sided, large sided) involve tactical, technical, physical, and psychological elements and are commonly implemented in soccer training. Although soccer sided-games research is plentiful, a meta-analytical synthesis of external load exposure during sided games is lacking. Objective The objective of this systematic review and meta-analysis was to: (1) synthesize the evidence on high-speed and sprint running exposure induced by sided games in adult soccer players, (2) establish pooled estimates and intra-individual reliability for high-speed and sprint running exposure, and (3) explore the moderating effects of game format and playing constraints. Methods A literature search was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses 2020 guidelines. Four databases (PubMed/MEDLINE, Scopus, SPORTDiscus, Web of Science Core Collection) were systematically searched up to 25 January, 2022. Eligibility criteria were adult soccer players (population); training programs incorporating sided games (intervention); game manipulations including number of players, pitch dimension, and game orientation (comparator); and high-speed, very high-speed, and sprint relative (m⋅min−1) running distances and associated intra-individual reliability (outcome). Eligible study risk of bias was evaluated using RoBANS. Pooled estimates for high-speed and sprint running exposure, and their intra-individual reliability, along with the moderating effect of tracking device running velocity thresholds, pitch dimension (i.e., area per player), and game orientation (i.e. score or possession), were determined via a multi-level mixed-effects meta-analysis. Estimate uncertainty is presented as 95% compatibility intervals (CIs) with the likely range of relative distances in similar future studies determined via 95% prediction intervals. Results A total of 104 and 7 studies met our eligibility criteria for the main and reliability analyses, respectively. The range of relative distances covered across small-sided games, medium-sided games, and large-sided games was 14.8 m⋅min−1 (95% CI 12.3–17.4) to 17.2 m⋅min−1 (95% CI 13.5–20.8) for high-speed running, 2.7 m⋅min−1 (95% CI 1.8–3.5) to 3.6 m⋅min−1 (95% CI 2.3–4.8) for very high-speed running, and 0.2 m⋅min−1 (95% CI 0.1–0.4) to 0.7 m⋅min−1 (95% CI 0.5–0.9) for sprinting. Across different game formats, 95% prediction intervals showed future exposure for high-speed, very high-speed running, and sprinting to be 0–46.5 m⋅min−1, 0–14.2 m⋅min−1, and 0–2.6 m⋅min−1, respectively. High-speed, very high-speed running, and sprinting showed poor reliability with a pooled coefficient of variation of 22.8% with distances being moderated by device speed thresholds, pitch dimension, and game orientation. Conclusions This review is the first to provide a detailed synthesis of exposure and intra-individual reliability of high-speed and sprint running during soccer sided games. Our estimates, along with the moderating influence of common programming variables such as velocity thresholds, area per player, and game orientation should be considered for informed planning of small-sided games, medium-sided games, and large-sided games soccer training. Clinical Trial Registration Open Science Framework available through
... Future work should compare these task-specific effects on MTU mechanics during various movements while rectify the aforementioned shortcoming of prior studies. Controversy exists regarding whether hamstring muscle strain injury occurs during swing or early stance (Kenneally-Dabrowski et al., 2019). Interestingly, all four hamstring muscles changed from eccentric to concentric contraction at the point of transition from swing to stance (i.e., foot-ground contact), as indicated by shortening fibre velocities in combination with substantial activations. ...
Muscle tendon unit fibre mechanics of hamstring and adductor strain injuries are not well studied, with factors such as fatigue promoted as risk factors in the absence of mechanistic evidence. In this study, musculoskeletal modelling was used to estimate fibre mechanics of four hamstring (biceps femoris long head, biceps femoris short head, semimembranosus and semitendinosus) and four adductor (adductor brevis, adductor longus, adductor magnus and gracilis) muscles during an anticipated cut task. The cut task was performed by 10 healthy elite male U20 basketball players both before and immediately after they played in one (of four) competitive basketball game. Biceps femoris long head produced significantly lower (p = 0.032) submaximal force post-game in the latter part of swing (30.7% to 35.0% of stride), though its peak force occurred later (37%) and remained unchanged. Semimembranosus produced significantly lower (p = 0.006) force post-game (32.9% to 44.9% of stride), which encompassed the instance of peak force (39%). Neither fibre velocity nor fibre length of the investigated muscles were significantly affected by game-play. These finding suggest that if fatigue is a factor in hamstring and adductor muscle strain injuries and is brought about by game-play, it is unlikely through the fibre mechanisms investigated in this study.
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Objective: To describe the injury mechanisms and magnetic resonance imaging (MRI) findings in acute hamstring injuries of male soccer players using a systematic video analysis. Design: Descriptive case series study of consecutive acute hamstring injuries from September 2017 to January 2022. Setting: Two specialized sports medicine hospitals. Participants: Professional male soccer players aged between 18 and 40 years, referred for injury assessment within 7 days after an acute hamstring injury, with an available video footage of the injury and positive finding on MRI. Independent variables: Hamstring injury mechanisms (specific scoring based on standardized models) in relation to hamstring muscle injury MRI findings. Main outcome measures: Hamstring injury mechanism (playing situation, player/opponent behavior, movement, and biomechanical body positions) and MRI injury location. Results: Fourteen videos of acute hamstring injuries in 13 professional male soccer players were analyzed. Three different injury mechanisms were seen: mixed-type (both sprint-related and stretch-related, 43%), stretch-type (36%), and sprint-type (21%). Most common actions during injury moments were change of direction (29%), kicking (29%), and running (21%). Most injuries occurred at high or very high horizontal speed (71%) and affected isolated proximal biceps femoris (BF) (36%). Most frequent body positions at defined injury moments were neutral trunk (43%), hip flexion 45-90 degrees (57%), and knee flexion <45 degrees (93%). Magnetic resonance imaging findings showed that 79% were isolated single-tendon injuries. Conclusions: According to a video analysis, most hamstring injuries in soccer occur during high-speed movements. Physicians should suspect proximal and isolated single-tendon-most often BF-hamstring injury, if represented injury mechanisms are seen during game play. In addition to sprinting and stretching, also mixed-type injury mechanisms occur.
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Proximal‐distal differences in muscle activity are rarely considered when defining the activity level of hamstring muscles. The aim of this study was to determine the inter‐muscular and proximal‐distal electromyography (EMG) activity patterns of hamstring muscles during common hamstring exercises. Nineteen amateur athletes without a history of hamstring injury performed 9 exercises while EMG activity was recorded along the biceps femoris long head (BFlh) and semitendinosus (ST) muscles using 15‐channel high‐density electromyography (HD‐EMG) electrodes. EMG activity levels normalized to those of a maximal voluntary isometric contraction (%MVIC) were determined for the eccentric and concentric phase of each exercise and compared between different muscles and regions (proximal, middle, distal) within each muscle. Straight‐knee bridge, upright hip extension and leg curls exhibited the highest hamstrings activity in both the eccentric (40‐54%MVIC) and concentric phases (69‐85%MVIC). Hip extension was the only BF‐dominant exercise (Cohen's d = 0.28 (eccentric) and 0.33 (concentric)). Within ST, lower distal than middle/proximal activity was found in the bent‐knee bridge and leg curl exercises (d range = 0.53‐1.20), which was not evident in other exercises. BFlh also displayed large regional differences across exercises (d range = 0.00–1.28). This study demonstrates that inter‐muscular and proximal‐distal activity patterns are exercise‐dependent, and in some exercises are affected by the contraction mode. Knowledge of activity levels and relative activity of hamstring muscles in different exercises may assist exercise selection in hamstring injury management. This article is protected by copyright. All rights reserved.
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According to the force-length-velocity relationships, the muscle force potential during locomotion is determined by the operating fibre length and velocity. We measured fascicle and muscle-tendon unit length and velocity as well as the activity of the human vastus lateralis muscle (VL) during walking and running. Furthermore, we determined the VL force-length relationship experimentally and calculated the force-length and force-velocity potentials (i.e. fraction of maximum force according to the force-length-velocity curves) for both gaits. During the active state of the stance phase, fascicles showed significantly (p < 0.05) smaller length changes (walking: 9.2 ± 4.7% of optimal length (L0); running: 9.0 ± 8.4%L0) and lower velocities (0.46 ± 0.36 L0/s; 0.03 ± 0.83 L0/s) compared to the muscle-tendon unit (walking: 19.7 ± 5.3%L0, -0.94 ± 0.32 L0/s; running: 34.5 ± 5.8%L0, -2.59 ± 0.41 L0/s). The VL fascicles operated close to optimum length (L0 = 9.4 ± 0.11 cm) in both walking (8.6 ± 0.14 cm) and running (10.1 ± 0.19 cm), resulting in high force-length (walking: 0.92 ± 0.08; running: 0.91 ± 0.14) and force-velocity (0.91 ± 0.08; 0.97 ± 0.13) potentials. For the first time we demonstrated that, in contrast to the current general conception, the VL fascicles operate almost isometrically and close to L0during the active state of the stance phase of walking and running. The findings further verify an important contribution of the series-elastic element to VL fascicle dynamics.
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Surface electromyography (sEMG) is a popular research tool in sport and rehabilitation sciences. Common study designs include the comparison of sEMG amplitudes collected from different muscles as participants perform various exercises and techniques under different loads. Based on such comparisons, researchers attempt to draw conclusions concerning the neuro- and electrophysiological underpinning of force production and hypothesize about possible longitudinal adaptations, such as strength and hypertrophy. However, such conclusions are frequently unsubstantiated and unwarranted. Hence, the goal of this review is to discuss what can and cannot be inferred from comparative research designs as it pertains to both the acute and longitudinal outcomes. General methodological recommendations are made, gaps in the literature are identified, and lines for future research to help improve the applicability of sEMG are suggested.
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Strength training is a valuable component of hamstring strain injury prevention programmes; however, in recent years a significant body of work has emerged to suggest that the acute responses and chronic adaptations to training with different exercises are heterogeneous. Unfortunately, these research findings do not appear to have uniformly influenced clinical guidelines for exercise selection in hamstring injury prevention or rehabilitation programmes. The purpose of this review was to provide the practitioner with an evidence-base from which to prescribe strengthening exercises to mitigate the risk of hamstring injury. Several studies have established that eccentric knee flexor conditioning reduces the risk of hamstring strain injury when compliance is adequate. The benefits of this type of training are likely to be at least partly mediated by increases in biceps femoris long head fascicle length and improvements in eccentric knee flexor strength. Therefore, selecting exercises with a proven benefit on these variables should form the basis of effective injury prevention protocols. In addition, a growing body of work suggests that the patterns of hamstring muscle activation diverge significantly between different exercises. Typically, relatively higher levels of biceps femoris long head and semimembranosus activity have been observed during hip extension-oriented movements, whereas preferential semitendinosus and biceps femoris short head activation have been reported during knee flexion-oriented movements. These findings may have implications for targeting specific muscles in injury prevention programmes. An evidence-based approach to strength training for the prevention of hamstring strain injury should consider the impact of exercise selection on muscle activation, and the effect of training interventions on hamstring muscle architecture, morphology and function. Most importantly, practitioners should consider the effect of a strength training programme on known or proposed risk factors for hamstring injury.
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Hamstring strain injury is one of most prevalent noncontact injuries in sports that involve high-speed running, such as sprinting, soccer, and rugby. 1 In order to optimize prevention strategies and injury rehabilitation, studies have been conducted to understand hamstring function during sprinting. 2-4 However, differences have long existed in the literature as to the cause of hamstring strain injuries. One of the most controversial topics is the debate over which phase of high-speed running is most associated with hamstring injuries. 5 Studies of running biomechanics indicate that the hamstrings are active for the entire gait cycle, with peaks in activation during the early stance and the late swing phases. 6,7 Mann and Sprague 3 reported that the highest torques of hip extension and knee flexion occur secondary to a peak value of the ground reaction forces (GRFs) during the initial stance phase. Based on this information, they concluded that the early stance was highly associated with hamstring strains. In contrast, many subsequent researchers held the view that the late swing phase of sprinting is the most hazardous. 4,6-9 These studies found that the hamstrings contract forcefully while reaching maximum length during the late swing phase. They ignored Mann's argument of high torques as an indicator of hamstring injury risk and preferred the hypothesis that hamstring strains occur during eccentric contractions. 10 However, most previous observers used treadmill sprinting rather than overground sprinting in their studies. 6,8,9 Although the treadmill is a convenient tool for assessment of running biomechanics, it has been shown that the biomechanics of treadmill running differ significantly from those of overground running, and thus may lead to erroneous conclusions about overground running. 11,12 Additionally, much of the previous research was aimed at investigating the kinematics of the hamstring during running alone. 7-9 Limited attempts have been made to measure the GRFs during overground sprinting and use these data to estimate the hamstring kinetics during stance. 3,4 To fill this gap, we investigated the loading conditions of the hamstring muscles during maximum-effort overground running. 2 Our results suggest that the hamstrings are most susceptible to injury during the swing and stance transitions of sprinting. We used a lower extremity intersegmental dynamics analysis for each body segment. 2,13 The intersegmental dynamics analysis we used allows for torques at each joint to be separated into 5 categories: gravitational torque (GTT), motion-dependent torque (MDT), external contact torque (EXT), generalized muscle torque (MST), and net joint torque (NET), which is the vector sum of the 4 previous components. Detailed interactions between the active muscle torques and the passive torque components could be quantified, giving us insight into how the hamstrings' function switches during the running cycle. Using this approach, we reached 3 main conclusions. First, the MST primarily countered the MDT during the swing phase for the knee and hip joints (Fig. 1A). In late swing, the leg was swinging forward due to its inertia, which cause a large hip-flexion MDT and a knee-extension MDT at the same time. Therefore, the hamstrings were active and started to extend the hip and flex the knee joints to counteract these passive effects for the subsequent ground contact (Fig. 1B). Further analysis of the components of the MDT showed that MDT at both joints was caused mainly by torques due to the leg angular acceleration. These passive torques applied stress to the hamstring muscles in the opposite direction of contraction at both joints. To counter this negative effect, the hamstrings encountered enormous loads, approximately 10 times the subjects' average body weight, to control the rapid leg rotation, which created conditions for hamstring injuries. Previous studies reported that the hamstrings stretch to their maximum length and the muscle force reaches its maximal value in this phase. 6-8 Our results confirmed these findings and showed how they happened. The key contributor to these high torques was the MDT created ScienceDirect mainly due to the leg angular acceleration. 2 Although there is debate as to whether eccentric muscle strain or muscle stress is the causative factor in muscle strain injuries, 1,10 it is known that an eccentric contraction occurs when the external force is greater than the muscle contraction force, that is, the eccentric muscle action is induced by an external force. During late swing, the leg angular acceleration led to a tremendous MDT, which caused the hamstring muscles to work eccentrically. This suggests that hamstring strains are associated with high loading caused by the inertial torque MDT. Second, the dominant passive torque switched to EXT in the transition from late swing to initial stance (Fig. 1C). We noticed that the GRFs passed anteriorly to the knee and hip joints during the initial stance phase, which generates a large extension torque at the knee and a flexion torque at the hip at the same time (Fig. 1D). As with the knee flexors and hip extensors in the late swing phase, the hamstring muscles serve both roles required to counteract the effect of the GRFs. It is likely that the hamstrings, which encounter at least 8 times the subjects' body weight in the initial stance phase, are susceptible to strain injury in this phase. This conclusion supports Mann's finding. 3 Additionally , we discovered that the external GRF passing anteriorly to the knee and hip generate the peak loads on the hamstrings. 2 As the early stance is a continuation of the late swing, the hamstrings were contracting concentrically after being fully extended. The muscles were suffering from enormous loads caused by 2 different factors (the inertia and the GRFs) throughout this eccentric-concentric transition. Chumanov et al. 6 indicated an increased loading for the hamstring muscles during the initial stance phase. However, they did not regard this phase as injurious because negative work (i.e., energy absorbed) during eccentric contraction has been shown to correlate best with muscle injuries in animal models. This is a widely held belief, despite experimental evidence of muscle strains being produced during concentric (shortening) contractions. 14 However, we currently cannot know Fig. 1. Averaged time-normalized graphs for joint torques at knee and hip joints during the swing (A) and stance (C) phases of sprinting. The top panels show positions of the lower extremity during the swing (A) and stance (C) phases. Data represent the group mean (lines) with 1SD (shading). (B) Diagram of sprinting during the late swing phase: the inertial loads (MDT) produced by segment motion at the knee and hip joints. (D) Diagram of sprinting during the initial stance phase: the GRF passes anteriorly to the knee and hip joints. EXT = external contact torque; GRF = ground reaction force; GTT = gravitational torque; MDT = motion-dependent torque; MST = muscle torque; NET = net torque. (Positive value indicates extension; negative value indicates flexion.) Adapted with permission. 2 134 Y. Liu et al. for certain if muscle strains are produced by the tremendous external forces during concentric contractions in the early stance of sprinting. In addition, we are aware of the evidence suggesting that loads on their own are not necessarily indicative of injury risk, but accumulated effects of biomechanical loads (i.e., musculotendon strain, velocity, force, power, and work) experienced by the hamstrings may result in hamstring strain injuries. We cannot state conclusively that high loading creates injury. However, we have evidence that the risk factors for hamstring injuries are high in both the late swing and the early stance phase for different loading mechanisms. Finally, unlike most previous research in which GRFs were not determined, 7-9 we took both kinematic and kinetic data into consideration 2 and examined overground sprinting at maximum effort in elite athletes. The average maximum speed in our study was 9.7 m/s, which approaches typical maximum sprinting speeds and associated enormous GRFs, and is higher than speeds achieved in previous studies. 4,6 It has been suggested that the hip and knee torques, which are estimated via the inverse dynamics approach, are particularly sensitive to the filter cutoff frequency, and the early portion of the stance phase is the most affected period. 15,16 Exaggerated fluctuations in the knee joint torques are data-processing artifacts rather than genuine characteristics of the joint kinetics. Therefore, it has been suggested that matched cutoff frequencies be used for both kinematic and kinetic data (i.e., 20-20 Hz) when applying inverse dynamics. Filtering at unmatched cutoff frequencies might affect, to some extent, the results obtained in our lab. However, one should not universally dismiss studies that use unmatched cutoff frequencies. Based on our results, the joint muscle torques counteract the EXT, which was caused by the GRFs during the stance phase. Careful examination of the raw curves of the GRFs reveal that the GRFs switch between passing in front and behind the knee joint during early stance. This phenomenon contributes to the fluctuations of the GRFs and affects the derivation of the joint muscle torque. Therefore, the peak values of the MST in early stance are not all artifacts. In addition, the aim of data filtering is to remove noise and reduce the attenuation of signals as much as possible. Data filtering must be based on the raw signals. To estimate if the filtered data are optimally processed, we need to compare the smoothed curve with the raw data curve. In the current study, we strictly followed the protocol for estimating optimum cutoff frequency. 17,18 The optimum cutoff frequency is not only a function of the residual between the filtered and unfiltered data but is also a function of the sampling frequency. Matched combinations of cutoff frequencies (i.e., 20-20 Hz) can potentially "over-smooth" the kinetic data, thereby removing crucial peak values of joint torques at the instant of foot strike, which explains why there were no fluctuations when using matched cutoff frequencies. Schache et al. 4 studied the mechanics of the hamstring muscles during overground sprinting, using an advanced mus-culoskeletal model accessed from OpenSim. They estimated the loads acting on individual muscles (semitendinosus, semimem-branosus, biceps femoris long head, and biceps femoris short head) based on the joint torques at the knee and hip obtained from inverse dynamics analysis. However, they did not find peak values during the early stance phase. Peak musculotendon forces for the bi-articular hamstrings would seem to have been underestimated in the early stance phase, and the authors attribute this to the limitations of the inverse dynamics-based static optimization combined with a minimum-stress performance criterion. However, in our opinion, this is a typical case in which over-filtered data were used for an inverse dynamics calculation. Compared with their previous results, which also indicated a peek knee flexion torque during the early stance phase, 19 the peak values might have been attenuated artificially. To sum up, during both the late swing and the initial stance phase, the large passive torques at the knee and hip joints acted to lengthen the hamstring muscles. The values of the flexion MST at the knee and the extension MST at the hip in those 2 phases were considerable, indicating that the knee flexors and hip extensors play an important role in sprint running, especially during the initial stance phase and the late swing phase. The active muscle torques generated mainly by the hamstrings counteracted the passive effects generated by the inertia of the leg (swing) and the external GRF (stance). Although different causes led to the high loads in the hamstrings in these 2 phases, we might think of these 2 phases as 1 period, the swing-stance transition period, because the motions of the lower-extremity are continuous and the hamstring muscles function to extend the hip and flex the knee throughout the entire phase. As a result, during sprinting or high-speed locomotion, the hamstring muscles may be more susceptible to strain injury during the swing-stance transition than during any other phase in sprint running. One limitation of our research is that the method for estimating muscle torques across a joint does not reveal an individual muscle's contributions to the joint torque. In addition, passive structures also contribute to the joint torques at the knee and hip. Because the hamstring muscles are the most injured muscles during sprinting 20 and are the only bi-articular muscles that flex the knee and extend the hip, we focused our MST-related discussion on the hamstring musculature. Future studies need to consider the role of other active and passive structures that cross the hip and knee joints.
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This study aimed to investigate activation characteristics of the biceps femoris long head (BFlh) and semitendinosus (ST) muscles during the acceleration and maximum-speed phases of sprinting. Lower-extremity kinematics and electromyographic (EMG) activities of the BFlh and ST muscles were examined during the acceleration sprint and maximum-speed sprint in 13 male sprinters during an overground sprinting. Differences in hamstring activation during each divided phases and in the hip and knee joint angles and torques at each time point of the sprinting gait cycle were determined between two sprints. During the early stance of the acceleration sprint, the hip extension torque was significantly greater than during the maximum-speed sprint, and the relative EMG activation of the BFlh muscle was significantly higher than that of the ST muscle. During the late stance and terminal mid-swing of maximum-speed sprint, the knee was more extended and a higher knee flexion moment was observed compared to the acceleration sprint, and the ST muscle showed higher activation than that of the BFlh. These results indicate that the functional demands of the medial and lateral hamstring muscles differ between two different sprint performances.
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The aim of this study was to quantify the contributions of lower extremity joint torques and the mechanical power of lower extremity muscle groups to further elucidate the loadings on hamstring and the mechanics of its injury. Eight national-level male sprinters performed maximum-velocity sprint running on a synthetic track. The 3D kinematic data and ground reaction force (GRF) were collected synchronously. Intersegmental dynamics approach was used to analyze the lower extremity joint torques and power changes in the lower extremity joint muscle groups. During sprinting, the GRF during the stance phase and the motion-dependent torques (MDT) during the swing phase had a major effect on the lower extremity movements and muscle groups. Specifically, during the stance phase, torque produced and work performed by the hip and knee muscles were generally used to counteract the GRF. During the swing phase, the role of the muscle torque changed to mainly counteract the effect of MDT to control the movement direction of the lower extremity. Meanwhile, during the initial stance and late swing phases, the passive torques, namely, the ground reaction torques and MDT produced by the GRF and the inertial movement of the segments of the lower extremity, applied greater stress to the hamstring muscles.
Recent studies suggest region-specific metabolic activity in hamstring muscles during injury prevention exercises, but the neural representation of this phenomenon is unknown. The aim of this study was to examine whether regional differences are evident in the activity of biceps femoris long head (BFlh) and semitendinosus (ST) muscles during two common injury prevention exercises. Twelve male participants without a history of hamstring injury performed the Nordic hamstring exercise (NHE) and stiff-leg deadlift (SDL) while BFlh and ST activity were recorded with high-density electromyography (HD-EMG). Normalised activity was calculated from the distal, middle, and proximal regions in the eccentric phase of each exercise. In NHE, ST overall activity was substantially higher than in BFlh (d = 1.06 ±0.45), compared to trivial differences between muscles in SDL (d = 0.19 ±0.34). Regional differences were found in NHE for both muscles, with different proximal-distal patterns: the distal region showed the lowest activity level in ST (regional differences, d range = 0.55 – 1.41) but the highest activity level in BFlh (regional differences, d range = 0.38 – 1.25). In SDL, regional differences were smaller in both muscles (d range = 0.29 – 0.67 and 0.16 – 0.63 in ST and BFlh, respectively) than in NHE. The use of HD-EMG in hamstrings revealed heterogeneous hamstrings activity during typical injury prevention exercises. High-density EMG might be useful in future studies to provide a comprehensive overview of hamstring muscle activity in other exercises and high-injury risk tasks. LINK FOR THE FULL TEXT AT THE COMMENTS BELOW
Background Although the vast majority of hamstring injuries in male soccer are sustained during high speed running, the association between sprinting kinematics and hamstring injury vulnerability has never been investigated prospectively in a cohort at risk. Purpose This study aimed to objectify the importance of lower limb and trunk kinematics during full sprint in hamstring injury susceptibility. Study Design Cohort study; level of evidence, 2. Methods At the end of the 2013 soccer season, three-dimensional kinematic data of the lower limb and trunk were collected during sprinting in a cohort consisting of 30 soccer players with a recent history of hamstring injury and 30 matched controls. Subsequently, a 1.5 season follow up was conducted for (re)injury registry. Ultimately, joint and segment motion patterns were submitted to retro- and prospective statistical curve analyses for injury risk prediction. Results Statistical analysis revealed that index injury occurrence was associated with higher levels of anterior pelvic tilting and thoracic side bending throughout the airborne (swing) phases of sprinting, whereas no kinematic differences during running were found when comparing players with a recent hamstring injury history with their matched controls. Conclusion Deficient core stability, enabling excessive pelvis and trunk motion during swing, probably increases the primary injury risk. Although sprinting encompasses a relative risk of hamstring muscle failure in every athlete, running coordination demonstrated to be essential in hamstring injury prevention.