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Background: Hamstring strain injuries (HSIs) are the most common injury type in Australian football, and the rate of recurrence has been consistently high for a number of years. Long-lasting neuromuscular inhibition has been noted in previously injured athletes, but it is not known if this influences the athlete's adaptive response to training. Purpose: To determine if elite Australian footballers with a prior unilateral HSI (previously injured group) display less improvement in eccentric hamstring strength during preseason training compared with athletes without a history of HSIs (control group). Study design: Cohort study; Level of evidence, 2. Methods: A total of 99 elite Australian footballers (17 with a history of unilateral HSIs in the previous 12-month period) participated in this study. Eccentric hamstring strength was assessed at the start and end of preseason training using an instrumented Nordic hamstring device. The change in eccentric strength across the preseason was determined in absolute terms and normalized to the start of preseason strength. The start of preseason strength was used as a covariate to control for differences in starting strength. Results: The left and right limbs in the control group showed no difference in absolute or relative change (left limb: 60.7 ± 72.9 N and 1.28 ± 0.34 N, respectively; right limb: 48.6 ± 83.8 N and 1.24 ± 0.43 N, respectively). Similarly, the injured and uninjured limbs in the previously injured group showed no difference in either absolute or relative change (injured limb: 13.1 ± 57.7 N and 1.07 ± 0.18 N, respectively; uninjured limb: 14.7 ± 54.0 N and 1.07 ± 0.22 N, respectively). The previously injured group displayed significantly less increase in eccentric hamstring strength across the preseason (absolute change, 13.9 ± 55.0 N; relative change, 1.07 ± 0.20 N) compared with the control group (absolute change, 54.6 ± 78.5 N; relative change, 1.26 ± 0.39 N) for both absolute and relative measures (P < .001), even after controlling for differences in the start of preseason eccentric hamstring strength, which had a significant effect on strength improvement. Conclusion: Elite Australian footballers with a unilateral history of HSIs displayed less improvement in eccentric hamstring strength across preseason training. The smaller improvements were not restricted to the previously injured limb as the contralateral limb also displayed similarly small improvements in eccentric strength. Whether this is the cause of or the result of an injury remains to be seen, but it has the potential to contribute to the risk of hamstring strain reinjuries.
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Medicine
The American Journal of Sports
http://ajs.sagepub.com/content/early/2014/11/13/0363546514556638
The online version of this article can be found at:
DOI: 10.1177/0363546514556638
published online November 14, 2014Am J Sports Med
David A. Opar, Morgan D. Williams, Ryan G. Timmins, Jack Hickey, Steven J. Duhig and Anthony J. Shield
During Preseason Training in Elite Australian Footballers
The Effect of Previous Hamstring Strain Injuries on the Change in Eccentric Hamstring Strength
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The Effect of Previous Hamstring Strain
Injuries on the Change in Eccentric
Hamstring Strength During Preseason
Training in Elite Australian Footballers
David A. Opar,
*
y
PhD, Morgan D. Williams,
z
PhD, Ryan G. Timmins,
y
Jack Hickey,
§
Steven J. Duhig,
||
and Anthony J. Shield,
||{
PhD
Investigation performed at the Queensland University of Technology,
Brisbane, Queensland, Australia
Background: Hamstring strain injuries (HSIs) are the most common injury type in Australian football, and the rate of recurrence
has been consistently high for a number of years. Long-lasting neuromuscular inhibition has been noted in previously injured ath-
letes, but it is not known if this influences the athlete’s adaptive response to training.
Purpose: To determine if elite Australian footballers with a prior unilateral HSI (previously injured group) display less improvement
in eccentric hamstring strength during preseason training compared with athletes without a history of HSIs (control group).
Study Design: Cohort study; Level of evidence, 2.
Methods: A total of 99 elite Australian footballers (17 with a history of unilateral HSIs in the previous 12-month period) participated
in this study. Eccentric hamstring strength was assessed at the start and end of preseason training using an instrumented Nordic
hamstring device. The change in eccentric strength across the preseason was determined in absolute terms and normalized to
the start of preseason strength. The start of preseason strength was used as a covariate to control for differences in starting
strength.
Results: The left and right limbs in the control group showed no difference in absolute or relative change (left limb: 60.7 672.9 N
and 1.28 60.34 N, respectively; right limb: 48.6 683.8 N and 1.24 60.43 N, respectively). Similarly, the injured and uninjured
limbs in the previously injured group showed no difference in either absolute or relative change (injured limb: 13.1 657.7 N and
1.07 60.18 N, respectively; uninjured limb: 14.7 654.0 N and 1.07 60.22 N, respectively). The previously injured group dis-
played significantly less increase in eccentric hamstring strength across the preseason (absolute change, 13.9 655.0 N; relative
change, 1.07 60.20 N) compared with the control group (absolute change, 54.6 678.5 N; relative change, 1.26 60.39 N) for both
absolute and relative measures (P\.001), even after controlling for differences in the start of preseason eccentric hamstring
strength, which had a significant effect on strength improvement.
Conclusion: Elite Australian footballers with a unilateral history of HSIs displayed less improvement in eccentric hamstring
strength across preseason training. The smaller improvements were not restricted to the previously injured limb as the contra-
lateral limb also displayed similarly small improvements in eccentric strength. Whether this is the cause of or the result of an injury
remains to be seen, but it has the potential to contribute to the risk of hamstring strain reinjuries.
Keywords: hamstring; muscle injury; eccentric strength; Nordic hamstring exercise
Over the past 20 seasons, hamstring strain injuries (HSIs)
have been the most prevalent injuries in Australian foot-
ball,
18
and they impose a significant financial burden on
athletes and their associated clubs.
9
While the rate of
recurrent HSIs in the elite Australian Football League
has fallen in recent years,
17
it still remains one of the
most common types of injury for recurrence.
18
In
Australian football, much like other sports,
1,8
the history
of HSIs is repeatedly identified as the primary risk factor
for future injuries
6,25
and is often considered a nonmodifi-
able risk factor (ie, it cannot be changed).
14
However,
a growing body of evidence indicates that neuromuscular
maladaptations associated with previous HSIs may be
responsible for the elevated risk of future injuries, despite
returning to play and ‘‘successful’’ rehabilitation.
15,16,21,22
Most notably, hamstring muscles that have previously sus-
tained a strain injury display signs of neuromuscular inhi-
bition during eccentric contractions when compared with
contralateral, uninjured hamstring muscles.
15,16,22
The
The American Journal of Sports Medicine, Vol. XX, No. X
DOI: 10.1177/0363546514556638
Ó2014 The Author(s)
1
AJSM PreView, published on November 14, 2014 as doi:10.1177/0363546514556638
at Queensland University of Tech on December 8, 2014ajs.sagepub.comDownloaded from
resultant deficits in eccentric knee flexor strength might
reasonably be expected to increase the likelihood of future
HSIs in this limb, given that lower levels of eccentric ham-
string strength increase the risk of future injuries.
4,23
A recent review
5
proposed a novel framework, suggest-
ing that persistent neuromuscular inhibition during eccen-
tric contraction after an HSI
15,16,22
could lead to continued
eccentric weakness and thus an elevated risk of reinju-
ries.
14
Based on the proposed framework, it would be
expected that this inhibition has the potential to limit
the extent of muscular adaptations in response to rehabil-
itative and prophylactic exercises, given the need for high
levels of activation to drive adaptation.
5,14
If this were the
case, athletes with a previous HSI might not only show def-
icits in eccentric hamstring strength in the previously
injured limb but may also show a suppressed response to
eccentric training interventions that are commonly uti-
lized in prophylactic programs. The effect of a prior HSI
on the adaptive capacity of a previously injured athlete
is, however, yet to be examined.
In the elite Australian Football League, the preseason
training period spans up to 4 months between November
and February.
24
It is a time in the training cycle when
teams focus on increasing physical fitness with an aim to
improve performance and avoid injuries.
24
From the per-
spective of preventing HSIs, it is common to target gains
in eccentric hamstring strength as one of the major out-
comes during the preseason period. Much of this philoso-
phy is based on evidence showing the preventive benefits
of eccentric hamstring strengthening during the preseason
in other sports.
2,4,19
There is currently no work that exam-
ines the improvements in eccentric hamstring strength
throughout the preseason training period in elite Austra-
lian footballers and whether a previous HSI affects the
athlete’s ability to improve eccentric hamstring strength.
The purpose of this investigation was to assess eccentric
hamstring strength changes during the preseason training
period in elite Australian footballers with and without
a history of unilateral HSIs. We hypothesized that athletes
with a history of HSIs would exhibit a minimal increase in
eccentric hamstring strength during the preseason train-
ing period compared with uninjured athletes.
MATERIALS AND METHODS
Sample Size Calculations
Based on a previous study,
22
which used a similar research
design, an a priori sample size of 15 for the previously
injured group and 75 for the control group was determined
using G*Power (v 3.1.7). The input parameters for the
power analysis were the following: independent ttest,
effect size (d) = 0.8, a= .05, b= .20, and allocation ratio
of 5:1. An independent ttest was selected because the
change in eccentric hamstring strength for both limbs
was expected to be averaged and then compared between
groups, as performed previously,
22
given that the adaptive
capacity would be centrally impaired and not limb spe-
cific.
5
A large effect size was anticipated based on Rhea
et al,
20
and the 5:1 sample ratio was based on typical ham-
string injury rates of 15% to 20%.
14
Participants
A total of 99 Australian footballers from 5 elite teams were
eligible to participate (from an overall pool of 210) in the
study, of whom 17 had a history of unilateral HSIs (previ-
ously injured group), confirmed by magnetic resonance
imaging (MRI), in the previous 12-month period. All partic-
ipants were free of injuries to the lower limbs (able to par-
ticipate fully in training), which would be expected to
influence knee flexor strength at the time of testing. Exclu-
sion criteria included any athlete with a history of bilateral
HSIs in the prior 12 months, any athlete with a history of
clinically diagnosed HSIs that were negative on MRI scans
in the prior 12 months, any athlete who sustained an HSI
during the preseason, and any athlete who had sustained
an anterior cruciate ligament rupture previously or who
had sustained an injury to the quadriceps, calf, or groin/
hip in the prior 12 months. All testing procedures were
approved by the university’s human research ethics com-
mittee, and participants gave informed written consent
before testing after having all procedures explained to
them.
Experimental Design
The current study employed a prospective cohort design.
All athletes reported for testing during the first and final
weeks of preseason training (November-February). On
each occasion, all athletes completed a submaximal
warm-up set of the Nordic hamstring exercise, followed
by a single set of 3 maximal repetitions of the Nordic ham-
string exercise, during which eccentric knee flexor forces of
the left and right limbs were recorded using a custom-
made device. All testing was performed after similar levels
(duration and intensity) of training completed in the days
previously.
*
Address correspondence to David A. Opar, PhD, School of Exercise Science, Australian Catholic University, 17 Young Street, Fitzroy, VIC, 3065, Aus-
tralia (e-mail: david.opar@acu.edu.au).
y
School of Exercise Science, Australian Catholic University, Melbourne, Victoria, Australia.
z
Faculty of Health, Sport and Science, University of South Wales, Pontypridd, Wales, UK.
§
MD Health Pilates, Melbourne, Victoria, Australia.
||
School of Exercise and Nutrition Science, Queensland University of Technology, Brisbane, Queensland, Australia.
{
Institute of Health and Biomedical Innovation, Queensland University of Technology, Brisbane, Queensland, Australia.
One or more of the authors has declared the following potential conflict of interest or source of funding: D.A.O. and A.J.S. are listed as co-inventors on
a patent application for the eccentric hamstring field testing device used in the current study. Funding was provided by Qutbluebox (http://www.qutblue
box.com.au/), the innovation and technology transfer affiliate of the Queensland University of Technology. Funding was provided to assist with the com-
pletion of the study and to cover costs associated with filing the patient application for the hamstring field testing device.
2Opar et al The American Journal of Sports Medicine
at Queensland University of Tech on December 8, 2014ajs.sagepub.comDownloaded from
Eccentric Knee Flexor Strength Assessment
The device used to determine eccentric knee flexor strength
during the Nordic hamstring exercise and its reliability
have been described previously and can be seen in Figures
1and2.
13
Participants knelt on a padded board, with the
ankles secured immediately superior to the lateral malleolus
by individual ankle braces that were attached to custom-
made uniaxial load cells (Delphi Force Measurement) with
wireless data acquisition capabilities (Mantracourt). The
ankle braces and load cells were secured to a pivot, which
allowedtheforcetoalwaysbemeasuredthroughthelong
axis of the load cells, with an individual load cell for both
the left and right limbs, allowing for separate measurements
from each limb. After a warm-up set, participants performed
1 set of 3 maximal repetitions of the bilateral Nordic ham-
string exercise. Instructionstoplayersweretogradually
lean forward at the slowest possible speed while maximally
resisting this movement with both limbs, while keeping the
trunk and hips held in a neutral position throughout and
the hands held across the chest.
13
Participants were loudly
exhorted to provide maximal effort throughout each repeti-
tion. A trial was deemed acceptable when the force output
reached a distinct peak (indicative of maximal eccentric
strength), followed by a rapid decline in force that occurred
whentheathletewasnolongerabletoresisttheeffectsof
gravity acting on the segment above the knee joint.
History of Injuries
For all athletes recruited who had sustained a unilateral
HSI in the 12 months before the first testing session,
details of their history of injuries were obtained from their
club clinician. Details obtained included which limb was
injured (dominant/nondominant limb), muscle injured
(long head of the biceps femoris/short head of the biceps
femoris/semimembranosus/semitendinosus), location of
injury (proximal/distal, muscle belly/muscle-tendon junc-
tion), activity type performed at the time of injury
(running/kicking, etc), and grade of injury (I, II, or III).
Importantly, all diagnoses were confirmed by MRI per-
formed 48 to 72 hours after the insult.
Preseason Training Programs
With regard to prophylactic programs for the prevention of
HSIs, all clubs utilized the Nordic hamstring exercise and
stiff-legged (or Romanian) deadlift as part of their training
regimen. Typical set and repetition ranges for the Nordic
hamstring exercise were 2 to 4 sets with 6 to 10 repetitions.
These prophylactic exercises were completed at least on
a weekly basis by all teams included in the study. In addition,
there was a strong focus on exercises that aimed to increase
eccentric hamstring strength using a combination of bilateral
and unilateral movements. Often, athletes with a history of
HSIs were prescribed additional eccentric exercises as part
of the effort to further reduce their risk of reinjuries.
Data Analysis
Force data for both limbs during the Nordic hamstring exer-
cise were logged into a personal computer at 100 Hz through
a wireless USB base station receiver (Mantracourt). For
both limbs (left/right for the control group or injured/
uninjured for the previously injured group), peak force for
each contraction was determined, and maximal force-
Figure 1. Performing the Nordic hamstring exercise using the novel device (progressing from left to right). The participant con-
trols the speed of the fall by forceful eccentric contraction of the knee flexors. After completion of the exercise, the participant
slowly returns to the starting position by pushing back up with both hands (not shown). The ankles are secured independently
in individual custom-made braces.
Figure 2. Close-up view of the ankle brace and load cell
organization with the participant’s limb in position during
the Nordic hamstring exercise.
Vol. XX, No. X, XXXX Eccentric Hamstring Strength in the AFL 3
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generating capacity was expressed as the mean of the peak
from 3 contractions (mean peak force). This method of anal-
ysis was chosen because it has displayed high test-retest
reliability (intraclass correlation coefficient, 0.85-0.89).
13
The change in eccentric strength across the preseason was
expressed in absolute units (newtons) as well as relative
to the early preseason strength measure by taking the quo-
tient of late preseason and early preseason strength.
Statistical Analysis
Data were screened, and all test assumptions were
assessed to confirm the appropriateness of the analyses.
The change in eccentric hamstring strength across the pre-
season was compared between the left and right limbs of
the control group and between the retrospectively injured
and uninjured limbs in the previously injured group using
a 2-tailed paired-samples ttest. As no within-group differ-
ences were noted, the 2 limbs for each group were aver-
aged. To compare between the control and previously
injured groups, a univariate general linear model was
employed, with eccentric knee flexor strength at the start
of the preseason used as a covariate, to control for differ-
ences in baseline strength because it was different between
groups. Statistical significance was set at P\.05 and the
Cohen dused to assess the magnitude of the effect. Data
are reported as mean differences 6SD or, if stated, 95%
CI. All statistical analyses and assumption testing were
performed using SPSS v 19.0.0.1 (IBM Corp).
RESULTS
Of the 17 athletes with a history of unilateral HSIs in the
prior 12 months, the injuries were distributed accordingly:
dominant limb (53%), long head of the biceps femoris
(76%), and the proximal muscle-tendon junction (53%)
(Table 1). Time since the most recent HSI ranged from
1.5 to 12 months (median time since injury, 4.4 months/
19 weeks), with the rehabilitation time ranging from 19
to 79 days (median rehabilitation time, 31 days). The dis-
tribution of these 17 athletes at each of the 5 participating
clubs was 5, 4, 4, 3, and 1, respectively. All athletes (and
associated medical staff) reported a strong emphasis on
eccentric conditioning and high-speed running during
late-stage rehabilitation and in the lead-up to return to
play.
Descriptive statistics for both groups with respect to
demographic data and absolute levels of eccentric ham-
string strength at the start and end of the preseason can
be found in Table 2. While the previously injured athletes
presented with generally higher levels of eccentric
strength compared with athletes in the control group, the
only significant difference was that the left limb in athletes
in the control group was weaker than the uninjured limb in
athletes with a previously injured limb (P= .020). With
respect to the change in eccentric hamstring strength
across the preseason, the left and right limbs of athletes
in the control group showed no difference in either absolute
or relative measures of change (left limb: 60.7 672.9 N and
1.28 60.34 N, respectively; right limb: 48.6 683.8 N and
1.24 60.43 N, respectively) (Table 3). Similarly, the
injured and uninjured limbs of athletes in the previously
injured group showed no difference in either absolute or
relative measures of change (injured limb: 13.1 657.7 N
and 1.07 60.18 N, respectively; uninjured limb: 14.7 6
54.0 N and 1.07 60.22 N, respectively) (Table 3).
Given that there were no differences in the change in
eccentric hamstring strength between the left and right
limbs of athletes in the control group (left vs right: absolute
TABLE 1
Details of Prior Hamstring Strain Injuries Sustained by Athletes in the Injured Group
a
Participant
Limb
Injured
Muscle
Injured
Location
of Injury
Activity at
Time of Injury
Rehabilitation Time for
Most Recent Injury, d
Time Between Most Recent Injury
and First Strength Testing Session, wk
1 D SM Proximal MTJ Running 62 14
2 D BFlh Distal MTJ Running 31 17
3 D BFlh Proximal MTJ Kicking 76 31
4 ND ST Muscle belly Running 25 24
5 ND BFlh Proximal MTJ Running 19 9
6 ND SM Proximal tendon Bending forward 79 30
7 D ST Distal MTJ Running 21 52
8 D BFlh Proximal MTJ Running 72 32
9 D BFlh Muscle belly Running/kicking 32 15
10 D BFlh Muscle belly Running 23 40
11 ND BFlh Muscle belly Not defined 26 25
12 ND BFlh Proximal MTJ Running 33 35
13 ND BFlh Proximal MTJ Running 60 16
14 ND BFlh Distal MTJ Running 23 19
15 D BFlh Proximal MTJ Bending forward 35 6
16 ND BFlh Proximal MTJ Running 21 12
17 D BFlh Proximal MTJ Running 19 13
a
BFlh, long head of the biceps femoris; D, dominant; MTJ, muscle-tendon junction; ND, nondominant; SM, semimembranosus; ST,
semitendinosus.
4Opar et al The American Journal of Sports Medicine
at Queensland University of Tech on December 8, 2014ajs.sagepub.comDownloaded from
change, P= .06, d= 0.15; relative change, P= .291, d=
0.10), the results of the 2 limbs were averaged to give
a mean control group change in eccentric hamstring
strength. Similarly, for the previously injured group, as
there was no difference between limbs (injured vs unin-
jured: absolute change, P= .88, d= 0.03; relative change,
P= .934, d= 0.00), the results of the injured and uninjured
limbs were also averaged to give a mean injured group
change in eccentric hamstring strength. Athletes in the
previously injured group displayed a significantly smaller
increase in eccentric hamstring strength across the presea-
son (absolute change, 13.9 655.0 N; relative change,
1.07 60.20 N) compared with those in the control group
(absolute change, 54.6 678.5 N; relative change, 1.26 6
0.39 N) for both absolute and relative measures, even after
controlling for differences in the start of preseason eccen-
tric hamstring strength. The start of preseason eccentric
hamstring strength had a significant effect (P\.001) on
both absolute and relative strength changes (Table 4).
DISCUSSION
The present study aimed to determine if elite Australian foot-
ballers with a history of unilateral HSIs (within the prior 12
months) would display a smaller increase in eccentric ham-
string strength across the preseason training period com-
pared with athletes without a history of HSIs. The major
finding was that the previously injured athletes displayed
smaller increases in eccentric hamstring strength compared
with the control group athletes, who had no history of HSIs
in the prior 12 months. Interestingly, the smaller increase
in eccentric strength across the preseason was not restricted
to the previously injured limb, as increases between injured
and uninjured limb strength did not differ.
This study is, to our knowledge, the first to examine the
change in eccentric hamstring strength across the presea-
son training period in elite Australian footballers. One
study, a randomized controlled trial (RCT) in elite Swedish
soccer players, examined the effect of augmented eccentric
training, via a flywheel ergometer, for the hamstrings
across 10 weeks of preseason training and reported an
approximately 19% increase in eccentric hamstring tor-
que.
2
The improvements in the control group in the present
study are similar in magnitude (15%-20%) to those
reported by Askling et al
2
in the training arm of their trial;
however, the effect of previous HSIs on eccentric strength
improvements was not examined.
The finding that athletes with a history of HSIs dis-
played a smaller increase in eccentric strength during
the preseason might have implications for recurrent inju-
ries. Given the retrospective nature of these observations,
it is impossible to determine whether a smaller increase in
eccentric strength is the result of injury and/or a predispos-
ing factor that leads to the initial insult. It is also possible
that a heavy focus on eccentric exercise during the late
stage of rehabilitation could influence the change in eccen-
tric hamstring strength during the subsequent preseason
training period. Regardless, given the established link
between prior HSIs and the increased risk of future inju-
ries in elite Australian football,
6,25
characteristics of previ-
ously injured athletes can help to identify variables that
warrant further investigation. Of interest from the current
data set is the possibility that athletes display variable
increases in eccentric hamstring strength (ie, high and
low responders) across preseason training. As eccentric
strengthening interventions
2,19
and smaller between-limb
eccentric strength imbalances
23
appear to reduce the risk
of HSIs, players with a reduced ability to increase eccentric
hamstring strength might be predisposed to a greater like-
lihood of future HSIs. Further work should consider the
implementation of a standard eccentric hamstring
strengthening intervention across a large participant pool
to determine the spectrum of strength increases, with
these participants followed prospectively to establish if
there is a causative relationship with HSIs.
It should also be acknowledged that rehabilitation pro-
cesses would likely play a critical role in the recovery of
eccentric strength after an HSI and might also influence
the adaptive response to eccentric exercise. It would be of
interest to examine increases in eccentric strength and
adaptive capacity in previously injured athletes who are
exposed to standardized rehabilitation protocols, such as
those reported previously.
3
It is also intriguing that the
injured athletes displayed smaller increases in eccentric
strength across the preseason but that there was no differ-
ence noted between the injured and uninjured limbs within
this group. This raises the possibility that persistent neu-
romuscular inhibition noted during eccentric contractions
after unilateral HSIs
15,16,22
may be mediated by central
mechanisms and as such have bilateral effects. Further-
more, it is possible that differences between the injured
and control groups with respect to eccentric hamstring
strength at the start of the preseason (ie, baseline
strength) may have affected the improvements seen in
TABLE 2
Demographic and Eccentric Knee Flexor
Strength Data for All Athletes
Variable Mean 6SD
Uninjured group (n = 82)
Age, y 22.6 63.3
Height, cm 188.3 67.6
Weight, kg 87.8 67.6
Early preseason eccentric strength, N
Left limb 271.9 674.8
Right limb 290.8 684.4
Late preseason eccentric strength, N
Left limb 327.7 673.5
Right limb 336.9 671.0
Previously injured group (n = 17)
Age, y 23.3 62.6
Height, cm 186.2 66.5
Weight, kg 85.9 66.6
Early preseason eccentric strength, N
Injured limb 297.9 689.6
Uninjured limb 310.9 682.7
Late preseason eccentric strength, N
Injured limb 311.0 682.6
Uninjured limb 325.6 682.0
Vol. XX, No. X, XXXX Eccentric Hamstring Strength in the AFL 5
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strength across the preseason. It might be argued that the
higher starting strength in the injured group would limit
the scope for improvement across the preseason; however,
ongoing subsequent work from our group suggests that
approximately 340 N is not close to the maximal strength
capacity of most elite Australian footballers, with scores
well in excess of 400 N noted in well-trained athletes.
When the start of preseason eccentric strength was con-
trolled for as a covariate in the analysis, differences
between the groups still persisted. It should also be noted
that when examining the increase in eccentric hamstring
strength in athletes from both groups in the bottom quar-
tile for eccentric strength at the start of the preseason, ath-
letes in the control group (mean start of preseason
strength, 195 N) displayed an approximately 55% increase
in eccentric strength while those in the previously injured
group (mean start of preseason strength, 194 N) increased
approximately 20%.
The limitation of eccentric hamstring strength gains in
the athlete with a previous HSI, as reported in the current
study, is intriguing as a large RCT has shown that the
implementation of the Nordic hamstring exercise during
the preseason in soccer players resulted in a significant
reduction in the rate of reinjuries.
19
It would be reasonable
to posit that the significant reduction in reinjuries was con-
ferred by an increase in eccentric hamstring strength after
the Nordic hamstring exercise intervention.
12
The results
from the current study suggest that eccentric strength
improvements may have been restricted in the previously
injured athletes; however, the cohort from the RCT
19
con-
sisted of soccer players without a history of eccentric train-
ing of the hamstrings before the intervention. This differs
significantly from the cohort of elite Australian footballers
who employ targeted eccentric exercise as part of the late
stages of rehabilitation and return to play and generally
for the prevention of HSIs. It remains to be seen if greater
magnitudes of, or larger improvements in, eccentric ham-
string strength, assessed during the performance of the
Nordic hamstring exercise, reduce the risk of future HSIs.
Besides a history of unilateral HSIs, other factors may
be responsible for the divergent responses between the 2
groups. First, the strong focus on eccentric exercise during
the late stages of rehabilitation has the potential to influ-
ence eccentric strength and the change in strength across
TABLE 4
Absolute and Relative Change in Eccentric Knee Flexor Strength Between Groups
Group
Absolute Change
in Eccentric Strength,
Mean 6SD, N
Between-Group
Difference
(95% CI), N PValue
Effect
Size
a
Relative Change in
Eccentric Strength,
Mean 6SD,
b
N
Between-Group
Difference
(95% CI), N PValue
Effect
Size
a
Previously injured
group (n = 17)
13.9 655.0 40.7 (1.0-80.4) .012
c
0.60 1.07 60.20 0.19 (0.0-0.38) .015
c
0.73
Uninjured group
(n = 82)
54.6 678.5 1.26 60.39
Covariate effect (early
preseason eccentric
strength)
\.001
d
\.001
d
a
Cohen dwas used to determine the effect size.
b
Change was determined as the quotient of late over early preseason eccentric hamstring strength.
c
Significance was set at P\.05, with the start of preseason eccentric strength employed as a covariate in a general linear model.
d
Early preseason strength had a significant effect as a covariate.
TABLE 3
Absolute and Relative Change in Eccentric Knee Flexor Strength Between Limbs
Group
Absolute Change in
Eccentric Strength,
Mean 6SD, N
Between-Limb
Difference
(95% CI), N PValue
Effect
Size
a
Relative Change in
Eccentric Strength,
Mean 6SD,
b
N
Between-Limb
Difference
(95% CI), N PValue
Effect
Size
a
Previously injured
group (n = 17)
Uninjured limb 14.7 654.0 1.6 (–37.4 to 40.6) .88 0.03 1.07 60.22 0.00 (–0.14 to 0.14) .934 0.00
Injured limb 13.1 657.7 1.07 60.18
Uninjured group
(n = 82)
Left limb 60.7 672.9 12.1 (–12.1 to 36.3) .06 0.15 1.28 60.34 0.04 (–0.08 to 0.16) .291 0.10
Right limb 48.6 683.8 1.24 60.43
a
Cohen dwas used to determine the effect size.
b
Change was determined as the quotient of late over early preseason eccentric hamstring strength.
6Opar et al The American Journal of Sports Medicine
at Queensland University of Tech on December 8, 2014ajs.sagepub.comDownloaded from
the preseason period. Indeed, the low between-limb
strength imbalance in the previously injured group at the
start of the preseason (1.3%), which was much smaller
than in previous reports using the current strength assess-
ment device (15%),
13
is suggestive that rehabilitation in
this cohort aimed to minimize any deficits in eccentric
strength. The influence of rehabilitation procedures, across
the spectrum of HSI severities, on long-lasting deficits in
function and response to training stimuli is an area of
great interest for future investigations. Second, the physi-
ological demands of Australian football require athletes at
the elite level to possess high aerobic and anaerobic fitness,
maximal sprint speeds, repeat sprint performance, and
strength and power qualities.
7
These diverse demands
require an intense training load for athletes, particularly
during preseason training. However speculative, it is pos-
sible that the multiple physiological demands of preseason
training might minimize improvements in certain perfor-
mance markers in some athletes.
10
If some athletes strug-
gle to improve strength/power qualities (such as eccentric
hamstring strength), then it would be reasonable to sug-
gest that their risk of HSI would be greater.
4
It is possible
that the athletes of the previously injured group in the cur-
rent work had, in prior seasons, improved eccentric ham-
string strength minimally because of the competing
demands of preseason training, predisposing them to inju-
ries, and that phenomenon (a low responder to strength
training) was measured here more so than the effect of
prior injuries. The complex interaction of the numerous
factors that can affect strength gains during preseason
training in elite athletes certainly requires greater focus,
particularly given the important role that strength plays
in injury prevention.
11
There are some limitations inherent to this study. The
investigators had no control over the preseason training
programs of any team involved (as is to be expected in an
elite sporting environment), as this study was purely
observational. While we were able to report general details
of the preseason HSI prophylactic program, we are not able
to comment as to whether differing training programs
between athletes and/or teams may have influenced the
findings. In spite of this, these observations were made
on 99 athletes across 5 elite Australian football teams, sug-
gesting that the results may be generalizable within this
sport. Furthermore, the history of HSIs was confined to
the previous 12 months to minimize reporting errors, and
this neglects HSIs that occurred before this time period.
Severe HSIs sustained more than 12 months ago may
have confounded the current findings. Importantly, how-
ever, all HSIs were confirmed by MRI to eliminate the
inclusion of athletes suffering referred pain from posterior
thigh injuries, and this is a strength of the current investi-
gation.
25
Finally, while the study was sufficiently powered
to detect between-group differences, given the relatively
small sample of previously injured athletes, it was under-
powered to explore the possible effect of time since injury,
the number and severity of previous HSIs, rehabilitation
type and length, and the possible role of other lower limb
injuries on improvements in eccentric hamstring strength
across the preseason. A larger study examining a more
homogeneous sample of HSIs, powered to include addi-
tional covariates, is warranted in the future. A larger sam-
ple would also allow for an analysis to control for cluster
effects by team, which was not possible with the current
sample size.
In conclusion, elite Australian footballers with a unilat-
eral history of HSIs within the previous 12 months dis-
played a greater baseline level of and a smaller increase
in eccentric hamstring strength through the preseason
training period compared with their control group counter-
parts. Interestingly, this diminished response was not con-
fined to the previously injured limb but was also observed
in the contralateral uninjured limb, which might suggest
that the effects of a prior HSI may be centrally mediated.
The existence of high and low responders to eccentric exer-
cise and the effect on the risk of future HSIs are worthy of
further examination.
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... In 0.9% of the injuries, 3 muscle heads were affected. All the injuries (including primary and secondary lesions) were described in 20 studies, for a total of 1068 hamstring injuries: 1,2,4,8,9,13,21,28,32,37,38,42,[44][45][46]48 (Figure 2 and Table 2). When 2 muscles were affected, BFLH (primary lesion) and ST (secondary lesion) was the most frequent combination (82.1%), others were less common (BFLH and BFSH, BFLH and SM, ST and SM, with 7.7%, 3.8%, and 6.4%, respectively). ...
... Regarding the type of tissue affected (described in 18 studies, for 1286 hamstring injuries 1,1,2,4,5,9,[11][12][13]18,28,38,42,43,48,[52][53][54][55], the MTJ was involved in more than half the cases (n = 668, 51.9%) ( Figure 2 and Table 2). ...
... When considering distal and proximal locations (described in 7 studies, for 713 hamstring injuries 1,3,11,13,32,37,55 ), the muscular lesions were mostly proximal (62.8%) ( Figure 2 and Table 2). When considering central and extensive lesions (described in 8 studies, for 344 hamstring injuries 8,23,38,42,43,50,52,54 ), the distribution looked homogenous, with 34.0%, 33.4%, and 32.6% for proximal, central, and distal locations, respectively. ...
Article
Context Hamstring muscle injury location using magnetic resonance imaging (MRI) is not so well described in the literature. Objective To describe the location of hamstring injuries using MRI. Data Sources PubMed, Web of Science, Scopus, SPORTDiscus, Cochrane Library. Study Selection The full text of studies, in English, had to be available. Case reports and reviews were excluded. Included studies must report the location of hamstring injuries using MRI within 8 days of the acute injury. Study Design Systematic review. Level of Evidence Level 4. Data Extraction A first screening was conducted based on title and abstract of the articles. In the second screening, the full text of the remaining articles was evaluated for the fulfillment of the inclusion criteria. Results From the 2788 references initially found in 5 databases, we included 34 studies, reporting a total of 2761 acute hamstring injuries. The most frequent muscle head involved was the long head of the biceps femoris (BFLH) (70%), followed by the semitendinosus (ST) (15%), generally associated with BFLH. The most frequent tissue affected was the myotendinous junction (MTJ) accounting for half the cases (52%). Among all lesions, the distribution between proximal, central, and distal lesions looked homogenous, with 34.0%, 33.4% and 32.6%, respectively. The stretching mechanism, while only reported in 2 articles, represented 3% of all reported mechanisms, appears responsible for a specific lesion involving the proximal tendon of the semimembranosus (SM), and leading to a longer time out from sport. Conclusion BFLH was the most often affected hamstring injuries and MTJ was the most affected tissue. In addition, the distal, central, and proximal locations were homogeneously distributed. We also noted that MRI descriptions of hamstring injuries are often poor and did not take full advantage of the MRI strengths. Systematic Review Registration Before study initiation, the study was registered in the PROSPERO International prospective register of systematic reviews (registration number CRD42018107580).
... Current results indicate that previously injured limbs displayed a possibly greater NHE strength than the uninjured contralateral limbs and greater than the uninjured limbs in the previously uninjured players. Previous work conducted with different athletic and nonathletic populations has found lower [24,25], similar [8,15], or greater [26] knee flexor eccentric strength in subjects with a history of hamstring strains. Due to the retrospective nature of the present study, it cannot be determined whether the observed knee flexor eccentric strength values were already present prior to the hamstring injury occurrence. ...
... As a practical application, the first step in secondary hamstring muscle injury prevention is to identify potential cohort-and/or individual-specific modifiable risk factors for the design of targeted injury prevention strategies. Despite all the research, attention, and support that knee flexor eccentric strength has recently gained [6,19,25,26], current results do not seem to support the idea that in our homogenous cohort of high-level football players, knee flexor strength weakness assessed using the NHE, is likely to be the main factor to counteract potential injury prevention interventions. If present results are transferred to a "real life" scenario and the teams' coaching and medical staffs are presented with current data, it would be questionable to prioritize the inclusion of NHE strength data in the previously injured group. ...
Article
Full-text available
The aim was to determine if players with a prior hamstring strain injury (HSI) exhibit bilateral deficits in knee flexor eccentric strength and hamstring muscle volume and differences in sprinting performance compared with players without a history of HSIs. Forty-six male professional soccer players participated in this study. Eccentric knee flexor strength, hamstring muscle volume (MRI), and a 20-m running sprint test (5- and 10-m split time) were assessed at the start of the preseason. Eccentric knee strength of the previously injured limbs of injured players was greater (ES: 1.18–1.36) than the uninjured limbs in uninjured players. Previously injured limbs showed possibly larger biceps femoris short heads (BFSh) and likely semitendinosus (ST) muscle volumes than the contralateral uninjured limbs among the injured players (ES: 0.36) and the limbs of the uninjured players (ES: 0.56), respectively. Players who had experienced a previous HSI were possibly slower in the 5-m (small ES: 0.46), while unclear differences were found in both the 10-m and 20-m times. Players with a prior HSI displayed greater eccentric knee flexor strength, possibly relatively hypertrophied ST and BFSh muscles, and possibly reduced 5-m sprinting performances than previously uninjured players. This can have implication for the design of secondary hamstring muscle injury prevention strategies.
... Changes to muscle structure (decreased fascicle length, 65,66 decreased muscle volume, 4,58,61 and development of scar tissue 60 ), along with ongoing neuromuscular inhibition, 27 and longterm deficits in muscle strength 14,49 have been demonstrated in previously injured quadriceps and hamstring muscles. 35,45,50 Quadriceps muscle architecture and angle of peak torque adapt in response to specific mechanical loads, 1,10,43 with reductions in rectus femoris fascicle length occurring following reduced exposure to eccentric load and periods of de-training from sport-specific tasks. 1 The reduced exposure to sport-specific stimuli following quadriceps or other lower-limb muscle injury may impact an athlete's ability to tolerate high levels of eccentric loading, especially kicking. While there was limited evidence for no association between MRI grading of index quadriceps muscle injury and recurrence, 31 there is evidence that MRI assessment of the specific location and severity of quadriceps injury may be associated with changes in the rehabilitation interval. ...
Article
Objective: To identify risk factors for quadriceps muscle strain injury in sport. Design: Risk factor systematic review. Literature search: A systematic search was conducted in the MEDLINECINAHL, Embase, AMED, AUSPORT, SPORTDiscus, PEDro, and Cochrane Library databases (from inception to September 2021). Study selection criteria: Studies reporting prospective data to evaluate risk factors related to index and/or recurrent quadriceps muscle strain injury. Data synthesis: A risk-of-bias assessment (using a modified Quality in Prognosis Studies tool) was performed, and we used best-evidence synthesis to qualitatively synthesize the data to quantify relationships between risk factors and quadriceps muscle injury. Results: Sixteen studies were included, capturing 2408 quadriceps injuries in 11 719 athletes. Meta-analyses were not performed due to clinical heterogeneity. The dominant kicking leg (over 3154 individuals, 1055 injuries), a previous history of quadriceps muscle injury (6208 individuals, 975 injuries), and a recent history of hamstring strain (4087 individuals, 581 injuries) were intrinsic factors associated with quadriceps injury. Extrinsic factors relating to the preseason period and competitive match play increased quadriceps injury risk; participating at higher levels of competition decreased quadriceps injury risk. Age, weight, and flexibility (intrinsic factors) had no association with quadriceps injury. Conclusion: Previous quadriceps injury, recent hamstring injury, the dominant kicking leg, and competitive match play were the strongest risk factors for future quadriceps muscle injury in sport. J Orthop Sports Phys Ther 2022;52(6):389-400. doi:10.2519/jospt.2022.10870.
... During all attempts, force data of each load cell (sample rate = 10 Hz) were simultaneously transferred via Bluetooth to a mobile smartphone. Both the peak force value obtained in each limb (Bourne et al., 2015;Buchheit et al., 2016;Opar et al., 2015;Ribeiro-Alvares et al., 2020) and the between-limb average peak force value (Bourne et al., 2015;Ribeiro-Alvares et al., 2021;Roe et al., 2018;Timmins et al., 2016) were considered for statistical analysis. The participants' age and anthropometric data (body mass and height) were provided by the respective clubs. ...
Article
The study aimed to verify how age, height, body mass and body mass index affects the eccentric knee flexor strength during the Nordic hamstring exercise (NHE) in male soccer players. Ten professional soccer clubs were included in this cross-sectional trial. Three hundred and eleven soccer players (192 from senior and 119 from under-20 teams) were assessed using a load-cell based device. Pearson’s correlation tests were performed between peak force values (measured in Newtons) and age, height, body mass and body mass index. The individual-limb strength (n = 622 limbs) and the between-limb average strength (n = 322 players) presented no correlation with age (r = 0.12 for both) and height (r = 0.13; r = 0,15), and fair correlations with body mass (r = 0.37; r = 0,41) and body mass index (r = 0.40; r = 0,43). In conclusion, the male soccer players’ eccentric knee flexor strength measured during the NHE execution is not affected by age or height. Body mass and body mass index play somewhat effect on strength, thus normalising absolute strength measures by body mass or body mass index can provide more accurate analysis in some contexts.
... These findings suggested that the chronic functional deficits of the injured limb and muscle lengthened the angle, which supported our study results. The injured limb demonstrated less improvement of strength and fascicle length after the intervention of preseason exercise and across the Australian football league season (Opar, Williams, Timmins, Hickey, Duhig, & Shield, 2015;Timmins et al., 2017), which may cause functional deficits of lengthened muscles after HSI. The deficits of function at the lengthened hamstring muscles may be due to structural (i.e., fascicle length) and neurological factors (i.e., EMG or neuromuscular inhibition) (Fyfe et al., 2013;Sole et al., 2011). ...
Article
Objective To investigate the effects of knee flexion angle on peak torque, rate of torque development (RTD) during isometric contraction and hamstring flexibility after hamstring strain injury (HSI). Design Cross-sectional. Setting Controlled laboratory research. Participants Fourteen male athletes with a history of HSI and 14 athletes without HSI (controls). Main outcome measures: Hamstring flexibility was evaluated using active knee extension test. Isometric knee flexion peak torque and RTD were determined at 30°, 60°, and 90° of knee flexion measured by an isokinetic dynamometer. Results Individuals with a history of HSI had statistically significant, moderate deficits in isometric peak torque at 30° of knee flexion (P = 0.037; effect size = 0.55) in the HSI limb than in the uninjured limb, but not at 60° and 90° of knee flexion. In the control group, no significant differences in isometric peak torque at any angle were found between limbs. No differences in peak RTD and flexibility were found between limbs in both groups. Conclusions Isometric peak torque at 30° of knee flexion was lower in the injured limb than in the uninjured limb. Isometric strength deficits after HSI tended to be affected by lengthened hamstring angles.
... However, previous studies have suggested that activation deficits (Opar et al., 2013) and insufficient muscle contraction persist despite successful rehabilitation and a return to preinjury levels of training and playing. This neuromuscular inhibition is considered to limit the extent of muscular adaptations in response to rehabilitative and prophylactic exercises by central mechanisms (Opar et al., 2015). Furthermore, another study has reported that an athlete with a previous hamstring strain injury that had occurred more than 5 years ago also demonstrated an activation deficit in the BFlh muscle during sprinting (Higashihara et al., 2019). ...
Conference Paper
Background Trunk movement is considered to be involved in lower extremity injuries. Hamstring strain injuries often occur when movements are unanticipatedly perturbed by the opponents. Objective To examine the neuromuscular responses of the hamstring and trunk muscles during unanticipated trunk perturbations in the athletes with and without a history of hamstring strain injury. Design Descriptive laboratory study. Setting College athletes. Participants Male college athletes were recruited, 11 with a history of unilateral hamstring strain injury and 10 without prior injury. Assessment In the kneeling position, the participants wore a chest harness attached to a cable that was pulled backward as a resisting force. They were instructed to resist the force isometrically and keep their initial position as possible as they could when the perturbations were applied. The force was released with a cue (CUE) and without cue (NoCUE). Trunk acceleration, three-dimensional kinematic data, and surface electromyography (EMG) signals of the erector spinae, internal oblique, gluteus maximus, biceps femoris, and semitendinosus muscles were measured. Main Outcome Measurements (1) Maximum trunk acceleration; (2) angular displacement of the trunk, pelvis, hip, and knee; (3) onset latency; (4) EMG activation in the 50-ms window before the perturbation; and (5) EMG activation in the 100ms after the perturbation. Results The maximum trunk acceleration and displacement were significantly greater during the NoCUE than during the CUE in both groups (p<0.05). The injured group demonstrated significantly delayed onset of the gluteus maximus and erector spinae muscles during the NoCUE compared with the CUE (p<0.05), while no difference was observed in the uninjured group. There was no difference in the phasic EMG activities between groups. Conclusions Athletes with a history of hamstring strain injury displayed reduction in the neuromuscular coordination of pelvis and trunk muscles when they controlled the unanticipated trunk movement.
... It has been suggested that a reduced ability to voluntarily activate the previously injured muscle may contribute to the deficits in the eccentric rate of torque development (Opar et al., 2013b) and strength (Opar et al., 2013a(Opar et al., , 2015Sole et al., 2011) observed following a hamstring injury. The current study expands upon previous findings (Opar et al., 2013b), by demonstrating that deficits in the rate of torque and sEMG development of previously injured limbs are specific to intended eccentric contractions. ...
Article
Persistent deficits in strength and voluntary activation have been observed in athletes with a history of hamstring strain injury. The mechanisms contributing to these deficits are poorly understood and consequently may not be appropriately addressed during rehabilitation. This study aimed to investigate the impact of intended knee flexor contraction mode (concentric, eccentric or isometric) on the rate of torque development and surface electromyography (sEMG) rise in athletes with and without a history of unilateral hamstring strain injury. The impact of the previous injury on hip extensor rate of torque development was also investigated. Previously injured limbs exhibited a slower rate of torque development (mean difference = −31%, p = 0.02, Cohen’s d = 0.62) and biceps femoris rate of sEMG rise (mean difference = −181% · s⁻¹, p = 0.003, Cohen’s d = 1.10) during intended eccentric knee flexor contractions compared with control limbs. Previously injured (mean difference = −29%, p = 0.01, Cohen’s d = 0.85) and contralateral uninjured limbs (mean difference = −31%, p = 0.007, Cohen’s d = 0.73) exhibited a slower rate of torque development during isometric hip extensor contractions compared with control limbs. These findings may highlight lower levels of descending input to hamstring motoneurons in previously injured athletes.
... However, previous studies have suggested that activation deficits (Opar et al., 2013) and insufficient muscle contraction persist despite successful rehabilitation and a return to preinjury levels of training and playing. This neuromuscular inhibition is considered to limit the extent of muscular adaptations in response to rehabilitative and prophylactic exercises by central mechanisms (Opar et al., 2015). Furthermore, another study has reported that an athlete with a previous hamstring strain injury that had occurred more than 5 years ago also demonstrated an activation deficit in the BFlh muscle during sprinting (Higashihara et al., 2019). ...
... However, previous studies have suggested that activation deficits (Opar et al., 2013) and insufficient muscle contraction persist despite successful rehabilitation and a return to preinjury levels of training and playing. This neuromuscular inhibition is considered to limit the extent of muscular adaptations in response to rehabilitative and prophylactic exercises by central mechanisms (Opar et al., 2015). Furthermore, another study has reported that an athlete with a previous hamstring strain injury that had occurred more than 5 years ago also demonstrated an activation deficit in the BFlh muscle during sprinting (Higashihara et al., 2019). ...
Article
Hamstring strain often occurs when an opponent unanticipatedly perturbs an athlete’s movements. We examined the neuromuscular responses of hamstring and trunk muscles during unanticipated trunk perturbations in athletes with and without a history of hamstring strain injury. Male college athletes (11 with a history of a unilateral hamstring injury and 10 without prior injury) knelt while wearing a chest harness attached to a cable that was pulled backward. They were instructed to resist the force isometrically and maintain their position when the perturbations were applied. The pressure was released with or without a cue (CUE or NoCUE). We measured trunk acceleration, three-dimensional kinematic data, and surface electromyography (EMG) signals of the erector spinae, internal oblique, gluteus maximus, biceps femoris long head, and semitendinosus muscles. Maximum trunk acceleration and displacement were greater with NoCUE in both groups (p < 0.05). EMG amplitude did not differ after perturbation of any investigated muscle. The injured group demonstrated a delayed onset of the gluteus maximus and erector spinae muscles in NoCUE versus CUE stimuli (p < 0.05). Athletes with a history of hamstring strain injury exhibited a reduced neuromuscular coordination of the lumbopelvic muscles in response to unanticipated trunk movement.
Article
The neuromuscular activity in the hamstring and quadriceps muscles is vital for rapid force control during athletic movements. This study aimed to investigate the recruitment properties of the corticospinal pathway of the biceps femoris long head (BFlh) and rectus femoris (RF) muscles. Thirty-two male subjects were participated in this study. Corticospinal excitability was investigated for BFlh and RF during the isometric knee flexion and extension tasks, respectively, using transcranial magnetic stimulation. A sigmoidal relationship was observed between the stimulus intensity and amplitude of motor-evoked potentials and characterized by a plateau value, maximum slope, and threshold. Compared with RF, BFlh had a significantly lower plateau value (P < 0.001, d = 1.17), maximum slope (P < 0.001, r = 0.79), and threshold (P = 0.003, d = 0.62). The results showed that the recruitment properties of the corticospinal pathway significantly differ between BFlh and RF. These results reveal that when a sudden large force is required during athletic movements, the RF can produce force through a rapid increase in the recruitment of motor units. The BFlh, on the other hand, requires larger or more synchronized motor commands for enabling the proper motor unit behavior to exert large forces. These differences in the neurophysiological factors between the hamstrings and quadriceps can have a substantial effect on the balance of force generation during athletic activities.
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Physical activity is important in both prevention and treatment of many common diseases, but sports injuries can pose serious problems. To determine whether physical activity exercises can reduce sports injuries and perform stratified analyses of strength training, stretching, proprioception and combinations of these, and provide separate acute and overuse injury estimates. PubMed, EMBASE, Web of Science and SPORTDiscus were searched and yielded 3462 results. Two independent authors selected relevant randomised, controlled trials and quality assessments were conducted by all authors of this paper using the Cochrane collaboration domain-based quality assessment tool. Twelve studies that neglected to account for clustering effects were adjusted. Quantitative analyses were performed in STATA V.12 and sensitivity analysed by intention-to-treat. Heterogeneity (I(2)) and publication bias (Harbord's small-study effects) were formally tested. 25 trials, including 26 610 participants with 3464 injuries, were analysed. The overall effect estimate on injury prevention was heterogeneous. Stratified exposure analyses proved no beneficial effect for stretching (RR 0.963 (0.846-1.095)), whereas studies with multiple exposures (RR 0.655 (0.520-0.826)), proprioception training (RR 0.550 (0.347-0.869)), and strength training (RR 0.315 (0.207-0.480)) showed a tendency towards increasing effect. Both acute injuries (RR 0.647 (0.502-0.836)) and overuse injuries (RR 0.527 (0.373-0.746)) could be reduced by physical activity programmes. Intention-to-treat sensitivity analyses consistently revealed even more robust effect estimates. Despite a few outlying studies, consistently favourable estimates were obtained for all injury prevention measures except for stretching. Strength training reduced sports injuries to less than 1/3 and overuse injuries could be almost halved.
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Reliability and case-control injury study. 1) To determine if a novel device, designed to measure eccentric knee flexors strength via the Nordic hamstring exercise (NHE), displays acceptable test-retest reliability; 2) to determine normative values for eccentric knee flexors strength derived from the device in individuals without a history of hamstring strain injury (HSI) and; 3) to determine if the device could detect weakness in elite athletes with a previous history of unilateral HSI. HSIs and reinjuries are the most common cause of lost playing time in a number of sports. Eccentric knee flexors weakness is a major modifiable risk factor for future HSIs, however there is a lack of easily accessible equipment to assess this strength quality. Thirty recreationally active males without a history of HSI completed NHEs on the device on 2 separate occasions. Intraclass correlation coefficients (ICCs), typical error (TE), typical error as a co-efficient of variation (%TE), and minimum detectable change at a 95% confidence interval (MDC95) were calculated. Normative strength data were determined using the most reliable measurement. An additional 20 elite athletes with a unilateral history of HSI within the previous 12 months performed NHEs on the device to determine if residual eccentric muscle weakness existed in the previously injured limb. The device displayed high to moderate reliability (ICC = 0.83 to 0.90; TE = 21.7 N to 27.5 N; %TE = 5.8 to 8.5; MDC95 = 76.2 to 60.1 N). Mean±SD normative eccentric flexors strength, based on the uninjured group, was 344.7 ± 61.1 N for the left and 361.2 ± 65.1 N for the right side. The previously injured limbs were 15% weaker than the contralateral uninjured limbs (mean difference = 50.3 N; 95% CI = 25.7 to 74.9N; P < .01), 15% weaker than the normative left limb data (mean difference = 50.0 N; 95% CI = 1.4 to 98.5 N; P = .04) and 18% weaker than the normative right limb data (mean difference = 66.5 N; 95% CI = 18.0 to 115.1 N; P < .01). The experimental device offers a reliable method to determine eccentric knee flexors strength and strength asymmetry and revealed residual weakness in previously injured elite athletes.J Orthop Sports Phys Ther, Epub 25 July 2013. doi:10.2519/jospt.2013.4837.
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Background Hamstring injury is the single most common injury in European professional football and, therefore, time to return and secondary prevention are of particular concern. Objective To compare the effectiveness of two rehabilitation protocols after acute hamstring injury in Swedish elite football players by evaluating time needed to return to full participation in football team-training and availability for match selection. Study design Prospective randomised comparison of two rehabilitation protocols. Methods Seventy-five football players with an acute hamstring injury, verified by MRI, were randomly assigned to one of two rehabilitation protocols. Thirty-seven players were assigned to a protocol emphasising lengthening exercises, L-protocol and 38 players to a protocol consisting of conventional exercises, C-protocol. The outcome measure was the number of days to return to full-team training and availability for match selection. Reinjuries were registered during a period of 12 months after return. Results Time to return was significantly shorter for the players in the L-protocol, mean 28 days (1SD±15, range 8–58 days), compared with the C-protocol, mean 51 days (1SD±21, range 12–94 days). Irrespective of protocol, stretching-type of hamstring injury took significantly longer time to return than sprinting-type, L-protocol: mean 43 vs 23 days and C-protocol: mean 74 vs 41 days, respectively. The L-protocol was significantly more effective than the C-protocol in both injury types. One reinjury was registered, in the C-protocol. Conclusions A rehabilitation protocol emphasising lengthening type of exercises is more effective than a protocol containing conventional exercises in promoting time to return in Swedish elite football.
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Background: The effect of prior strain injury on myoelectrical activity of the hamstrings during tasks requiring high rates of torque development has received little attention. Purpose: To determine if recreational athletes with a history of unilateral hamstring strain injury will exhibit lower levels of myoelectrical activity during eccentric contraction, rate of torque development (RTD), and impulse (IMP) at 30, 50, and 100 milliseconds after the onset of myoelectrical activity or torque development in the previously injured limb compared with the uninjured limb. Study design: Case control study; Level of evidence, 3. Methods: Twenty-six recreational athletes were recruited. Of these, 13 athletes had a history of unilateral hamstring strain injury (all confined to biceps femoris long head), and 13 had no history of hamstring strain injury. Following familiarization, all athletes undertook isokinetic dynamometry testing and surface electromyography (integrated EMG; iEMG) assessment of the biceps femoris long head and medial hamstrings during eccentric contractions at -60 and -180 deg·s(-1). Results: In the injured limb of the injured group, compared with the contralateral uninjured limb, RTD and IMP was lower during -60 deg·s(-1) eccentric contractions at 50 milliseconds (RTD: injured limb, 312.27 ± 191.78 N·m·s(-1) vs uninjured limb, 518.54 ± 172.81 N·m·s(-1), P = .008; IMP: injured limb, 0.73 ± 0.30 N·m·s vs uninjured limb, 0.97 ± 0.23 N·m·s, P = .005) and 100 milliseconds (RTD: injured limb, 280.03 ± 131.42 N·m·s(-1) vs uninjured limb, 460.54 ± 152.94 N·m·s(-1), P = .001; IMP: injured limb, 2.15 ± 0.89 N·m·s vs uninjured limb, 3.07 ± 0.63 N·m·s, P < .001) after the onset of contraction. Biceps femoris long head muscle activation was lower at 100 milliseconds at both contraction speeds (-60 deg·s(-1), normalized iEMG activity [×1000]: injured limb, 26.25 ± 10.11 vs uninjured limb, 33.57 ± 8.29, P = .009; -180 deg·s(-1), normalized iEMG activity [×1000]: injured limb, 31.16 ± 10.01 vs uninjured limb, 39.64 ± 8.36, P = .009). Medial hamstring activation did not differ between limbs in the injured group. Comparisons in the uninjured group showed no significant between limbs difference for any variables. Conclusion: Previously injured hamstrings displayed lower RTD and IMP during slow maximal eccentric contraction compared with the contralateral uninjured limb. Lower myoelectrical activity was confined to the biceps femoris long head. Regardless of whether these deficits are the cause of or the result of injury, these findings could have important implications for hamstring strain injury and reinjury. Particularly, given the importance of high levels of muscle activity to bring about specific muscular adaptations, lower levels of myoelectrical activity may limit the adaptive response to rehabilitation interventions and suggest that greater attention be given to neural function of the knee flexors after hamstring strain injury.
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Background The injury risk in football is high, but little is known about causes of injury. Purpose To identify risk factors for football injuries using a multivariate model. Study Design Prospective cohort study. Methods Participants were 306 male football players from the two highest divisions in Iceland. Before the 1999 football season started, the following factors were examined: height, weight, body composition, flexibility, leg extension power, jump height, peak O2 uptake, joint stability, and history of previous injury. Injuries and player exposure were recorded throughout the competitive season. Results Older players were at higher risk of injury in general (odds ratio [OR] = 1.1 per year, P = 0.05). For hamstring strains, the significant risk factors were age (OR = 1.4 [1 year], P < 0.001) and previous hamstring strains (OR = 11.6, P < 0.001). For groin strains, the predictor risk factors were previous groin strains (OR = 7.3, P = 0.001) and decreased range of motion in hip abduction (OR = 0.9 [1°], P = 0.05). Previous injury was also identified as a risk factor for knee (OR = 4.6) and ankle sprains (OR = 5.3). Conclusions Age and previous injury were identified as the main risk factors for injury among elite football players from Iceland.
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Hamstring strain injuries (HSIs) have remained the most prevalent injury in the Australian Football League (AFL) over the past 21 regular seasons. The effect of HSIs in sports is often expressed as regular season games missed due to injury. However, the financial cost of athletes missing games due to injury has not been investigated. The aim of this report is to estimate the financial cost of games missed due to HSIs in the AFL. Data were collected using publicly available information from the AFL's injury report and the official AFL annual report for the past 10 competitive AFL seasons. Average athlete salary and injury epidemiology data were used to determine the average yearly financial cost of HSIs for AFL clubs and the average financial cost of a single HSI over this time period. Across the observed period, average yearly financial cost of HSIs per club increased by 71% compared with a 43% increase in average yearly athlete salary. Over the same time period the average financial cost of a single HSI increased by 56% from $A25 603 in 2003 to $A40 021 in 2012, despite little change in the HSI rates during the period. The observed increased financial cost of HSIs was ultimately explained by the failure of teams to decrease HSI rates, but coupled with increases in athlete salaries over the past 10 season. The information presented in this report highlights the financial cost of HSIs and other sporting injuries, raising greater awareness and the need for further funding for research into injury prevention strategies to maximise economical return for investment in athletes.
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Background: Injuries are common in all professional football codes (including soccer, rugby league and union, American football, Gaelic football, and Australian football). Purpose: To report the epidemiology of injuries in the Australian Football League (AFL) from 1992-2012 and to identify changes in injury patterns during that period. Study design: Descriptive epidemiology study. Methods: The AFL commenced surveying injuries in 1992, with all teams and players included since 1996. An injury was defined as "any physical or medical condition that causes a player to miss a match in the regular season or finals (playoffs)." Administrative records of injury payments (which are compulsory as part of salary cap compliance) to players who do not play matches determined the occurrence of an injury. The seasonal incidence was measured in units of new injuries per club (of 40 players) per season (of 22 matches). Results: There were 4492 players listed over the 21-year period who suffered 13,606 new injuries/illnesses and 1965 recurrent injuries/illnesses, which caused 51,919 matches to be missed. The lowest seasonal incidence was 30.3 new injuries per club per season recorded in 1993, and the highest was 40.3 recorded in 1998. The injury prevalence (missed matches through injury per club per season) varied from a low of 116.3 in 1994 to a high of 157.1 in 2011. The recurrence rate of injuries was highest at 25% in 1992 and lowest at 9% in 2012 and has steadily fallen across the 21 years (P < .01). The most frequent and prevalent injury was hamstring strain (average of 6 injuries per club per season, resulting in 20 missed matches per club per season; recurrence rate, 26%), although the rate of hamstring injuries has fallen in the past 2 seasons after a change to the structure of the interchange bench (P < .05). The rate of knee posterior cruciate ligament injuries fell in the years after a rule change to prevent knee-to-knee collisions in ruckmen (P < .01). Conclusion: Annual public reporting (by way of media release and reports available freely online) of injury rates, using units easily understood by laypeople, has been well received. It has also paved the way for rule changes with the primary goal of improving player safety.
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The aim of this study was to determine if athletes with a history of hamstring strain injury display lower levels of surface EMG (sEMG) activity and median power frequency in the previously injured hamstring muscle during maximal voluntary contractions. Recreational athletes were recruited, 13 with a history of unilateral hamstring strain injury and 15 without prior injury. All athletes undertook isokinetic dynamometry testing of the knee flexors and sEMG assessment of the biceps femoris long head (BF) and medial hamstrings (MHs) during concentric and eccentric contractions at ±180 and ±60°s(-1). The knee flexors on the previously injured limb were weaker at all contraction speeds compared to the uninjured limb (+180°s(-1)p=0.0036; +60°s(-1)p=0.0013; -60°s(-1)p=0.0007; -180°s(-1)p=0.0007) whilst sEMG activity was only lower in the BF during eccentric contractions (-60°s(-1)p=0.0025; -180°s(-1)p=0.0003). There were no between limb differences in MH sEMG activity or median power frequency from either BF or MH in the injured group. The uninjured group showed no between limb differences in any of the tested variables. Secondary analysis comparing the between limb difference in the injured and the uninjured groups, confirmed that previously injured hamstrings were mostly weaker (+180°s(-1)p=0.2208; +60°s(-1)p=0.0379; -60°(-1)p=0.0312; -180°s(-1)p=0.0110) and that deficits in sEMG were confined to the BF during eccentric contractions (-60°s(-1)p=0.0542; -180°s(-1)p=0.0473). Previously injured hamstrings were weaker and BF sEMG activity was lower than the contralateral uninjured hamstring. This has implications for hamstring strain injury prevention and rehabilitation which should consider altered neural function following hamstring strain injury.