Motor control and strength as predictors of hamstring injury

in elite players of Australian football

Matt Camerona,*, Roger Adamsb, Christopher Maherb

aPhysiotherapist, Sydney Swans Football Club, P.O. Box 173, Paddington, NSW 2021, AustraliabSchool of Physiotherapy, University of Sydney, Sydney, Australia

Abstract

Objectives. To investigate motor control as a possible contributing factor in hamstring injuries using a test movement similar to the injury-

critical phase during running. Leg swing movement discrimination (MD), thigh muscle strength as tested by Cybex isokinetic dynamometry

and previous hamstring injury history were all assessed to determine any association with subsequent hamstring injury.

Design. Prospective and retrospective observational and analytical cohort study.

Setting. A professional football club and a university laboratory.

Participants. Twenty elite level players of Australian football.

Results. In the 2 years prior to testing, 7 subjects had sustained a hamstring injury. At the time of testing, there were no significant differences

between subjects never injured and those previously injured. In the two years following testing, 6 players sustained a hamstring injury. Mean

MD score and concentric hamstring-to-quadriceps strength ratio of subsequently injured subjects were significantly worse than those

uninjured ðp , 0:05Þ: Optimum cutoffs on these measures were determined.

Conclusion. Both a lower MD ability score for the backward swinging leg and an imbalance of thigh muscle strength were predictive of

hamstring injury.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Hamstrings; Injury; Motor control; Movement discrimination

1. Introduction

Hamstring strain injuries are common in all football

codes and sports involving sprinting, and are the most

frequently occurring and recurring of all injuries in

Australian football. At the elite level, hamstring injuries

occur at a rate of 6.2 injuries per club per season, and result

in 21.2 missed player games per club per season (Orchard

and Seward, 2002). These rates are the highest of all the elite

level football codes in Australia (Seward et al., 1993).

Hamstring injuries have the highest recurrence rate of all

football injuries and notwithstanding the best rehabilitation

attempts, more than one in three (34%) injuries recur within

the same season (Orchard and Seward, 2002). Risk has been

shown to be increased following previous hamstring injury

(Bennell et al., 1998; Garrett, 1996; Orchard, 2001), calf

strain injury (Orchard, 2001), and serious knee and groin

injuries (Verral et al., 2001). Risk has also been shown to be

increased in players of Australian football older than

23 years of age (Orchard, 2001).

Despite the magnitude of this problem in Australian

football, the aetiology of such injuries is unclear. Factors

that have been suggested to predispose an athlete to

hamstring muscle strain injury include muscle weakness,

muscle imbalance, poor flexibility, fatigue, inadequate

warm-up, poor neuromuscular control and poor running

technique (Agre, 1985). There is little evidence for poor

flexibility as a hamstring injury predictor (Bennell et al.,

1999), and apart from muscle weakness there is no empirical

support for any of the other suggested factors (Orchard,

2001).

The hamstring muscle group reaches peak elongation and

acts eccentrically at the hip and knee during the late swing

phase of the running cycle (Frigo et al., 1979; Simonsen

et al., 1985). Kinetic and EMG studies reveal that the

hamstrings are most active and develop the greatest torques

at the hip and knee during late swing through to the mid-

stance phase of running (Mann and Sprague, 1980;

Montgomery et al., 1994). It is during these parts of the

running cycle that the hamstrings are under the greatest

1466-853X/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S1466-853X(03)00053-1

Physical Therapy in Sport 4 (2003) 159–166

www.elsevier.com/locate/yptsp

* Corresponding author. Tel.: þ61-2-9339-9135; fax: þ61-2-9339-9101.

E-mail address: mcameron@cyberone.com.au (M. Cameron).

demands and injury most likely (Mann and Sprague, 1980;

Stanton and Purdam, 1989). Given the high forces involved,

it would seem that hamstring weakness might predispose an

athlete to injury, however, to date, there have not been

adequate findings to support either hamstring muscle

weakness or hamstring-quadriceps strength imbalance as

risk factors (Orchard, 2001).

Muscle weakness has been the most extensively

investigated of all proposed predisposing factors for ham-

string injury. Retrospective studies have suggested a

relationship between muscle weakness and hamstring injury

(Croisier and Crielaard, 2000; Crosier et al., 2002; Heiser

et al., 1984), however, retrospectivity can confound the

causes and effects of injury. The literature also contains a

small number of prospective studies, which allow firmer

conclusions to be drawn regarding the factors involved in

the prediction of injury, but the results to date are

inconsistent. Two studies using isometric hamstring

strength assessment demonstrated an association between

injury and a side-to-side deficit of 10% (Burkett, 1970) and

a lower hamstring-to-quadriceps ratio (Yamamoto, 1993).

The latter finding was supported by a study using isokinetic

dynamometry at 60 8/s, which linked injury to a hamstring-

to-quadriceps ratio below 0.60 (Orchard et al., 1997). In

contrast, Liemohn (1978) did not find any relationship

between injury and isometric hamstring-to-quadriceps

strength ratio, nor did a larger study examining concentric

and eccentric isokinetic strength reveal any association

between injury and similar strength ratios (Bennell et al.,

1998).

One possible causative factor of hamstring injury, that

has not been previously examined, is the accuracy of

neuromuscular control of the leg during running. Through-

out the running cycle there are many challenging neuro-

muscular events in which the hamstring acts, for example, to

control hip and knee motion in late swing and to provide hip

extensor torque in early stance. During sprinting, these

events occur over a very short period of time, and if the

control and coordination are inadequate, then muscle strain

injury may result (Agre, 1985; Bennell et al., 1999). Control

of a limb requires that information be obtained and

integrated from proprioceptors of the entire limb, and that

control will be influenced in part by activity in the opposite

limb. Joint or single segment proprioception has been

assessed by various techniques that employ joint position

testing, kinesthaesia testing or sense of effort testing, often

in non-weight-bearing postures (Lephart and Fu, 2000).

Waddington and Adams (1999) assessed movement dis-

crimination (MD) at the ankle based on functional move-

ment principles, whereby subjects performed ankle

inversion movements in standing and made judgments

regarding the extent of these movements without visual

input about the movement. This requires the processing of

both afferent and efferent information about the lower limb

being tested, and performance reflects a subject’s ability to

do this accurately. An association was demonstrated

between poor discrimination ability and previous ankle

sprain injury. This task was later extended to enable

assessment of MD of the knee during weight-bearing

flexion (Waddington et al., 2000).

Utilising the same functional movement principles for

the current study on hamstring function, a similar apparatus

may be employed to assess the discrimination of movement

extent using the backward swinging leg, whilst weight-

bearing on the other side, in order to create a functional

movement as close as possible to the movement at injury.

The position of the player while being tested is selected to

recreate the proposed movement region in which hamstring

injury occurs: between the late swing to mid-stance phases

of the running cycle (Fig. 1). Ability to accurately

discriminate swinging leg movements can be seen as

reflecting use of proprioception from the lower limbs, and

is integral to the motor control of the leg in this region of the

running cycle where hamstring injury is likely to occur.

The aim of this study was to assess swinging leg MD,

isokinetic hamstring and quadriceps strength, and history of

previous hamstring injury, in order to determine any

association these factors have with respect to subsequent

hamstring injuries in a group of elite Australian football

players.

2. Method

2.1. Participants

Twenty players of Australian football were recruited for

this study from the training squad of one professional

Australian Football League (AFL) team. All subjects were

male and the mean (SD) age was 23.6 years (3.2), height

Fig. 1. Left leg late swing phase of the running cycle.

M. Cameron et al. / Physical Therapy in Sport 4 (2003) 159–166160

185.5 cms (8.5), weight 87.8 kg (9.1), and the mean number

of AFL training years was 4.7 (3.34). Subjects were

excluded if any significant lower limb injury was sustained

in the twelve weeks immediately prior to assessment.

Approval for the study was obtained from the University of

Sydney Human Ethics Committee, and all subjects signed

an informed consent.

2.2. Tests

2.2.1. Movement discrimination testing protocol

Lower limb motor control was assessed with a purpose-

built active movement extent discrimination apparatus, the

AMEDA (Waddington and Adams, 1999). This apparatus

consists of a vertical contact plate attached to the end of the

motor-driven shaft, which sets the stop for ending leg

swinging movements at different positions. A laptop

computer controlled the Programmable Stepper Motor1

driving the shaft, with end-point re-positioning accuracy

manufacturer-specified at 0.01 mm. Five stop settings were

used to generate the five leg swing movement extents, and

these were each 5.6 mm apart. The stop positions were

located from 20–43 mm behind the posterior margin of the

heels in standing, which corresponded to an angle of the

lower limb to vertical of approximately 2 8 and represented

the position of the leg at early to mid-stance phase of

running (see Fig. 2).

Testing of leg swing movement commenced with

subjects in normal weight-bearing stance astride the

apparatus, heads up and eyes focused on a point on the

wall opposite, so that they had no direct vision of their foot

or the contact plate. The contact plate was set to one of the

five positions and on command, subjects transferred weight

to one leg, actively lifted the tested leg to touch the start bar,

before swinging the limb backward toward the plate until

contact was made, and then returning to standing (Fig. 2).

After this movement the subject identified which one of the

five stop settings (i.e. movement extents) they had just

experienced.

Each subject was given a series of trial leg swing

movements on the AMEDA in order to familiarize them

with the feel of each stop setting prior to data collection.

Following this, all of the five stop positions were presented

eight times in random order. Subjects were allowed only one

leg swing movement for each trial. After each movement

the subject was asked which of the five stop numbers they

felt corresponded to the movement extent performed. The

reported stop position was recorded for the forty trials, and

no feedback as to reporting accuracy was provided. Both

legs of each subject were tested and the side first tested was

randomly determined. From this testing, a measure of a

subject’s ability to differentiate between different move-

ment extents was obtained.

2.2.2. Muscle strength testing protocol

Strength measurements were conducted on all players in

the pre-season of 2000 at the Sport Science Department of

the New South Wales Academy of Sport, University of

Sydney, Australia. Knee flexion and extension concentric

strength were assessed with a Cybex II Isokinetic

Dynamometer2 using a protocol which most players had

experienced at previous screening sessions. Warm-up prior

to data collection consisted of five minutes cycling on a

stationary bike and stretching of the lower limb mus-

culature. For isokinetic testing, subjects were positioned in

sitting with straps around the thigh, waist and chest, with

arms folded across the chest. The axis of rotation of the

dynamometer was aligned with the centre of the lateral

femoral condyle as outlined by Perrin (1993). The shin pad

of the dynamometer shaft was placed 2 cm proximal to the

lateral malleolus. After set-up, subjects completed several

submaximal knee flexion/extension trials. Torque and

displacement were determined at an angular velocity of

60 8/s, on both legs, and for both knee flexion and extension.

At each direction, subjects were required to exert maximum

effort for three repetitions, and the mean of the two highest

torque values was recorded. A minimum two-minute

recovery period was imposed between repeated trials. The

order of leg testing was randomised, however, the quad-

riceps was tested prior to the hamstrings.

The Cybex II dynamometer was computer-interfaced and

torque and angular displacement data were collected for 5 s

at a sampling rate of 1000 Hz. This information was stored

to disk for later processing, which included the determi-

nation of peak torque that in all instances occurred after the

impact spike (Kannus, 1994). Given this protocol, ramping

and damping procedures were considered unnecessary.

Prior to, and after testing, the torque output of the Cybex

was calibrated by measuring a range of known torques

according to the manufacturer’s specification. Peak torque

values were normalised for body weight for each subject to

Fig. 2. A subject standing on the active movement discrimination apparatus

and swinging the right leg to contact the start bar (A) and then the stop

plate (B).

1 RS Components Pty Ltd, 129 – 137 Beaconsfield Street, 2141

Silverwater, NSW, Australia.

2 Cybex International Inc, 10 Trotter Drive, Medway, MA 02053,

USA.

M. Cameron et al. / Physical Therapy in Sport 4 (2003) 159–166 161

allow comparison of knee flexion and extension concentric

strength between subjects.

2.2.3. Procedure

Previous hamstring injury occurrence in the two years

prior to the study was determined by questionnaire and

review of club injury records, and an injury was deemed to

be significant if any missed matches resulted. All subjects

were tested prior to the 2000 football season, and monitored

throughout the 2000 and 2001 seasons for significant

hamstring injury. The club medical officer made a diagnosis

of significant hamstring injury upon satisfying all of the

following criteria

(a) acute onset of pain in the posterior thigh during

training or a match

(b) posterior thigh pain reproduced with stretch, contrac-

tion and palpation of the hamstring muscle

(c) hamstring muscle injury demonstrated by magnetic

resonance imaging (MRI)

(d) at least one match missed due to hamstring injury

The medical officer was blind to the results of MD and

isokinetic testing.

2.2.4. Statistical analysis

The reported stop positions obtained from discrimination

testing were cast into stimulus-response matrices, and non-

parametric signal detection analysis was used to obtain MD

scores, defined as the area under the Receiving Operating

Characteristic (ROC) curves (McNicol, 1972). ROC curves

were used to plot the probability of correctly identifying the

extreme stop positions against the probability of incorrectly

labeling them as other stop positions. The area under a ROC

curve provides an overall measure of stop position

discrimination ability in this task, and was calculated with

the ROC subroutine in SPSS 10.0 for Windows. An area of

0.5 corresponds to chance whereas 1.0 corresponds to

perfect discriminability (Maher and Adams, 1995). From

the strength data, peak concentric torque of the hamstring

and quadriceps muscle groups were used in data analysis,

and a hamstring-to-quadriceps strength ratio (H/Q) was

calculated for each leg of each subject. These data were

analysed with SPSS 10.0 for Windows software. Prospec-

tive and retrospective injured/uninjured grouping structures

were created by the occurrence of hamstring injuries in the

two-year period following measurement, and by the past

history of hamstring injuries in the two years preceding

measurement. Thus determined, separate ANOVAs were

conducted on the muscle strength and MD variables, with

factors Injury Status (injured/uninjured) and Side (domi-

nant/non-dominant).

Previous injury, MD, and hamstrings and quadriceps

muscle strength variables were evaluated by the use of ROC

curves as to their performance of predicting prospective

injury status grouping. Use of ROC curves, originally

developed to measure the ability of a subject to differentiate

between different stimuli (Green and Swets, 1966), has been

extended to medical research, where the curves have been

used to examine a diagnostic test’s discriminative capability

for determining presence of disease or injury (Swets et al.,

2000). Applied in this way, curves were generated by

plotting the true-positive rate (sensitivity) and false-positive

rate (1-specificity) along the vertical and horizontal axes for

each of the predictor variables. The area under the ROC

curve is the most useful comparative measure of the

performance of screening tests (Hanley and McNeil, 1982;

Park et al., 2002), and a value over 0.8 can be interpreted as

a test with good predictive power (Meijer et al., 2002).

Areas under the ROC curve were calculated using SPSS

10.0 for Windows, and comparisons between variables and

calculations of cut-off points were performed with Predic-

tion program version 3.0.

3. Results

In the two seasons following testing, six subjects

sustained one or more significant hamstring muscle strains.

In the two seasons prior to testing, seven subjects had

experienced hamstring muscle strains, and two of these

subjects were in the group of six injured in the subsequent

period. There was no past history of hamstring injury in 4 of

the 6 subsequently injured subjects.

Mean MD scores and thigh concentric strength variables

for the groups are given in Table 1. With respect to the

prospective analyses, MD scores were significantly worse in

those subsequently injured compared to the subsequently

uninjured group ðFð1; 18Þ ¼ 9:44; p ¼ 0:007Þ: No player

with a MD score above the group mean subsequently injured

a hamstring, but six of the ten players with scores below the

mean did incur injury. Subsequently injured subjects also

had significantly lower hamstrings-to-quads (H/Q) strength

ratios ðFð1; 18Þ ¼ 8:75; p ¼ 0:008Þ and significantly greater

quadriceps strength adjusted for their body weight

ðFð1; 18Þ ¼ 6:13; p ¼ 0:02Þ than uninjured subjects. Ham-

string muscle strength values, however, showed no

Table 1

Mean (SD) movement discrimination score (MD), relative peak torques

(N m/kg) and hamstring-to-quadriceps (H/Q) ratio for previously and

subsequently injured and uninjured players

Previously Subsequently

Injured

ðn ¼ 7Þ

Uninjured

ðn ¼ 13Þ

Injured

ðn ¼ 6Þ

Uninjured

ðn ¼ 14Þ

MD score 0.75 (.15) 0.80 (.16) 0.69 (.10) 0.82 (.15)a

Hamstring 2.05 (.30) 2.02 (.24) 1.95 (.15) 2.08 (.30)

Quadriceps 3.15 (.22) 3.03 (.32) 3.30 (.24) 2.99 (.29)a

H/Q ratio 0.64 (.07) 0.67 (.09) 0.59 (.03) 0.69 (.09)a

a Statistically significant difference p , 0:05:

M. Cameron et al. / Physical Therapy in Sport 4 (2003) 159–166162

significant differences between groups ðFð1; 18Þ ¼

0:83; p ¼ 0:37Þ:

When the group was separated into previously injured

and uninjured subjects, there were no significant differences

on any of the variables. No differences were found between

dominant and non-dominant sides in either the prospective

or retrospective analyses, and there were no interactions

between side and injury status. Because there are strong

temporal and spatial relationships between the lower limbs

during locomotion (Shapiro et al., 1981), indicating a high

level of inter-limb coordination, the two legs during running

have been considered a single coordinative structure

(Sherwood, 1989). Accordingly, the two limbs for each

subject were averaged to give a single score for MD and for

each strength assessment. These form the data presented in

Table 1. To examine the injury-predictive capability of the

tested variables, mean MD score, hamstring and quadriceps

strength, and H/Q ratio across limbs were calculated for

each player. The ability of each variable to predict

hamstring injury was evaluated by calculating the area

under the relevant ROC curve. These values, level of

significance and confidence intervals are presented in

Table 2.

Movement discrimination score, quadriceps strength and

H/Q ratio were significant predictors of hamstring injury

based on the area under the ROC curve. Pair-wise

comparisons revealed no significant difference between

MD score and quadriceps strength in terms of their value as

predictors ðz ¼ 0:3278; p ¼ 0:3715Þ or H/Q ratio ðz ¼

0:1038; p ¼ 0:4587Þ; or between quadriceps strength

and H/Q ratio ðz ¼ 0:4107; p ¼ 0:3406Þ: There was no

significance difference between hamstring and quadriceps

strength ðz ¼ 1:1230; p ¼ 0:1307Þ as injury predictors,

however, there was a significant difference in hamstring

injury predictive value between hamstring strength and H/Q

ratio ðz ¼ 2:8558; p ¼ 0:0021Þ and between hamstring

strength and MD score ðz ¼ 1:9295; p ¼ 0:0268Þ: Ham-

string strength and previous hamstring injury were indivi-

dually not significant predictors of subsequent hamstring

injury, and there were no significant differences between

them. The sample size was too small to permit the develop-

ment of an equation combining the predictor variables.

To develop cut-points or decision thresholds (Swets et al.,

2000) for the significant hamstring injury predictor

variables, sensitivity and specificity values and a summary

measure, Youden’s index, were calculated for each variable,

and are presented in Table 3. Youden’s index is the best

Table 3

Sensitivity (SENS), specificity (SPEC), and Youden’s index (YI) for movement discrimination (MD), hamstring-to-quadriceps strength ratio (H/Q) and

relative quadriceps (Q) peak torque (Nm/kg) cut-point values

MD score H/Q score QUAD peak torque

SCORE SENS SPEC YI SCORE SENS SPEC YI SCORE SENS SPEC YI

0.94 1 0.07 0.07 0.82 1 0.14 0.14 2.44 1 0.07 0.07

0.93 1 0.14 0.14 0.76 1 0.21 0.21 2.64 1 0.14 0.14

0.91 1 0.21 0.21 0.75 1 0.29 0.29 2.86 1 0.29 0.29

0.90 1 0.29 0.29 0.72 1 0.43 0.43 2.88 1 0.43 0.43

0.88 1 0.36 0.36 0.69 1 0.50 0.50 3.00 0.83 0.50 0.33

0.87 1 0.43 0.43 0.68 1 0.57 0.57 3.02 0.83 0.71 0.55

0.80 1 0.50 0.50 0.67 1 0.64 0.64

0.79 1 0.57 0.57 0.66 1 0.71 0.71 3.18 0.67 0.79 0.45

0.79 1 0.64 0.64 3.26 0.67 0.86 0.52

0.78 1 0.71 0.71 0.65 0.83 0.71 0.55 3.28 0.50 0.86 0.36

0.62 0.83 0.86 0.69 3.34 0.50 0.93 0.43

0.77 0.83 0.71 0.55 0.61 0.67 0.86 0.52 3.36 0.33 0.93 0.26

0.77 0.83 0.79 0.62 0.6 0.50 0.86 0.36 3.40 0.17 0.93 0.10

0.75 0.67 0.79 0.45 0.59 0.33 0.86 0.19 3.50 0.17 1 0.17

0.74 0.67 0.86 0.52 0.58 0.33 1 0.33 3.56 0 1 0

0.73 0.50 0.86 0.36 0.57 0.17 1 0.17

0.73 0.33 0.86 0.19 0.55 0 1 0

0.72 0.33 0.93 0.26

0.71 0.33 1 0.33

0.68 0.17 1 0.17

0.50 0 1 0

Table 2

Area under the ROC curve (Area), standard error (SE), asymptotic

significance ðpÞ and 95% confidence interval (CI) for hamstring injury

predictor variables: movement discrimination score (MD), relative peak

hamstring (H) and quadriceps (Q) torque, hamstring-to-quadriceps ratio

(H/Q), and previous hamstring injury (Prev Inj)

Variable Area SE p CI

MD score 0.869 0.080 0.011 0.712–1.026

H 0.607 0.124 0.458 0.354–0.851

Q 0.827 0.099 0.023 0.634–1.020

H/Q 0.881 0.077 0.008 0.730–1.032

Prev Inj 0.488 0.144 0.934 0.206–0.770

M. Cameron et al. / Physical Therapy in Sport 4 (2003) 159–166 163

summary measure of a diagnostic test’s ability (Biggerstaff,

2000), and is calculated for a given cut-point by adding the

sensitivity and specificity values and subtracting one.

4. Discussion

Three measures—backward leg swing MD, hamstring

strength relative to quadriceps strength and quadriceps

strength relative to body weight—were found to predict

hamstring injury in a subsequent two-season period. The

number of players sustaining a hamstring injury in this study

was equivalent to a seasonal rate of 15% and is similar to the

rate of 14% described by a previous study of injuries in

Australian football at this level (Seward et al., 1993).

First, on a test utilizing a movement similar to the action

at the most likely time of injury, subsequently hamstring-

injured players had below average MD ability, and this

would suggest that it is players with poor lower limb

proprioception and motor control who are at risk of

hamstring injury. If an error is made in the control of the

swinging lower leg at a time in the running cycle when high

hamstring tissue forces exist, then a strain injury is possible.

The performance of swinging leg MD testing as a

screening tool for hamstring injury was assessed using ROC

curve analysis. Movement discrimination testing is a good

predictor of subsequent hamstring injury, as indicated by

obtaining an area under the ROC curve of 0.869. A range of

MD scores were assessed as the cut-point or decision

threshold for this measure, whereby players scoring below

this point are deemed to have a positive test and predicted to

sustain a hamstring injury, and scores above this point are

deemed negative and not predicted to injure. Consequently,

sensitivity and specificity values were calculated for each

decision threshold and are listed in Table 3. The Youden’s

index for each MD score acting as the decision threshold is

also indicated in Table 3. A MD score of 0.78 corresponds

to the decision threshold with the highest Youden’s index of

0.71, having 100% sensitivity and 71.4% specificity. These

values suggest that the MD test is better at identifying those

players unlikely to injure, and that some players testing

positive are able to avoid injury for other reasons. As with

any predictive test, the cut-point can be varied according to

the requirements of the clinician. In professional football

where hamstring injuries are one of the most frequently

occurring and recurring injuries, and players do have the

opportunity to engage in injury prevention programs, the

identification of any player at risk is worthwhile. In this

situation, the cost of false positives is less than the cost of

false negatives, (i.e. not identifying players who sub-

sequently sustain a hamstring injury). However, leg swing

MD testing lacks clinical usefulness without an intervention

program to rectify any deficit, and this is a direction for

future research.

In relation to the second significant injury predictor, over

the two-year study period, those players who sustained

a hamstring injury also had a lower H/Q strength ratio when

measured using isokinetic dynamometry at 60 8/s. The

magnitude of the difference in H/Q ratio between injured

and uninjured groups is similar to a previous study of elite

footballers (Orchard et al., 1997). However, from the third

significant injury predictor identified, it would seem that it is

the increased quadriceps strength rather than a decrease in

hamstring strength that is responsible for the reduced H/Q

ratio in the injured group of this study. This is a finding that

has not been reported previously. The increased quadriceps

strength of injured subjects suggests that footballers who

develop excessive quadriceps strength increase the risk of

hamstring injury, despite otherwise adequate hamstring

strength. The hamstring and quadriceps muscles co-contract

during the early stance phase of running as the knee flexes

after ground contact then extends. If quadriceps force

development during this co-contraction is in excess of the

hamstring muscle group’s capacity, then hamstring injury

may be possible. There is further co-contraction later in

stance and a shift from hip extension and knee flexion

torques, to hip flexion and knee extension dominance in

mid-stance (Mann and Sprague, 1980). It is at this point that,

Yamamoto (1993) suggests an imbalance between the

hamstrings and the hip flexor rectus femoris may result in

injury. The development of additional quadriceps strength

may result in an athlete running at a speed or with a

technique that predisposes the insufficient hamstring muscle

to injury (Muckle, 1982).

Players of Australian football would be aware of the

common and recurring nature of hamstring injuries, and

may be purposefully increasing their hamstring strength

training. This may involve increasing the number of lower

limb strength exercises generally, and as a consequence, the

quadriceps muscles may be strengthened to a point that

places the hamstrings at further injury risk, despite any

previous strength gains.

From the data here, low H/Q ratio and excessive

quadriceps strength are two strength factors that are

associated with an increased hamstring injury risk in

Australian football. Both factors can be considered to be

good predictors of hamstring injury with areas under the

ROC curve of 0.881 and 0.827, respectively. Table 3

contains the sensitivity and specificity values, and Youden’s

index for the corresponding H/Q ratios found in this study.

The traditional H/Q ratio of 0.60, when used as a cut-point

for identifying at-risk players, results in this test variable as

having 50% sensitivity, 85.7% specificity and Youden’s

index of 0.36. If 0.60 is used, half of the hamstring-injured

players would be missed, however, if the cut-point is raised

to 0.66 then sensitivity increases to 100%, specificity falls to

71.4%, and Youden’s index nearly doubles to 0.71. None of

the players in this study with a H/Q ratio above 0.66

sustained a hamstring injury in the two years subsequent to

testing. These findings suggest that a higher H/Q ratio of

0.66 should be used as the threshold for detecting elite

players of Australian football at-risk of hamstring injury.

M. Cameron et al. / Physical Therapy in Sport 4 (2003) 159–166164

The hamstring injury risk associated with high quad-

riceps strength relative to body weight poses an interesting

clinical dilemma in professional football. Intervention to

reduce quadriceps strength may involve reducing leg weight

training, however, this may have performance repercussions

and be undesirable in a large proportion of players. A high

quadriceps strength cut-off will decrease the number of

players involved in any intervention program that may harm

performance, however, it will increase the number of

injuries missed i.e. false negatives. Considering the

consequences of injury and intervention, the relationship

between excessive quadriceps strength and hamstring injury

deserves to be further investigated.

A ratio of eccentric hamstring and concentric

isokinetic strength has been suggested as useful in

assessment of hamstring-injured subjects (Aagaard et al.,

1995; Croisier and Crielaard, 2000). A significant

difference in such a ratio in previously hamstring-injured

athletes was demonstrated in a retrospective investigation

(Crosier et al., 2002), but not in a prospective study

(Bennell et al., 1998). There are no prospectively

designed studies supporting an eccentric/concentric ratio

identifying hamstring injury risk athletes, however, there

is with a concentric only ratio (Orchard et al., 1997). The

current study was also limited to only concentric strength

and ratios, however, these findings suggest that the early

stance phase of running involving concentric hamstring

activity, rather than the eccentric late swing phase, may

be a more likely region of hamstring injury. The role of

eccentric strength and the region of the running cycle of

hamstring injury deserve further investigation.

Eccentric strength assessment involves some injury risk,

particularly in hamstring muscles (Orchard et al., 2001),

therefore a concentric thigh strength assessment is rec-

ommended for the pre-season screening of hamstring injury

risk in participants of Australian football. The motor control

test did not place any stress on capacity to generate force, on

range, or on speed, but it did challenge the capacity of the

players to make fine discriminations between different

extents of backward leg swinging movements. The motor

control measure does not produce muscle pain or fatigue, is

less disruptive to training, and is a useful screening tool for

hamstring injury risk at any time of the season.

The results of this study support an association between

hamstring injury and both a low MD score and a high degree

of hamstring and quadriceps strength imbalance. Crosier

et al. (2002) argue that the relationship between muscle

imbalance and injury has always been a logical assumption,

however, we would argue that the relationship between poor

accuracy in relevant movement control and hamstring injury

is equally logical. One possible explanation for the pattern

of predictors observed here is that weight training to

develop leg strength can improve quadriceps strength

relatively more easily than hamstring strength, yet without

the level of motor control needed for injury-free perform-

ance with a stronger system. One implication is that there

may exist an interaction between thigh muscle strength and

leg swing MD such that a deficit in one variable can be

compensated by a higher ability in the other. The decision

threshold for each variable and any possible interaction need

to be validated in a subsequent group of footballers. Future

research should also be directed at training methods that can

reliably improve MD in backward leg swing, in order to

reduce the risk of hamstring injury.

5. Conclusion

The findings of this study suggest that poor leg

neuromuscular control may be a significant contributor to

hamstring injury. Data in this study has also supported the

injury risk of a low hamstring-to-quadriceps ratio. Investi-

gations into only a single contributing factor are likely to

lack agreement due to the multi-factorial nature of ham-

string strain injury. This study has linked two variables—

poor leg MD and thigh muscle strength imbalance—with an

increased risk of hamstring injury. At this point, it is

recommended that players be screened for hamstring injury

risk with leg motor control testing and thigh muscle strength

measurements, so that changes obtained in these values

could be used as a basis for designing effective injury

prevention programs.

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