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Objective The aim of this study was to explore expert opinion to identify the components of sprinting technique they believed to be risk factors for hamstring strain injuries (HSI). Design Mixed-method research design. Methods The Concept Systems groupwisdom™ web platform was used to analyse and collect data. Participants brainstormed, sorted and rated the components of sprinting technique to consider in a HSI prevention strategy. Results Twenty-three experts (academic/researcher, physiotherapist, strength and conditioning coaches and sprint coaches) brainstormed 66 statements that were synthesised and edited to 60 statements. Nineteen participants sorted the statements into clusters and rated them for relative importance and confidence they could be addressed in a hamstring injury prevention program. Multidimensional scaling and cluster analysis identified a 8-cluster solution modified to a 5-cluster solution by the research team: Training prescription (10 statements, mean importance: 3.79 out of 5 and mean confidence: 3.79); Neuromuscular and tendon properties (9, 3.09, 3.08); Kinematics parameters/Technical skills (27, 2.99, 2.98); Kinetics parameters (10, 2.85, 2.92); and Hip mechanics (4, 2.70, 2.63). The statement: “low exposure to maximal sprint running” located in the cluster “Training prescription” received the highest mean importance (4.55) and confidence ratings (4.42) of all statements. Conclusion The five clusters of components of sprinting technique believed to be risk factors for HSIs in order of most to least important were: training prescription, neuromuscular and tendon properties, kinematics parameters/technical drills, kinetics parameters and hip mechanics.

Noncontact lower limb injuries are common within the Australian Football League (AFL) and National Rugby League (NRL). Smaller (8.5 cm2) lumbar multifidus at the fifth vertebra (LM L5) and larger (8.2 cm2) quadratus lumborum (QL) cross-sectional area (CSA) have been associated with increased noncontact lower limb injury risk in AFL players. These associations have not been explored in an NRL cohort. This study will attempt to replicate previous research findings by confirming that muscle morphology is associated with noncontact lower limb injury.AFL (n = 87) and NRL (n = 151) players underwent LM L2-L5 and QL CSA ultrasound measures during preseason. Each club's medical staff reported all noncontact lower limb injuries sustained in the subsequent regular season. LM and QL CSA, age, body mass index, and noncontact lower limb injuries were analyzed using multivariable logistic regression.Seventy-two players sustained a noncontact lower limb injury in the 2020 regular season (AFL = 21, NRL = 51). The multivariable logistic regression (odds ratio (OR) = 1.36; 95% confidence interval (CI), 1.02-1.85; P = 0.038) identified AFL players with larger QL CSA at increased risk of sustaining a noncontact lower limb injury during the regular season, but no relationship was found for LM CSA and noncontact lower limb injuries in the AFL (OR = 1.01; 95% CI, 0.36-2.78; P = 0.591) or NRL (OR = 0.63; 95% CI, 0.29-1.33; P = 0.149).AFL players who sustained regular season noncontact lower limb injuries had larger QL CSA in preseason tests. No significant associations between either LM L5 CSA or LM L5 to QL ratio and regular season noncontact lower limb injuries were found.

ObjectivesThis study aimed to investigate short-interval intracortical inhibition (SICI) and muscle function in the Triceps surae of runners with mid-portion Achilles tendinopathy (AT).MethodsRunners with (n=11) and without (n=13) AT were recruited. Plantar flexor isometric peak torque and rate of torque development (RTD) were measured using an isokinetic dynamometer. Triceps surae endurance was measured as single leg heel raise (SLHR) to failure test. SICI was assessed using paired-pulse transcranial magnetic stimulation during a sustained contraction at 10% of plantar flexor isometric peak torque.ResultsTriceps surae SICI was 14.3% (95%CI: -2.1 to 26.4) higher in AT than control group (57.9%, 95%CI: 36.2, to 79.6; and 43.6% 95%CI:16.2 to 71.1; p=0.032) irrespective of the tested muscle. AT performed 16 (95%CI: 7.9 to 23.3, pPerspectiveOur data suggest greater intracortical inhibition for the Triceps surae muscles for the AT group accompanied by reduced SLHR endurance, without deficits in isometric peak torque or RTD. The increased SICI observed in the AT group could be negatively influencing Triceps surae endurance; thus, rehabilitation aiming to reduce intracortical inhibition should be considered to improve patient outcomes. Furthermore, SLHR is a useful clinical tool to assess plantar flexor function in AT patients.

Cortical mechanisms may contribute to weakness in participants with previous hamstring strain injury. This study aims to examine intra-cortical inhibition (SICI) and corticospinal excitability in previously injured participants. In this cross-sectional study, TMS was used to examine SICI, silent period, silent period: MEP ratios and area under the stimulus response curve in the biceps femoris and medial hamstrings. Comparisons were made between participants with (n = 10) and without (n = 10) previous hamstring strain injury. Motor threshold and isometric knee flexor strength were also compared between participants and the relationship between strength and SICI in control and previously injured participants was examined. Isometric knee flexor strength was lower in previously injured limbs compared with control limbs (mean difference = − 41 Nm (− 26%) [95% CI = − 80 to − 2 Nm], p = 0.04, Cohen’s d = − 1.27) and contralateral uninjured limbs (mean difference = − 23 Nm (− 17%), [95% CI = − 40 to − 6 Nm], p = 0.01, Cohen’s d = − 0.57). Previously injured limbs exhibited smaller responses to paired pulse stimulation (i.e. greater levels of SICI) in the biceps femoris compared with control limbs (mean difference = − 19%, [95% CI = − 34 to − 5%], p = 0.007, Cohen’s d = − 1.33). Isometric knee flexor strength was associated with the level of SICI recorded in the biceps femoris in previously injured participants (coefficient = 23 Nm [95% CI = 7–40 Nm], adjusted R2 = 0.31, p = 0.01). There were no differences in markers of corticospinal excitability between previously injured and control limbs (all p > 0.24, all Cohen’s d

The aim of this study was to determine the impacts of anterior cruciate ligament reconstruction (ACLR) and recent (

Background Lower limb muscle strain injury is highly prevalent in running-based sports and is considered a risk factor for recurrent injury. It is possible that differences in muscle activity and activation in previously strain-injured limbs may contribute to the elevated risk of reinjury. Objectives To systematically review available literature investigating whether muscle activity and/or activation is different in previously strain-injured muscles compared to contralateral uninjured muscles or uninjured controls. Methods A systematic review of literature in SPORTDiscus, MEDLINE Complete, CINAHL and Web of Science was conducted. Full-text English articles which compared indicators of neuromuscular function between injured and uninjured contralateral limbs or control groups in those with a history of muscle strain injury were included. Results Twelve studies were included in the review after eligibility criteria were applied. A best evidence synthesis revealed moderate to limited evidence suggesting differences in surface electromyography (sEMG) amplitude, integrated sEMG amplitude, inter-muscle sEMG ratios and voluntary activation in injured limbs, most often during eccentric contractions. Studies utilising sprinting assessments demonstrated conflicting evidence when comparing late swing phase biceps femoris sEMG amplitude between limbs with a history of hamstring strain injury and uninjured contralateral limbs. Conclusions Differences in muscle activity and activation were observed between injured and uninjured limbs across a variety of strength assessments. The evidence supporting these differences was most often moderate or limited and was generally observed during eccentric contractions. Mostly conflicting or limited evidence was found to suggest that participants with previous hamstring strain injury demonstrate no differences in muscle activity during running tasks when compared with their uninjured counterparts or contralateral limbs. Trial Registry PROSPERO (ID, CRD42019135681).

Objectives To assess if swimming practice results in changes in supraspinatus tendon thickness, acromiohumeral distance, and occupational ratio in shoulders of elite swimmers with and without a history of shoulder pain. Design Case-Control study. Methods A convenience sample of fifty elite swimmers (14-22 years) were recruited for this study. Groups were defined by the presence (history of pain, N = 37) or absence (pain free, N = 63) of significant interfering shoulder pain within the previous 6 months. The current study analyzed supraspinatus tendon thickness, acromiohumeral distance, and the occupational ratio, through the use of ultrasound. Measures were taken prior to swim practice; immediately after practice; and 6 hours post-practice. Results No statistically significant difference in supraspinatus tendon thickness, acromiohumeral distance or ratio between shoulders with and without a history of pain were found at rest. Following a swimming practice, both shoulders with and without a history of pain had a significant increase in tendon thickness (0.27 & 0.17 mm; P ≤ .001 &

There is a lack of definitive evidence supporting deficits in voluntary activation in participants with prior hamstring injury; moreover, it remains unknown if spinal mechanisms contribute to suspected deficits.This study aimed to determine the effect of prior hamstring strain injury on knee flexor concentric and eccentric strength, voluntary activation, surface electromyographic (sEMG) activity, and stretch and tendon reflex amplitudes.Twenty-five participants were recruited, 12 with a history of unilateral hamstring strain injury of at least moderate severity. Voluntary activation, strength, and sEMG activity were recorded during maximal eccentric and concentric knee flexor contractions at 60°·s. Stretch and tendon reflexes were also recorded at rest.Previously injured limbs exhibited lower levels of voluntary activation (mean difference = -24.1%, 95% confidence interval [CI] = -34.1% to -14.0%, P0.001), strength (mean difference vs control group = -0.37 Nm·kg, 95% CI = -0.71% to -0.03 N·m·kg, P = 0.03), and normalized sEMG (mean difference = -17%, 95% CI = -32% to -2%, P = 0.02) during maximal eccentric knee flexor contractions compared with control group. No such differences were seen in concentric contractions. Stretch reflexes (mean difference = -3.8%, 95% CI = -6.8 to -0.8, P = 0.02) and tendon reflexes (mean difference = -13%, 95% CI = -26% to 0%, P = 0.04) were also lower in previously injured compared with control biceps femoris muscles.Moderate to severe hamstring strain injury is associated with long-term deficits in voluntary activation during maximal eccentric contraction. Hamstring injury history is also associated with deficits in stretch reflex and tendon reflex amplitude.

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

Background Hamstring injuries remain a major burden in football while the effective prevention exercise the Nordic Hamstring is poorly adopted, despite the added positive effects on performance. Better understanding of hamstring function during Nordic Hamstring compared to other exercises may provide better insight to the physiological adaptations of different types of hamstring curls. Purpose This cross-sectional study therefore aimed to compare the Nordic Hamstring curl with a conventional prone Leg Curl at different loads, and novel high velocity Hamstring Catches; in terms of peak normalized electromyographical activity (nEMG) and rate of electromyographic rise (RER) of Biceps Femoris long head, and angular velocity of the knee. Study design Cross-sectional study. Methods Out of 28 participants enrolled, the final sample included 23 recreationally active male participants who attended a session for determining RM (repetition maximum) to establish loading (8 and 16RM for Hamstring Catches, and 8, 16 and 24RM for Leg Curl) and to familiarize themselves with the three different exercises (Nordic Hamstring, Leg Curl and Hamstring Catch), and a testing session >4 days after during which EMG data were collected during 3 repetitions of each exercise performed in a random order. Results The Nordic Hamstring evoked higher RER (1091.8 nEMG/s) than Hamstring Catches (mean difference: 421 nEMG/s, p Conclusion The Nordic Hamstring displayed the highest level muscle activity and most explosive recruitment characteristics with early and high rate of electromyographic rise, compared to even high velocity exercises, thus providing a possible mechanism by which it may increase performance and reduce injuries. Levels of evidence 3 What is known about the subject Early phase force and muscle recruitment have been linked to both performance and hamstring-related inhibition and fatiguability. However, the potential for different hamstring exercises to elicit explosive recruitment is unknown. What this study adds to existing knowledge Early phase recruitment was higher and faster during the Nordic Hamstring exercise compared with conventional hamstring Leg Curl exercises with different loads and a high-velocity hamstring exercise. Clinical Relevance The surprisingly fast and explosive recruitment characteristics during the Nordic Hamstring exercise suggests the possibility that this exercise have the potential to improve the rate of force development and perhaps counter the effects of hamstring-related inhibition and fatigue.

This study examined the spatial patterns of hamstring and gluteal muscle activation during high-speed overground running in limbs with and without aprior hamstring strain injury. Ten active males with arecent (ST>BFLH>GMED>SM>BFSH) and may exhibit altered intramuscular hamstring activation after returning to sport from BFLH strain injury.

Hamstring strain injury (HSI) is a common and costly injury in many sports such as the various professional football codes. Most HSIs have been reported to occur during high intensity sprinting actions. This observation has led to the suggestion that a link between sprinting biomechanics and HSIs may exist. The aim of this literature review was to evaluate the available scientific evidence underpinning the potential link between sprinting biomechanics and HSIs. A structured search of the literature was completed followed by a risk of bias assessment. A total of eighteen studies were retrieved. Sixteen studies involved retrospective and/or prospective analyses, of which only three were judged to have a low risk of bias. Two other case studies captured data before and after an acute HSI. A range of biomechanical variables have been measured, including ground reaction forces, trunk and lower-limb joint angles, hip and knee joint moments and powers, hamstring muscle–tendon unit stretch, and surface electromyographic activity from various trunk and thigh muscles. Overall, current evidence was unable to provide a clear and nonconflicting perspective on the potential link between sprinting biomechanics and HSIs. Nevertheless, some interesting findings were revealed, which hopefully will stimulate future research on this topic.

The aims of this study were to investigate changes in selected performance measures during an off-season period, their association, and the potential role of age and previous hamstring injury in semi-professional and amateur football players. Seventy-four male players (age: 25 ± 4 years, stature: 178.0 ± 6.6 cm, body mass: 74.9 ± 8.1 kg) were assessed at the beginning and end of the off-season summer-period for sprint, change-of-direction performance and eccentric hamstring strength. Small to medium increases in sprint times were observed at 5 (d = 0.26, p = 0.057), 10 (d = 0.42, p < 0.001) and 30 m (d = 0.64, p < 0.001). Small (d = −0.23, p = 0.033) improvements were observed for COD performance, and no changes in eccentric hamstring strength (d = 0.10, p = 0.317). The changes in the outcomes were not affected by age (p = 0.449 to 0.928) or previous hamstring injury (p = 0.109 to 0.995). The impaired sprint performance was not related to changes in eccentric hamstring strength (r = −0.21 to 0.03, p = 0.213 to 0.856), instead, changes in COD performance were associated with changes in eccentric hamstring strength (r = −0.42, p = 0.008).

Objectives To compare the change in supraspinatus tendon thickness (STT) following a high volume (HV) and high intensity (HI) swimming practice in shoulders of elite swimmers. Design Cohort Study. Setting Non-clinical, state swim team training facility. Participants A convenience sample of eight non-injured state and national level swimmers from a regional swim team were recruited for this study. Main outcome measures Ultrasound measures of STT were collected in response to the two swimming practice sessions. Measures were taken prior to each swim practice; immediately after practice; 6-hours post practice and 24-hours post practice. Results A significant increase in STT resulted from both the HI and HV (p < 0.05) practice immediately post practice. For the HI practice, the STT remained significantly thicker than pre-practice measures at the 6-hour post practice test (p < 0.05) however no longer significant 24-hours post practice. The difference in the change in STT between the HI and HV practice was significantly different immediately post practice and 6-hours post practice (p < 0.05) however no longer significant 24-hour post practice. Conclusion Ultrasound measures of STT following different swimming volumes and intensities may provide information on shoulder tendon loads.

Purpose Harvest of the semitendinosus (ST) tendon for anterior cruciate ligament reconstruction (ACLR) causes persistent hypotrophy of this muscle even after a return to sport, although it is unclear if hamstring activation patterns are altered during eccentric exercise. It was hypothesised that in comparison with contralateral control limbs, limbs with previous ACLR involving ST grafts would display (i) deficits in ST activation during maximal eccentric exercise; (ii) smaller ST muscle volumes and anatomical cross-sectional areas (ACSAs); and (iii) lower eccentric knee flexor strength. Methods Fourteen athletes who had successfully returned to sport after unilateral ACLR involving ST tendon graft were recruited. Median time since surgery was 49 months (range 12–78 months). Participants underwent functional magnetic resonance imaging (MRI) of their thighs before and after the Nordic hamstring exercise (NHE) and percentage change in transverse (T2) relaxation time was used as an index of hamstring activation. Muscle volumes and ACSAs were determined from MRI and distal ST tendons were evaluated via ultrasound. Eccentric knee flexor strength was determined during the NHE. Results Exercise-induced T2 change was lower for ST muscles in surgical than control limbs (95% CI − 3.8 to − 16.0%). Both ST muscle volume (95% CI − 57.1 to − 104.7 cm3) and ACSA (95% CI − 1.9 to − 5.0 cm2) were markedly lower in surgical limbs. Semimembranosus (95% CI 5.5–14.0 cm3) and biceps femoris short head (95% CI 0.6–11.0 cm3) volumes were slightly higher in surgical limbs. No between-limb difference in eccentric knee flexor strength was observed (95% CI 33 N to − 74 N). Conclusion ST activation is significantly lower in surgical than control limbs during eccentric knee flexor exercise 1–6 years after ACLR with ST graft. Lower levels of ST activation may partially explain this muscle’s persistent hypotrophy post ACLR and have implications for the design of more effective rehabilitation programs. Level of evidence IV.

Hamstring strain injuries are endemic in running-based sports. Given the economic and performance implications of these injuries, a significant body of research has emerged in recent years in an attempt to identify risk factors and develop or optimise injury prevention strategies. Surveys of injury prevention practices among medical and conditioning staff in elite sport suggest that many sporting clubs invest significant efforts in eccentric hamstring conditioning and lumbo-pelvic or trunk stability programmes. The purpose of this narrative review was to critically evaluate the evidence underpinning these practices. Single-exercise eccentric training interventions have proven effective in the prevention of primary and recurrent hamstring strains, when compliance is adequate. However, despite its almost universal acceptance, the authors are aware of only one, very recent, prospective risk factor study examining the effect of lumbo-pelvic motion during sprinting on hamstring injury risk. Furthermore, the interventions exploring the effect of lumbo-pelvic training on hamstring injury rates have not measured stability in any way. An improved understanding of the evidence underpinning commonly employed hamstring injury prevention practices may enable clinicians and coaches to better prioritise effective strategies in the increasingly complex environment of elite sport.

Strength training is a valuable component of hamstring strain injury prevention programmes; however, in recent years a significant body of work has emerged to suggest that the acute responses and chronic adaptations to training with different exercises are heterogeneous. Unfortunately, these research findings do not appear to have uniformly influenced clinical guidelines for exercise selection in hamstring injury prevention or rehabilitation programmes. The purpose of this review was to provide the practitioner with an evidence-base from which to prescribe strengthening exercises to mitigate the risk of hamstring injury. Several studies have established that eccentric knee flexor conditioning reduces the risk of hamstring strain injury when compliance is adequate. The benefits of this type of training are likely to be at least partly mediated by increases in biceps femoris long head fascicle length and improvements in eccentric knee flexor strength. Therefore, selecting exercises with a proven benefit on these variables should form the basis of effective injury prevention protocols. In addition, a growing body of work suggests that the patterns of hamstring muscle activation diverge significantly between different exercises. Typically, relatively higher levels of biceps femoris long head and semimembranosus activity have been observed during hip extension-oriented movements, whereas preferential semitendinosus and biceps femoris short head activation have been reported during knee flexion-oriented movements. These findings may have implications for targeting specific muscles in injury prevention programmes. An evidence-based approach to strength training for the prevention of hamstring strain injury should consider the impact of exercise selection on muscle activation, and the effect of training interventions on hamstring muscle architecture, morphology and function. Most importantly, practitioners should consider the effect of a strength training programme on known or proposed risk factors for hamstring injury.

Objectives: To examine the effect of 100 drop punt kicks on isokinetic knee flexor strength and surface electromyographic activity of bicep femoris and medial hamstrings. Design: Randomized control study. Methods: Thirty-six recreational footballers were randomly assigned to kicking or control groups. Dynamometry was conducted immediately before and after the kicking or 10 min of sitting (control). Results: Eccentric strength declined more in the kicking than the control group (p < 0.001; d = 1.60), with greater reductions in eccentric than concentric strength after kicking (p = 0.001; d = 0.92). No significant between group differences in concentric strength change were observed (p = 0.089; d = 0.60). The decline in normalized eccentric hamstring surface electromyographic activity (bicep femoris and medial hamstrings combined) was greater in the kicking than the control group (p < 0.001; d = 1.78), while changes in concentric hamstring surface electromyographic activity did not differ between groups (p = 0.863; d = 0.04). Post-kicking reductions in surface electromyographic activity were greater in eccentric than concentric actions for both bicep femoris (p = 0.008; d = 0.77) and medial hamstrings (p < 0.001; d = 1.11). In contrast, the control group exhibited smaller reductions in eccentric than concentric hamstring surface electromyographic activity for bicep femoris (p = 0.026; d = 0.64) and medial hamstrings (p = 0.032; d = 0.53). Reductions in bicep femoris surface electromyographic activity were correlated with eccentric strength decline (R = 0.645; p = 0.007). Conclusions: Reductions in knee flexor strength and hamstring surface electromyographic activity are largely limited to eccentric contractions and this should be considered when planning training loads in Australian Football.

Objectives To investigate whether five-weeks of concentric (CON) or eccentric (ECC) hamstring strength training have different effects on recovery from sprint running, eccentric strength and architecture of the biceps femoris long head (BFLH). Design Cohort study. Methods Thirty males (age, 22.8 ± 4.1 y; height, 180.1 ± 6.4 cm; weight, 85.2 ± 14.6 kg) were allocated into either a CON or ECC group, both performing nine sessions of resistance training. Prior to and immediately after the five-week intervention, each participant’s BFLH fascicle length (FL), pennation angle (PA), muscle thickness (MT), peak isometric KF torque and Nordic eccentric strength were assessed. Post-intervention, participants performed two timed sprint sessions (10 × 80 m) 48 h apart. Blood samples and passive KF torques were collected before, immediately after, 24 h and 48 h after the first sprint session. Results After five-weeks of strength-training, fascicles lengthened in the ECC (p < 0.001; d = 2.0) and shortened in the CON group (p < 0.001; d = 0.92), while PA decreased for the ECC (p = 0.001; d = 0.52) and increased in the CON group (p < 0.001; d = 1.69). Nordic eccentric strength improved in both ECC (p < 0.001; d = 1.49) and CON (p < 0.001; d = 0.95) groups. No between-group differences were observed in peak isometric strength (p = 0.480), passive KF torques (p = 0.807), sprint performance decrements between sprint sessions (p = 0.317) and creatine kinase (p = 0.818). Conclusions Despite inducing significant differences in BFLH muscle architecture, there were no significant between group differences in sprint performance decrements across two sprint sessions.

Background: The architectural characteristics of muscle (fascicle length, pennation angle muscle thickness) respond to varying forms of stimuli ( eg, training, immobilisation and injury ). Architectural changes following injury are thought to occur in response to the restricted range of motion experienced during rehabilitation and the associated neuromuscular inhibition. However, it is unknown if these differences exist prior to injury, and had a role in injury occuring ( prospectively ), or if they occur in response to the incident itself ( retrospectively ). Considering that the structure of a muscle will influence how it functions, it is of interest to understand how these architectural variations may alter how a muscle acts with reference to the force-length and force-velocity relationships. Objectives: Our narrative review provides an overview of muscle architectural adaptations to training and injury. Specifically, we (1) describe the methods used to measure muscle architecture; (2) detail the impact that architectural alterations following training interventions, immobilisation and injury have on force production and (3) present a hypothesis on how neuromuscular inhibition could cause maladaptations to muscle architecture following injury.

This study aimed to determine: (a) the spatial patterns of hamstring activation during the Nordic hamstring exercise (NHE); (b) whether previously injured hamstrings display activation deficits during the NHE; and (c) whether previously injured hamstrings exhibit altered cross-sectional area (CSA). Ten healthy, recreationally active men with a history of unilateral hamstring strain injury underwent functional magnetic resonance imaging of their thighs before and after six sets of 10 repetitions of the NHE. Transverse (T2) relaxation times of all hamstring muscles [biceps femoris long head (BFlh); biceps femoris short head (BFsh); semitendinosus (ST); semimembranosus (SM)] were measured at rest and immediately after the NHE and CSA was measured at rest. For the uninjured limb, the ST's percentage increase in T2 with exercise was 16.8%, 15.8%, and 20.2% greater than the increases exhibited by the BFlh, BFsh, and SM, respectively (P

Purpose: This study aimed (i) to determine the reliability of two-dimensional ultrasonography for the assessment of biceps femoris long head (BFlh) architectural characteristics and (ii) to determine whether limbs with a history of strain injury in the BFlh display different architecture and eccentric strength compared to uninjured limbs. Methods: This case–control study (control [n = 20], injured group [n = 16], males) assessed the BFlh architecture at rest and during graded isometric contractions using two-dimensional ultrasonography. The control group were assessed three times (>24 h apart) to determine reliability. Previously injured individuals were evaluated once. Results: The assessment of BFlh architecture was highly reliable (intraclass correlations >0.90). Fascicle length (P < 0.001; d range = 0.67–1.34) and fascicle length relative to muscle thickness (P < 0.001; d range = 0.58–0.85) of the previously injured BFlh were significantly less than the contralateral uninjured BFlh at all intensities. Pennation angle of the previously injured BFlh was significantly greater (P < 0.001; d range = 0.62–0.88) than the contralateral uninjured BFlh at all intensities. Eccentric strength in the previously injured limb was significantly lower than that in the contralateral limb (−15.4%; −52.5 N, 95% confidence interval = −76.2 to −28.4; P < 0.001, d = 0.56). Conclusions: These data indicate that two-dimensional ultrasonography is reliable for assessing BFlh architecture at rest and during graded isometric contractions. Fascicle length, fascicle length relative to muscle thickness, and pennation angle are significantly different in previously injured BFlh compared to an uninjured contralateral BFlh. Eccentric strength of the previously injured limb is also significantly lower than that of the uninjured contralateral limb. These findings have implications for rehabilitation and injury prevention practices, which should consider altered architectural characteristics.

Background: Few studies have examined acute injuries in track and field in both elite and subelite athletes. Purpose: To observe the absolute number and relative rates of injury in track and field athletes across a wide range of competition levels and ages during 3 years of the Penn Relays Carnival to assist with future medical coverage planning and injury prevention strategies. Study Design: Descriptive epidemiology study. Methods: Over a 3-year period, all injuries treated by the medical staff were recorded on a standardized injury report form. Absolute number of injuries and relative injury rates (number of injuries per 1000 competing athletes) were determined and odds ratios (ORs) of injury rates were calculated between sexes, competition levels, and events. Injuries were also broken down into major or minor medical or orthopaedic injuries. Results: Throughout the study period, 48,473 competing athletes participated in the Penn Relays Carnival, and 436 injuries were sustained. For medical coverage purposes, the relative rate of injury subtypes was greatest for minor orthopaedic injuries (5.71 injuries per 1000 participants), followed by minor medical injuries (3.42 injuries per 1000 participants), major medical injuries (0.69 injuries per 1000 participants), and major orthopaedic injuries (0.18 injuries per 1000 participants). College/elite athletes displayed the lowest relative injury rate (7.99 injuries per 1000 participants), which was significantly less than that of high school (9.87 injuries per 1000 participants) and masters athletes (16.33 injuries per 1000 participants). Male athletes displayed a greater likelihood of having a minor orthopaedic injury compared with female athletes (OR, 1.36 [95% CI, 1.06-1.75]; χ2 = 5.73; P = .017) but were less likely to sustain a major medical injury (OR, 0.33 [95% CI, 0.15-0.75]; χ2 = 7.75; P = .005). Of the 3 most heavily participated in events, the 4 × 400-m relay displayed the greatest relative injury rate (13.6 injuries per 1000 participants) compared with the 4 × 100-m and 4 × 200-m relays. Conclusion: Medical coverage teams for future large-scale track and field events need to plan for at least 2 major orthopaedic and 7 major medical injuries per 10,000 participants. Male track and field athletes, particularly masters male athletes, are at greater risk of injury compared with other sexes and competition levels.

Background: The architectural and morphological adaptations of the hamstrings in response to training with different exercises have not been explored. Purpose: To evaluate changes in biceps femoris long head ( BFLH ) fascicle length and hamstring muscle size following 10-weeks of Nordic hamstring exercise ( NHE ) or hip extension ( HE ) training. Methods: 30 recreationally active male athletes ( age, 22.0±3.6 years; height, 180.4±7 cm; weight, 80.8±11.1 kg ) were allocated to 1 of 3 groups: ( 1 ) HE training ( n=10 ), NHE training ( n=10 ), or no training ( control, CON ) ( n=10 ). BFLH fascicle length was assessed before, during ( Week 5 ) and after the intervention with a two-dimensional ultrasound. Hamstring muscle size was determined before and after training via MRI. Results: Compared with baseline, BFLH fascicles were lengthened in the NHE and HE groups at mid-training ( d=1.12–1.39, p < 0.001 ) and post-training ( d=1.77–2.17, p < 0.001 ) and these changes did not differ significantly between exercises ( d=0.49–0.80, p=0.279–0.976 ). BFLHvolume increased more for the HE than the NHE ( d=1.03, p=0.037 ) and CON ( d=2.24, p < 0.001 ) groups. Compared with the CON group, both exercises induced significant increases in semitendinosus volume ( d=2.16–2.50, ≤0.002 ) and these increases were not significantly different ( d=0.69, p=0.239 ). Conclusion: NHE and HE training both stimulate significant increases in BFLH fascicle length; however, HE training may be more effective for promoting hypertrophy in the BFLH.

Purpose: Three of the most commonly identified hamstring strain injury (HSI) risk factors are age, previous HSI, and low levels of eccentric hamstring strength. However, no study has investigated the ability of these risk factors to predict the incidence of HSI in elite Australian footballers. Accordingly, the purpose of this prospective cohort study was to investigate the predictive ability of HSI risk factors using machine learning techniques. Methods: Eccentric hamstring strength, demographic and injury history data were collected at the start of preseason for 186 and 176 elite Australian footballers in 2013 and 2015, respectively. Any prospectively occurring HSI were reported to the research team. Using various machine learning techniques, predictive models were built for 2013 and 2015 within-year HSI prediction and between-year HSI prediction (2013 to 2015). The calculated probabilities of HSI were compared with the injury outcomes and area under the curve (AUC) was determined and used to assess the predictive performance of each model. Results: The minimum, maximum, and median AUC values for the 2013 models were 0.26, 0.91, and 0.58, respectively. For the 2015 models, the minimum, maximum and median AUC values were, correspondingly, 0.24, 0.92, and 0.57. For the between-year predictive models the minimum, maximum, and median AUC values were 0.37, 0.73, and 0.52, respectively. Conclusions: Although some iterations of the models achieved near perfect prediction, the large ranges in AUC highlight the fragility of the data. The 2013 models performed slightly better than the 2015 models. The predictive performance of between-year HSI models was poor however. In conclusion, risk factor data cannot be used to identify athletes at an increased risk of HSI with any consistency.

The aim of this study was to determine whether declines in knee flexor strength following overground repeat sprints were related to changes in hamstrings myoelectrical activity. Seventeen recreationally active men completed maximal isokinetic concentric and eccentric knee flexor strength assessments at 180°/s before and after repeat sprint running. Myoelectrical activity of the biceps femoris (BF) and medial hamstrings (MHs) was measured during all isokinetic contractions. Repeated measures mixed model [fixed factors = time (pre- and post-repeat sprint) and leg (dominant and nondominant), random factor = participants] design was fitted with the restricted maximal likelihood method. Repeat sprint running resulted in significant declines in eccentric, and concentric, knee flexor strength (eccentric = 26 ± 4 Nm, 15% P

Australian rules football is a challenging contact sport requiring high levels of fitness and skill. In the last two decades, hamstring strain injuries have remained an ongoing problematic issue, constituting a large proportion of soft tissue injuries sustained in the elite Australian Football League (AFL).1 The predominant injury mechanism for hamstring strain injuries is sprinting,2 …

This study aimed to observe the incidence rates of hamstring strain injuries (HSIs) across different competition levels and ages during the Penn Relays Carnival. Over a 3-year period, all injuries treated by the medical staff were recorded. The type of injury, anatomic location, event in which the injury occurred, competition level, and demographic data were documented. Absolute and relative HSI (per 1000 participants) were determined, and odds ratios (ORs) were calculated between sexes, competition levels, and events. Throughout the study period 48 473 athletes registered to participate in the Penn Relays Carnival, with 118 HSIs treated by the medical team. High school girls displayed lesser risk of HSI than high school boys (OR = 0.55, P = 0.021), and masters athletes were more likely than high school- (OR = 4.26, P

BackgroundTo investigate the association between running exposure and the risk of hamstring strain injury (HSI) in elite Australian footballers.MethodsElite Australian footballers (n=220) from 5 different teams participated. Global positioning system (GPS) data were provided for every athlete for each training session and match for the entire 2015 season. The occurrences of HSIs throughout the study period were reported. Receiver operator characteristic curve analyses were performed and the relative risk (RR) of subsequent HSI was calculated for absolute and relative running exposure variables related to distance covered above 10 and 24 km/hour in the preceding week/s.Results30 prospective HSIs occurred. For the absolute running exposure variables, weekly distance covered above 24 km/hour (>653 m, RR=3.4, 95% CI 1.6 to 7.2, sensitivity=0.52, specificity=0.76, area under the curve (AUC)=0.63) had the largest influence on the risk of HSI in the following week. For the relative running exposure variables, distance covered above 24 km/hour as a percentage of distance covered above 10 km/hour (>2.5%, RR=6.3, 95% CI 1.5 to 26.7, sensitivity=0.93, specificity=0.34, AUC=0.63) had the largest influence on the risk of HSI in the following week. Despite significant increases in the RR of HSI, the predictive capacity of these variables was limited.ConclusionsAn association exists between absolute and relative running exposure variables and elite Australian footballers' risk of subsequent HSI, with the association strongest when examining data within 7–14 days. Despite this, the use of running exposure variables displayed limited clinical utility to predict HSI at the individual level.

Objective - To determine which strength training exercises selectively activate the biceps femoris long head (BFLongHead) muscle. Methods - We recruited 24 recreationally active men for this two-part observational study. Part 1: We explored the amplitudes and the ratios of lateral (BF) to medial hamstring (MH) normalised electromyography (nEMG) during the concentric and eccentric phases of 10 common strength training exercises. Part 2: We used functional MRI (fMRI) to determine the spatial patterns of hamstring activation during two exercises which (1) most selectively and (2) least selectively activated the BF in part 1. Results - Eccentrically, the largest BF/MH nEMG ratio occurred in the 45° hip-extension exercise; the lowest was in the Nordic hamstring (Nordic) and bent-knee bridge exercises. Concentrically, the highest BF/MH nEMG ratio occurred during the lunge and 45° hip extension; the lowest was during the leg curl and bent-knee bridge. fMRI revealed a greater BF(LongHead) to semitendinosus activation ratio in the 45° hip extension than the Nordic (p

Background/Aim - To investigate the role of eccentric knee flexor strength, between-limb imbalance and biceps femoris long head (BFlh) fascicle length on the risk of a future hamstring strain injury (HSI). Methods - Elite soccer players (n=152) from eight different teams participated. Eccentric knee flexor strength during the Nordic hamstring exercise and BFlh fascicle length were assessed at the beginning of pre-season. The occurrences of a HSI following this were recorded by the team medical staff. Relative risk (RR) was determined for univariate data, and logistic regression was employed for multivariate data. Results - Twenty-seven new HSIs were reported. Eccentric knee flexor strength below 337N (RR = 4.4; 95% CI = 1.1 to 17.5) and BFlh fascicles shorter than 10.56cm (RR = 4.1; 95% CI=1.9 to 8.7) significantly increased the risk of a subsequent HSI. Multivariate logistic regression revealed significant effects when combinations of age, previous history of HSI, eccentric knee flexor strength and BFlh fascicle length were explored. From these analyses the likelihood of a future HSI in older athletes or those with a previous HSI history was reduced if high levels of eccentric knee flexor strength and longer BFlh fascicles were present. Conclusions - The presence of short BFlh fascicles and low levels of eccentric strength in elite soccer players increase the risk of a future HSI. The greater risk of a future HSI in older players or those with a previous HSI is reduced when they possess longer BFlh fascicles and high levels of eccentric strength.

Background Hamstring strain injuries (HSIs) are common within the Australian Football League (AFL) with most occurring during high-speed running (HSR). Therefore, this study investigated possible relationships between mean session running distances, session ratings of perceived exertion (s-RPE) and HSIs within AFL footballers. Methods Global positioning system (GPS)-derived running distances and s-RPE for all matches and training sessions over two AFL seasons were obtained from one AFL team. All HSIs were documented and each player's running distances and s-RPE were standardised to their 2-yearly session average, then compared between injured and uninjured players in the 4 weeks (weeks −1, −2, −3 and −4) preceding each injury. Results Higher than ‘typical’ (ie, z=0) HSR session means were associated with a greater likelihood of HSI (week −1: OR=6.44, 95% CI=2.99 to 14.41, p < 0.001; summed weeks −1 and −2: OR=3.06, 95% CI=2.03 to 4.75, p < 0.001; summed weeks −1, −2 and −3: OR=2.22, 95% CI=1.66 to 3.04, p < 0.001; and summed weeks −1, −2, −3 and −4: OR=1.96, 95% CI=1.54 to 2.51, p < 0.001). However, trivial differences were observed between injured and uninjured groups for standardised s-RPE, total distance travelled and distances covered whilst accelerating and decelerating. Increasing AFL experience was associated with a decreased HSI risk (OR=0.77, 95% CI 0.57 to 0.97, p=0.02). Furthermore, HSR data modelling indicated that reducing mean distances in week −1 may decrease the probability of HSI. Conclusions Exposing players to large and rapid increases in HSR distances above their 2-yearly session average increased the odds of HSI. However, reducing HSR in week −1 may offset HSI risk.

Purpose: This study aimed to determine whether limbs with a history of anterior cruciate ligament (ACL) injury reconstructed from the semitendinosus display different biceps femoris long head (BFlh) architecture and eccentric strength, assessed during the Nordic hamstring exercise, compared with the contralateral uninjured limb. Methods: The architectural characteristics of the BFlh were assessed at rest and at 25% of a maximal voluntary isometric contraction (MVIC) in the control group (n = 52) and in the group who had previous ACL injury (n = 15) using two-dimensional ultrasonography. Eccentric knee flexor strength was assessed during the Nordic hamstring exercise. Results: Fascicle length was shorter (P = 0.001; d range, 0.90–1.31) and pennation angle (P range, 0.001–0.006; d range, 0.87–0.93) was greater in the BFlh of the ACL-injured limb compared with those in the contralateral uninjured limb at rest and during a 25% MVIC. Eccentric strength was lower in the ACL-injured limb when compared with the contralateral uninjured limb. Fascicle length, MVIC, and eccentric strength were not different between the left and right limb in the control group. Conclusions: Limbs with a history of ACL injury reconstructed from the semitendinosus have shorter fascicles and greater pennation angles in the BFlh compared with those of the contralateral uninjured side. Eccentric strength during the Nordic hamstring exercise of the ACL-injured limb is significantly lower than that of the contralateral side. These findings have implications for ACL rehabilitation and hamstring injury prevention practices, which should consider altered architectural characteristics.

We thank Prof Li and Mr Syno for their interest in our paper. We agree that our study would have been more informative had it included an assessment of hip muscle activations but the correspondents’ call for a broader study fails to appreciate the goals of the current one (assessing hamstring activation) and the limitations encountered when undertaking time consuming and expensive research. For example, when embarking on this and the other studies that were part of our broader project (Bourne et al., 2017a, 2017b & 2017c) we had to consider the pressures placed on a PhD candidate (MB) for timely completion and the budget limitations which precluded scanning a larger volume of the lower limb. However, we do hope to assess hip muscle activation in future studies and we extend an invitation to Professor Li and Mr Syno to join us in that endeavour...

Hamstring strain injuries are the predominant injury in many sports, costing athletes and clubs a significant financial and performance burden; therefore, the ability to identify and intervene with individuals who are considered at high risk of injury is important. One measure that has grown in popularity as an outcome variable following hamstring intervention/prevention studies and rehabilitation is the angle of peak knee flexor torque. This current opinion article will firstly introduce the measure and the processes behind it. Second, the article will summarise how the angle of peak knee flexor torque has been suggested to measure hamstring strain injury risk. Finally, the numerous limitations will be presented and the article will outline how these limitations may influence the usefulness of the angle of peak knee flexor torque measure. These include the lack of muscle specificity, the common concentric contraction mode of assessment, reliability of the measure, various neural contributions (such as rate of force development and neuromuscular inhibition), as well as the lack of prospective data showing any predictive value in the measure.

Background: Hamstring strain injuries (HSIs) represent the most common cause of lost playing time in rugby union. Eccentric knee flexor weakness and between-limb imbalance in eccentric knee flexor strength are associated with a heightened risk of HSIs in other sports; however, these variables have not been explored in rugby union. Purpose: To determine if lower levels of eccentric knee flexor strength or greater between-limb imbalance in this parameter during the Nordic hamstring exercise are risk factors for HSIs in rugby union. Study Design: Cohort study; Level of evidence, 2. Methods: This prospective study was conducted over the 2014 Super Rugby and Queensland Rugby Union seasons. In total, 178 rugby union players (mean age, 22.6 ± 3.8 years; mean height, 185.0 ± 6.8 cm; mean weight, 96.5 ± 13.1 kg) had their eccentric knee flexor strength assessed using a custom-made device during the preseason. Reports of previous hamstring, quadriceps, groin, calf, and anterior cruciate ligament injuries were also obtained. The main outcome measure was the prospective occurrence of HSIs. Results: Twenty players suffered at least 1 HSI during the study period. Players with a history of HSIs had a 4.1-fold (95% CI, 1.9-8.9; P = .001) greater risk of subsequent HSIs than players without such a history. Between-limb imbalance in eccentric knee flexor strength of ≥15% and ≥20% increased the risk of HSIs by 2.4-fold (95% CI, 1.1-5.5; P = .033) and 3.4-fold (95% CI, 1.5-7.6; P = .003), respectively. Lower eccentric knee flexor strength and other prior injuries were not associated with an increased risk of future HSIs. Multivariate logistic regression revealed that the risk of reinjuries was augmented in players with strength imbalances. Conclusion: Previous HSIs and between-limb imbalance in eccentric knee flexor strength were associated with an increased risk of future HSIs in rugby union. These results support the rationale for reducing imbalance, particularly in players who have suffered a prior HSI, to mitigate the risk of future injuries.

Purpose: Are eccentric hamstring strength and between-limb imbalance in eccentric strength, measured during the Nordic hamstring exercise, risk factors for hamstring strain injury (HSI)? Methods: Elite Australian footballers (n = 210) from five different teams participated. Eccentric hamstring strength during the Nordic exercise was obtained at the commencement and conclusion of preseason training and at the midpoint of the season. Injury history and demographic data were also collected. Reports on prospectively occurring HSI were completed by the team medical staff. Relative risk (RR) was determined for univariate data, and logistic regression was employed for multivariate data. Results: Twenty-eight new HSI were recorded. Eccentric hamstring strength below 256 N at the start of the preseason and 279 N at the end of the preseason increased the risk of future HSI 2.7-fold (RR, 2.7; 95% confidence interval, 1.3 to 5.5; P = 0.006) and 4.3-fold (RR, 4.3; 95% confidence interval, 1.7 to 11.0; P = 0.002), respectively. Between-limb imbalance in strength of greater than 10% did not increase the risk of future HSI. Univariate analysis did not reveal a significantly greater RR for future HSI in athletes who had sustained a lower limb injury of any kind within the last 12 months. Logistic regression revealed interactions between both athlete age and history of HSI with eccentric hamstring strength, whereby the likelihood of future HSI in older athletes or athletes with a history of HSI was reduced if an athlete had high levels of eccentric strength. Conclusion: Low levels of eccentric hamstring strength increased the risk of future HSI. Interaction effects suggest that the additional risk of future HSI associated with advancing age or previous injury was mitigated by higher levels of eccentric hamstring strength.

The twitch interpolation technique is commonly employed to assess the completeness of skeletal muscle activation during voluntary contractions. Early applications of twitch interpolation suggested that healthy human subjects could fully activate most of the skeletal muscles to which the technique had been applied. More recently, however, highly sensitive twitch interpolation has revealed that even healthy adults routinely fail to fully activate a number of skeletal muscles despite apparently maximal effort. Unfortunately, some disagreement exists as to how the results of twitch interpolation should be employed to quantify voluntary activation. The negative linear relationship between evoked twitch force and voluntary force that has been observed by some researchers implies that voluntary activation can be quantified by scaling a single interpolated twitch to a control twitch evoked in relaxed muscle. Observations of non-linear evoked-voluntary force relationships have lead to the suggestion that the single interpolated twitch ratio can not accurately estimate voluntary activation. Instead, it has been proposed that muscle activation is better determined by extrapolating the relationship between evoked and voluntary force to provide an estimate of true maximum force. However, criticism of the single interpolated twitch ratio typically fails to take into account the reasons for the non-linearity of the evoked-voluntary force relationship. When these reasons are examined, it appears that most are even more challenging to the validity of extrapolation than they are to the linear equation. Furthermore, several factors that contribute to the observed non-linearity can be minimised or even eliminated with appropriate experimental technique. The detection of small activation deficits requires high resolution measurement of force and careful consideration of numerous experimental details such as the site of stimulation, stimulation intensity and the number of interpolated stimuli. Sensitive twitch interpolation techniques have revealed small to moderate deficits in voluntary activation during brief maximal efforts and progressively increasing activation deficits (central fatigue) during exhausting exercise. A small number of recent studies suggest that resistance training may result in improved voluntary activation of the quadriceps femoris and ankle plantarflexor muscles but not the biceps brachii. A significantly larger body of evidence indicates that voluntary activation declines as a consequence of bed-rest, joint injury and joint degeneration. Twitch interpolation has also been employed to study the mechanisms by which caffeine and pseudoephedrine enhance exercise performance.

Background The Nordic hamstring exercise (NHE) is employed to increase eccentric strength and reduce the risk of recurrent hamstring strain injury. Currently, the prescription of the NHE offering most benefit is unknown. Objective To assess force production, muscle activation and kinematics during two NHE prescriptions. It was hypothesized that a 5*6 repetitions protocol would yield greater average forces, smaller force drops and greater muscle activation per set compared to a 3*10 protocol. Additionally, it was hypothesised that during both protocols, peak muscle activation would occur at shorter muscle lengths than peak force. Design Randomised cross-over study. Setting Recreationally trained and sub-elite male athletes. Participants Thirteen healthy males (age 23±4 years, stature 178±6.5 cm and weight 75±17 kg). Interventions Participants performed 3*10 and 5*6 repetitions of the NHE (on separate occasions), 3 minutes rest was given between sets. Main Outcome Measurements Load cells were used to record knee flexor force, electromyography (EMG) was used to measure hamstring activation and a goniometer was used to measure knee flexion angles during each repetition. Results Average force (666.44N during 3*10 vs. 697.95N during 5*6) and force drop per set (−54.48N during 3*10 and −51.73N during 5*6) was not different between protocols. Additionally, normalised biceps femoris (87 vs. 79% of max isometric EMG during 3*10 and 5*6 respectively) and medial hamstring (111 vs. 98% of max isometric EMG during 3*10 and 5*6 respectively) EMG per set was not different between protocols. For both protocols a significant difference between the angle of peak torque and angle of peak biceps femoris EMG was observed (p Conclusions Force output and muscle activation during the NHE is not affected by prescription. Peak activation of the biceps femoris appears to occur at shorter muscle lengths than peak force, suggesting inhibition of this muscle at long lengths.

Background The primary mechanism for hamstring strain injury is high speed running however insults also occur during kicking in which they are more severe. Objective To examine the effect of 100 drop punt kicks on isokinetic knee flexor strength and surface electromyographic (sEMG) activity of biceps femoris (BF) and medial hamstrings (MH). Design Randomised control study. Setting Community level Australian football. Participants Thirty-six recreational footballers were recruited. Interventions Participants were randomly assigned to kicking or control groups. Dynamometry was conducted immediately before and after the kicking or 10 minutes of sitting (control). Main Outcome Measurements Knee flexor strength and biceps femoris and medial hamstring surface electromyography. Results Eccentric strength declined more in the kicking than the control group (p Conclusions Reductions in knee flexor strength and hamstring sEMG are largely limited to eccentric contractions and this should be considered when planning training loads in Australian Football.

Background Hamstring strain injuries (HSIs) are the most common cause of time loss from training and competition in elite football. Anterior cruciate ligament (ACL) ruptures are also prevalent and typically require surgery from hamstring grafts, which often results in chronic deficits in eccentric knee flexor strength, a known risk factor for HSI. Objective We aimed to determine the effect of prior ACL reconstruction (ACLR) on 1) eccentric knee flexor strength during the Nordic hamstring exercise (NHE) and; 2) the risk of future HSI. We hypothesised that athletes with a prior ACLR would display 1) lower levels of eccentric knee flexor strength than athletes without such history and; 2) an increased risk of future HSI. Design Prospective study. Setting Secondary analysis of 3 studies previously completed in Australian sport. Patients (or Participants) 541 elite to sub-elite male footballers had their eccentric knee flexor strength assessed using the NHE. Reports of HSIs and any history of ACL injuries were obtained. All participants were categorized into injury history groups. Interventions (or Assessment of Risk Factors) Injury history, eccentric strength. Main Outcome Measurements Prospective HSI occurrence. Results Athletes with history of ACLR were 39N (95% CI, 14.2–67.1; P Conclusions ACLR injured limbs are significantly weaker than uninjured limbs. Athletes with prior ACLR are significantly more likely to suffer future HSI, particularly if they have a recent history of HSI. This is the first adequately powered study to have assessed the effect of prior ACLR on future HSI risk.