Explosive hamstrings strength asymmetry persists despite maximal hamstring strength recovery following anterior cruciate ligament reconstruction using hamstring tendon autografts.

PURPOSE: To investigate the differences in maximal (isometric and concentric peak torque) and explosive (rate of torque development (RTD)) hamstring and quadriceps strength symmetry between males and females during early- and late-phase rehabilitation after anterior cruciate ligament reconstruction (ACLR) using hamstring tendon (HT) autografts and to determine the interaction of time and sex on maximal and explosive strength symmetry. METHODS: A total of 38 female and 51 male participants were assessed during early (3-6 months post-operative) and late (7-12 months post-operative) phases of rehabilitation following ACLR. Maximal (concentric and isometric peak torque) and explosive (isometric RTD) hamstring and quadriceps strength were assessed and presented as limb symmetry index (LSI). RESULTS: Maximal concentric hamstrings asymmetry (Early: 86 ± 14; Late 92 ± 13; p = 0.005) as well as maximal concentric (Early, 73 ± 15; Late 91 ± 12; p < 0.001) and explosive (Early: 82 ± 30; Late: 92 ± 25; p = 0.03) quadriceps asymmetry decreased from early to late rehabilitation. However, there were no significant changes in maximal isometric quadriceps strength and explosive isometric hamstring strength in the same time period. Females had a larger asymmetry in maximal concentric (Females: 75 ± 17; Males: 81 ± 15; p = 0.001) and explosive (Females: 81 ± 32; Males: 89 ± 25; p = 0.01) quadriceps strength than males throughout rehabilitation. There were no sex differences in maximal and explosive hamstring strength. There were no sex by time interactions for any variables. CONCLUSION: Explosive hamstring strength asymmetry did not improve despite recovery of maximal hamstring strength during rehabilitation following ACLR with HT autografts. While sex did not influence strength recovery, females had larger maximal and explosive quadriceps strength asymmetry compared to males throughout rehabilitation following ACLR. LEVEL OF EVIDENCE: Level III.

Assessing isometric kicking force and post-match responses using the Kicker test.

This study examined the inter-session reliability of force output from a novel isometric strength assessment protocol (the Kicker); and its suitability to monitor soccer player's combined hip flexion and knee extension force capacity over 72-h post-competitive matches. Reliability (Part-A) testing was completed over three sessions on 20 individuals participating in various sports at a recreational level or higher. Post-match strength response (Part-B) data were collected for 72-h after a game (24-h (+24), 48-h (+48) and 72-h (+72) post-match) in 17 male academy soccer players. After familiarisation, Kicker force for each limb showed high inter-session reliability (ICC >0.95; typical error <14 N, CV <6%); minimum detectable change at a 95% confidence interval <40 N). Across the 72-h post-match period, Kicker force for each limb was suppressed compared to baseline (force loss range = -5.8% to -12.5%; effect sizes range = -0.26 to -0.43) at all time points. The Kicker assessment protocol measures combined isometric hip flexor and knee extensor force capacity with high inter-session reliability. The proof of concept that the protocol can be used as a monitoring tool was evidenced by sustained suppression of baseline force capacity in both kicking limbs for 72-h post soccer matches.

Hamstring and gluteal activation during high-speed overground running: Impact of prior strain injury.

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 (<18 month) unilateral biceps femoris long head (BFLH) strain injury underwent functional magnetic resonance imaging before and immediately after arepeat-sprint running protocol. Transverse relaxation (T2) time, an index of muscle activation, of the BFLH and short head (BFSH), semitendinosus (ST), semimembranosus (SM), gluteus maximus (GMAX) and medius (GMED) was assessed pre-post exercise. No significant between-limb differences in running-induced mean T2 changes were observed (p = 0.949), however, decision tree induction revealed that previously injured limbs were characterised by highly variable intramuscular activation of the ST (SD5.3). T2 times increased more for GMAX than all other muscles (all p< 0.001, d= 0.5-2.5). Further, T2 changes were greater for ST than BFSH, SM, GMED, and BFLH (all p≤ 0.001, d= 0.5-2.9); and were greater for BFLH than BFSH, SM, and GMED (all p< 0.001, d= 1.2-1.6). Athletes display heterogenous patterns of posterior thigh activation when sprinting (GMAX>ST>BFLH>GMED>SM>BFSH) and may exhibit altered intramuscular hamstring activation after returning to sport from BFLH strain injury.

Hamstring muscle activation and morphology are significantly altered 1-6 years after anterior cruciate ligament reconstruction with semitendinosus graft.

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.

Razor hamstring curl and Nordic hamstring exercise architectural adaptations: Impact of exercise selection and intensity.

OBJECTIVES: To investigate knee flexor strength and biceps femoris long head (BFlh) architectural adaptations following two different Nordic hamstring exercise (NHE) interventions and one razor hamstring curl (RHC) intervention. METHODS: Thirty recreationally active males performed a total of 128 reps of NHEbodyweight (n = 10), NHEweighted (n = 10), or RHCweighted training (n = 10) across 6 weeks. Following the intervention, participants avoided any eccentric training for 4 weeks (detraining period). Strength results during the NHE and RHC were recorded pre- and post-intervention, as well as following detraining. Architectural characteristics of the BFlh were assessed weekly throughout the intervention and detraining periods. RESULTS: For the NHEweighted group, NHE strength increased (+81N, P = 0.044, d = 0.90) and BFlh fascicles lengthened (+1.57 cm, P < 0.001, d = 1.41) after 6 weeks of training. After 1 week of detraining, BFlh fascicle lengths shortened, with the largest reductions seen in the NHEweighted group (-0.96 cm, P = 0.021, d = -0.90). Comparatively, BFlh fascicle length and NHE strength responses were moderate in the NHEbodyweight group and negligible in the RHCweighted group. The greatest RHC strength changes (+82N, P = 0.038, d = 1.15) were seen in the RHCweighted group. CONCLUSIONS: NHEweighted interventions induce large BFlh fascicle lengthening responses and these adaptations decay after just 1 week of detraining. NHEbodyweight training has a moderate impact on BFlh architecture while the RHCweighted group has the least. Weighted NHE and RHC training promoted exercise-specific increases in strength. These findings suggest that exercise selection and intensity should be considered when prescribing exercises aiming to increase eccentric strength and BFlh fascicle length.

Running exposure is associated with the risk of hamstring strain injury in elite Australian footballers.

BACKGROUND: To investigate the association between running exposure and the risk of hamstring strain injury (HSI) in elite Australian footballers. METHODS: Elite 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. RESULTS: 30 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. CONCLUSIONS: An 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.

Impact of exercise selection on hamstring muscle activation.

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<0.001). The T2 increase after hip extension for BFLongHead, semitendinosus and semimembranosus muscles was greater than that for BFShortHead (p<0.001). During the Nordic, the T2 increase was greater for the semitendinosus than for the other hamstring muscles (p≤0.002). SUMMARY: We highlight the heterogeneity of hamstring activation patterns in different tasks. Hip-extension exercise selectively activates the long hamstrings, and the Nordic exercise preferentially recruits the semitendinosus. These findings have implications for strategies to prevent hamstring injury as well as potentially for clinicians targeting specific hamstring components for treatment (mechanotherapy).

Impact of the Nordic hamstring and hip extension exercises on hamstring architecture and morphology: implications for injury prevention.

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). BFLH volume 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.

Architectural adaptations of muscle to training and injury: a narrative review outlining the contributions by fascicle length, pennation angle and muscle thickness.

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.

Hamstring strength and flexibility after hamstring strain injury: a systematic review and meta-analysis.

OBJECTIVE: To systematically review the evidence base related to hamstring strength and flexibility in previously injured hamstrings. DESIGN: Systematic review and meta-analysis. DATA SOURCES: A systematic literature search was conducted of PubMed, CINAHL, SPORTDiscus, Cochrane Library, Web of Science and EMBASE from inception to August 2015. INCLUSION CRITERIA: Full-text English articles which included studies which assessed at least one measure of hamstring strength or flexibility in men and women with prior hamstring strain injury within 24 months of the testing date. RESULTS: Twenty-eight studies were included in the review. Previously injured legs demonstrated deficits across several variables. Lower isometric strength was found <7 days postinjury (d=-1.72), but this did not persist beyond 7 days after injury. The passive straight leg raise was restricted at multiple time points after injury (<10 days, d=-1.12; 10-20 days, d=-0.74; 20-30 days, d=-0.40), but not after 40-50 days postinjury. Deficits remained after return to play in isokinetically measured concentric (60°/s, d=-0.33) and Nordic eccentric knee flexor strength (d=-0.39). The conventional hamstring to quadricep strength ratios were also reduced well after return to play (60:60°/s, d=-0.32; 240:240°/s, d=-0.43) and functional (30:240°/s, d=-0.88), but these effects were inconsistent across measurement methods. CONCLUSIONS: After hamstring strain, acute isometric and passive straight leg raise deficits resolve within 20-50 days. Deficits in eccentric and concentric strength and strength ratios persist after return to play, but this effect was inconsistent across measurement methods. Flexibility and isometric strength should be monitored throughout rehabilitation, but dynamic strength should be assessed at and following return to play.

Short biceps femoris fascicles and eccentric knee flexor weakness increase the risk of hamstring injury in elite football (soccer): a prospective cohort study.

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 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 preseason. The occurrences of HSIs 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 337 N (RR=4.4; 95% CI 1.1 to 17.5) and possessing BFlh fascicles shorter than 10.56 cm (RR=4.1; 95% CI 1.9 to 8.7) significantly increased the risk of a HSI. Multivariate logistic regression revealed significant effects when combinations of age, 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 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 knee flexor strength in elite soccer players increases the risk of future HSI. The greater risk of a future HSI in older players or those with a previous HSI is reduced when they have longer BFlh fascicles and high levels of eccentric strength.

The financial cost of hamstring strain injuries in the Australian Football League.

BACKGROUND: Hamstring strain injuries (HSIs) have remained the most prevalent injury in the Australian Football League (AFL) over the past 21 regular seasons. The effect of HSIs in sports is often expressed as regular season games missed due to injury. However, the financial cost of athletes missing games due to injury has not been investigated. The aim of this report is to estimate the financial cost of games missed due to HSIs in the AFL. METHOD: Data were collected using publicly available information from the AFL's injury report and the official AFL annual report for the past 10 competitive AFL seasons. Average athlete salary and injury epidemiology data were used to determine the average yearly financial cost of HSIs for AFL clubs and the average financial cost of a single HSI over this time period. RESULTS: Across the observed period, average yearly financial cost of HSIs per club increased by 71% compared with a 43% increase in average yearly athlete salary. Over the same time period the average financial cost of a single HSI increased by 56% from $A25,603 in 2003 to $A40,021 in 2012, despite little change in the HSI rates during the period. CONCLUSIONS: The observed increased financial cost of HSIs was ultimately explained by the failure of teams to decrease HSI rates, but coupled with increases in athlete salaries over the past 10 season. The information presented in this report highlights the financial cost of HSIs and other sporting injuries, raising greater awareness and the need for further funding for research into injury prevention strategies to maximise economical return for investment in athletes.

Reviewing current knowledge in snatch performance and technique: the need for future directions in applied research.

This is a review of current research trends in weightlifting literature relating to the understanding of technique and its role in successful snatch performance. Reference to the world records in the snatch from the 1960s onwards indicates little progress across all weight categories. With such mediocre advances in performance at the International level, there is a need to better understand how snatch technique can improve performance even if only by a small margin. Methods of data acquisition for technical analysis of the snatch have involved mostly 2-dimensional barbell and joint kinematics. Although key variables which play a role in the successful outcome of a snatch lift have been heavily investigated, few studies have combined variables relating both the barbell and the weightlifter in their analyses. This suggests the need for a more detailed approach integrating both barbell-related and weightlifter-related data to enhance understanding of the mechanics of a successful lift. Currently, with the aid of technical advances in motion analysis, data acquisition, and methods of analysis, a more accurate representation of the movement can be provided. Better ways of understanding the key characteristics of technique in the snatch could provide the opportunity for more effective individualized feedback from the coach to the athlete, which should in turn lead to improved performance in competition.

Assessment of conditioning-specific movement tasks and physical fitness measures in talent identified under 16-year-old rugby union players.

Preparedness to train was assessed using a battery of conditioning-specific movement tasks (CSMTs) on a group of talent identified rugby union players (n = 156; age = 15 ± 7 years; stature = 176 ± 7 cm; and mass = 74 ± 14 kg). In addition to explore the link between movement competency and performance, a series of standard fitness tests was conducted. Overall the group's CSMTs competency ratings were low, but task dependent. The proportion of competent players ranged from 14% for a single leg squat to 70% for a double to single leg landing. Players were subsequently grouped based on their CSMTs ratings using cluster analysis. This analysis classified players on features of the CSMT battery that distinguished between groups rather than an arbitrary score. Fitness test scores were then compared between the 3 groups identified. The "general low competency" group jumped 9.1 cm lower (p = 0.0218), sprinted slower across 10, 20 and 40 m (range, p = 0.0126-0.0018) and covered 389 m less (p = 0.0105) Yo-Yo intermittent recovery level 1 distance compared with the "squat competent group." In summary, at this important time before academy selection, most players could not competently perform the CSMTs that underpin rugby conditioning and may not be prepared for the transition into the "training to compete" stage of the suggested long-term athlete development model. For this sample of players, the athlete development process may therefore be unnecessarily inhibited. Moreover, our observations that competency in some CSMTs may explain better running and jumping performances in some players suggest that a focus on monitoring and addressing movement competencies during the training to train stage of player development should be considered.

Hamstring Strain Injury Risk Factors in Australian Football Change over the Course of the Season.

BACKGROUND/AIM: This study aimed to determine which factors were most predictive of hamstring strain injury (HSI) during different stages of the competition in professional Australian Football. METHODS: Across two competitive seasons, eccentric knee flexor strength and biceps femoris long head architecture of 311 Australian Football players (455 player seasons) were assessed at the start and end of preseason and in the middle of the competitive season. Details of any prospective HSI were collated by medical staff of participating teams. Multiple logistic regression models were built to identify important risk factors for HSI at the different time points across the season. RESULTS: There were 16, 33, and 21 new HSIs reported in preseason, early in-season, and late in-season, respectively, across two competitive seasons. Multivariate logistic regression and recursive feature selection revealed that risk factors were different for preseason, early in-season, and late in-season HSIs. A combination of previous HSI, age, height, and muscle thickness were most associated with preseason injuries (median area under the curve [AUC], 0.83). Pennation angle and fascicle length had the strongest association with early in-season injuries (median AUC, 0.86). None of the input variables were associated with late in-season injuries (median AUC, 0.46). The identification of early in-season HSI and late in-season HSI was not improved by the magnitude of change of data across preseason (median AUC, 0.67). CONCLUSIONS: Risk factors associated with prospective HSI were different across the season in Australian Rules Football, with nonmodifiable factors (previous HSI, age, and height) mostly associated with preseason injuries. Early in-season HSI were associated with modifiable factors, notably biceps femoris long head architectural measures. The prediction of in-season HSI was not improved by assessing the magnitude of change in data across preseason.

Hamstring and knee injuries are associated with isometric hip and trunk muscle strength in elite Australian Rules and Rugby League players.

OBJECTIVES: This study investigated relationships between isometric trunk and hip extensor strength, lumbar muscle morphology, and the risk of hamstring and knee ligament injuries in Australian Football League and National Rugby League players. DESIGN: Prospective cohort study. METHODS: Trunk and hip extensor strength, multifidus and quadratus lumborum cross-sectional area were measured during the 2020 pre-season. Logistic regressions and decision trees were employed to explore associations between maximum strength, strength endurance, multifidus and quadratus lumborum cross-sectional area, age, previous injuries, and hamstring and knee ligament injury risk. RESULTS: Greater strength endurance [odds ratio = 0.42 (0.23-0.74), p = 0.004] and maximum strength [odds ratio = 0.55 (0.31-0.94), p = 0.039] reduced hamstring injury risk. Increased risk of knee ligament injuries was associated with larger multifidus [odds ratio = 1.66 (1.14-2.45), p = 0.008] and higher multifidus to quadratus lumborum ratio (odds ratio = 1.57 (1.13-2.23), p = 0.008]. Decision tree models indicated that low strength endurance (< 99 Nm) characterised hamstring strains, while high (≥ 1.33) multifidus to quadratus lumborum ratio mitigated risk. Knee ligament injuries were associated with larger (≥ 8.49 cm2) multifidus, greater (≥ 1.25) multifidus to quadratus lumborum ratio, and lower maximum strength (< 9.24 N/kg). CONCLUSIONS: Players with lower trunk and hip extensor maximum strength and strength endurance had increased risk of hamstring injuries, while knee ligament injury risk was elevated with larger multifidus cross-sectional area, higher multifidus to quadratus lumborum ratio, and lower maximum trunk and hip extensor strength.

Sprinting technique and hamstring strain injuries: A concept mapping study.

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.

Screening Hamstring Injury Risk Factors Multiple Times in a Season Does Not Improve the Identification of Future Injury Risk.

PURPOSE: To determine if eccentric knee flexor strength and biceps femoris long head (BFlh) fascicle length were associated with prospective hamstring strain injury (HSI) in professional Australian Football players, and if more frequent assessments of these variables altered the association with injury risk. METHODS: Across two competitive seasons, 311 Australian Football players (455 player seasons) had their eccentric knee flexor strength during the Nordic hamstring exercise and BFlh architecture assessed at the start and end of preseason and in the middle of the competitive season. Player age and injury history were also collected in preseason. Prospective HSIs were recorded by team medical staff. RESULTS: Seventy-four player seasons (16%) sustained an index HSI. Shorter BFlh fascicles (<10.42 cm) increased HSI risk when assessed at multiple time points only (relative risk [RR], 1.9; 95% confidence interval [CI], 1.2-3.0). Neither absolute (N) nor relative (N·kg-1) eccentric knee flexor strength was associated with HSI risk, regardless of measurement frequency (RR range, 1.0-1.1); however, between-limb imbalance (>9%), when measured at multiple time points, was (RR, 1.8; 95% CI, 1.1-3.1). Prior HSI had the strongest univariable association with prospective HSI (RR, 2.9; 95% CI, 1.9-4.3). Multivariable logistic regression models identified a combination of prior HSI, BFlh architectural variables and between-limb imbalance in eccentric knee flexor strength as optimal input variables; however, their predictive performance did not improve with increased measurement frequency (area under the curve, 0.681-0.726). CONCLUSIONS: More frequent measures of eccentric knee flexor strength and BFlh architecture across a season did not improve the ability to identify which players would sustain an HSI.

Anterior Cruciate Ligament Reconstruction Increases the Risk of Hamstring Strain Injury Across Football Codes in Australia.

OBJECTIVE: The aim of this study was to determine the impacts of anterior cruciate ligament reconstruction (ACLR) and recent (< 12 months) hamstring strain injury (HSI) on (1) future HSI risk, and (2) eccentric knee flexor strength and between-limb imbalance during the Nordic hamstring exercise. A secondary goal was to examine whether eccentric knee flexor strength was a risk factor for future HSI in athletes with prior ACLR and/or HSI. METHODS: In this prospective cohort study, 531 male athletes had preseason eccentric knee flexor strength tests. Injury history was also collected. The main outcome was HSI occurrence in the subsequent competitive season. RESULTS: Overall, 74 athletes suffered at least one prospective HSI. Compared with control athletes, those with a lifetime history of ACLR and no recent HSI had 2.2 (95% confidence interval [CI] 1.1-4.4; p = 0.029) times greater odds of subsequent HSI while those with at least one HSI in the previous 12 months and no history of ACLR had 3.1 (95% CI 1.8-5.4; p < 0.001) times greater odds for subsequent HSI. Only athletes with a combined history of ACLR and recent HSI had weaker injured limbs (p = 0.001) and larger between-limb imbalances (p < 0.001) than uninjured players. An exploratory decision tree analysis suggested eccentric strength may protect against HSI after ACLR. CONCLUSION: ACLR and recent HSI were similarly predictive of future HSI. Lower levels of eccentric knee flexor strength and larger between-limb imbalances were found in athletes with combined histories of ACLR and recent HSI. These findings may have implications for injury rehabilitation.