Biceps femoris long head muscle and aponeurosis geometry in males with and without a history of hamstring strain injury.

OBJECTIVES: Hamstring strain injuries (HSIs) commonly affect the proximal biceps femoris long head (BFlh) musculotendinous junction. Biomechanical modeling suggests narrow proximal BFlh aponeuroses and large muscle-to-aponeurosis width ratios increase localized tissue strains and presumably risk of HSI. This study aimed to determine if BFlh muscle and proximal aponeurosis geometry differed between limbs with and without a history of HSI. METHODS: Twenty-six recreationally active males with (n = 13) and without (n = 13) a history of unilateral HSI in the last 24 months underwent magnetic resonance imaging of both thighs. BFlh muscle and proximal aponeurosis cross-sectional areas, length, volume, and interface area between muscle and aponeurosis were extracted. Previously injured limbs were compared to uninjured contralateral and control limbs for discrete variables and ratios, and along the relative length of tissues using statistical parametric mapping. RESULTS: Previously injured limbs displayed significantly smaller muscle-to-aponeurosis volume ratios (p = 0.029, Wilcoxon effect size (ES) = 0.43) and larger proximal BFlh aponeurosis volumes (p = 0.019, ES = 0.46) than control limbs with no history of HSI. No significant differences were found between previously injured and uninjured contralateral limbs for any outcome measure (p = 0.216-1.000, ES = 0.01-0.36). CONCLUSIONS: Aponeurosis geometry differed between limbs with and without a history of HSI. The significantly larger BFlh proximal aponeuroses and smaller muscle-to-aponeurosis volume ratios in previously injured limbs could alter the strain experienced in muscle adjacent to the musculotendinous junction during active lengthening. Future research is required to determine if geometric differences influence the risk of re-injury and whether they can be altered via targeted training.

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.

Bilateral Corticomotor Reorganization and Symptom Development in Response to Acute Unilateral Hamstring Pain: A Randomized, Controlled Study.

Accumulating evidence demonstrates that pain induces adaptations in the corticomotor representations of affected muscles. However, previous work has primarily investigated the upper limb, with few studies examining corticomotor reorganization in response to lower limb pain. This is important to consider, given the significant functional, anatomical, and neurophysiological differences between upper and lower limb musculature. Previous work has also focused on unilateral corticomotor changes in response to muscle pain, despite an abundance of literature demonstrating that unilateral pain conditions are commonly associated with bilateral motor dysfunction. For the first time, this study investigated the effect of unilateral acute hamstring pain on bilateral corticomotor organization using transcranial magnetic stimulation (TMS) mapping. Corticomotor outcomes (TMS maps), pain, mechanical sensitivity (pressure pain thresholds), and function (maximal voluntary contractions) were recorded from 28 healthy participants at baseline. An injection of pain-inducing hypertonic (n = 14) or pain-free isotonic (n = 14) saline was then administered to the right hamstring muscle, and pain ratings were collected every 30 seconds until pain resolution. Follow-up measures were taken immediately following pain resolution and at 25, 50, and 75 minutes post-pain resolution. Unilateral acute hamstring pain induced bilateral symptom development and changes in corticomotor reorganization. Two patterns of reorganization were observed-corticomotor facilitation and corticomotor depression. Corticomotor facilitation was associated with increased mechanical sensitivity and decreased function bilaterally (all P < .05). These effects persisted for at least 75 minutes after pain resolution. PERSPECTIVE: These findings suggest that individual patterns of corticomotor reorganization may contribute to ongoing functional deficits of either limb following acute unilateral lower limb pain. Further research is required to assess these adaptations and the possible long-term implications for rehabilitation and reinjury risk in cohorts with acute hamstring injury.

The Effect of Eccentric or Isometric Training on Strength, Architecture, and Sprinting across an Australian Football Season.

PURPOSE: This study aimed to investigate the effect of an isometric (ISO) or Nordic hamstring exercise (NHE) intervention, alongside a sprint training program on hamstring strength, architecture, and sprinting performance in Australian footballers. METHODS: Twenty-five male athletes undertook NHE ( n = 13) or ISO ( n = 12) training across a 38-wk period (including preseason and in season). Biceps femoris long head (BFlh) architecture, ISO, and eccentric knee flexor strength were assessed at baseline, at the end of preseason (14 wk), and at the conclusion of the intervention. Sprint times and force-velocity profiles were determined at baseline and at the end of preseason. RESULTS: After the intervention, both groups had significant improvements in BFlh fascicle length (NHE: 1.16 cm, 95% CI = 0.68 to 1.63 cm, d = 1.88, P < 0.001; ISO: 0.82 cm, 95% CI = 0.57 to 1.06 cm, d = 1.70, P < 0.001), muscle thickness (NHE: 0.11 cm, 95% CI = 0.01 to 0.21 cm, d = 0.51, P = 0.032; ISO: 0.21 cm, 95% CI = 0.10 to 0.32 cm, d = 0.86, P = 0.002), and eccentric strength (NHE: 83 N, 95% CI = 53 to 114 N, d = 1.79, P < 0.001; ISO: 83 N, 95% CI = 17 to 151 N, d = 1.17, P = 0.018). Both groups also finished the intervention weaker isometrically than they started (NHE: -45 N, 95% CI = -81 to -8 N, d = -1.03, P = 0.022; ISO: -80 N, 95% CI = -104 to -56 N, d = -3.35, P < 0.001). At the end of preseason, the NHE group had improved their 5-m sprint time by 3.3% ± 2.0%), and their maximum horizontal velocity was 3% ± 2.1% greater than the ISO group who saw no changes. CONCLUSIONS: Both ISO and NHE training with a periodized sprinting program can increase BFlh fascicle length, thickness, and eccentric strength in Australian footballers. NHE training also improves 5-m sprint time and maximum velocity. However, both interventions reduced ISO strength. These findings provide unique, contextually relevant insights into the adaptations possible in semiprofessional athletes.

Validity of Inertial Measurement Units to Measure Lower-Limb Kinematics and Pelvic Orientation at Submaximal and Maximal Effort Running Speeds.

Inertial measurement units (IMUs) have been validated for measuring sagittal plane lower-limb kinematics during moderate-speed running, but their accuracy at maximal speeds remains less understood. This study aimed to assess IMU measurement accuracy during high-speed running and maximal effort sprinting on a curved non-motorized treadmill using discrete (Bland-Altman analysis) and continuous (root mean square error [RMSE], normalised RMSE, Pearson correlation, and statistical parametric mapping analysis [SPM]) metrics. The hip, knee, and ankle flexions and the pelvic orientation (tilt, obliquity, and rotation) were captured concurrently from both IMU and optical motion capture systems, as 20 participants ran steadily at 70%, 80%, 90%, and 100% of their maximal effort sprinting speed (5.36 ± 0.55, 6.02 ± 0.60, 6.66 ± 0.71, and 7.09 ± 0.73 m/s, respectively). Bland-Altman analysis indicated a systematic bias within ±1° for the peak pelvic tilt, rotation, and lower-limb kinematics and -3.3° to -4.1° for the pelvic obliquity. The SPM analysis demonstrated a good agreement in the hip and knee flexion angles for most phases of the stride cycle, albeit with significant differences noted around the ipsilateral toe-off. The RMSE ranged from 4.3° (pelvic obliquity at 70% speed) to 7.8° (hip flexion at 100% speed). Correlation coefficients ranged from 0.44 (pelvic tilt at 90%) to 0.99 (hip and knee flexions at all speeds). Running speed minimally but significantly affected the RMSE for the hip and ankle flexions. The present IMU system is effective for measuring lower-limb kinematics during sprinting, but the pelvic orientation estimation was less accurate.

Lower Patellofemoral Joint Contact Force During Side-Step Cutting After Return-to-Sports Clearance Following Anterior Cruciate Ligament Reconstruction.

BACKGROUND: Low patellofemoral joint (PFJ) contact force has been associated with PFJ osteoarthritis. Quadriceps force and knee flexion angles, which are typically altered after an anterior cruciate ligament reconstruction (ACLR), primarily influence PFJ contact forces. It is still inconclusive whether differences in PFJ contact forces are present during high knee flexion tasks such as side-step cutting after clearance to return to sports (RTS) after ACLR. PURPOSE: To explore PFJ contact forces in the ACLR limb and compare them with those of the contralateral and control limbs during side-step cutting tasks after clearance to RTS. STUDY DESIGN: Controlled laboratory study. METHODS: A total of 26 male athletes with ACLR who were previously cleared to RTS were matched with 23 healthy men serving as the control group. Three-dimensional motion capture and force plate data were collected while both groups performed anticipated side-step cutting tasks. Joint kinematics, kinetics, muscle forces, and PFJ contact forces were calculated using musculoskeletal modeling. RESULTS: Peak PFJ force was lower in the ACLR limbs compared with the contralateral limbs (mean difference [MD], 5.89 body weight [BW]; 95% CI, 4.7-7.1 BW; P < .001) and the control limbs (MD, 4.44 BW; 95% CI, 2.1-6.8 BW; P < .001). During peak PFJ force, knee flexion angle was lower in ACLR limbs compared with the contralateral (MD, 4.88°; 95% CI, 3.0°-6.7°; P < .001) and control (MD, 6.01°; 95% CI, 2.0°-10.0°; P < .002) limbs. A lower quadriceps force compared with the contralateral (MD, 4.14 BW; 95% CI, 3.4-4.9 BW; P < .001) and control (MD, 2.83 BW; 95% CI, 1.4-4.3 BW; P < .001) limbs was also found. CONCLUSION: Lower PFJ contact forces and a combination of quadriceps force deficits and smaller knee flexion angle were found in the ACLR compared with the contralateral and control limbs even after clearance to RTS. CLINICAL RELEVANCE: Despite rehabilitation and subsequent clearance to RTS, differences in PFJ contact forces are present after ACLR. Current rehabilitation and RTS battery may not be effective and sensitive enough to identify and address these differences.

Common High-Speed Running Thresholds Likely Do Not Correspond to High-Speed Running in Field Sports.

ABSTRACT: Freeman, BW, Talpey, SW, James, LP, Opar, DA, and Young, WB. Common high-speed running thresholds likely do not correspond to high-speed running in field sports. J Strength Cond Res 37(7): 1411-1418, 2023-The purpose of this study was to clarify what percentage of maximum speed is associated with various running gaits. Fifteen amateur field sport athletes (age = 23 ± 3.6 years) participated in a series of 55-meter running trials. The speed of each trial was determined by instructions relating to 5 previously identified gait patterns (jog, run, stride, near maximum sprint, and sprint). Each trial was filmed in slow motion (240 fps), whereas running speed was obtained using Global Positioning Systems. Contact time, stride angle, and midstance free-leg knee angle were determined from video footage. Running gaits corresponded with the following running speeds, jogging = 4.51 m·s -1 , 56%Vmax, running = 5.41 m·s -1 , 66%Vmax , striding = 6.37 m·s -1 , 78%Vmax, near maximum sprinting = 7.08 m·s -1 , 87%Vmax, and sprinting = 8.15 m·s -1 , 100%Vmax. Significant ( p < 0.05) increases in stride angle were observed as running speed increased. Significant ( p < 0.05) decreases were observed in contact time and midstance free-leg knee angle as running speed increased. These findings suggest currently used thresholds for high-speed running (HSR) and sprinting most likely correspond with jogging and striding, which likely underestimates the true HSR demands. Therefore, a higher relative speed could be used to describe HSR and sprinting more accurately in field sports.

The dose-response of pain throughout a Nordic hamstring exercise intervention.

The Nordic hamstring exercise (NHE) reduces hamstring injury incidence. Compliance to large exercise volumes of the NHE is poor, with exercise related soreness often seen as a contributing factor. We investigated the dose-response of NHE exposure with delayed onset muscle soreness (DOMS) and non-DOMS pain. Forty males were randomized to a 6-week intervention of four different NHE dosages: Group 1: very low volume; Group 2: low volume; Group 3: initial high to low volume; Group 4: low to high volume. Group 4 experienced more DOMS (p < 0.05) and non-DOMS pain (p = 0.030) than other groups. High volumes of NHE increase DOMS and non-DOMS pain while lower volume protocols have lesser DOMS and non-DOMS pain responses.

Reliability of corticospinal excitability and intracortical inhibition in biceps femoris during different contraction modes.

This study aimed to determine the test-retest reliability of a range of transcranial magnetic stimulation (TMS) outcomes in the biceps femoris during isometric, eccentric and concentric contractions. Corticospinal excitability (active motor threshold 120% [AMT120%] and area under recruitment curve [AURC]), short- and long-interval intracortical inhibition (SICI and LICI) and intracortical facilitation (ICF) were assessed from the biceps femoris in 10 participants (age 26.3 ± 6.0 years; height 180.2 ± 6.6 cm, body mass 77.2 ± 8.0 kg) in three sessions. Single- and paired-pulse stimuli were delivered under low-level muscle activity (5% ± 2% of maximal isometric root mean squared surface electromyography [rmsEMG]) during isometric, concentric and eccentric contractions. Participants were provided visual feedback on their levels of rmsEMG during all contractions. Single-pulse outcomes measured during isometric contractions (AURC, AMT110%, AMT120%, AMT130%, AMT150%, AMT170%) demonstrated fair to excellent reliability (ICC range, .51 to .92; CV%, 21% to 37%), whereas SICI, LICI and ICF demonstrated good to excellent reliability (ICC range, .62 to .80; CV%, 19 to 42%). Single-pulse outcomes measured during concentric contractions demonstrated excellent reliability (ICC range, .75 to .96; CV%, 15% to 34%), whereas SICI, LICI and ICF demonstrated good to excellent reliability (ICC range, .65 to .76; CV%, 16% to 71%). Single-pulse outcomes during eccentric contractions demonstrated fair to excellent reliability (ICC range, .56 to .96; CV%, 16% to 41%), whereas SICI, LICI and ICF demonstrated good to excellent (ICC range, .67 to .86; CV%, 20% to 42%). This study found that both single- and paired-pulse TMS outcomes can be measured from the biceps femoris muscle across all contraction modes with fair to excellent reliability. However, coefficient of variation values were typically greater than the smallest worthwhile change which may make tracking physiological changes in these variables difficult without moderate to large effect sizes.

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.

Muscle function during single leg landing.

Landing manoeuvres are an integral task for humans, especially in the context of sporting activities. Such tasks often involve landing on one leg which requires the coordination of multiple muscles in order to effectively dissipate kinetic energy. However, no prior studies have provided a detailed description of the strategy used by the major lower limb muscles to perform single-leg landing. The purpose of the present study was to understand how humans coordinate their lower limb muscles during a single-leg landing task. Marker trajectories, ground reaction forces (GRFs), and surface electromyography (EMG) data were collected from healthy male participants performing a single-leg landing from a height of 0.31 m. An EMG-informed neuromusculoskeletal modelling approach was used to generate neuromechanical simulations of the single-leg landing task. The muscular strategy was determined by computing the magnitude and temporal characteristics of musculotendon forces and energetics. Muscle function was determined by computing muscle contributions to lower limb net joint moments, GRFs and lower limb joint contact forces. It was found that the vasti, soleus, gluteus maximus and gluteus medius produced the greatest muscle forces and negative (eccentric) mechanical work. Downward momentum of the centre-of-mass was resisted primarily by the soleus, vasti, gastrocnemius, rectus femoris, and gluteus maximus, whilst forward momentum was primarily resisted by the quadriceps (vasti and rectus femoris). Flexion of the lower limb joints was primarily resisted by the uni-articular gluteus maximus (hip), vasti (knee) and soleus (ankle). Overall, our findings provide a unique insight into the muscular strategy used by humans during a landing manoeuvre and have implications for the design of athletic training programs.

Early introduction of high-intensity eccentric loading into hamstring strain injury rehabilitation.

OBJECTIVES: This study aimed to investigate the number of days following hamstring strain injury (HSI) taken to introduce high-intensity eccentric loading (HIEL) into rehabilitation based on exercise-specific progression criteria, and whether pain resolution during isometric knee flexion strength testing occurred before or after this milestone. DESIGN: Cohort study. METHODS: We included 42 men (mean ±â€¯sd; age = 26 ±â€¯5 years; height = 181 ±â€¯8 cm; mass = 86 ±â€¯12 kg) with HSIs, who performed fully supervised rehabilitation twice per week until they met return to play clearance criteria. Isometric knee flexion strength testing was completed before every rehabilitation session and HIEL was introduced via the Nordic hamstring exercise and unilateral slider once participants could perform a bilateral slider through full eccentric knee flexion range of motion. We reported the median (IQR) number of days following HSI taken to introduce HIEL, along with participant's pain rating during isometric knee flexion strength testing before that rehabilitation session. We also reported the median (IQR) number of days following HSI taken for participants to achieve pain resolution during isometric knee flexion. RESULTS: HIEL was introduced 5 (2-8) days following HSI, despite 35/42 participants reporting pain during isometric knee flexion strength testing immediately prior to that rehabilitation session, which was rated as 3.5 (3-5) on a 0-10 numeric rating scale. Pain resolution during isometric knee flexion strength testing was achieved 11 (9-13) days following HSI. CONCLUSION: HIEL can be safely introduced into early HSI rehabilitation based on exercise-specific progression criteria, without needing to wait for pain resolution during isometric knee flexion strength testing before doing so.

Mechanical, Material and Morphological Adaptations of Healthy Lower Limb Tendons to Mechanical Loading: A Systematic Review and Meta-Analysis.

BACKGROUND: Exposure to increased mechanical loading during physical training can lead to increased tendon stiffness. However, the loading regimen that maximises tendon adaptation and the extent to which adaptation is driven by changes in tendon material properties or tendon geometry is not fully understood. OBJECTIVE: To determine (1) the effect of mechanical loading on tendon stiffness, modulus and cross-sectional area (CSA); (2) whether adaptations in stiffness are driven primarily by changes in CSA or modulus; (3) the effect of training type and associated loading parameters (relative intensity; localised strain, load duration, load volume and contraction mode) on stiffness, modulus or CSA; and (4) whether the magnitude of adaptation in tendon properties differs between age groups. METHODS: Five databases (PubMed, Scopus, CINAHL, SPORTDiscus, EMBASE) were searched for studies detailing load-induced adaptations in tendon morphological, material or mechanical properties. Standardised mean differences (SMDs) with 95% confidence intervals (CIs) were calculated and data were pooled using a random effects model to estimate variance. Meta regression was used to examine the moderating effects of changes in tendon CSA and modulus on tendon stiffness. RESULTS: Sixty-one articles met the inclusion criteria. The total number of participants in the included studies was 763. The Achilles tendon (33 studies) and the patella tendon (24 studies) were the most commonly studied regions. Resistance training was the main type of intervention (49 studies). Mechanical loading produced moderate increases in stiffness (standardised mean difference (SMD) 0.74; 95% confidence interval (CI) 0.62-0.86), large increases in modulus (SMD 0.82; 95% CI 0.58-1.07), and small increases in CSA (SMD 0.22; 95% CI 0.12-0.33). Meta-regression revealed that the main moderator of increased stiffness was modulus. Resistance training interventions induced greater increases in modulus than other training types (SMD 0.90; 95% CI 0.65-1.15) and higher strain resistance training protocols induced greater increases in modulus (SMD 0.82; 95% CI 0.44-1.20; p = 0.009) and stiffness (SMD 1.04; 95% CI 0.65-1.43; p = 0.007) than low-strain protocols. The magnitude of stiffness and modulus differences were greater in adult participants. CONCLUSIONS: Mechanical loading leads to positive adaptation in lower limb tendon stiffness, modulus and CSA. Studies to date indicate that the main mechanism of increased tendon stiffness due to physical training is increased tendon modulus, and that resistance training performed at high compared to low localised tendon strains is associated with the greatest positive tendon adaptation. PROSPERO registration no.: CRD42019141299.

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.

Muscle Force Contributions to Anterior Cruciate Ligament Loading.

Anterior cruciate ligament (ACL) injuries are one of the most common knee pathologies sustained during athletic participation and are characterised by long convalescence periods and associated financial burden. Muscles have the ability to increase or decrease the mechanical loads on the ACL, and thus are viable targets for preventative interventions. However, the relationship between muscle forces and ACL loading has been investigated by many different studies, often with differing methods and conclusions. Subsequently, this review aimed to summarise the evidence of the relationship between muscle force and ACL loading. A range of studies were found that investigated muscle and ACL loading during controlled knee flexion, as well as a range of weightbearing tasks such as walking, lunging, sidestep cutting, landing and jumping. The quadriceps and the gastrocnemius were found to increase load on the ACL by inducing anterior shear forces at the tibia, particularly when the knee is extended. The hamstrings and soleus appeared to unload the ACL by generating posterior tibial shear force; however, for the hamstrings, this effect was contingent on the knee being flexed greater than ~ 20° to 30°. The gluteus medius was consistently shown to oppose the knee valgus moment (thus unloading the ACL) to a magnitude greater than any other muscle. Very little evidence was found for other muscle groups with respect to their contribution to the loading or unloading of the ACL. It is recommended that interventions aiming to reduce the risk of ACL injury consider specifically targeting the function of the hamstrings, soleus and gluteus medius.

Hamstring Strain Injury Rehabilitation.

Hamstring strain injuries are common among athletes and often require rehabilitation to prepare players for a timely return to sport performance while also minimizing reinjury risk. Return to sport is typically achieved within weeks of the injury; however, subsequent athlete performance may be impaired, and reinjury rates are high. Improving these outcomes requires rehabilitation practitioners (eg, athletic trainers and physical therapists) to understand the causes and mechanisms of hamstring strain injury, know how to perform a thorough clinical examination, and progress loading to the site of injury safely and effectively. This narrative review discusses current clinical concepts related to these aspects of rehabilitation for hamstring strain injury, with the aim of helping practitioners improve athletes' outcomes. Collectively, this knowledge will inform the implementation of evidence-based rehabilitation interventions.

The Dose-Response of the Nordic Hamstring Exercise on Biceps Femoris Architecture and Eccentric Knee Flexor Strength: A Randomized Interventional Trial.

PURPOSE: To examine the dose-response of the Nordic hamstring exercise (NHE) on biceps femoris long head (BFlh) architecture and eccentric knee flexor strength. DESIGN: Randomized interventional trial. METHODS: Forty recreationally active males completed a 6-week NHE training program consisting of either intermittent low volumes (group 1; n = 10), low volumes (group 2; n = 10), initial high volumes followed by low volumes (group 3; n = 10), or progressively increasing volumes (group 4; n = 10). A 4-week detraining period followed each program. Muscle architecture was assessed weekly during training and after 2 and 4 weeks of detraining. Eccentric knee flexor strength was assessed preintervention and postintervention and after 2 and 4 weeks of detraining. RESULTS: Following 6 weeks of training, BFlh fascicle length (FL) increased in group 3 (mean difference = 0.83 cm, d = 0.45, P = .027, +7%) and group 4 (mean difference = 1.48 cm, d = 0.94, P = .004, +14%). FL returned to baseline following detraining in groups 3 and 4. Strength increased in group 2 (mean difference = 53.6 N, d = 0.55, P = .002, +14%), group 3 (mean difference = 63.4 N, d = 0.72, P = .027, +17%), and group 4 (mean difference = 74.7, d = 0.83, P = .006, +19%) following training. Strength returned to baseline following detraining in groups 2 and 3 but not in group 4. CONCLUSIONS: Initial high volumes of the NHE followed by lower volumes, as well as progressively increasing volumes, can elicit increases in BFlh FL and eccentric knee flexor strength. Low volumes of the NHE were insufficient to increase FL, although as few as 48 repetitions in 6 weeks did increase strength.

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.

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.

Prediction of Hamstring Injuries in Australian Football Using Biceps Femoris Architectural Risk Factors Derived From Soccer.

BACKGROUND: Hamstring strain injuries are the most common injuries in team sports. Biceps femoris long head architecture is associated with the risk of hamstring injury in soccer. To assess the overall predictive ability of architectural variables, risk factors need to be applied to and validated across different cohorts. PURPOSE: To assess the generalizability of previously established risk factors for a hamstring strain injury (HSI), including demographics, injury history, and biceps femoris long head (BFlh) architecture to predict HSIs in a cohort of elite Australian football players. STUDY DESIGN: Cohort study; Level of evidence, 3. METHODS: Demographic, injury history, and BFlh architectural data were collected from elite soccer (n = 152) and Australian football (n = 169) players at the beginning of the preseason for their respective competitions. Any prospectively occurring HSIs were reported to the research team. Optimal cut points for continuous variables used to determine an association with the HSI risk were established from previously published data in soccer and subsequently applied to the Australian football cohort to derive the relative risk (RR) for these variables. Logistic regression models were built using data from the soccer cohort and utilized to estimate the probability of an injury in the Australian football cohort. The area under the curve (AUC) and Brier score were the primary outcome measures to assess the performance of the logistic regression models. RESULTS: A total of 27 and 30 prospective HSIs occurred in the soccer and Australian football cohorts, respectively. When using cut points derived from the soccer cohort and applying these to the Australian football cohort, only older athletes (aged ≥25.4 years; RR, 2.7 [95% CI, 1.4-5.2]) and those with a prior HSI (RR, 2.5 [95% CI, 1.3-4.8]) were at an increased risk of HSIs. Using the same approach, height, weight, fascicle length, muscle thickness, pennation angle, and relative fascicle length were not significantly associated with an increased risk of HSIs in Australian football players. The logistic regression model constructed using age and prior HSIs performed the best (AUC = 0.67; Brier score = 0.14), with the worst performing model being the one that was constructed using pennation angle (AUC = 0.53; Brier score = 0.18). CONCLUSION: Applying cut points derived from previously published data in soccer to a dataset from Australian football identified older age and prior HSIs, but none of the modifiable HSI risk factors, to be associated with an injury. The transference of HSI risk factor data between soccer and Australian football appears limited and suggests that cohort-specific cut points must be established.