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.

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.

The Effect of a Functional Strength Training Intervention on Movement Quality and Physical Fitness in Adolescents.

This study compared a 12 week Functional Strength Training (FST) program on functional movement and physical performance to typical physical education (PE) classes for middle school (MS) and high school (HS) students. We randomly assigned 266 participants (M age = 14.35, SD = 0.57 years; M height = 164.82, SD = 6.13 cm; M mass = 55.09, SD = 12.19 kg; M BMI = 20.11, SD = 3.54 kg/m2) into an FST or control group. The FST group trained in flexibility and stability, functional movement patterns, and health-related functional strength. The control group continued regular physical education (PE) classes. Each group trained three-times/week in 45 minute sessions for 12 weeks. Outcome measures included the Functional Movement Screen protocol and seven physical performance tests, assessed every four weeks over a 12 week period. We employed a mixed model ANOVA with Bonferroni post-hoc tests to examine differences between and within groups. Compared to the control group, the FST group significantly (p < 0.01) improved Functional Movement Screen total scores (25.7%), curl-ups (70.4%), pull-ups (281.6%), and flexibility (83.6%). We suggest including the FST program in the MS and HS PE curriculum.

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.

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.

Predicting Noncontact Lower Limb Injury Using Lumbar Morphology in Professional Australian Football and Rugby League Players.

INTRODUCTION: 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. METHODS: 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. RESULTS: 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). CONCLUSIONS: 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.

Effect of concentric and eccentric hamstring training on sprint recovery, strength and muscle architecture in inexperienced athletes.

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±4.1y; height, 180.1±6.4cm; weight, 85.2±14.6kg) 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 (10×80m) 48h apart. Blood samples and passive KF torques were collected before, immediately after, 24h and 48h 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.

Drop punt kicking induces eccentric knee flexor weakness associated with reductions in hamstring electromyographic activity.

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 10min 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.

Eccentric hamstring strength and hamstring injury risk in Australian footballers.

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 effect of previous hamstring strain injuries on the change in eccentric hamstring strength during preseason training in elite Australian footballers.

BACKGROUND: Hamstring strain injuries (HSIs) are the most common injury type in Australian football, and the rate of recurrence has been consistently high for a number of years. Long-lasting neuromuscular inhibition has been noted in previously injured athletes, but it is not known if this influences the athlete's adaptive response to training. PURPOSE: To determine if elite Australian footballers with a prior unilateral HSI (previously injured group) display less improvement in eccentric hamstring strength during preseason training compared with athletes without a history of HSIs (control group). STUDY DESIGN: Cohort study; Level of evidence, 2. METHODS: A total of 99 elite Australian footballers (17 with a history of unilateral HSIs in the previous 12-month period) participated in this study. Eccentric hamstring strength was assessed at the start and end of preseason training using an instrumented Nordic hamstring device. The change in eccentric strength across the preseason was determined in absolute terms and normalized to the start of preseason strength. The start of preseason strength was used as a covariate to control for differences in starting strength. RESULTS: The left and right limbs in the control group showed no difference in absolute or relative change (left limb: 60.7 ± 72.9 N and 1.28 ± 0.34 N, respectively; right limb: 48.6 ± 83.8 N and 1.24 ± 0.43 N, respectively). Similarly, the injured and uninjured limbs in the previously injured group showed no difference in either absolute or relative change (injured limb: 13.1 ± 57.7 N and 1.07 ± 0.18 N, respectively; uninjured limb: 14.7 ± 54.0 N and 1.07 ± 0.22 N, respectively). The previously injured group displayed significantly less increase in eccentric hamstring strength across the preseason (absolute change, 13.9 ± 55.0 N; relative change, 1.07 ± 0.20 N) compared with the control group (absolute change, 54.6 ± 78.5 N; relative change, 1.26 ± 0.39 N) for both absolute and relative measures (P < .001), even after controlling for differences in the start of preseason eccentric hamstring strength, which had a significant effect on strength improvement. CONCLUSION: Elite Australian footballers with a unilateral history of HSIs displayed less improvement in eccentric hamstring strength across preseason training. The smaller improvements were not restricted to the previously injured limb as the contralateral limb also displayed similarly small improvements in eccentric strength. Whether this is the cause of or the result of an injury remains to be seen, but it has the potential to contribute to the risk of hamstring strain reinjuries.