Sex-Specific Muscle Activity and Kinematics of the Posterior Kinetic Chain Targeting Hamstring Strengthening Exercises and Sprinting: A Cross-Sectional Study

# Background Neuromuscular and kinematic peculiarities in the lower extremities between the sexes could influence the way the posterior kinetic chain interacts during exercises and sprinting, potentially altering hamstring strengthening exercise selection. # Purpose This study compared sex-differences in four exercises (the rocker, perpetuum mobile fast and slow [PMfast and PMslow] and the Nordic Hamstring exercise) on their potential to simulate sprint-like muscle activity and kinematics. # Methods Muscle activity of the posterior kinetic chain (biceps femoris [BFlh], semitendinosus [ST], gluteus maximus [GMax] and gastrocnemius medialis [GCM]) was measured with surface electromyography (sEMG) during each exercise and treadmill running at 75% of the individual maximal sprint velocity. sEMG data was normalized to maximal sprinting. 3D-motion capture was employed to assess hip and knee angles. # Results Eight male and 12 female athletes were included (age: 22.4 years ± 3.1; body mass: 61.0 kg ± 7.1; height: 1.67 m ± 0.07). Peak (MD: 31, p= 0.003, [95%CI: 14-48], SMD: 1.1) and average hamstring activity (MD: 16, p< 0.001, [95%CI: 8-24], SMD: 1.4) was greater in females for every examined task. A sex and muscle interaction showed that females displayed significantly greater peak (MD: 39, p=0.002, [95%CI: 21-60], SMD: 1.3) and mean activity (MD: 20, p< 0.001, [95%CI: 12-29], SMD: 1.5) for ST but not BFlh. Joint angles of peak muscle activity showed significantly greater hip angles during the PMfast in females (MD: 30°, p= 0.014, [95%CI: 15-46°], SMD: 2.1) and an overall significantly smaller knee flexion angle for the ST compared to the BFlh (MD: 7°, p= 0.02, [95%CI: 1-12°], SMD: 0.4). Neither hamstring elongation stress nor muscle selectivity showed significant sex differences. # Discussion Aside from overall display of greater hamstring, specifically ST activity in all tasks for females, only minor kinematic and no synergist muscle sex-differences were found. Further research exploring sex-specific hamstring neuromuscular strategies is needed. # Level of evidence 3b