Previous Hamstring Injury Does Not Impact Biceps Femoris Long Head Fascicle Length and Eccentric Knee Flexion Strength Changes Following a Low-Volume Resistance Training Intervention in Elite Under-20-Year-Old Male Gaelic Footballers

This study reports biceps femoris long head (BFlh) fascicle length and eccentric knee flexion strength changes following a 9-week in-season low-volume resistance training intervention in elite under-20-year-old (U20) male Gaelic footballers with and without previous hamstring injury. We included 26 male Gaelic footballers (age = 18.5 ± 0.5 years; height = 183.1 ± 5.1 cm; body mass = 79.2 ± 7.9 kg) in this study, with 10 of these participants reporting previous hamstring injuries within the last 12 months. Participants performed two sets of four Nordic hamstring exercise (NHE) repetitions and three sets of six staggered-stance Romanian deadlift (RDL) repetitions once per week for nine weeks. Before and after the intervention, BFlh fascicle length was assessed via ultrasound and eccentric knee flexion strength was measured during the NHE. We ran separate linear mixed-effects models for BFlh fascicle length and eccentric knee flexion strength, with fixed-effects of time (baseline and follow-up), limb-type (previously injured and contralateral uninjured limbs of previously injured participants and between-limb average of uninjured participants) and the time*limb-type interaction. The linear mixed-effects model for BFlh fascicle length revealed a significant main effect of time (β = 0.31 cm; SE = 0.15 cm; p = 0.04), while the time*limb-type interaction was not statistically significant. The linear mixed-effects model for eccentric knee flexion strength revealed no significant increase in eccentric knee flexion strength (β = 9.53 N, SE = 12.97 N, p = 0.47), while the time*limb-type interaction was not statistically significant. These findings suggest changes to BFlh fascicle length and eccentric knee flexion strength from baseline to follow-up were comparable across all limb-types.