Semitendinosus and biceps femoris long head active stiffness response until failure in professional footballers with vs. without previous hamstring injury

This study sought to examine the active stiffness of semitendinosus (ST) and biceps femoris long head (BFlh) during a knee flexor isometric contraction at 20% of maximal voluntary isometric contraction until failure in elite footballers (n = 50, age: 22.3 ± 5.3 years; height: 1.82 ± 0.08 m; body mass: 74.7 ± 9.0 kg). Active stiffness was assessed using ultrasound-based shear wave elastography by means of shear modulus quantification. Comparisons were performed between limbs with (n = 11) vs. without (n = 89) previous hamstring injury. A similar time until failure in the knee flexor fatigue task was observed between groups (p =.401). At the start of the task, lower limbs with previous hamstring injury showed a lower BFlh active stiffness (31.0.1 ± 10.4 kPa, p =.023) and BFlh/ST active stiffness ratio (0.50 ± 0.29), and no differences for ST (72.8 ± 26.8 kPa, p =.221) compared to lower limbs without previous hamstring injuries (BFlh: 38.0 ± 9.6 kPa; ST: 64.0 ± 18.4 kPa; BFlh/ST: 0.65 ± 0.27). During the task, the ST active stiffness in both groups decreased from 80% of task time (p =.032), in the absence of changes in BFlh active stiffness (p =.534), resulting in an increase in BFlh/ST active stiffness from 80% of task time (p =.029). No differences between limbs were observed during the fatigue task for all parameters (p >.099). Future research is warranted to verify if the differences found represent an increased risk of hamstring injury. Highlights The hamstring's active stiffness response to a fatigue protocol in soccer athletes with a history of injury is unknown. Athletes with previous injury showed less active stiffness in the biceps femoris long head. Similar response to fatigue was observed between athletes with and without hamstring injury history.