Martínez-Serrano, A, Radaelli, R, Trindade de Freitas, T, Alcaraz, PE, and Freitas, SR. Hamstring muscle stiffness during isometric contractions until task failure in footballers with and without injury history. J Strength Cond Res XX(X): 000-000, 2024-Despite various proposed prevention strategies, the incidence of hamstring injuries in modern soccer is still elevated. Recent research has focused on exploring how muscle tissue stiffness behaves under fatigue conditions as a potential risk factor. This study aimed to examine the active stiffness of biceps femoris long head (BFlh) and semitendinosus (ST) muscles using ultrasound-based shear wave elastography (SWE) during a knee flexors' submaximal contraction until exhaustion in highly trained national-level male footballers, comparing previously injured and noninjured limbs. A case-control study was performed including 94 highly trained male footballers. Using SWE, the passive and active stiffness of the BFlh and ST were assessed at rest and during a knee flexors' submaximal isometric contraction at 40% of maximal voluntary isometric contraction (MVIC) until exhaustion. Differences in stiffness patterns between previously injured and noninjured limbs were analyzed, along with passive muscle stiffness, knee flexors' MVIC, and endurance capacity. No statistically significant differences in the active stiffness of BFlh and ST between previously injured and noninjured limbs throughout the contraction task were found (p >
0.05
0-100% contraction time). Similarly, there were no statistically significant differences in BFlh (mean difference [mean diff.] = 0.2 kPa
p >
0.05) and ST (mean diff. = 0.9 kPa
p >
0.05) passive stiffness, knee flexors' MVIC (mean diff. = -8.5 Nm
p >
0.05), or time to exhaustion (mean diff. = 6.95 seconds
p >
0.05). Load-sharing between the BFlh and ST did not change significantly throughout the contraction (p >
0.05
0-100% contraction time). These results suggest that players with a history of hamstring injuries may retain similar mechanical properties and coordination strategies as noninjured players.