The Time-Course of Hamstring Muscle Passive Stiffness and Perceived Tightness After Strenuous Exercise: Is It Time to Start Using Localized Stiffness Measurements?

Abstract: Pieters, D, Witvrouw, E, Steyaert, A, Vanden Bossche, L, Schuermans, J, Freitas, S, and Wezenbeek, E. The time-course of hamstring muscle passive stiffness and perceived tightness after strenuous exercise: Is it time to start using localized stiffness measurements? J Strength Cond Res XX(X): 000-000, 2025-Hamstring injuries are inherent to high-speed running sports. Postexercise muscle stiffness is suggested as a potential risk factor, yet an objective measurement tool to monitor its time-course was lacking. Interestingly, shear wave elastography (SWE) is an objective measurement of tissue stiffness. Surprisingly, the time-course of postexercise hamstring muscle passive stiffness (measured with SWE) and how this relates to the subjective perception of tightness have insufficiently been examined. Therefore, the aim was to determine and compare the time-course of the localized passive stiffness of the hamstring muscles (using SWE) with the perceived overall hamstring tightness after an exhaustive exercise. Twenty-four subjects underwent a prone-leg-curl exercise until full exhaustion. At 6 moments in time (baseline, 0, 24, 48, 72, and 96 h postexercise), the passive stiffness was quantified in each hamstring head through SWE and the perceived hamstring tightness through a numeric rating scale. Mixed models were used to investigate the effect of maximal exercise on the behavior of both the passive muscle stiffness and perceived tightness of the hamstrings. The level of significance was set at α = 0.05. Concerning the stiffness behavior of the semimembranosus and biceps femoris long head muscles, no significant changes were found over time (p > 0.05). However, the shear modulus of the semitendinosus muscle increased significantly, reaching its maximum 48 h postexercise (+9.7%, p = 0.012) followed by a significant decrease (-7.8%, p = 0.024) to baseline level after 96 h. In contrast, the perceived hamstring tightness peaked at 24 h (p < 0.001), significantly decreased between 24 and 48 h (p = 0.036), with a normalization after 96 h (p < 0.001). Thus, the time-course of the athlete's perceived hamstring tightness after strenuous exercise did not follow the localized hamstring passive stiffness, suggesting that precaution should be taken when designing training and recovery programs.