RESUMEN
OBJECTIVE: the purpose of this study was to identify differences in hamstring passive stiffness between the pre-season and in-season periods. METHODS: Hamstring strength and passive stiffness were measured in professional male soccer players before and after the pre-season (4 weeks), and after the in-season (6 weeks) periods using an isokinetic dynamometer. Muscle passive stiffness was determined from the slope of the passive torque-angle relationship. External loads (acceleration and jumps) were monitored by GPS and internal loads by questionnaire. RESULTS: Hamstring passive stiffness increased after 10 weeks of training and matches, without changes in passive peak torque and range of motion. The hamstring passive stiffness modifications were associated with the volume and intensity of accelerations and jumps. The individual data analysis also provided some support for the suppression of the biomechanical adaptation in the subjects with relatively large external load. CONCLUSIONS: Regular training and match workouts increase hamstring passive stiffness in professional soccer players but the adaptation of muscle-tendon unit passive elements might not occur if players experience excessive mechanical stress.