The purpose of this study was to investigate the effects of active warm-up (stationary cycling) and passive warm-up (heat pack) on the mechanical properties of the hamstring musculature in vivo. Passive tension measurements were recorded during the passive knee extensions of ten men before and after an active warm-up, passive warm-up, and control treatments. The results of the present investigation revealed no effect of active warm-up or passive warm-up on the passive tension variables of energy absorbed, energy returned, peak torque, average stiffness, or peak stiffness of the hamstring musculature. These findings suggest that the mechanical properties of the connective tissue were unaltered by the warm-up techniques utilized in this study. Although these results do not support the use of active or passive warm-up to facilitate increased muscle tendon unit compliance to stretching activity, we do not recommend discontinuation of these practices before athletic competition, exercise, physical recreational activity, or stretching exercise.