Nowadays, athletes are increasingly exposed to high environmental temperatures. While many studies investigated the effects of heat stress on human physiology, the effects of acute or repeated heat exposure on muscle-tendon properties and function are not fully understood in vivo. Therefore, the purpose of this work is to describe the responses of the neuromuscular and musculotendinous system (i.e., voluntary and electrically-evoked maximal and explosive force production, force-velocity and force-length relationships and elastic properties) in vivo, using ultrafast ultrasound, under heat stress. The experimental part of this work is based on three studies, focused on gastrocnemius medialis muscle-tendon unit. The first study aimed to determine the effects of passive heat exposure on muscle-tendon unit properties. Our results showed an acceleration of rate of force development in the early phase, while soft tissue stiffness decreased. The second study investigated the acute effects of heat exposure on muscle-tendon interactions and fascicle dynamics in active participants (i.e., during running). We demonstrated that muscle-tendon unit properties and operating fascicle lengths during running were unaffected by environmental temperatures up to 40 min of running at 10 km.h^-1. The third study measured the impact of active heat acclimation on muscle-tendon unit properties. While the training protocol (i.e., repeated low-intensity cycling sessions) induced effective physiological adaptations, the properties of muscle-tendon unit assessed and the performance in vertical jump were unchanged. These findings offer the opportunity to improve our understanding of human motor skills responses to heat stress and to provide practical recommendations to coaches and athletes exposed to hot environments.