**1. Introduction**

Elastic strain energy is stored in tendons and is used in various motor tasks. It permits to minimize energy costs of muscle contraction and increase power output of muscle-tendon units (MTU). Therefore, specific mechanical properties of different tendons can influence MTU behavior and, ultimately, mechanical power and muscle efficiency of movements [1].

It was suggested that there was a relationship between the structure of various mammalian tendons and their role in effective transmission of force or energy conservation during elastic deformation [2]. The relevance of this idea lies in the context of improving results in various sport movements due to conformity of MTU functioning with requirements and specific features of a sport discipline.

This article presents research data on various strategies used for power production by MTU of the lower extremities in drop jumps. According to the hypothesis [3], to perform a motor task, a muscle develops power that should be adequate to

mechanical requirements of the task in order to produce a movement. Any movement is determined by timing, sequence and amplitude of muscle activation. Two elastic mechanisms contribute to the efficiency of power production in multi-joint movements of the lower extremities during a take-off: pre-stretch of skeletal muscles and mechanical energy transfer via biarticular muscles. It is assumed that the extent to which each of the mechanisms is used exerts effect on the strategy of organization of multi-joint movements.

Preliminary stretching of skeletal muscles is the first mechanism to increase power of a take-off. It is known that pre-stretch of the muscle-tendon complex (MTU) amplifies strength of its subsequent contraction [4, 5]. This mechanism manifests itself in various movements, including vertical jumps [6–8].

The second mechanism of increasing power of a take-off (mechanical energy transfer) was described in a few works related to running, hopping and vertical jumping [3–5, 9–12]. The mechanism of mechanical energy transfer via barticular muscles is also associated with the effects occurring in the MTU [13, 14].

Enhancement of MTU contraction can be reached with the help of three mechanisms: accumulation and release of elastic deformation energy, activation of spinal reflexes, and muscle potentiation. Many studies related to this topic were carried out in both animals and humans. They showed that several mechanisms could be activated simultaneously in the stretch-contraction cycle in order to enhance the MTU contraction. Those mechanisms could vary depending on external conditions and motor tasks [15–23]. For example, in running and hopping it is necessary to maintain muscle strength that is achieved by the use of MTU elastic strain energy; in accelerations, starts and jumps the catapult mechanism is used; and in landing energy is absorbed due to muscle compliance [24]. Such variability in performance of different motor tasks is achieved by modulating the muscles' intrinsic mechanical properties due to spinal reflexes, which can increase or decrease muscle stiffness [25–27]. So, it is possible to use the elastic deformation mechanism in muscles or, conversely, the muscles can act as a damper in landing. It is assumed that the choice of this or that mechanism depends on the character and requirements of the sport exercise, and athletes from different sports may demonstrate difference in organization of MTU control.

We hypothesized that to perform the same motor task athletes could employ different ways of movement organization. An athlete's preference when choosing this or that mechanism for enhancing power of muscle contraction depended on specific features of sport discipline, notably, requirements to the main sport exercise.

From a practical perspective, our research was aimed at obtaining data that could be used in training of top athletes. Strength training is an integral part of athletic training in sport of top results, and it is effective only when it is based on individual characteristics of elite athletes. The results of this study will help coaches develop individual training plans for athletes, in particular strength training exercises targeting specific muscle groups.
