In the paper we substantiated biomechanical aspects of athletes' working positions in interphase transitions of an exercise as boundary positions in which the extremes of the program control are reached: flexion-extention movements in joints. It is shown that flexion-extention movements of the athlete in joints are elementary motor actions which are not exposed to further biomechanically grounded division and therefore have to be learnt as a complete method. The authors consider the formation conditions of motor representations of the athlete in the context of biomechanics of motor actions from psychology-pedagogical positions. The sports experts in the issue of training quite often address to the theory of activity and the concept based on it of "systematic and stage-by-stage formation of actions", developed by P.Ya. Galperin and his disciples. Motor representations of the athlete on the studied sports exercise are formed generally on the basis of the semantic idea of technical, system and structural, parametrical properties of movement and, in particular, are created by the visual idea of kinematics of the links of the performer's body. The main provisions poses of the athlete in the boundary provisions defining borders of the phase structure of the exercise or the extreme of the operating functions can be marker points of the trajectory of the biomechanical system, in this case. The structural division of the sports exercise on the phase structure, allocation of the boundary provisions corresponding to the working poses of the performer in the main support areas allow to allocate the operating movements realizing flexion-extention movements in joints in motor action. Achievement of maximum or minimum bending or extension (extremes) in joints can be simultaneous or occur consistently. As flexion-extention movement in joints is an elementary motor action, it is not subject to further biomechanical division and, therefore, it should be mastered as a complete method. Therefore, the definition of the phase of sports exercises and its pedagogical structuring is an urgent task in the process of teaching gymnastic exercises, as it allows to reveal the major support areas in the orienting basis of activities and plan the content side of the learning process of sports exercises. The results of these studies allow to assert that major support areas in the orienting basis of actions are based on objective biomechanical indicators of the studied gymnastic exercise.

Обосновываются биомеханические аспекты использования рабочих положений спортсмена в межфазовых переходах упражнения в качестве граничных положений, в которых достигаются экстремальные значения программного управления: сгиба-тельно-разгибательные движения в суставах. Показано, что сгибательно-разгибательные движения спортсмена в суставах являются элементарными двигательными действиями, которые не подвергаются дальнейшему биомеханически обоснованному дроблению и поэтому должны осваиваться целостным методом.