The shoulder muscles also have a more pronounced symmetry of the averaged EMG responses with respect to the ascending and descending phases of force waves, while demonstrating a lower degree of antagonist cocontraction. It has been shown that the concentric and eccentric differences in the responses of the shoulder muscles are more clearly distinguishable than those in the elbow muscles. On the other hand, the data obtained indicate that the movement kinematics has a strong modulating effect on the activation synergy, dividing it into concentric and eccentric subtypes, in accordance with the known non-linear features of the muscle dynamics. The data obtained allows to suggest that for two-joint movements of the forelimbs, it is sufficient to consider the force and activation synergies (patterns of simultaneous activity in different muscles), ignoring at the first stage the effects associated with kinematic synergy.

The EMG waves related to the respective force waves were strongly dependent on the predominant direction of the muscle length changes within the correspondent force wave locations: the EMG intensities were higher for the shortening muscle movements ( concentric contractions) and lower during muscle lengthening ( eccentric contractions). The averaged EMG traces in muscles of both joints show their close correspondence to the related force traces, however, the co-activation patterns of activity in agonists and antagonists were also often encountered. Additionally, for one of the subjects, there was carried out MRI identification and 3D printing of the bones of the forelimb, shoulder, scapula and collarbone, which made it possible to reconstruct for him the length and force traces of all the muscles under study. The positional changes in the averaged EMGs of the elbow and shoulder muscles were compared for all combinations of direction of movement and generated force. The movement tests were analyzed in a group of eight subjects, which were asked to develop tangential force using visual biofeedback while performing slow, externally imposed, circular movements of right hand holding a moving handle operated by a computerized mechatronic system. This study examines some of the non-linear effects of signal transduction in the human motor system, with particular emphasis on muscle hysteresis. 4Faculty of Physical Education, Health and Tourism, Institute of Physical Culture, Kazimierz Wielki University, Bydgoszcz, Poland.3Institute of Traumatology and Orthopedics, National Academy of Medical Sciences of Ukraine, Kyiv, Ukraine.2Department of Physical Education, Gdansk University of Physical Education and Sport, Gdansk, Poland.1Department of Movement Physiology, Bogomoletz Institute of Physiology, National Academy of Sciences, Kyiv, Ukraine.

Shushuiev 1, Mariusz Zasada 4 and Serhii S.

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