![]() From the perspective of data analysis, note that the muscle synergy patterns can be affected by biomechanical task constraints, selected muscles, and specification of electromyographic (EMG) data processes such as filtering and scaling. The composition of muscle synergies quantifies the muscles that are co-activated to achieve specific motor goals required for the given motor tasks, whereas the activation coefficient indicates the timing and activation magnitude of the synergies. The number of muscle synergies indicates the complexity of intermuscular coordination. The information obtained from muscle synergy analysis includes the number, composition, and activation coefficients of muscle synergies. According to the muscle synergy hypothesis, the human body simplifies intermuscular coordination to manipulate the numerous degrees of freedom in the musculoskeletal system by modulating the activation of a limited number of muscle synergies (i.e., consistent co-activation patterns of muscle groups) rather than by controlling individual muscles. Intermuscular coordination has been quantitatively evaluated in the form of muscle synergies for the last few decades. Previous studies have reported that abnormal intermuscular coordination produces the reduction of reaching distance and the horizontal work area of the hand as well as the torque coupling between upper limb joints. In the case of upper limbs, two classical patterns of abnormal intermuscular coordination are the coupling of shoulder abduction and elbow flexion (termed 'flexor synergy’) and the coupling of shoulder adduction and elbow extension (called 'extensor synergy’). ![]() In addition to spasticity and muscle weakness, abnormal and stereotypical intermuscular coordination contributes to the limited motor capability of the stroke-affected limb. Motor impairments following a stroke affect the daily lives, social participation activities, and the quality of life of stroke survivors. We postulated the effect of biomechanical constraints on the intermuscular coordination and suggested a possible intermuscular coordination-based rehabilitation protocol that provides the biomechanical constraint appropriate to a trainee throughout the progress of rehabilitation. Compared with free (i.e., unconstrained) movement, exercise under biomechanical constraints including the isokinetic constraint might promote the activation of muscle synergies independently in stroke survivors. Stroke-affected upper limbs seemed to modularize the activation of the shoulder and elbow muscles in a fairly similar way to that of neurologically intact individuals during isokinetic movements. The severity of motor impairments was negatively correlated with the similarity of the post-stroke synergies with respect to the mean control synergies. ![]() Alternatively, the modulation of synergy activation coefficients was altered after a stroke. When the number of muscle synergies between the groups matched, the comparable composition of muscle synergies was observed in both groups. The composition of muscle synergies was comparable between the groups, except that the three heads of the deltoid muscle were co-activated and formed one synergy in the stroke group, whereas those muscles formed two synergies in the control group. ![]() Resultsįour and five muscle synergies in the stroke and control groups were observed, respectively. The correlation between the alteration of muscle synergies and the level of motor impairment was investigated. Then, we compared the number, composition, and activation coefficients of muscle synergies and the end-point force between the groups. Intermuscular coordination of the stroke survivors and the control participants was quantified in the form of muscle synergies. End-point forces and electromyographic activities of the shoulder and elbow muscles were measured while the participants performed isokinetic upper limb movements in a three-dimensional space. Sixteen chronic stroke survivors and eight neurologically intact individuals were recruited. In this study, we investigated upper limb intermuscular coordination after a stroke during isokinetic movements. By evaluating the intermuscular coordination in the affected limb under various biomechanical task constraints, the impact of a stroke on motor control can be analyzed and intermuscular coordination-based rehabilitation strategies can be developed. Abnormal intermuscular coordination limits the motor capability of stroke-affected upper limbs. ![]()
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