Effect of Hand-Held Flexi-Bar on Shoulder Functional Tonic and Phasic Time-Varying Muscle Synergies

An open research question is how the central nervous system (CNS) to find a solution for the problem of redundancy or degree of freedom in the human shoulder motion control. We used time-varying synergy theory in which assumed that the relative activation between muscles is time-varying, to investigate the combination of activation patterns of muscles in twelve 3-D hand-held exercises by Flexi-Bar using Anybody Technology software (A/S, Aalborg, Denmark). Using activation of 12 muscles and the moment across the joint as an input matrix for the optimization procedure to extract functional time-varying synergies, time delays and amplitude coefficients, the achieved 5 tonic and phasic synergies explained 79% of the data variation. Matching pursuit procedure and non-negative least square used to find timing shifts and amplitude coefficients respectively. Considering a new exercise out of the primary database, 60% of the activation patterns reconstructed using time-varying synergies. Although the extracted synergies seem to be directionally tuned, the results show that due to the same velocity in all exercises and also because the torque which that was applied due to the weight of the bar and arm on the joint is not significant, both timing shifts and separation phasic-tonic parts of the activation patterns provide no further explanation on CNS behavior and finding them causes unnecessary computational cost. Future study can focus on the comparison of synergies between two or more groups of exercises by the Flexi-Bar such as holding the bar vertically or horizontally with swinging it up and down or back and forth.