Muscular synergies during motor corrections: investigation of the latencies of muscle activities.

To reduce the complexity of muscular control, a small number of muscular activations are combined to produce an infinity of movements. This concept of muscle synergies has been widely investigated, mainly by means of principal component analyses (PCA) in the case of unperturbed movements. However, reaching movements can be altered at any time if the target location is changed during their execution. In this case, PCA does not precisely measure the latencies of muscles activities. We develop here a simple method to investigate how a random target jump toward a single location induced motor corrections in the whole musculature by precisely determining the latencies of muscle activities during a complex pointing movement. Our main result demonstrated that both initiation times together as well as correction times together were strongly correlated for some pairs of muscles, independently of their occurrences during the motor sequence and independently of the location of the muscles at the anatomical level. This study thus provides a simple method to investigate the latencies of muscular activities and the way they are correlated between certain muscles to stress the muscular synergies involved in the movement. It also suggests that the CNS re-programs a new synergy after the target jumps in order to correct the on-going reaching movement. This latter corrective synergy involves the control of more muscles together compared to that used to initiate the movement. At the level of the Primary Motor Cortice (M1), muscles appear to be controlled as a coupled functional system, rather than individually and separately.