The concept of motor equivalent combinations of arm muscles, or M-modes, was investigated during reaching to insert a pointer into a cylindrical target with and without an elbow perturbation. Five M-modes across 15 arm/scapula muscles were identified by principal component analysis with factor extraction. The relationship between small changes in the M-modes and changes in the position/orientation of the pointer were investigated by linear regression analyses. The results revealed a motor equivalent organization of the M-modes for perturbed compared with nonperturbed reaches, both with respect to hand position and orientation, especially in the first 100-ms postperturbation. Similar findings were obtained for motor equivalence computed based on changes in the joint configuration, although the kinematically defined motor equivalence was stronger for pointer orientation. The results support the hypothesis that the nervous system organizes muscles into M-modes and flexibly scales M-mode activation to preserve stable values of variables directly related to performance success.
Mattos is with the Biomechanics and Movement Science Program, University of Delaware, Newark, DE. Kuhl is with the Department of Biology, University of Delaware, Newark, DE. Scholz is with the Biomechanics and Movement Science Program, and the Department of Physical Therapy, University of Delaware, Newark, DE. Latash is with the Department of Kinesiology, Pennsylvania State University, University Park, PA.