The Movement- and Load-Dependent Differences in the EMG Patterns of the Human Arm Muscles during Two-Joint Movements (A Preliminary Study)

Cocontraction (the simultaneous activation of antagonist muscles around a joint) provides the nervous system with a way to adapt the mechanical properties of the limb to changing task requirements-both in statics and during movement. However, relatively little is known about the conditions under which the motor system modulates limb impedance through cocontraction. The goal of this study was to test for a possible relationship between cocontraction and movement accuracy in multi-joint limb movements. The electromyographic activity of seven single- and double-joint shoulder and elbow muscles was recorded using surface electrodes while subjects performed a pointing task in a horizontal plane to targets that varied randomly in size. Movement speed was controlled by providing subjects with feedback on a trial-to-trial basis. Measures of cocontraction were estimated both during movement and during a 200-ms window immediately following movement end. We observed an inverse relationship between target size and cocontraction: as target size was reduced, cocontraction activity increased. In addition, trajectory variability decreased and endpoint accuracy improved. This suggests that, although energetically expensive, cocontraction may be a strategy used by the motor system to facilitate multi-joint arm movement accuracy. We also observed a general trend for cocontraction levels to decrease over time, supporting the idea that cocontraction and associated limb stiffness are reduced over the course of practice.

Because humans have limited ability to independently control the many joints of the hand, a wide variety of hand shapes can be characterized as a weighted combination of just two or three main patterns of covariation in joint rotations, or "postural synergies." The present study sought to align muscle synergies with these main postural synergies and to describe the form of membership of motor units in these postural/muscle synergies. Seventeen joint angles and the electromyographic (EMG) activities of several hand muscles (both intrinsic and extrinsic muscles) were recorded while human subjects held the hand statically in 52 specific shapes (i.e., shaping the hand around 26 commonly grasped objects or forming the 26 letter shapes of a manual alphabet). Principal-components analysis revealed several patterns of muscle synergy, some of which represented either coactivation of all hand muscles, or reciprocal patterns of activity (above and below average levels) in the intrinsic index finger and thumb muscles or (to a lesser extent) in the extrinsic four-tendoned extensor and flexor muscles. Single- and multiunit activity was generally a multimodal function of whole hand shape. This implies that motor-unit activation does not align with a single synergy; instead, motor units participate in multiple muscle synergies. Thus it appears that the organization of the global pattern of hand muscle activation is highly distributed. This organization mirrors the highly fractured somatotopy of cortical hand representations and may provide an ideal substrate for motor learning and recovery from injury.

Compared with the control of precision grips involving the thumb and one or two fingers, the control of grasping using the entire hand involves a larger number of degrees of freedom that has to be controlled simultaneously, and it introduces indeterminacies in the distribution of grip forces suitable for holding an object. We studied the control of five-digit grasping by measuring contact forces when subjects lifted, held, and replaced a manipulandum. This study focused primarily on the patterns of coordination among the normal forces exerted by each of the digits, assessed by varying the center of mass of the manipulandum. The force patterns during the lift and hold phases were modulated as a function of the location of the center of mass. A frequency domain analysis revealed a consistent temporal synergy by which digits tended to exert normal forces in phase with each other across all experimental conditions. This tendency for in-phase covariations by the normal forces exerted by the digits extended over the entire functional frequency range (up to 10 Hz). When the effect of thumb force was removed, a second synergy was revealed in which forces in two fingers could be modulated 180 degrees out of phase (also prevailing throughout the range of frequencies studied). The first synergy suggests the presence of a "common drive" to all of the extrinsic finger muscles, whereas the second one suggests another input, ultimately resulting in a reciprocally organized pattern of activity of some of these muscles.