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

Abstract

Slow circular movements of the hand with a fixed wrist joint that were produced in a horizontal plane under visual guidance during conditions of action of the elastic load directed tangentially to the movement trajectory were studied. The positional dependencies of the averaged surface EMGs in the muscles of the elbow and shoulder joints were compared for four possible combinations in the directions of load and movements. The EMG intensities were largely correlated with the waves of the force moment computed for a corresponding joint in the framework of a simple geometrical model of the system: arm - experimental setup. At the same time, in some cases the averaged EMGs exit from the segments of the trajectory restricted by the force moment singular points (FMSPs), in which the moments exhibited altered signs. The EMG activities display clear differences for the eccentric and concentric zones of contraction that are separated by the joint angle singular points (JASPs), which present extreme at the joint angle traces. We assumed that the modeled patterns of FMSPs and JASPs may be applied for an analysis of the synergic interaction between the motor commands arriving at different muscles in arbitrary two-joint movements.

Theoretical Analysis of the Force and Position Synergies in Two-Joint Movements

“…As was shown in [11], waveforms of the averaged EMG activities of the elbow and shoulder muscles are closely related to the correspondent sectors of the movement trajectories between neighboring FSPs, in which the force moments acting around the correspondent joints change their directions. Waves of the activity in these sectors are alternated in flexors and extensors; the activation patterns are reversed with changes in the loading direction.…”

“…As a result, not all fundamental synergies can be identified experimentally in multi-joint movements of the limbs. In previous experimental studies of our group [9][10][11], we proposed an approach allowing one to analyze quantitatively the simplest form of the synergy effects in circular movements of the subject's right arm. The obtained results were used to find the functional relationships between basic mechanical parameters of two-joint movements and the related central commands.…”

“…The obtained results were used to find the functional relationships between basic mechanical parameters of two-joint movements and the related central commands. In order to determine inter-joint muscle interactions for these movements, we proposed a simplified classification of the synergy effects [11]; the same classification is used in the present study. In accordance with definitions accepted in the above study, we will further use the terms of position, force, and activation synergies, which are based on temporal changes in the following parameters, correspondingly: (i) joint angles, (ii) force moments at the joints, and (iii) activities of the muscles participating in a given movement.…”

“…The locations of the FSPs and PSPs were defined in [11] by computation of the time courses of the force moments and joint angles. At present, graphical methods begin to be widely used for the analysis of the synergy effects (see, e.g., [12]).…”

“…In this study, we propose a graphical method for a theoretical definition of the force and position synergies for planar two-joint arm movements. In accordance with the methods described in [11], the axis of the proximal joint in our model is assumed to be in a fixed position, while the distal end of the other limb segment moves with a small constant velocity along the circular trajectory; the movements are produced under the action of constant loadings directed tangentially with respect to the trajectory. Afterwards, we extrapolated this consideration on the ellipsoidal and linear movement traces.…”