Variances of Hand Positions and Arm Configurations During Arm Movements Under External Load and Without External Load

• Published in: Motor control 2007-01, Vol.11, p.S127-S127
• Main Authors: Laczko, J, Keresztenyi, Z
• Format: Article
• Language: English
• ISSN: 1087-1640

The variances of hand positions, and angular arm configurations are studied to investigate how the variances and arm muscle activities are effected if a motor task is performed under external load. Subjects lay on a plate of weight bench and held a bar in their hands above their chest while the elbows were fully flexed. They were asked to lift the bar by fully extending their arms and then replacing the bar to the original position. The subjects executed this extension-flexion movement 10 times without load and 10 times with a bar that was loaded by 160 N. Nine young healthy subjects participated in the measurements. The coordinates of six points of the arm were recorded using a ZEBRIS movement analyzing system: one above the shoulder, two in the elbow, two in the wrist and one on the hand. From these coordinates the angular arm configuration was computed. Simultaneously, muscle activities (EMG) of the biceps brachii and triceps brachii muscles were recorded. The variance of the hand positions and the variance of the angular configurations were computed for two conditions: with load and without load. Time normalized variances were computed separately for the lifting and for the replacing phase. The averaged EMG amplitudes have been computed in the lifting phase and in the reposing phase separately with load and without load and we computed the ratio of the values obtained from these two conditions for both phases. Results: During the lifting phase the variance of the hand position was higher for movements without load than with load. This was true for the variances of the angular configurations as well. During the reposing phase neither the variance of the hand position nor the angular variance differed significantly for the two conditions. The ratio of the EMG values of the triceps was larger for the reposing phase than for lifting. There was no significant difference in the ratios for the biceps. Conclusion: We conclude that during the lifting phase the load (inducing higher muscle activities) helps to stabilize hand position and angular configuration. In the reposing phase the load has a larger effect on the triceps activity that helps to slow down and insure the smooth reposing movement but it does not enhance the stability. The method for studying the effect of load on kinematic variances is planned to discern how the level of muscle activities relate to movement stability.