Effect of Age, Elbow Muscle Co-Contraction Level, and Elbow Moment Loading Charachteristics on Elbow Angle Positional Variability in Postural Tasks.

Abstract

Hand positional variability during a task confounds many activities of daily living, such as inserting a key into a lock, pouring hot water into a cup, and eating with a spoon. Current dogma suggests that older adults use increased co-contraction to attenuate hand positional variability; however, this has not been proven. This thesis proposes that elbow muscle co-contraction significantly affects elbow angle positional variability (PV), which affects hand positional variability, and that older adults can reduce their PV by decreasing their co-contraction level. Variable muscle stiffness was incorporated into the Signal Dependent Noise Theory to model the effect of elbow muscle co-contraction on predicted PV during a quasistatic elbow flexion task. The results demonstrated an optimal level of co-contraction that minimized the positional variability for both variable and constant loading paradigms. The first experiment tested the effect of greater than natural elbow muscle co-contraction levels on PV. Fourteen younger and 14 older healthy adults resisted both constant and variable external elbow extension moments while co-contracting at different levels. Increasing co-contraction was found to increase positional variability for both younger and older adults (p < 0.005). Older adults naturally co-contracted at higher levels and had lower positional variability than younger adults (p < 0.005). The second experiment tested the effect of below-natural elbow muscle co-contraction levels on PV in 14 younger and 14 older healthy adults. The effect of age was not significant in predicting positional variability at the lowered co-contraction level. In the third experiment, 12 younger and 14 older healthy adults poured water through different sized container openings. Older subjects co-contracted at higher levels and had lower PV than younger adults. While co-contraction increased with decreasing target size, increased co-contraction within a pouring target size increased PV (p < 0.05) and decreased pouring accuracy (p < 0.001). We conclude that increasing elbow muscle co-contraction increases PV and hinders the performance of tasks, such as pouring, that require low positional variability. This counters the current belief that high co-contraction levels are beneficial for task performance, and suggests that people who struggle with these tasks may benefit from reduced co-contraction.