Muscle co-contraction in elderly people change due to postural stability during single-leg standing

BACKGROUND: Muscle co-contraction is the simultaneous contraction of agonist and antagonist muscles crossing a joint, and it increases with age. This study primarily aimed to clarify the difference in the effect of a light fingertip contact to stationary surface on postural sway and muscle co-contraction during single-leg standing (SLS) between young and elderly groups; the secondary aim was to reveal the quantitative difference in the muscle co-contraction of the ankle joint among the three different support structure conditions in the elderly group. METHODS: This study included eight young adults (age 23.4 ± 2.6 years) and nine community dwelling older adults (age 74.7 ± 3.4 years). The task was SLS under the following conditions: (1) no supporting structure, FR; (2) light index fingertip contact to a stationary supporting structure (to touch in force < 1 N), LT; and (3) dependence on a supporting structure for stabilization as desired, DO. Center of pressure (COP) variables [root-mean-square distance (RDIST), total excursion (TOTEX), mean velocity (MVELO), and standard deviation area (AREA-SD)] and the co-contraction index (CI) between the tibialis anterior and soleus were measured using surface electromyography. RESULTS: With regard to the effect of the light fingertip contact to stationary surface, in the young group, TOTEX, MVELO, AREA-SD, and CI during SLS were smaller under the LT condition than under the FR condition. However, in the elderly group, only AREA-SD and CI were smaller under the LT condition than under the FR condition. No significant difference was observed in COP variables and CI under the DO condition between the young and elderly groups. CONCLUSION: Both young and elderly groups could decrease muscle co-contraction using the light fingertip contact. On the other hand, in the elderly group, COP variables showed a limited effect from the light fingertip contact; only the “sway” domain measure (AREA-SD). Both young and elderly groups showed the smallest CI under the DO condition. Therefore, the elderly group could decrease muscle co-contraction of the ankle joint depending on postural stability.