Are sex differences in force steadiness due to dissimilar motor unit activity between men and women?

• Main Author: Brown, Ruth Emily
• Language: English
• Published: University of British Columbia 2011
• Online Access: http://hdl.handle.net/2429/33740

Force steadiness (FS) is the ability to maintain a constant isometric contraction around a given force level and is expressed as the coefficient of variation (CV) of force. Although it has recently been shown that men are steadier than women in the elbow flexor muscles, the primary mechanism for this difference has yet to be determined. Motor unit activity is thought to influence FS, in particular motor unit discharge rate variability (MUDRV). The purpose of this thesis was to determine if motor unit activity differs between men and women, and if this is a possible mechanism to explain sex differences in FS. Eight young men (25 ± 3.6 years) and 11 young women (21.2 ± 3.2 years) performed a 7.5 second steady isometric elbow flexion contraction in the neutral wrist position at 5%, 10%, 25% and 50% of maximum voluntary contraction (MVC). Single motor unit activity, force, and surface electromyography (EMG) were measured simultaneously during the 7.5s contraction from the short head (SH) and long head (LH) of the biceps brachii (BB) of the left, non-dominant arm. Results indicate that men were significantly (p < 0.05) stronger than women as well as steadier at all force levels. Motor unit discharge rate (MUDR) and surface EMG did not differ between men and women at the same relative force levels. However, women exhibited greater MUDRV than men (p = 0.02). When data for men and women were combined, significant relationships were observed between CV of force with MVC (r² = 0.42) (p < 0.001), CV of force with MUDR (r² = 0.05) (p = 0.01), but CV of force and MUDRV were not related. When men and women were assessed independently, there was no relationship between FS and MVC, MUDR, or MUDRV for men, yet significant relationships were observed between FS and MVC (r² = 0.35) (p < 0.001) and FS and MUDR (r² = 0.25) (p < 0.001) for women. Partial correlations revealed that maximum strength (-0.71) and MUDR (-0.37) contribute the most to FS. Although MUDRV was greater in women than men during isometric elbow flexion, MUDRV does not appear to be a factor contributing to FS in young adults. Maximum strength seems to be the largest contributor to sex differences in FS, in that the weaker the muscle the less steady it is.