Sex-specific effects of a repetitive fatiguing task on stability: Analysis with motor equivalence model

Although studies showed that several internal factors affect task-specific stability, the sex-specific effects of fatigue on whole-limb stability during a semi-cycle repetitive pointing task remain unidentified. Synergy and Motor Equivalence concepts in the UCM framework have been developed to explain task-specific stability. The motor equivalence model quantifies the amount of deviation in the space of elemental variables that occurs in two directions; one that preserves the performance variable (good variance), and the other that affects it (bad variance). Synergy index (the difference between good and bad variance divided by the total variance > 0) represent stability in performing a task. Healthy adults (n = 26, 13F; age: 35.3 ± 10.6 yrs.) performed an RPT by moving their dominant arm between a proximal target and a distal target in a standing position until near fatigue (Borg CR10 rating 8/10). Tridimensional kinematics of trunk, upper arm, forearm, and hand segments were captured by high-resolution cameras every minute, and joint angles were extracted according to the ZX′Y″ Euler sequence. Results showed the synergy > 0 for both women and men, reflecting synergies stabilizing the endpoint coordinate in both Non-Fatigue and Fatigue conditions. Statistics (ANOVA) showed a significant Condition * Sex effect (p = 0.01), with higher good (by 0.19 ± 0.1 rad) and bad variances (by 0.15 ± 0.09 rad) in women compared to men after fatigue. Higher good and bad variability, with no change in women's performance could represent a less stable strategy, leading to the development of risk factors for neck-shoulder disorders.