Amount and structure of force variability during short, ramp and sustained contractions in males and females.

The aim of this study was to investigate the effect of gender differences on force variability as assessed by means of linear and nonlinear estimators during short duration, ramp and sustained isometric elbow flexions. Ten males and ten females performed elbow flexion receiving visual feedback from the direction of force exertion. Isometric elbow flexions were performed during: maximum voluntary contraction (MVC before and after endurance test), short contraction at 10-90% MVC with 10% increment for 5s, ramp contraction from 5% to 50% MVC over 30s, and endurance contraction at 20% MVC. Standard deviation (SD), coefficient of variations (CV), and sample entropy (SaEn) were computed from the force signals recorded in 3D. During short and ramp contraction, SD increased with contraction level while SaEn followed an inverted U-shape function (p<.01). During endurance test, SD and CV increased with contraction time (p<.01). SD and SaEn were consistently higher in males than females while it was opposite for CV (p<.05). Separate control and compensatory mechanisms could be responsible for the observed changes in the amount and structure of task-related and tangential forces variability. Moreover, gender differences most likely point towards gender-dependent force control mechanisms. The lower magnitude and structure of variability observed in females may increase the risk of muscle overload and damage.