Purpose: The purpose of the present study was to evaluate inter-arm and inter-gender differences in fractal dimension (FD) and conduction velocity (CV) obtained from multichannel surface electromyographic (sEMG) recordings during sustained fatiguing contractions of the biceps brachii.
Methods: A total of 20 recreationally active males (24±6 years) and 18 recreationally active females (22±9 years) performed two isometric contractions at 120 degrees elbow joint angle: (1) at 20% maximal voluntary contraction (MVC) for 90 s, and (2) at 60% MVC until exhaustion the time to perform the task has been measured. Signals from sEMG were detected from the biceps brachii using bidimensional arrays of 64 electrodes and initial values and rate of change of CV and FD of the sEMG signal were calculated.
Results: No difference between left and right sides and no statistically significant interaction effect of sides with gender were found for all parameters measured. A significant inter-gender difference was found for MVC (p<0.0001). Initial values of CV were higher in females than in males at both force levels (20% MCV: p<0.0001; 60% MCV: p<0.05) whereas a lower initial estimate of FD was observed in females compared to males (20% MCV: p<0.05; 60% MCV: p<0.0001). No difference in CV and FD slopes was found at 20% MVC between genders. At 60% MVC significantly lower CV and FD slopes (CV and FD: p<0.05) and a more protracted time to exhaustion were found in females than in males (p<0.0001). When considering time to exhaustion at both levels of contraction no difference in percentage change (Δ%) of CV and FD slopes was found between genders (p>0.05). During the sustained 60% MVC no statistical correlation was found between MVC and CV or FD initial estimates nor between MVC and CV or FD slopes both in males and females whereas. A significant positive correlation between CV and FD slopes was found in both genders (males: r = 0,61; females: r = 0,55).
Conclusions: Fatigue determines changes in FD and CV values in biceps brachii during sustained contractions at 60% MVC. In particular males show greater increase in the rate of change of CV and FD than females whereas no difference in percentage change of these sEMG descriptors of fatigue was found. A significant correlation between FD and CV slopes found in both genders highlights that central and peripheral myoelectric components of fatigue may interact during submaximal isometric contractions.