Differences in electromechanical delay components induced by sex, age and physical activity level: new insights from a combined electromyographic, mechanomyographic and force approach

Electromyographic (EMG), mechanomyographic (MMG) and force (F) signals combined analysis represents an interesting approach to partition the electrochemical and mechanical events contributing to total electromechanical delay, i.e., the time lag existing between the muscle activation and the onset of force generation. The study sought to assess the differences in electromechanical delay due to sex, age, and physical activity level. Electromechanical components were assessed on vastus lateralis muscle during a maximum voluntary contraction and electrically evoked contractions in 180 participants. During each contraction, the EMG, MMG and F signals were recorded. Electromechanical delays and its two components (Δt EMG-MMG, mainly electrochemical component; and Δt MMG-F, mainly mechanical component) were computed. Measurements’ reliability (intraclass correlation coefficient, ICC) and sensitivity (minimum detectable changes at 95% confidence as a percentage, MDC95%) were also calculated. ICC spanned from 0.89 to 0.97 with a percentage change of the standard error of the measurement (SEM%) ranging from 1.6 to 4.9%. MDC95% values ranged between 3.1 and 9.8%. Longer electromechanical delay values were observed in: (1) women compared to men; (2) 40–45 years old compared to 30–35 years and 20–25 years; and (3) sedentary than active participants. Differences were accompanied by increments in Δt MMG-F but not in Δt EMG-MMG values. The alterations in the whole electromechanical delay induced by sex, age, and physical activity level could be ascribed to the difference in the duration of the mechanical events included in the electromechanical delay, possibly due to modifications in the muscle–tendon unit characteristics.