Electromyographic and Mechanomyographic Time and Frequency Responses During Fatiguing, Submaximal, Isokinetic Muscle Actions of the Biceps Brachii

The purpose of the present investigation was to examine the time-course of changes in electromyographic (EMG) and mechanomyographic (MMG), time and frequency domain responses during repeated, submaximal, concentric, isokinetic, forearm flexion muscle actions. Twelve men (mean age ± SD = 22.6 ± 2.2 yrs; body weight = 84.0 ± 8.3 kg; height = 178.6 ± 8.3 cm) performed 50 repeated, submaximal (65% of concentric peak torque), concentric muscle actions of the dominant forearm flexors on an isokinetic dynamometer at 60°·s-1. Surface EMG and MMG signals were simultaneously recorded from the biceps brachii muscle. Polynomial regression analyses (first, second, and third order) were used to examine the composite patterns of responses for EMG amplitude (AMP), EMG mean power frequency (MPF), MMG AMP, and MMG MPF across the fatiguing workbout. The results indicated that across the fatiguing workbout EMG AMP increased linearly (r² = 0.961), while EMG MPF decreased quadratically (R² = 0.771), and MMG AMP and MMG MPF decreased linearly (r² = 0.747 and r² = 0.575, respectively). The increase in EMG AMP, but decreases in EMG MPF and MMG MPF may have reflected the fatigue-induced recruitment of higher-threshold motor units with lower firing rates (as described by the Onion-Skin Scheme) due to the buildup of metabolic byproducts which interfere with contractile properties of the activated muscle fibers. Despite potential increases in motor unit recruitment, MMG AMP decreased which may have been due to decreased muscle compliance. In addition to the Onion-Skin Scheme, it is also possible that the decrease in MMG MPF could be described by the Muscle Wisdom Theory which optimizes force production. Collectively, in the present study the increase in EMG AMP and decrease in MMG MPF may have reflected an increase in motor unit recruitment, but a decrease in motor unit firing rate which suggested that the maintenance in torque could be explained by both the Onion-Skin Scheme as well as the Muscle Wisdom Theory. Advisor: Terry J. Hous