An assessment of the motor control strategies and effects of fatigue specific to isometric, concentric, and eccentric muscle actions

The purpose of this study was twofold: (1) to examine the motor control strategies associated with fatigue across 30 repeated maximal isometric (ISO), concentric (CON), and eccentric (ECC) muscle actions of the leg extensors; and (2) to compare the effects of fatiguing ISO, CON, and ECC repetitions on muscle activation, motor unit recruitment, firing rate, and peak torque. Twelve females performed repeated maximal ISO, CON, and ECC muscle actions of the leg extensors on an isokinetic dynamometer. Surface electromyographic (EMG) and mechanomyographic (MMG) signals were recorded simultaneously from the vastus lateralis muscle. The results indicated that across the ISO muscle actions, there were significant decreases in torque (r2 = 0.95), EMG amplitude (R2 = 0.44), EMG MPF (r2 = 0.62), and MMG MPF (r2 = 0.48), but no change in MMG amplitude (r 2 = 0.07). In addition, during the CON fatigue protocol, there were significant decreases in torque (R2 = 0.97), EMG amplitude (R2 = 0.46), EMG MPF (R2 = 0.86), MMG amplitude (R2 = 0.44), and MMG MPF (R2 = 0.80). During the ECC fatigue protocol, however, there were decreases in torque (R 2 = 0.60), EMG MPF (r2 = 0.19), and MMG MPF (r 2 = 0.41), but increases in MMG amplitude (r2 = 0.81) and no change in EMG amplitude (r2 = 0.001). The ISO and CON fatigue protocols resulted in significant decreases in torque and EMG MPF during the ISO, CON, and ECC muscle actions. These findings were attributed to the accumulation of metabolic byproducts associated with fatiguing ISO and CON muscle actions. The ECC fatigue protocol, however, resulted in significant decreases in ISO and CON torque, but not ECC torque. These findings were likely the result of the muscle damage and mechanical efficiency related to repeated ECC muscle actions. In conjunction, these findings indicated that there were different motor control strategies and effects of fatigue during ISO, CON, and ECC muscle actions