Age-Related Differences in Maximal Strength and Muscle Fatigue: Association with Motor Unit Activation Patterns

The aim of this study was to investigate age-related differences in motor unit (MU) recruitment patterns and firing rates (MUFRs) during maximal volitional isometric contractions (MVC) of the knee extensors and during a fatiguing maximal contraction. Methods: Twenty men (22±3yr) and 20 boys (10±1yr) completed a ramped MVC and a 50s maximal contraction. Surface electromyography (sEMG) was recorded from the vastus lateralis and decomposed into individual MU action potential trains. Results: MVC relative to lean body mass was greater (p<.001) in men (4.3±0.9 Nm/kg) than in boys (2.6±0.7 Nm/kg). During the ramped MVC, MUFR was higher (p<.001) and the MUFR-recruitment threshold (RT) slope was steeper in men than in boys (p=.005). During the fatiguing contraction, torque decreased more in men (52±24%) than in boys (40±22%) but the difference was not significant (p=.13). Both groups experienced a significant decline in MUFRs over the course of the fatiguing contraction (p<.001). Furthermore, there was a significant age x time interaction (p<.001), wherein MUFRs decreased linearly in boys, but remained initially stable with a steep decrease in the last 20s in men. Conclusion: Our findings suggest that children have different neuromotor strategies to complete maximal and fatiguing tasks compared with adults, possibly related to children’s lower maximal strength and greater fatigue-resistance.