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

UNLABELLED: 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.