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

Abstract

Differences in muscle strength and fatigue between children and adults are well established, but the physiological mechanisms explaining these differences remain unclear. Beyond smaller muscle size, differences in motor unit (MU) activation may contribute to age- related differences in muscle performance. However, research examining age-related differences in discrete MU behaviour is limited and inconsistent. Existing studies have focused exclusively on submaximal contractions, which may be insufficient to recruit the higher-threshold MUs, and none have explored MU activation patterns during fatigue. Thus, this study aimed to examine age-related differences in discrete MU properties during maximal volitional isometric contractions (MVC) of the knee extensors and during a maximal fatiguing contraction. Twenty boys (10.2 ± 1.4 years) and 20 men (21.9 ± 2.6 years) completed familiarization and experimental sessions. During the experimental session, MVC torque of the knee extensors and flexors was first determined (Biodex System 3). Participants then completed a ramped MVC at 33%MVC/s, held for 20s. Following a 30-min recovery, participants performed a 50s sustained maximal, fatiguing contraction. To assess recovery, four additional MVCs were performed immediately, 1-, 3- and 10-minutes post-task termination. Surface electromyography (sEMG) was recorded from the vastus lateralis (VL) and decomposed into individual MU action potential trains (Trigno®, Galileo, Delsys Inc) from which recruitment thresholds (RT) and firing rates (MUFRs) were calculated. Absolute and lean body mass-relative MVC torque was greater (p=.001) in men (277.9±68.9 Nm; 4.3±0.9 Nm/kg LBM respectively) than boys (73.4±25 Nm; 2.6±0.7 Nm/kg LBM, respectively). During the ramped MVC, the MUFR-RT relationship showed a higher yintercept (p<.001) and a steeper slope in the men (p=.005). The maximal RTs at which MUs were identified were significantly higher in the men (p=.032), while the initial RTs did not differ between groups. Throughout the fatiguing contraction there was no difference in the decrease in relative torque between groups, but the pattern of torque decline, as reflected in the torque trace, differed (p=.18). MUFRs were significantly lower in boys than in men (p=.028) and significantly decreased over time in both groups (p<.001), but the temporal pattern differed (p<.001). Furthermore, boys’ torque returned to near-baseline values within 1-minute post-task termination, whereas men were unable to recover after 10-minutes. In conclusion, age-related differences in muscle performance, fatigue and recovery may be partly explained by lower MU firing rates and different activation patterns during contraction. However, further research is needed to elucidate potential age-related differences in MU recruitment and the developmental factors which may affect them.