Volitional muscle activation and its reliability in boys and men

Abstract

Background Maximal torque production largely depends on the ability to activate the available motor unit (MU) pool. Using the interpolated twitch technique (ITT), early studies reported lower volitional MU activation (VA) in children compared with adults, while several recent studies suggested no age-related differences. The reliability of VA determination has been studied to a limited extent in adults but has not been examined in children. Purpose To assess age-related VA difference and its reliability in boys and men. Methods Eleven boys (8‒12 years) and 12 men (18‒30 years) completed two identical test sessions (following habituation session) which included 10 x 5-s knee-extension MVCs, with 2-min rest intervals. Each contraction was immediately followed by an evoked twitch (Tc). A superimposed twitch (SiT) was applied only to the last five MVCs each day. Age-related VA differences were determined using a repeated measures ANOVA. ITT reliability (SiT, Tc, VA) was assessed in 7 boys and 12 men, using intraclass correlation coefficients (ICC), derived from a fully nested ANOVA model. Results Maximal knee extension torque was significantly lower in boys compared with men (84.4±18.5 vs. 267.8±67.6 N.m, respectively), even after correcting for body mass (2.3±0.5 vs. 3.3±0.6 N.m.kg-1, respectively). VA was significantly lower in boys than in men (visit 2: 92.6±4.5 vs. 95.2±2.0 %, respectively; visit 3: 93.5±3.4 vs. 96.2±2.8%, respectively, group effect = 0.04), ii with no difference between visits nor group-by-visit interaction. A similar pattern was observed for the SiT and Tc. The ICC for VA was higher in men than in boys (ICC=0.80 vs. 0.33, respectively). In both groups, most of the variance in VA stemmed from inter-trial variability (58.2% and 59.7% of total variance for boys and men, respectively), indicating inconsistency in both groups. In boys, large day-to-day variance (32.5%) indicated poor stability. The SiT reliability was moderate in both groups (ICC=0.69 and 0.47 for boys and men, respectively). The Tc reliability was high for boys and men (r = 0.96 and 0.85, respectively). Conclusions In congruence with early previous findings, boys’ knee extensors VA was lower than that of the men. Contradictory reports of age-related differences in VA in the literature may be due to lack of reliability, and specifically, lack of trial-to-trial consistency using the ITT.