Author(s): FARAGHER, M., MCKIEL, A., JEVTIC, N., HOLMES, M., BEAUDETTE, S., FALK, B., Institution: BROCK UNIVERSITY, Country: CANADA, Abstract-ID: 1737

INTRODUCTION:
Differences in muscle strength and fatigue between children and adults are well documented, but the mechanisms explaining these differences are not understood. Beyond smaller muscles, differences in motor unit (MU) activation may also explain age-related differences in muscle performance. Studies of age-related differences in discrete MU activation are limited and inconsistent. Thus, the aim of this study was to investigate age-related differences in discrete MU recruitment patterns and firing rates (MUFRs) during maximal volitional isometric contractions (MVIC) of the knee extensors and during a fatiguing maximal contraction.
METHODS:
Following a habituation session, MVIC was determined for 20 men (22±3yr) and 20 boys (10±1yr). Participants then completed a ramped MVIC at 33%/s, held for 20s. After a 30-min recovery, participants completed a 50s maximal contraction. sEMG from the vastus lateralis were recorded and decomposed into individual MU action potential trains (Trigno®, Galileo, Delsys Inc.). Recruitment thresholds (RT) and MUFRs were calculated for each MU. Boys-men differences in the MUFR-RT relationship during the ramped contraction were assessed using a linear mixed-effects model. A mixed-model ANOVA was used to assess group differences in MUFR during the fatiguing contraction.
RESULTS:
MVIC 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). MUFR was lower in boys than in men in both the ramped MVIC and the fatiguing contraction (Age effects, p<.01). During the ramped MVIC the MUFR-RT slope was steeper in men than in boys (Age-RT interaction, p=.005). The initial RT at which MUs were identified were not different between groups (p=0.2); however, the maximum detected RTs were greater in men compared with boys (71±13 and 60±17 %MVC, respectively, p=.03). During the 50s fatigue contraction, torque decreased more in men (52±24%) than in boys (40±22%) but differences were not statistically significant (p=.13). Both groups experienced a significant decline in MUFRs over the course of the contraction (p<.001). Furthermore, there was a significant age-time interaction (p < .001), wherein MUFRs decreased linearly in boys and remained initially stable with a steep decrease in the last 20s in men.
CONCLUSION:
Our results demonstrate that boys display lower MUFRs compared with men during maximal and fatiguing isometric tasks. In maximal contractions, boys’ RT range appears smaller. Additionally, the pattern of decrease in MUFR during fatigue differs between boys and men. It is suggested that children have different neuromotor strategies to complete maximal and fatiguing tasks compared with adults and support the notion that children may have a lesser ability to activate their higher-threshold MUs. The findings provide further insight into the mechanisms underlying age-related differences in muscle performance, and neuromuscular control.