Muscle strength, size, and neuromuscular function before and during adolescence.

Purpose: To compare measurements of muscle strength, size, and neuromuscular function among pre-adolescent and adolescent boys and girls with distinctly different strength capabilities.

Methods: Fifteen boys (mean age ± confidence interval: 13.0 ± 1.0 years) and 13 girls (12.9 ± 1.1 years) were categorized as low strength (LS, n = 14) or high strength (HS, n = 14) based on isometric maximal voluntary contraction strength of the leg extensors. Height (HT), seated height, and weight (WT) determined maturity offset, while percent body fat and fat-free mass (FFM) were estimated from skinfold measurements. Quadriceps femoris muscle cross-sectional area (CSA) was assessed from ultrasound images. Isometric ramp contractions of the leg extensors were performed while surface electromyographic amplitude (EMG) and mechanomyographic amplitude (MMG) were recorded for the vastus lateralis (VL). Neuromuscular efficiency from the EMG and MMG signals (NME and NME, respectively) and log-transformed EMG and MMG vs. torque relationships were also used to examine neuromuscular responses.

Results: HS was 99-117% stronger, 2.3-2.8  years older, 14.0-15.7 cm taller, 20.9-22.3 kg heavier, 2.3-2.4 years more biologically mature, and exhibited 39-43% greater CSA than LS (p ≤ 0.001). HS exhibited 74-81% higher NME than LS (p ≤ 0.022), while HS girls exhibited the highest NME (p ≤ 0.045). Even after scaling for HT, WT, CSA, and FFM, strength was still 36-90% greater for HS than LS (p ≤ 0.031). The MMG patterns in the LS group displayed more type I motor unit characteristics.

Conclusions: Neuromuscular adaptations likely influence strength increases from pre-adolescence to adolescence, particularly when examining large, force-producing muscles and large strength differences explained by biological maturity, rather than simply age.