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 RMS ) and mechanomyographic amplitude (MMG RMS ) were recorded for the vastus lateralis (VL). Neuromuscular efficiency from the EMG and MMG signals (NME EMG and NME MMG , 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 EMG than LS ( p ≤ 0.022), while HS girls exhibited the highest NME MMG ( 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 RMS 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.