Muscle cross-sectional area and motor unit properties of the medial gastrocnemius and vastus lateralis in normal weight and overfat children

New Findings What is the central question of this study?Are differences in muscle size and motor unit properties between normal weight and overfat children muscle specific? What is the main finding and its importance?Muscle cross-sectional area and motor unit action potential amplitudes and firing rates were similar between overfat and normal weight children for both the medial gastrocnemius and vastus lateralis muscles. There was no evidence that the chronic mechanical overload provided by the greater body mass resulted in significant hypertrophy of contractile tissue or motor units that would be used during lower-to-moderate intensity activities. This study examined the possible differences in muscle cross-sectional area (mCSA), motor unit action potential amplitudes (MUAP(AMPS)) and interspike intervals (ISIs) of the firing instances of the medial gastrocnemius (MG) and vastus lateralis (VL) between normal weight (NW) and overfat (OF) children aged 7-10 years. Fourteen NW (age = 8.6 +/- 1.1 years, BMI = 15.8 +/- 1.4 kg m(-2)) and 12 OF (age = 8.8 +/- 0.9 years, BMI = 21.8 +/- 2.4 kg m(-2)) children performed isometric trapezoidal muscle actions at 40% of maximal voluntary contraction of the plantar flexors and knee extensors. Surface electromyography was recorded from the MG and VL and decomposed into the firing events of motor units (MUs). Statistical procedures were performed on the composite recruitment thresholds (RTs), ISIs and MUAP(AMPS) of recorded MUs collapsed across subjects and the y-intercepts and slopes calculated from each subject's ISI and MUAP(AMP) vs. RT relationships. Ultrasound was used to assess mCSA, echo intensity (mEI), and subcutaneous fat (sFAT) of the MG and VL. The OF had greater mCSAs, mEI and sFAT (P = 0.004-0.024), but there were no differences in mCSA when accounting for mEI for the MG (P = 0.506) and VL (P = 0.326). The NW children had significantly greater composite MUAP(AMPS) for the VL and MG (P < 0.001), but only significantly larger MUAP(AMPS) of the VL remained for the NW (P < 0.001) when subjects were matched for sFAT. There were no differences between groups for the ISI or MUAP(AMP) vs. RT relationships (P > 0.05). These findings suggest that the OF children did not undergo significant muscle or MU hypertrophy that would be routinely activated during activities of daily living.