Examination of muscle composition and motor unit behavior of the first dorsal interosseous of normal and overweight children

We examined differences between normal weight (NW) and overweight (OW) children aged 8-10 yr in strength, muscle composition, and motor unit (MU) behavior of the first dorsal interosseous. Ultrasonography was used to determine muscle cross-sectional area (CSA), subcutaneous fat (sFAT), and echo intensity (EI). MU behavior was assessed during isometric muscle actions at 20% and 50% of maximal voluntary contraction (MVC) by analyzing electromyography amplitude (EMG(RMS)) and relationships between mean firing rates (MFR). recruitment thresholds (RT), and MU action potential amplitudes (MUAP(size)) and durations (MUAP(time)). The OW group had significantly greater EI than the NW group (P = 0.002; NW, 47.99 +/- 6.01 AU; OW, 58.90 +/- 10.63 AU, where AU is arbitrary units) with no differences between groups for CSA (P = 0.688) or MVC force (P = 0.790). MUAP(size) was larger for NW than OW in relation to RT (P = 0.002) and for MUs expressing similar MFRs (P = 0.011). There were no significant differences (P = 0.279- 0.969) between groups for slopes or y-intercepts from the MFR vs. RT relationships. MUAP(time) was larger in OW (P = 0.015) and EMG Rms was attenuated in OW compared with NW (P = 0.034); however, there were no significant correlations (P = 0.133-0.164, r = 0.270-0.291) between sFAT and EMG(RMS) . In a muscle that does not support body mass, the OW children had smaller MUAP(size) as well as greater El, although anatomical CSA was similar. This contradicts previous studies examining larger limb muscles. Despite evidence of smaller MUs, the OW children had similar isometric strength compared with NW children. NEW & NOTEWORTHY Ultrasound data and motor unit action potential sizes suggest that overweight children have poorer muscle composition and smaller motor units in the first dorsal interosseous than normal weight children. Evidence is presented that suggests differences in action potential size cannot be explained by differences in subcutaneous fat alone.