Renaud Halin | PubMed Health

PubMed2003

Neuromuscular response of young boys versus men during sustained maximal contraction.

PURPOSE: This study was designed to compare neuromuscular response between boys and men during sustained maximal voluntary contraction (MVC). METHODS: Fifteen boys (YB, 10.5 +/- 0.9 yr) and 12 men (AM, 21.5 +/- 4.5 yr) participated in the experiment. Arm's cross sectional area (CSA) and maximal force (F(max)) of elbow flexor were measured before subjects performed a 30-s sustained MVC. Mean power frequency (MPF) and muscle fiber conduction velocity (MFCV) were calculated from myoelectric signals of the biceps brachii. F(max)/CSA, MPF, and MFCV changes were expressed by slopes of linear regressions. Maximal MPF (I-MPF) and MFCV (I-MFCV) were derived from the intercept of each regression. RESULTS: AM had significantly greater F(max)/CSA (P < 0.05), I-MPF (P < 0.05), and I-MFCV (P < 0.01) than YB. F(max)/CSA (P < 0.001), MPF (P < 0.001), and MFCV (P < 0.01) declined significantly more for AM than YB. MPF/MFCV ratio increased, i.e., MPF decreased more than MFCV, for both groups but this was significantly (P < 0.001) more pronounced for AM. CONCLUSION: Taken together, those results suggest that more fatigable Type II motor units are involved in men, resulting in greater lactic acid and ions accumulations during fatigue. This difference in muscle's metabolic and ionic state could be responsible for a greater reflex-induced decrease of motor units firing rates in men compared with boys. This firing rate decrease could be explained using the "muscular wisdom" hypothesis and would express a nervous command adaptation to sustain a maximal contraction.

Adolescent↗

PubMed2002

Differences in strength and surface electromyogram characteristics between pre-pubertal gymnasts and untrained boys during brief and maintained maximal isometric voluntary contractions.

The present study was aimed at investigating differences of maximal strength (F(max)) of the elbow flexors and characteristics of the surface electromyogram (EMG) between six gymnasts (G) and six untrained (UT) 10-year-old boys during brief and maintained maximal voluntary isometric contraction (MVC). The F(max) was estimated during 5 s MVC (maximal test, MT) and normalized to the cross sectional area (CSA) of the arm. The EMG signal of the biceps brachii was recorded during MT and during a 25 s maintained MVC (fatigue test). Values were calculated for root-mean-square (rms(MT)) and mean power frequency (MPF(MT)) of the EMG signal for the duration of the MT. For the fatigue test, MPF were normalized to the initial value (MPF(n)) and kinetics were expressed by the slope coefficient of linear regression. Although F(max) and F(max)/CSA tended to be higher for G than UT, the differences did not reach significance. The MPF(MT) was significantly higher for G [mean (SD)][136 (8) Hz] than for UT [125 (9) Hz]. The MPF(n) slope coefficients were significantly greater for G than for UT [-1.0 (0.2) and -0.5 (0.3), respectively]. When all the children were considered, F(max) was significantly correlated to MPF(MT) (r = 0.61). These results showed that gymnasts tend to have higher F(max) and F(max)/CSA accompanied by a significantly higher MPF(MT) and a steeper MPF downshift. Moreover, children with greater strength tended to have higher MPF(MT). It is suggested that spatial and/or temporal recruitment of more fatigable fast motor units could have been enhanced in G and more generally, that it could be a mechanism that would explains, in part, the level of force production in children.

Child↗