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


CONCLUSIONS
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.
Halin
Renaud
R
Laboratory of Motor Performance, Department of Ergonomic Conception and Motor Activity, U.F.R. S.T.A.P.S, University of Orléans, France. Renaud.Halin@univ-orleans.fr
Germain
Philippe
P
Bercier
Stephane
S
Kapitaniak
Bronislaw
B
Buttelli
Olivier
O
D000293:Adolescent; D000328:Adult; D000367:Age Factors; D002648:Child; D004550:Elbow / Q000502:physiology; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018763:Muscle Fatigue; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology*; D018482:Muscle, Skeletal / Q000254:growth & development / Q000294:innervation* / Q000502:physiology*; D014891:Weight Lifting / Q000502:physiology