Effects of 6 mo of heavy-resistance training combined with explosive exercises on neural activation of the agonist and antagonist leg extensors, muscle cross-sectional area (CSA) of the quadriceps femoris, as well as maximal and explosive strength were examined in 10 middle-aged men (M40; 42 +/- 2 yr), 11 middle-aged women (W40; 39 +/- 3 yr), 11 elderly men (M70; 72 +/- 3 yr) and 10 elderly women (W70; 67 +/- 3 yr). Maximal and explosive strength remained unaltered during a 1-mo control period with no strength training. After the 6 mo of training, maximal isometric and dynamic leg-extension strength increased by 36 +/- 4 and 22 +/- 2% (P < 0. 001) in M40, by 36 +/- 3 and 21 +/- 3% (P < 0.001) in M70, by 66 +/- 9 and 34 +/- 4% (P < 0.001) in W40, and by 57 +/- 10 and 30 +/- 3% (P < 0.001) in W70, respectively. All groups showed large increases (P < 0.05-0.001) in the maximum integrated EMGs (iEMGs) of the agonist vastus lateralis and medialis. Significant (P < 0.05-0.001) increases occurred in the maximal rate of isometric force production and in a squat jump that were accompanied with increased (P < 0.05-0. 01) iEMGs of the leg extensors. The iEMG of the antagonist biceps femoris muscle during the maximal isometric leg extension decreased in both M70 (from 24 +/- 6 to 21 +/- 6%; P < 0.05) and in W70 (from 31 +/- 9 to 24 +/- 4%; P < 0.05) to the same level as recorded for M40 and W40. The CSA of the quadriceps femoris increased in M40 by 5% (P < 0.05), in W40 by 9% (P < 0.01), in W70 by 6% (P < 0.05), and in M70 by 2% (not significant). Great training-induced gains in maximal and explosive strength in both middle-aged and elderly subjects were accompanied by large increases in the voluntary activation of the agonists, with significant reductions in the antagonist coactivation in the elderly subjects. Because the enlargements in the muscle CSAs in both middle-aged and elderly subjects were much smaller in magnitude, neural adaptations seem to play a greater role in explaining strength and power gains during the present strength-training protocol.

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