Changes in electromyographic activity, muscle fibre and force production characteristics during heavy resistance/power strength training in middle-aged and older men and women

K Häkkinen, W J Kraemer, R U Newton, M Alen
Acta Physiologica Scandinavica 2001, 171 (1): 51-62
The effects of a 6-month resistance training (2 day/week) designed to develop both strength and power on neural activation by electromyographic activity (EMG) of the agonist and antagonist knee extensors, muscle fibre proportion and areas of type I, IIa, and IIb of the vastus lateralis (VL) as well as maximal concentric one repetition maximum (1 RM) strength and maximal and explosive isometric strength of the knee extensors were examined. A total of 10 middle-aged men (M40; 42 +/- 2), 11 middle-aged women (W40; 39 +/- 3), 11 elderly men (M70; 72 +/- 3) and 10 elderly women (W70; 67 +/- 3) served as subjects. Maximal and explosive strength values remained unaltered during a 1-month control period. After the 6-month training maximal isometric and 1RM strength values increased in M40 by 28 +/- 14 and 27 +/- 7% (P < 0.001), in M70 by 27 +/- 17 and 21 +/- 9% (P < 0.001), in W40 by 27 +/- 19 and 35 +/- 14% (P < 0.001) and in W70 by 26 +/- 14 and 31 +/- 14% (P < 0.001), respectively. Explosive strength improved in M40 by 21 +/- 41% (P < 0.05), in M70 by 21 +/- 24% (P < 0.05), in W40 by 32 +/- 45% (NS) and in W70 by 22 +/- 28% (P < 0.05). The iEMGs of the VL and vastus medialis (VM) muscles increased during the training in M40 (P < 0.001 and 0.05), in M70 (P < 0.001 and 0.05), in W40 (P < 0.001 and 0.05) and in W70 (P < 0.001 and 0.05). The antagonist biceps femoris (BF) activity during the isometric knee extension remained unaltered in M40, in W40, and in M70 but decreased in W70 (from 42 +/- 34 to 32 +/- 26%; P < 0.05) during the first 2 months of training. Significant increases occurred during the training in the mean fibre areas of type I in W70 (P < 0.05) and of overall type II along with a specific increase in IIa in both W40 (P < 0.05) and in W70 (P < 0.05), while the changes in the male groups were not statistically significant. The individual percentage values for type II fibres at pretraining correlated with the individual values for 1 RM strength in both W70 (r=0.80; P < 0.05) and M70 (r=0.61; P < 0.05) and also at post-training for maximal isometric torque in W70 (r=0.77, P < 0.05). The findings support the concept of the important role of neural adaptations in strength and power development in middle-aged and older men and women. The muscle fibre distribution (percentage type II fibres) seems to be an important contributor on muscle strength in older people, especially older women. Women of both age groups appear to be hypertrophically responsive to the total body strength training protocol performed two times a week including heavier and lower (for fast movements) loads designed for both maximal strength and power development, while such a programme has limited effects on muscle hypertrophy in men.

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