Muscle cross-sectional area, force production and relaxation characteristics in women at different ages

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Thirty women, divided among three different age groups, i.e. 30 years (range 26–35;n = 10), 50 years (range 46–55;n = 10) and 70 years (range 66–75;n = 10) volunteered as subjects for examination of the characteristics of the muscle cross-sectional area (CSA), maximal voluntary isometric force, isometric force-time and relaxation-time of their leg extensor muscles. The CSA of the quadriceps femoris muscle in the youngest age group was slightly larger (NS) than in the middle-aged group and much larger (P<0.41) than in the oldest age group whose CSA was markedly smaller (P<0.01) than the middle-aged group. Maximal force in the youngest group was slightly greater (NS) than in the middle-aged group and much greater (P<0.01) than in the oldest group whose values were markedly smaller (P<0.05) than the middle-aged group. The individual values in CSA correlated with maximal force both in the total subject sample (r=0.82;P<0.001) and in the three age groups separately (r=0.72;P<0.01;r=0.86;P<0.01 andr=0.67;P<0.05, respectively). When the force values were related to the CSA of the muscle, the mean values of 45.4N·cm−2, SD 5.6, 47.6 N·cm−2, SD 5.0 and 46.8 N· cm−2, SD 7.0 for the three groups did not differ significantly from each other. The isometric force-time curves differed among the groups, so that the times to produce the same absolute as well as relative force levels were shorter in the 30-year age group (P<0.05) than in the 70-year age group. The times of relaxation did not differ significantly among the groups. The present results suggested that the decline in maximal force in females with age could well be related to the decline in the cross-sectional area of the muscle. However, the time taken in the production of explosive force may worsen even more than maximal strength especially at older ages. This indicated that atrophying effects of aging may be greater on fast than on slow twitch muscle fibres and/or that the rate of neural activation of the muscles could also be influenced by aging.