BILATERAL AND UNILATERAL NEUROMUSCULAR FUNCTION AND MUSCLE CROSS-SECTIONAL AREA IN MIDDLE-AGED AND ELDERLY MEN AND WOMEN

Forty-eight healthy men (M) and women (W), divided into two different age groups, i.e., M50 yrs (range 44-57; n = 12), W50 yrs (range 43-57; n = 12), M70 yrs (range 59-75; n = 12), and W70 yrs (range 62-75; n = 12), volunteered as subjects for examination of muscle cross-sectiona l area (CSA), maximal voluntary forces, force-time curves, and electro myographic activity of the knee extensor muscles during bilateral and unilateral isometric contractions. The maximal bilateral knee extensio n force and the average CSA values in M50 were greater (p < .05) than in M70 and in W50 were greater (p < .05-.001) than those recorded for W70. The early forces in the force-time curve were greater (p < .05) f or M50 than for M70 and in W50 were greater (p < .05) than in W70. The individual values of the CSA of the left and right quadriceps femoris (QF) correlated (p < .05-.001) with the individual values of maximal unilateral knee extension forces in M50 (r = .87 and r = .87), M70 (r = .61 and r = .80), W50 (r = .79 and r = .58), and W70 (r = .56 and r = .54). When the force values were related to the CSA of the muscle, W 70 demonstrated a lower (p < .05) value than the other three groups. M aximal voluntary bilateral forces didn't differ from those of the summ ed unilateral forces, and the maximal integrated EMG values during the bilateral and unilateral contractions of the same leg were also the s ame. The results suggest that the decline in maximal strength with inc reasing age could be related to the decline in the CSA of the muscle, but in older people, especially women, strength decreases seemed to be multifactorial, including possibly a decrease in voluntary neural dri ve or changes in ''qualitative'' characteristics of the muscle tissue. Explosive strength may decrease with aging even more than maximal str ength, suggesting that atrophying effects of aging may be greater on f ast-twitch muscle fibers than on slow-twitch fibers and/or that the ra te of neural activation of the muscles may also be influenced by aging . On the other hand, the central nervous system in a simple single joi nt isometric force production of the knee extensors seems to be capabl e of activation of the two bilateral QF muscle groups to the same degr ee in comparison to that of the unilateral activation only.