Effects of aging on muscle fibers and collagen content of the diaphragm: a comparison with the rectus abdominis muscle.

Age-related changes of muscle fiber and collagen content of the diaphragm (DIA) and rectus abdominis (RA) muscles were examined in rats aged 1, 4, 8 and 18 months. The cross-sectional areas of all muscle fiber types (I, IIa, IIb) and the amount of collagen in both DIA and RA increased up to the age of 8 months after which a decrease in growth following a parabolic curve was found. The older DIA showed an increase in the percentage of type I fibers while in RA there was a high percentage of type IIb fibers. The aged DIA became more resistant to fatigue but at the expense of slow contraction while older RA is faster and stronger. Aged DIA and RA muscles showed a low amount of collagen with great concentration of cross-linkings leading to a tissue with low viscoelastic properties and reduced compliance.