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 diaphr agm (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 IIA inc reased up to the age of 8 months after which a decrease in growth foll owing a parabolic curve was found. The older DM showed an increase in the percentage of type I fibers while in RA there was a high percentag e of type IIb fibers. The aged DIA became more resistant to fatigue bu t at the expense of slow contraction while older RA is faster and stro nger. Aged DIA and RA muscles showed a low amount of collagen with gre at concentration of cross-linkings leading to a tissue with low viscoe lastic properties and reduced compliance.