Effects of Age and Exercise on Fiber Type and Total Fiber Number in Skeletal Muscle

Abstract

There have been varied reports of the effects of age and exercise on fiber type composition in skeletal muscle. To elucidate these discrepancies, we conducted a one year endurance exercise study in male FBN rats, starting with rats age 24 mo and continuing daily until age 36 mo. Rats were divided into 3 groups; high intensity (H), and moderate intensity (M) treadmill exercise, and sedentary (S). M and H animals trained at 13 m/min for 30 min/day, with the H group at 5% incline. At the end of the training protocol the plantaris muscle was removed, frozen and sectioned. Fiber type was determined by IHC using an anti‐type I MHC antibody (BA‐D5s). Total fiber number and number of type I v. type II fibers were determined from high resolution images of muscle cross sections. Total plantaris fiber number declined with age (decrease of 34% in 36 mo compared to 24 mo) but fiber number was not significantly different between the 36 mo exercise groups and the sedentary animals. The proportion of fibers that were type I fibers was 15% in 24 mo, and then 18%, 20% and 18% in S, M, and H respectively. None of these percentages were significantly different from one another, indicating no shift in fiber type with either aging or exercise. These results indicate that total fiber number decreases with age, but this fiber loss may not be preferential to specific fiber types, and also suggest that exercise is unable to minimize fiber loss.Supported by: NIH AG030423