The muscle fibers of M. rectus abdominis from 3 sportsmen (Weight-lifting, Boxing and Body-building) were examined histologically and compared with the mean of 22 human control subjects. Following results were obtained.
1. Compared with the M, rectus abdominis of control subjects, the sportsmen's rectus abdominis muscle area was about 4 times the cross-sectional area of muscle belly and the number of muscle fibers per sq, mm, were 1/3 that of the control specimens. The total number and mean size of sportsmen muscle fibers corresponded to the respective maxima of the controls, and the muscle fiber density in sportsmen corresponded to the minimum control value.
2. Comparing the myofibrous organization of 3 specimens, the muscle of the boxer was largest in cross-sectional area, in frequency of muscle fibers with large diameter, and in volume of interstitial connective tissue. It was the smallest of the 3 specimens in the number of muscle fibers per sq. mm. We found that the muscle of the weight-lifter was intermediate in these four categories.