- BACKGROUND AND PURPOSE: The purpose of this study was to determine the effect of a swimming endurance training program on changes in percentages and sizes of fiber types in different regions of the hamster diaphragm. METHODS: Adult male golden Syrian hamsters were randomly assigned to a control group (n = 9) or a swimming group (n = 10). Hamsters in the swimming group swam for 80 minutes per session, 5 days per week, for 13 weeks. Fiber-type percentages and sizes were determined for the costal region and for the abdominal and thoracic surfaces of the crural region of the diaphragm from cross sections processed for myofibrillar adenosine triphosphatase. RESULTS: Muscle fibers in the thoracic surface of the crural region were smaller in the swimming group than in the control group. Fiber-type percentages in the diaphragm, however, were not different between groups. CONCLUSION AND DISCUSSION: Swim training may have improved the endurance of the thoracic/crural region by decreasing cross-sectional area and thus decreasing the distance for oxygen to diffuse to the internal regions of the muscle fibers.