Single muscle fiber adaptations with marathon training

The purpose of this investigation was to characterize the effects of marathon training on single muscle fiber contractile function in a group of recreational runners. Muscle biopsies were obtained from the gastrocnemius muscle of seven individuals (22 +/- 1 yr, 177 +/- 3 cm, and 68 +/- 2 kg) before, after 13 wk of run training, and after 3 wk of taper. Slow-twitch myosin heavy chain [(MHC) I] and fast-twitch ( MHC IIa) muscle fibers were analyzed for size, strength (P-o), speed (V-o), and power. The run training program led to the successful completion of a marathon ( range 3 h 56 min to 5 h 35 min). Oxygen uptake during submaximal running and citrate synthase activity were improved ( P < 0.05) with the training program. Muscle fiber size declined ( P < 0.05) by similar to 20% in both fiber types after training. Po was maintained in both fiber types with training and increased ( P < 0.05) by 18% in the MHC IIa fibers after taper. This resulted in > 60% increase ( P < 0.05) in force per cross- sectional area in both fiber types. Fiber Vo increased ( P < 0.05) by 28% in MHC I fibers with training and was unchanged in MHC IIa fibers. Peak power increased ( P < 0.05) in MHC I and IIa fibers after training with a further increase ( P < 0.05) in MHC IIa fiber power after taper. These data show that marathon training decreased slow-twitch and fast-twitch muscle fiber size but that it maintained or improved the functional profile of these fibers. A taper period before the marathon further improved the functional profile of the muscle, which was targeted to the fast- twitch muscle fibers.