Fitts, R. H., Costill, D. L., & Gardetto, P. R. (1989). Effect of swim exercise training on human muscle fiber function. Journal of Applied Physiology, 66, 465-475.

The effects of a typical collegiate swim-training program and an intensified 10-day training program on deltoid muscle fibers were evaluated.

A 10-week training program produced an almost twofold increase in the mitochondrial marker enzyme, citrate synthase, in the muscle fibers. Peak tension of single fibers was not altered by either the usual or intensified training experiences. No differences in peak tension were observed between type I (slow-twitch) fibers or type II (fast-twitch) fibers. Fast-twitch fibers contracted almost five times faster than slow-twitch fibers.

Normal training increased the contractile speed of the slow-twitch fibers and decreased that of the fast-twitch fibers. The increased training load further and significantly decreased the contractile speed of the fast-twitch fibers. After a period of detraining, both fiber types returned to control/normal levels.

Implication. Normal/tolerable swimming training does not alter the force-velocity relation in either type I or II fibers. Intensified training reduces the contractile velocity of type II fibers further. The harder a swimmer trains, the poorer will become sprint-swimming performances. Tapering will produce a recovery from depressed sprinting performances.

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