Changes in the contractile properties of both type I and IIa (fast-twitch oxidative) fibers of the deltoid muscle in swimmers (N = 6) were observed prior to and following a 21 -day taper. - Swimming times improved by an average of 4%. - There was no change in type I fiber diameter but type IIa fibers were 7% larger. - Peak tension did not change in type I fibers but increased by 15% in type IIa fibers. - Shortening velocity increased in both sets of fibers by 37% and 55% in type I and IIa fibers respectively. - Type I fiber power was unaltered but type IIa increased by 114%. Type IIa fibers are more affected by a taper than are type I fibers. Implication: This is basic physiological evidence to support Costill's often said proposal that taper affects power more than it does endurance. Since this study only assessed one feature of a taper it should not be concluded as being the only feature to change.
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