The relationship of forearm flexion isometric strength, endurance, and fiber composition; and the effect of heating.

Forearm flexion isometric strength and endurance and fiber composition of the biceps brachii were examined in ten college-age men. The isometric endurance exercise regimens were assessed at room temperature and in a heated water bath at 45 degrees C. On each session four baseline isometric strength trials (MVCs) were measured followed by three isometric endurance bouts with 11 min rest between bouts. Post-heating MVCs were assessed prior to the endurance bouts. Two types of isometric endurance times were measured: holding a weight amounting to (1) 50% of maximal strength (MVC) and (2) 80% MVC. The heat treatment significantly reduced both the baseline MVCs and the 50% and 80% MVC endurance times. Pre- and post-heating MVC measures did not correlate significantly with fiber type but did correlate with body weight and biceps girth. In the nonheated condition, endurance times were unrelated to fiber type or body size. However, in the heated condition, both 50% and 80% endurance times correlated negatively with percent fast-twitch fiber area, implying a selective influence of heat on muscle fiber types.