The present invention relates to a composition for muscle function improvement or exercise ability enhancement, the composition including kirenol, a Siegesbeckia herba extract, or a fraction of the Siegesbeckia herba extract as an active ingredient. Kirenol, the Siegesbeckia herba extract containing kirenol, or the Siegesbeckia herba extract fraction according to the present invention has an excellent effect of increasing muscle mass by increasing the protein expression of p-mTOR, which is a major gene concerned with muscle functions. Also, kirenol, the Siegesbeckia herba extract containing kirenol, or the Siegesbeckia herba extract fraction according to the present invention has an effect of remarkably enhancing exercise ability by increasing the protein expression of PGC-1α, which is a major gene concerned with exercise ability. In addition, the present invention is a natural product, and thus is safe for use without side effects and can be used as a medicine or food.
