Among individuals of comparable size with normal lungs, short-term breathing capacity varies widely for unknown reasons. If the velocity of ventilatory muscle contraction limits airflow, then persons with a high proportion of fast twitch fibers in the ventilatory muscles should have large ventilatory capacities. As one test of this hypothesis, we correlated ventilatory capacity with vertical jump, an index of leg and thus of diaphragmatic muscle fiber composition. We found significant positive correlations between vertical jump and the 12-second maximal voluntary ventilation in both women (n =19, r = 0.6; p < 0.01) and men (n = 25; r = + 0.4; p = 0.06). Both correlations were independent of body weight or vital capacity. These results are consistent with the hypothesis that short-term ventilatory capacity is influenced by the contractile characteristics of the ventilatory muscles.