Effects of the menstrual cycle on respiratory muscle function.

To investigate the effect of the menstrual cycle on respiratory muscle function, respiratory muscle and pulmonary functions were measured and compared in the midfollicular phase and in the midluteal phase of the menstrual cycle in 30 healthy adult women. Respiratory muscle strength was indicated by maximal static inspiratory and expiratory pressures. Inspiratory muscle endurance was determined by the product of pressure load and the sustained time, i.e., pressure-time index, while the subject breathed against an inspiratory pressure load on a modified Nickerson-Keens device. The results showed that the inspiratory muscle endurance was greater in the midluteal phase than in the follicular phase (815 +/- 43 versus 649 +/- 62 cmH2O.min, respectively), whereas the respiratory muscle strength and pulmonary function were unchanged. We conclude that inspiratory muscle endurance is affected by the menstrual cycle, i.e., higher in the midluteal phase and lower in the midfollicular phase.