Cortisol levels before and after isometric contractions are not influenced by the menstrual cycle

Low-intensity, long-duration isometric contractions produce significant exercise-induced analgesia. There are also sex differences in pain perception before and after isometric contractions. To understand the mechanisms involved in the sex difference of exercise-induced analgesia, we investigated the influence of cortisol levels during different phases of the menstrual cycle before and after low-intensity and long-duration isometric contractions. Twenty female subjects (20.9 +/- 1.0 years) participated in three experimental sessions that were separated by ~one week. In session 1, subjects were familiarized to the pain device, instructed on how to use the ovulation kit, and performed 3 maximal voluntary contractions (MVC). Sessions 2 and 3 were randomly assigned according to the subject's phase of the menstrual cycle. One session took place during the mid-follicular phase (5-8 days past menses) and the other session during the mid-luteal phase (7-9 days past ovulation). During these last two sessions, pressure-induced pain ratings and thresholds were assessed before and after a sustained isometric contraction at 25% MVC force held until task failure. Salivary cortisol levels were measured at four different time points: 1) baseline, 2) 20 min following the first pain test, 3) immediately after the isometric contraction, and 4) 20 min following the static contraction. Pain perception and salivary cortisol levels did not differ during the different phases of the menstrual cycle (p> 0.05). Pain thresholds increased and pain ratings decreased following the performance of the static contractions (p= 0.0001 and 0.001, respectively). Salivary cortisol levels did not change following the pain test or the isometric fatiguing contraction (p> 0.05). There was no association between the absolute or relative changes in cortisol levels with pain ratings or pain threshold (p> 0.05). In conclusion, changes in cortisol levels are not associated with exercise-induced analgesia or phases of the menstrual cycle. Supported by a grant from APS (MHB).