INFLUENCE OF THE MENSTRUAL-CYCLE AND ORAL-CONTRACEPTIVES ON THERMOREGULATORY RESPONSES TO EXERCISE IN YOUNG-WOMEN

Thermoregulatory responses to exercise in relation to the phase of the menstrual cycle were studied in ten women taking oral contraceptives (P) and in ten women not taking oral contraceptives (NP). Each subject was tested for maximal aerobic capacity (VO2max) and for 50% VO2max e xercise in the follicular (F) and luteal (L) phases of the menstrual c ycle. Since the oral contraceptives would have prevented ovulation a q uasi-follicular phase (q-F) and a quasi-luteal phase (q-L) of the mens trual cycle were assumed for P subjects. Exercise was performed on a c ycle ergometer at an ambient temperature of 24-degrees-C and relative air humidity of 50%. Rectal (T(re)), mean skin (T(sk)BAR), mean body ( T(b)BAR) temperatures and heart rate (f(c)) were measured. Sweat rate was estimated by the continuous measurement of relative humidity of ai r in a ventilated capsule placed on the chest, converted to absolute p ressure (PH2O(chest)). Gain for sweating was calculated as a ratio of increase in PH2O(chest) to the appropriate increase in T(re) for the w hole period of sweating (G) and for unsteady-state (G(u)) separately. The VO2max did not differ either between the groups of subjects or bet ween the phases of the menstrual cycle. In P, rectal temperature thres hold for sweating (T(re,td)) was 37.85-degrees-C in q-L and 37.60-degr ees-C in q-F (P<0.01) and corresponded to a significant difference fro m T(re) at rest. The T(re), T(sk)BAR, T(b)BAR and f(c) increased simil arly during exercise in q-F and q-L. No menstrual phase-related differ ences were observed either in the dynamics of sweating or in G. In NP, T(re,td) was shorter in L than in F (37.70 vs 37.47-degrees-C, P<0.02 ) with a significantly greater value from T(re), at rest. The dynamics and G for sweating were also greater in L than in F. The G(u) was 36. 8 versus 16.6 kPa.degrees-C-1 (P<0.01) while G was 6.4 versus 3.8 kPa. degrees-C-1 (P<0.05), respectively. The T(re), T(sk)BAR, T(b)BAR and f (c) increased significantly more in phase F than in phase L. It was co ncluded that in these women performing moderate exercise, there was a greater temperature threshold and larger gains for sweating in phase L than in phase F. Intake of oral contraceptives reduced the difference s in the gains for sweating making the thermoregulatory responses to e xercise more uniform.