CHANGES IN MUSCLE STRENGTH, RELAXATION RATE AND FATIGUABILITY DURING THE HUMAN MENSTRUAL-CYCLE

1. The effect of the different phases of the menstrual cycle on skelet al muscle strength, contractile properties and fatiguability was inves tigated in ten young, healthy females. Results were compared with a si milar group on the combined (non-phasic) oral contraceptive pill (OC). Cycle phases were divided into the early and mid-follicular, midcycle (ovulatory) and mid- and late luteal. Circle phases were estimated fr om the first day of the menstrual bleed. 2. Subjects were studied week ly through two complete cycles. Measurements included quadriceps and h andgrip maximum voluntary isometric force and the relaxation times, fo rce-frequency relationship and fatigue index of the quadriceps during percutaneous stimulation at a range of frequencies from 1 to 100 Hz. 3 . In the women not taking the OC there was a significant increase of a bout 11% in quadriceps and handgrip strength at mid-cycle compared wit h both the follicular and luteal phases. Accompanying the increases in strength there was a significant slowing of relaxation and increase i n fatiguability at mid-cycle. No changes in any parameter were found i n the women taking the OC. 4. The changes in muscle function at mid-cy cle may be due to the increase in oestrogen that occurs prior to ovula tion.