The relationship of static muscle function to use of oral contraceptives

The handgrip endurance time (ET) and total force output (FO) at 50% maximum voluntary contraction (MVC) of 26 females (18–33 yr) were measured during the follicular phase (day 8 ± 2) and luteal phase (day 21 ± 3) of one menstrual cycle. Three groups were utilized: 1) normals (non–users of oral contraceptive agents [OCA]); 2) users of OCA only (users of OCA but not vitamin B–6); and 3) users of OCA and vitamin B–6. Vitamin B–6 status was assessed by measurement of 24–h urine xanthurenic acid (XA) excretion following a 2–g loading dose of L–tryptophan. Recent vitamin B–6 intake was assessed by measurement of 24–h urine 4–pyridoxic acid (4–PA) excretion. There were no significant among-group differences in MVC, however, MVC during the follicular phase was significantly (P<0.05) greater than during the luteal phase. The ET and FO of normal subjects was significantly (P<0.05) greater than that of the combined group of users of OCA only, and users of OCA and vitamin B–6. There was no significant difference between users of OCA only and users of OCA and vitamin B–6 in either ET or FO, nor between the follicular and luteal phases of the menstrual cycle. Levels of 4–PA confirmed that the subjects who reported being users of OCA and vitamin B–6 had substantially larger intakes of vitamin B–6 than did the other subjects. Levels of XA indicated that only two subjects, both users of OCA only, were vitamin B℃6 deficient. Compared to other users of OCA only, the mean MVC of the vitamin B℃6 deficient subjects was lower, the mean ET higher, and the mean FO approximately equal. Therefore, we have concluded that users of OCA appear to exhibit lower static muscle endurance than normal subjects, and their vitamin B–6 status was not a mediating factor.