Abstract
Background: Menstrual cycle is a major biological process for women during their reproductive years. Female sex hormones (estrogen and progesterone) fluctuate radically throughout the menstrual cycle. In women, there is a strong link between these fluctuating levels of hormones and their influence on muscle performance throughout the menstrual cycle. Estrogen has a beneficial effect on muscle efficiency. The decrease in muscle strength and early fatigability in muscle coincides with decrease in estrogen in females during certain periods of menstrual cycle, indicating the need for special care to avoid injuries during those periods. In the present study, the duration of the onset of muscle fatigue was assessed and compared to study the muscle performance during the four different phases of menstrual cycle in females of 18–25 years for 3 consecutive months.

Aims and Objectives: This study aims to assess the duration of the onset of muscle fatigue (seconds) using surface electromyography and to compare the duration of the onset of muscle fatigue during four different phases of menstrual cycle for 3 consecutive months.

Materials and Methods: Fifty apparently healthy female subjects selected for the study. Duration of the onset of fatigue in muscle was assessed on the following days of the menstrual cycle for 3 consecutive months: F1 phase (2nd day), F2 phase (12th day), L1 phase (20th day), and L2 phase (26th day). LH kit (luteinizing hormone (LH)-ovulation predictor) was used to confirm for the occurrence of ovulation during that particular menstrual month. Onset of the duration of fatigue was recorded using surface EMG on the right/left biceps muscle out. A total of six comparisons were made between F1 and F2, F1 and L1, F1 and L2, F2 and L1, F2 and L2, and L1 and L2. Student’s t-test (two tailed, dependent) has been used to find the significance of study parameters on continuous scale within each phase of menstrual cycle.

Results: Onset of the duration of fatigue (seconds) significantly occurred late during the F2 (71.68 ± 15.61, 70.26 ± 14.68, and 68.06 ± 12.79) followed by L1 phase (59.02 ± 12.92, 59.22 ± 11.49, and 62.02 ± 11.05) and the onset of fatigue significantly occurred early during L2 (42.02 ± 8.81, 39.24 ± 8.83, and 39.42 ± 6.66) and F1 phases (37.58 ± 9.21, 37.26 ± 8.64, and 35.70 ± 5.86) of all the 3 months with P < 0.001.

Conclusions: Muscle performance was highest with duration of the onset of fatigue occurring late in muscle during F2 phase of menstrual cycle which corresponds with estrogen peak during ovulation, followed by L2 phase wherein there is another estrogen peak after ovulation during early luteal phase. The muscle performance declined as fatigue occurred very early during F1 and L2 phase of menstrual cycle as the estrogen levels decline accordingly. Estrogen has a positive effect on muscle tissue, with its two peaks occurring in its concentration levels during the pre-ovulatory and post-ovulatory (midluteal) phase of menstrual cycle and decline in muscle efficiency with drop in its concentration levels during the menses and premenstrual phase.

Key words: Menstrual Cycle; Duration of Onset of Fatigue in Muscle; Surface Electromyography; Estrogen