Muscle Strength and Endurance Do Not Significantly Vary Across 3 Phases of the Menstrual Cycle in Moderately Active Premenopausal Women *†Cecilia Fride ´n, RPT, ‡Angelica Linde ´n Hirschberg, MD, PhD, and *To ¨ nu Saartok, MD, PhD *Section of Sports Medicine, Division of Surgical Sciences, Karolinska Institutet, †Stockholm University College of Physical Education and Sports, and ‡Department of Obstetrics and Gynaecology, Karolinska Hospital, Stockholm, Sweden. Objective: To investigate muscle strength and muscle en- durance in women during 3 well-determined phases of the men- strual cycle: early follicular phase, ovulation phase, and mid- luteal phase. Design: Prospective, within-woman analysis was per- formed of muscle strength and muscle endurance by repeated measures analysis of variance in 3 hormonally verified phases of 2 consecutive menstrual cycles. Participants: Fifteen female subjects with moderate physi- cal activity level and regular menstrual cycles volunteered to participate in the study. Analyses are based on 10 subjects who completed 2 consecutive menstrual cycles with hormonally verified phases. Main Outcome Measurements: Handgrip strength, 1-leg hop test, isokinetic muscle strength, and muscle endurance were measured in 2 consecutive menstrual cycles in the early follicular phase, in the ovulation phase, and in the midluteal phase. Isokinetic muscle strength and endurance were tested with knee extension exercise on a standard instrument. Men- strual cycle phases were determined by analysis of sex hor- mone levels in serum, and ovulation was detected by luteiniz- ing hormone surge in urine. Results: No significant variation in muscle strength or muscle endurance could be detected during different well- determined phases of the menstrual cycle. Conclusions: This study detected no significant variation in muscle strength and muscle endurance during the menstrual cycle. In contrast to other studies showing variations in strength and endurance during the menstrual cycle, the present study was hormonally validated and was repeated in 2 consecutive menstrual cycles. However, it is unknown whether these data in moderately active university students would be relevant to the highly trained woman athlete. Key Words: estrogen, progesterone, hormones, muscle per- formance Clin J Sport Med 2003;13:238–241. INTRODUCTION The influence of the female sex steroids (estrogens and progesterone [P-4]) on muscle strength, muscle en- durance, and performance has lately been discussed. It has been suggested that estrogen has an anabolic effect on the muscle, while P-4 possibly has a catabolic effect. 1 However, it is still not clear how the hormonal fluctua- tions during the menstrual cycle influence muscle strength and muscle endurance. Davies et al 2 reported an increase in explosive muscle strength during the early follicular phase (cycle day 1–4, low levels of estradiol [E 2 ] and P-4) compared with the late follicular phase (cycle day 12–14, high levels of E 2 ) and the luteal phase (cycle day 19–21, high levels of E 2 and P-4). The men- strual cycle phases were determined by counting days in the menstrual cycle. The authors speculated that E 2 and P-4 inhibit skeletal muscle performance, whereas low levels of these female sex steroids during the early fol- licular phase enhance muscle strength. In contrast, other reports have suggested that muscle strength is increased by E 2 . Phillips et al 3 recorded an increase in isometric muscle strength of the adductor pollicis muscle around ovulation (cycle day 14). Another study by the same group 4 related low levels of E 2 to the decrease of muscle strength in postmenopausal women and showed that this decrease may be prevented by hor- mone replacement therapy. Sawar et al 5 confirmed this effect of estrogen in a study measuring maximum vol- untary isometric force in quadriceps and handgrip during the menstrual cycle. They showed significantly higher values of these parameters during ovulation (cycle day 12–18) compared with the other phases. In this study, they also showed a significantly slower muscle relax- ation time and an increase in muscle fatigability during ovulation compared with the other phases. 5 The phases of the menstrual cycle were determined by counting days in the studies of Phillips et al 3 and Sawar et al. 5 No hormonal analyses were performed. Received for publication September 2002; accepted January 2003. Reprints: Cecilia Fride ´n, RPT, Division of Surgical Sciences, Sec- tion of Sports Medicine, Karolinska Hospital, SE-171 76 Stockholm, Sweden. E-mail: Cecilia.friden@telia.com Supported by the Swedish National Center for Research in Sports and Swedish Medical Research Council (No. 13142), Karolinska In- stitutet. Clinical Journal of Sport Medicine, 13:238–241 © 2003 Lippincott Williams & Wilkins, Inc., Philadelphia 238