An Examination of Changes in Muscle Thickness, Isometric Strength, and Body Water Throughout the Menstrual Cycle

Women are often excluded from research studies interested in changes in muscle size and strength due to confounding influence that the menstrual cycle may have on these variables. Although there is considerable data showing that strength may fluctuate throughout the menstrual cycle, the thesis that muscle size changes throughout the cycle is based on the premise that changes in body water throughout the menstrual cycle may influence the size of the muscle. Despite this suggestion there is no experimental data demonstrating that the menstrual cycle has any influence on muscle size. Purpose: The purpose of this study is to examine the changes in muscle size and strength throughout the menstrual cycles in females and to compare these values to those to a control group of time-matched males. Methods: 12 males and 16 females visited the laboratory on four separate occasions. The first visit consisted of paperwork, familiarization with cycle tracking and isometric strength assessment. During the second visit, which was 1-7 days after the first day of the menstrual phase, measurements of muscle thickness, isometric strength, and body water were recorded. The third visit took place during the ovulation phase. This phase typically occurs between 6-10 days after menstruation, ovulation was predicted through the Flo App. Visit four took place during the luteal phase which typically occurs between days 22-25 of the menstrual cycle. Males scheduled their second visit 1-7 days following visit 1. This visit represented the “menstrual phase” for male participants. Hereafter, male participants followed a mock menstrual cycle via the Flo app. During all visits, muscle thickness, isometric strength of the biceps and body water composition were measured. In addition to this, the participants were asked to complete 4 sets of biceps curls at 70% of 1RM to volitional failure, in one arm during visits 2,3 and 4. Additional measures of muscle thickness were taken following exercise. All participants were instructed to continue normal eating and water consumption habits during this study. A food log was used to ensure that the same food and drink are consumed 24 hours prior to each visit. Results: For muscle thickness there was no interaction (p = 0.73). In addition, there was no main effect for time (p = 0.93). However, there was a main effect for sex. Males had higher muscle thickness values at all time points compared to females (p \u3c 0.001). For acute changes in muscle thickness (swelling)there was no interaction (p = 0.28). In addition, there was no main effect for time (p = 0.12). However, there was a main effect for sex. Males had higher acute changes in muscle thickness all time points compared to females (p \u3c 0.001). For total body water there was no interaction (p = 0.66). In addition, there was no main effect for time (p = 0.97). However, there was a main effect for sex (p \u3c 0.001). Males had greater total body water at all time points compared to females. For isometric strength there was no interaction (p = 0.23). In addition, there was no main effect for time (p = 0.73). However, there was a main effect for sex. Males had higher isometric strength values at all time points compared to females (p \u3c 0.001). Conclusions: Phase of the menstrual cycle does not appear to influence muscle thickness, isometric strength or total body water