The study's statistical population included female athlete students in Tehran, Iran. Statistical software (G*Power software vs. 3.1) was used to calculate the sample size. Given the study repeated measured ANOVA (1 group and three repeated measures), a medium overall effect size f = 0.25, an α-error = 0.05, and a desired power (1-ß error) = 0.8, the total sample size resulted in thirty-five participants. A total of 37 female athletes participated in the study. Table 1 summarizes the individuals' demographic characteristics. The subjects were selected based on the purposive convenience sampling method among students with a three-year continuous activity in one of the high-risk sports for ACL injury (futsal, volleyball, basketball, handball, and martial arts). The inclusion criteria were female athletes aged 18–25 years, undergraduate students, and a maximum menstrual cycle duration of 30 days.
Further, the other criteria can be mentioned as having no injury in the lower limb, not suffering from premenstrual syndrome, and not consuming contraceptives, dietary supplements and steroid drugs. The exclusion criteria included experiencing premenstrual syndrome due to nervous pressure or disease, taking hormonal medicine, lower-limb injury during the study, disruption in the menstrual cycle, and inappropriate mental conditions to perform strength tests (14, 18). Before the study began, all subjects were notified of the potential risks and benefits of participating in the research. All subjects signed an informed consent form to participate in the project. Before the start of the study, the Ethics Committee of Shahid Beheshti University approved the study. The study followed the Helsinki Declaration recommendations of Human Ethics in Research.
Demographic characteristics of the subject (mean ± standard deviation)
Body mass (kg)
The present descriptive causal-comparative study was based on a single-group design. In this regard, the isometric and isokinetic strength of all subjects' knee joint flexor and extensor muscles was measured in three stages during the different phases of the menstrual cycle on an isokinetic system at an angular velocity of 60˚/s.
After selecting the eligible athletes among the statistical population, they were invited to attend the pathology laboratory at the Faculty of Sport Science and Health of Shahid Beheshti University at the appointed time. The individuals who participated in the laboratory were given full explanations of how to conduct tests. They were first subjected to anthropometric measurements.
The data were collected during a one-month process according to the menstrual cycle of the subjects. Each student attended the laboratory three times at the follicular (1–9 days), ovulatory (10–14 days), and luteal (15–28 days) phases (14). It is worth noting that the exact timing of the phases was obtained through interviewing the individuals and their self-reporting about their cycle phases. The Biodex system 4 isokinetic dynamometer is a reliable instrument with high intra class correlation (ICC) (37) and was utilized to assess the isometric and isokinetic strength of the flexor and extensor muscles of the dominant leg, as well as the ratio of the two strengths at the angular velocity of 60˚/s (6). The student performed a 15-minute warm up on a cycle ergometer at desired speed at each session. Afterwards, participants were asked to sit on the device's seat to perform the isokinetic tests (Fig. 1). The seat back was regulated at 70–85˚, and the rotation axis of the device arm was exactly placed in front of the center of the lateral epicondyle of dominant-leg thigh. After fixing the trunk and thigh to the seat, the upper part of the lateral malleolus was connected to the system's rotation axis using special cushions. The range of motion was 0–90˚, and the test velocity was set at 60 ˚/s (6). The procedure mentioned above was repeated in the same way in the three measurements, followed by comparing the mean data of the three phases. Additionally, the evaluations of all three phases were conducted at a fixed time (10–12 a.m.) to minimize the adverse effect of daily activities on the results (18).
The Biodex 4 system is a reliable instrument with high intra class correlation (ICC) for collecting the isokinetic strength data of knee joint muscles (6, 19). The subjects performed a 15-minute warm-up on a cycle ergometer at desired speed at each session. Afterwards, participants were asked to sit on the device's seat to perform the isokinetic tests (6). The seat back was regulated at 70–85˚, and the rotation axis of the device arm was exactly placed in front of the center of the lateral epicondyle of dominant-leg thigh. After fixing the trunk and thigh to the seat, the upper part of the lateral malleolus was connected to the system's rotation axis using unique cushions. The range of motion was 0–90˚, and the test velocity was set at 60 ˚/s (6).
Further, the female students executed knee flexion and extension movements several times before the primary test to be familiar with the test. Then, the quadriceps and hamstring muscles were contracted in three stages with a 20-second rest interval at an angular velocity of 60˚/s. These contractions were performed as an eccentric-concentric cycle in the muscles. The functional ratio of the muscle isokinetic strength was calculated by dividing the maximum torque of the hamstring muscle during the eccentric contraction by that of the quadriceps one during this contraction (20).
Regarding the isometric strength, the dynamometer was fixed at an angle of 60˚ based on the device manual. The isometric strength of the hamstring and quadriceps muscles was examined by performing three five-second contractions with a 20-second rest interval. The muscle strength was expressed in Nm according to the maximum generated torque (21).
The Leven test determined the homogeneity of variables in the research groups, and the data were normalized using the Shapiro–Wilk test. Then, inferential tests were executed. Changes between the three-time points were assessed using a repeated-measures analysis of variance (ANOVA), followed by the Bonferroni post hoc test for pairwise comparisons. Data analysis was performed using SPSS software (23.0; IBM SPSS Inc., Chicago, IL, USA), and the significance level was set at p < 0.05 at all stages.