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Measuring Performance during the Menstrual Cycle: A Model Using Oral Contraceptives

REDMAN, LEANNE M.1; WEATHERBY, ROBERT P.2

Medicine & Science in Sports & Exercise: January 2004 - Volume 36 - Issue 1 - p 130-136
doi: 10.1249/01.MSS.0000106181.52102.99
APPLIED SCIENCES: Physical Fitness and Performance
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REDMAN, L. M., and R. P. WEATHERBY. Measuring Performance during the Menstrual Cycle: A Model Using Oral Contraceptives. Med. Sci. Sports Exerc., Vol. 36, No. 1, pp. 130–136, 2004.

Purpose The aim of the study was to use a low-dose oral contraceptive (OC) pill to generate consistent estrogen and progestogen concentrations and investigate the relationship between steroid hormone concentrations during the OC cycle and anaerobic performance.

Methods Five female rowers taking a low-dose OC performed tests of anaerobic power (10-s all-out effort) and capacity (1000-m row) on the Concept IIC rowing ergometer at two time points in each of three OC cycles. These time points corresponded to high estrogen and high progestogen (pill day 16–18; TDH) and low estrogen and low progestogen (pill day 26-28; TDL). Blood samples were collected at rest and postexercise for the quantification of 17β-estradiol (E2), progesterone (P4), glucose, triglyceride, and lactate concentrations.

Results Endogenous E2 and P4 concentrations were not significantly different between testing days or OC cycles (P > 0.05). Peak power output was higher (P < 0.05) and 1000-m rowing ergometer time faster (P < 0.05) at TDL. Pre- and postexercise glucose concentrations were increased (P < 0.05) at TDL, whereas rest and postexercise plasma triglyceride concentrations were significantly (P < 0.05) lower during this time.

Conclusions This study found that alterations in anaerobic performance throughout the OC cycle occurred with improved performances corresponding to low estrogen and progestogen concentrations. The OC provided a consistent hormonal milieu reducing inter- and intra-individual variations in sex steroids and standardized all performance and metabolic variables across each OC cycle tested. Given that OC use has a high prevalence among female athletes and provides a controlled hormonal environment, it serves as a good model in which the acute effects of female sex steroids on exercise performance can be studied.

1Exercise Physiology Laboratory, Department of Physiology, University of Adelaide, Adelaide, SA, AUSTRALIA; and

2School of Exercise Science and Sport Management, Southern Cross University, Lismore, NSW, AUSTRALIA

Address for correspondence: Leanne M. Redman, Department of Biological Sciences, Ohio University, Athens, OH 45701-2979; E-mail: redman@ohio.edu.

Submitted for publication January 2003.

Accepted for publication September 2003.

Although the potential impact of the menstrual cycle phase on aerobic performance has been the focus of several investigations (2,12,19), the potential effect of the menstrual cycle on anaerobic performance has received little attention. There is no consensus on whether there is a relationship between hormone changes during the menstrual cycle and the level of anaerobic performance. Some studies have concluded that anaerobic performance is unaffected by cycle phase (10,11,23,26,31), whereas others have reported menstrual cycle phase differences in anaerobic performances (5,9,28,34). Both Brooks-Gunn et al. (5) and Davies and coworkers (9) found performance to be improved during menstruation, whereas an earlier study (34) found performance worse at this time point.

The lack of consensus may in part be due to a lack of adequate experimental controls and the wide variation in the types of methods used to determine the phase of the menstrual cycle (e.g., body temperature vs hormonal assay) and the timing of the exercise tests (e.g., menstruation vs early and/or mid-follicular phase vs ovulation vs mid and/or late luteal phase (LP)) during the menstrual cycle. The wide inter- and intra-individual variation in estradiol (E2) and progesterone (P4) concentrations within the natural menstrual cycle and varying lengths of the menstrual cycle between individuals make valid interpretations of previous investigations difficult. In addition, the subject selection criteria in some studies were not clearly defined (with regard to age, menstrual cycle history, fitness status, menstrual cycle length, and gynecological problems), and there has often been limited standardization of preexercise status including controlled dietary intakes and activity levels.

Estrogens and progestogens are known to influence several physiological processes such as regulation of energy metabolism (21), body water (14,33), respiration (25), and temperature (32) throughout the course of a menstrual cycle. Short duration, high intensity exercise, which is dependent on intramuscular stores of ATP, CP, and glycogen, and the simultaneous production of lactate could potentially be influenced by the level of ovarian steroid hormones. Estradiol has been shown to increase the relative contribution of fat metabolism during the LP of the menstrual cycle (4,7,18,23,35) and could therefore by limiting carbohydrate metabolism cause a restriction of anaerobic metabolism. However, it could also be postulated that the increased glycogen storage recorded in response to exogenous estrogens (21) and during the LP of the menstrual cycle (when the levels of estradiol are high) (16,17) could result in increased substrate availability enhancing the anaerobic production of ATP and thereby increasing anaerobic performance. Additionally, given the antagonistic relationship of progesterone to aldosterone at the aldosterone receptor (33), and that falling levels of progesterone on transition from the luteal to follicular phases results in fluid and electrolyte retention (14), one could expect an improvement in buffering capacity and a greater capacity for anaerobic performance at this time point.

Oral contraceptives (OC) suppress normal menstrual cycle levels of E2 and P4 by inhibiting the pituitary secretion of gonadotropins (FSH and LH) (30) and provide consistent pharmacological control of the reproductive cycle by systematically controlling concentrations of endogenous sex hormones. Thus, the inter- and intra-individual variations in circulating endogenous sex hormone concentrations typical within the menstrual cycle of the non-OC user can be limited. A multi-phasic OC pill contains a changing dosage of both an estrogen and progestogen throughout three phases of the OC cycle. This reliable pattern of administration begins with 21-d “active” steroid administration followed by 7-d nonsteroid administration in which a placebo is given (Fig. 1). Given that endogenous production of E2 and P4 is suppressed during OC usage, the serum concentration of active sex steroids are directly related to the OC dosage administered (13).

FIGURE 1

FIGURE 1

Despite the wide incidence of OC pill use in the community since its development in the 1950s, reports in the early 1980s indicated that OC pill use was not as predominant among athletic females (20). However, since then, with the introduction of the lower-dose preparations, it is estimated that OC pill use in female athletes is more widespread and matches the prevalence of OC use within the general community (29).

There is a growing body of evidence in support of the notion that aerobic capacity is impaired by OC pill use (8,23,27), which has not be explained by changes in stroke volume, muscle blood flow, or hemoglobin levels but could be due to blunting of the sympathetic nervous system by the high ovarian hormone concentrations characteristic of OC use (8). It is interesting, given these findings, that few studies have sought to determine the impact of the OCP on anaerobic performance.

To better understand the relationship between the menstrual cycle and anaerobic performance, this study used a low-dose OC pill to pharmacologically generate consistently low and high circulating equivalent E2 and P4 concentrations. It was hypothesized that anaerobic performance would be altered in cyclic patterns throughout the OC cycle with the best performances observed when the level of exogenous hormone administration was low and these levels of female sex steroids might facilitate carbohydrate metabolism and improve buffering capacity.

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METHODS

Approach to the problem and experimental design.

Research concerned with examining the impact of steroid hormones on exercise performance has been complicated by wide inter- and intra-individual variations of endogenous estrogen and progesterone concentrations within the natural course of the human menstrual cycle. Further, inconsistent menstrual cycle lengths make the accurate determination and validation of exercise testing days between subjects almost impossible. Triphasic OC are combined estrogen-progestogen preparations whereby the regime consists of three phases, each with a different progestogen dose and some with increased estrogen in the second phase. Triphasic OC closely mimic the endogenous or natural menstrual cycle given the regular changes in hormone concentrations. Because the OC pill regains control over the hypothalamic-pituitary-ovarian axis and the amount of the “active” circulating sex steroids are related to the daily dosage administered (13), the triphasic OC pill would be a reliable experimental model to use in these studies where the aim is to assess the impact of female sex steroids on performance outcomes.

The aim of this study was to use a triphasic OC pill to pharmacologically generate consistently low and high levels of estrogens and progestogens to study the impact of low and high ovarian steroid concentrations on anaerobic performance. To assess the efficacy of the OC as the model for this research, anaerobic performances were assessed at two hormonally different time points in each of three consecutive OC pill cycles. The independent variables for this study were level of exogenous hormone concentration (high E and high P vs low E and low P) and OC pill cycle (cycle 1, cycle 2, and cycle 3), whereas the dependent variables were anaerobic performance (power and capacity) and biochemical indicators of metabolism (glucose, triglycerides, and lactate).

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Subjects.

Elite and sub-elite female rowers (18–25 yr) were invited to participate in this study; all had a O2max > 50 mL·kg−1·min−1 and were currently competing at an Australian national or state level, and had done so for the previous 2–6 yr. All subjects were in the same rowing squad and were completing a preparatory-training phase at the time of the study that involved six to eight rowing sessions (100–150 km) per week as well as five to six additional sessions of cross-training (e.g., weights and cycling). Subjects were required to have a gynecological age greater than 3 yr and to have been taking a low-dose OC pill continuously throughout the past 12 months. Twelve potential participants completed a subject selection questionnaire that assessed gynecological health, general health, and OC history, and six women taking Triphasil-28 or equivalent (hormonally similar to 50- to 125-μg levonorgestrel and 30- to 40-μg ethinylestradiol) were chosen to participate in the study. Subjects did not smoke, were free from illness and/or medication, dietary interventions, and menstrual abnormalities. Physical characteristics of the subject group are displayed in Table 1. Before the commencement of the study, one subject was excluded due to illness and a subsequent antibiotic that presented potential contraindications to exercise testing.

TABLE 1

TABLE 1

Subjects (N = 5) were required to continue taking the OC pill at the same time each day as specified for OC usage. The number of hours from pill ingestion to the exercise test was estimated to be between 2 and 8 h but was constant for each subject. Subjects were informed of the nature of the study, testing protocols, possible risks, or discomforts and signed a written informed consent. The Human Research Ethics Committee of Southern Cross University approved the study.

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Experimental overview.

All subjects completed a test of anaerobic power (10-s all-out effort) and anaerobic capacity (1000-m row) at two different time points throughout the OC cycle. These time points corresponded to a day of low estrogen and low progestogen and a day of high estrogen and high progestogen. All performance tests were conducted on a rowing ergometer (Concept II Inc., Morrisville, VT) and were repeated for three consecutive OC cycles. Blood samples were collected at rest and postexercise for the quantification of glucose, triglyceride, and lactate concentrations.

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Determination of testing days.

Exercise tests were conducted at two hormonally different days during the OC cycle, and test days were chosen based on daily hormone dosages within the low-dose triphasic OC administration (Fig. 1). A day of high estrogen and high progestogen (TDH), pill days 16–18 and a day of low estrogen and low progestogen applicable to pill days 26–28 (TDL) were used. Exercise testing took place between 0800 and 1600 h on pill days 16 ± 1 (TDH) and 26 ± 0 (TDL) and time of day for each testing session was kept constant for each subject. Testing commenced at either TDH or TDL, depending on when the subject was recruited into the study and three subjects first commenced testing during TDH while two began at TDL.

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Preexercise controls.

On the day of testing, subjects reported to the laboratory following a 4-h fast (water only) having abstained from alcohol, caffeine, and strenuous physical activity for the previous 24 h. Subjects were required to keep a daily record of menstrual cycle symptomatology and physical activity profiles. A menstrual cycle diary was used to assess individual perception and presence of common menstrual cycle symptomatology such as menstrual bleeding, bloatedness/fluid retention, breast tenderness, headache, backache, and tiredness and to confirm OC compliance throughout the study. There were no reports of missed OC pills throughout the study, and subjects reported taking the pill at the same time each day (± 1.5 h). Daily physical activity was monitored using the training diary adapted from the Queensland Academy of Sport Performance Enhancement Center to assess physical activity, training volume, and intensity throughout the study and specifically in the 48 h before each experimental session. Preexercise dietary intakes were recorded 48 h before the first testing session. This diet was replicated in the 48 h preceding each subsequent testing session and was analyzed for compositions of carbohydrate, fat, and protein and total energy intake using SERVE Nutritional Management System for Windows (M. H. Williams Pty. Ltd, 1995).

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Anaerobic performance tests.

On the subjects’ arrival at the laboratory, the menstrual cycle diary, training diary, and dietary logs were collected. Anthropometric measures including body mass (Wedderburn Precision Scales, Australia) and skinfold thickness at nine sites: bicep, tricep, subscapular, mid-axilla, abdominal, supraspinale, suprailiac, anterior thigh, and medial calf (Harpenden Skinfold Caliper, British Indicators Ltd.) were recorded. After 5 min of seated rest, subjects’ blood samples were drawn for analysis of basal sex hormone concentrations (17β-estradiol and progesterone) and metabolite (plasma glucose, triglyceride and lactate) concentrations.

The two assigned anaerobic protocols were chosen for its sports specificity in these athletes and were designed to mimic different components of a 2000-m rowing event, a standard race distance at Australian and International Rowing Regattas. The 10-s all-out row replicates the explosive power executed by these athletes at the onset of a race, whereas the 1000 m was chosen to assimilate the standard 2000-m race distance. To avoid the likelihood of improved performances with learning, all subjects were familiarized with both exercise protocols before the commencement of the study and the order of testing, that is, TDL followed by TDH was completed in two subjects whereas TDH followed by TDL was completed in three subjects.

The test retest reliability between trials at TDH was determined by the coefficient of variation (CV). The CV for the peak power test was 2.2% and the likely range 1.3–6.4 (%), and the CV for the 1000-m row was 1.5% which ranged between 0.9 and 4.3 (%).

All exercise tests were performed on the Concept IIC rowing ergometer. After 10 min of seated rest, subjects completed a 10-s all-out row. Subjects were seated on the ergometer, and feet were secured to the foot stretcher. After two to three familiarization strokes and a 3-s countdown, subjects were required to exert maximal effort for 10 s or until peak power (W) was attained and performance (power output) declined. Power output (W) was recorded each stroke with the highest power output defined as the peak value.

After 10 min of rest, subjects were seated on the rowing ergometer and completed a 1000-m simulated row (anaerobic capacity test). Once the breathing apparatus and mouthpiece were fitted and feet were secured in the foot stretcher, subjects performed a few light strokes to ensure the comfort of the breathing equipment. The nose-clip was attached and the subject took hold of the ergometer handle in preparation for the start of the exercise and remained stationary for 1 min while resting data were collected. Throughout the 1000-m ergometer test open spirometry techniques were used to measure O2, CO2, and E (unreported data) (MedGraphics CPX/D™ gas analysis system, MedGraphics Corp. Minneapolis, MN). Heart rate was monitored continuously using a Polar electro sports tester unit (Kempele, Finland). Heart rate, stroke rate, and time (s) were recorded at each 100-m interval. Blood samples were drawn at rest and at several intervals postexercise (1.25, 2.5, 5, 7.5, and 10 min) for blood lactate measurements. At 5 min postexercise, a venous blood sample was extracted for a postexercise analysis of glucose and triglyceride concentrations.

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Blood sampling.

Before exercise venous blood samples were collected via a venipuncture of an antecubital vein (Venoject 21G × 1.5 needle, Terumo Corporation, Belgium) for evaluation of plasma 17β-estradiol, progesterone, glucose, and triglyceride and immediately postexercise for plasma glucose and triglyceride. Blood lactate concentration was determined from capillary blood samples collected from an arterialized earlobe (Finalgon®, Boehringer Mannheim) at rest and postexercise.

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Hormone and metabolic analyses.

Basal 17β-estradiol and progesterone concentrations were measured from a 5-mL venous blood sample collected in a plain-coated silicon tube. The sample was refrigerated (4°C) until the completion of the exercise session (1 h) when it was immediately taken to the Chemical Pathology Department at the Brisbane Mater Hospital for analysis. Hormones were analyzed by an automated immunoassay system (AIA-1200 TOSOH Medics Inc.) using commercially available immunoassay kits that have been validated for measurement in human serum (TOSOH Medics Inc.). The intra- and interassay coefficients of variation (CV) for estradiol ranged from 2.25 to 7.17% and 2.9 to 9.8%, respectively, and progesterone ranged from 6.5 to 9.2% and 4.5 to 9.1%, respectively (TOSOH Medics Inc.). Pre- and postexercise plasma glucose and triglyceride concentrations were determined from 4 mL of venous blood. The samples were collected in lithium heparin-coated tubes, immediately centrifuged, and the plasma stored at −18°C until the analysis was conducted (approximately 4 wk). Blood glucose and triglyceride concentrations were analyzed using the Kodak Ektachem DT60 model (Eastman Kodak Company) and DT Vitros slides for glucose and triglycerides (Johnson & Johnson Clinical Diagnostics). Fifty microliters of capillary blood was collected into a heparinized capillary tube and the blood lactate concentrations analyzed using the YSI2700 Lactate Analyzer (Yellow Springs Instruments).

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Statistical analysis.

Data are reported as mean ± SEM. All data were subject to descriptive statistics and repeated measures analysis of variance (RMANOVA) using SPSS for Windows version 8.0. A two-factor RMANOVA (cycle, test day, cycle × test day) were used to compare hormonal and performance responses over time. Three-factor RMANOVA (cycle, test day, pre/post, cycle × test day × pre/post) were used to compare metabolic variables pre- and postexercise and between testing days over time. Significance was assumed when α ≤ 0.05. The relationship between test and retest was determined using the coefficient of variation on the log transformation of raw data.

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RESULTS

Longitudinal monitoring.

There were slight variations in body mass noted throughout the study; however, there were no differences between cycle phase (TDH: 70.2 ± 1.8 kg, TDL: 70.3 ± 1.8 kg; P = 0.4). Skinfold thickness (sum of nine sites) was not significantly different between testing days (TDH: 155.1 ± 11.4 mm; TDL: 158.9 ± 12.6 mm; P = 0.091) or across the three OC cycles (P = 0.428). Training diaries confirmed that preexperimental training status was consistent throughout the study. There was no significant difference in preexperimental training distance rowed between testing days (TDH: 17.3 ± 0.9 km; TDL: 17.1 ± 0.8 km; P = 0.869) or training time (TDH: 78.3 ± 5.0 min; TDL: 76.6 ± 5.8 min; P = 0.823).

The presence and severity of menstrual symptoms varied considerably throughout the OC cycle. The highest prevalence of symptoms was reported from days 21–28 corresponding to menstrual bleeding and the placebo pill. The common symptoms reported at TDL were menstrual bleeding, abdominal pain, and bloatedness, whereas headache and tiredness were the most frequently reported symptoms at TDH. Given the large interindividual variation in perception of menstrual cycle symptoms, statistical analysis was not carried out on these data.

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Endogenous hormonal profile.

Endogenous 17β- estradiol and progesterone concentrations were not significantly different between testing days (17β-estradiol: TDH, 60.3 ± 10.1 pmol·L−1; TDL, 60.5 ± 7.6 pmol·L−1; P = 0.99; progesterone: TDH, 1.10 ± 0.21 nmol·L−1; TDL, 0.37 ± 0.14 nmol·L−1; P = 0.20) or across the three OC pill cycles tested.

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Biochemical responses.

A summary of the pre- and postexercise blood chemistry at TDH and TDL are included in Table 2. With TDL, the pre- and postexercise plasma glucose concentrations were found to be significantly higher (P < 0.01), 37% and 8%, respectively, than during TDH.

TABLE 2

TABLE 2

The triglyceride concentrations increased in response to exercise at both hormonal situations (Table 2, P < 0.01). The pre- and postexercise triglyceride concentrations at TDH were significantly greater than (P < 0.01) triglyceride concentrations at TDL (31% and 8%, respectively). No significant difference in triglyceride concentrations at TDH and TDL between the OC cycles was shown (P = 0.57).

Blood lactate concentrations at TDH and TDL are displayed in Table 2. Although blood lactate concentrations significantly increased in response to anaerobic exercise (P = 0.00) at both time points, there was no significant difference in concentrations between TDH and TDL (P = 0.16) or between OC cycles (P = 0.45).

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Anaerobic performance.

The relationship between peak power and hormone status is illustrated in Figure 2. The average peak power was greater at TDL (448.73 ± 5.80 W) compared with TDH (433.07 ± 5.28 W). The RMANOVA revealed a significant difference in peak power scores between TDL and TDH (P < 0.05), and results were consistent across all OC cycles. During each OC pill cycle tested the 1000-m simulated row times (Fig. 3) were faster (P < 0.05) at TDL (226.5 ± 1.3 s) than TDH (230.6 ± 1.4 s).

FIGURE 2

FIGURE 2

FIGURE 3

FIGURE 3

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DISCUSSION

This study found that OC administration consistently suppressed circulating endogenous sex steroid hormone concentrations in OC users to clinical deficiency levels (30). The OC synthetically generated high or low hormone environments similar to the normal menstrual cycle to enable performance changes during the menstrual cycle to be studied, and therefore the only effects seen were from the administration of the OC hormones.

Anaerobic performance was not constant during the triphasic OC cycle with improved performances noted at TDL (pill days 26–28) in each of the three OC cycles tested. Increased performances during this time point were associated with significantly lower rest and postexercise triglyceride concentrations, higher glucose concentrations, and a tendency toward lower lactate concentrations after exercise compared with TDH (pill days 16–18).

These results indicate that, provided the OC is taken as directed, it could be used as an experimental model creating a more controlled environment for this area of study. Orally administered sex steroids act to indirectly reduce the natural production of estrogens and progestogens allowing the menstrual cycle to be manipulated by daily exogenous hormone concentrations. This “synthetic” menstrual cycle as shown in this study is less susceptible to inter- and intra-individual variations in basal hormone levels and menstrual cycle lengths. The OC pill, therefore, represents an ideal research tool for investigating daily acute ovarian steroid hormone changes and their effect on performance. Decreased inter- and intra-individual variation in cycle length and endogenous hormone concentrations allows more accurate, reliable, and repeatable testing sessions to be used. The OC therefore could be used to rectify these common inconsistencies and limitations identified by previous investigations (14,23,24).

There is no consensus in the literature whether anaerobic performance is affected by changes in sex steroid concentrations typical throughout the normal menstrual cycle and the synthetic menstrual cycle of the OC user. Results of the current investigation found that both anaerobic power and anaerobic capacity were increased at TDL, whereas circulating concentrations of ethinyl estradiol and levonorgestrel (E2 and P4 equivalents) are low.

Four earlier studies have reported menstrual cycle phase differences in anaerobic performance. Consistent with the findings of the current investigation are three studies (5,9,34) that report improved anaerobic performances during menstruation, menstrual phase or early follicular phase (FP), time points consistent with TDL in the current investigation. Specifically, Brooks-Gunn et al. (5) compared swimming (100-yard freestyle performance and 100-yard preferred stroke) performances in six young adolescent swimmers during menstruation, and mid-follicular and premenstruation phases. Regardless of stroke, performance times were fastest during the menstruation phase (during menstrual flow), whereas the slowest performances were associated with the premenstrual phase (4 d before onset of menses). In contrast, one group (28) demonstrated higher peak and mean power output during the mid-FP (days 7–9) compared with the mid-luteal (days 13–17) and menstruation (days 1–2) phases in a modified Wingate protocol. In addition, several studies (10,11,15,23,26,31) involving normally menstruating women have demonstrated no cycle phase differences in anaerobic performance.

To our knowledge, only two studies (10,15) have investigated changes in anaerobic performance during an OC cycle. Both studies found no differences in either anaerobic endurance (10) or anaerobic power (15) throughout the OC cycle or in comparison with normal menstrual cycle controls. These earlier studies, however, investigated performance changes throughout a single OC cycle unlike the current investigation where performance changes throughout three consecutive OC cycles were studied.

Most of the inconsistencies between studies are likely to be the result of varied methods for determining menstrual cycle phase, definition of different menstrual cycle phases, and hence the choice of exercise testing days. In addition, the sample sizes are small and have only measured performance changes during a single menstrual or OC cycle.

Enhanced anaerobic performance in this study could be the result of the secondary cellular effects of E2 and P4 equivalents on substrate utilization and buffering capacity. High concentrations of E2 and P4 typical during the LP have been reported to elicit a sparing of glycogen (4,6,23) both at rest and during exercise with the concurrent inhibition of gluconeogenesis and glycogenolysis (6,18). Thus, the decrease in plasma glucose and increase in plasma triglyceride concentrations observed under high E2 and P4 equivalent concentrations (TDH) could be due in part to a reduction in glucose formation and increased storage of glycogen in the liver and muscle tissues (1) and hence favored lipid metabolism. Lipid oxidation and utilization rates are reported to be increased during the LP in comparison with the follicular phase (15,16,18). Therefore, improved performance at TDH when sex steroid concentrations are high could therefore restrict carbohydrate metabolism and the availability of glucose as substrate for the production of ATP. There is no evidence in the literature to suggest that muscle phosphate stores (ATP and CP) and utilization during exercise is affected by menstrual cycle phase.

Enhanced anaerobic performance at TDL could also be attributed to the secondary effects of progesterone on buffering capacity. Progesterone acts as a competitive antagonist at the aldosterone receptor site in the distal tubule of the kidney (22,33). During the LP, high concentrations of progesterone result in water and electrolyte loss that stimulates a concurrent increase in aldosterone concentration during this phase. Therefore, the rapid reduction of circulating progesterone concentrations on transition from the luteal to FP results in excess aldosterone concentrations leading to the commonly observed premenstrual increase in water and electrolyte retention (14) during the normal menstrual cycle. Fluid retention or bloatedness is also a common side effect during OC usage (24) and was reported at TDL by subjects in this study. Increased water and electrolyte stores have been associated with improved plasma volume maintenance (14), which could potentially promote improved buffering capacity and cellular alkalosis. Improved buffering capacity, thought to be a determinant of muscular fatigue, could enhance anaerobic capabilities during the late LP and early FP of the normal menstrual cycle and at TDL of the artificial cycle created by the OC.

This study provided evidence that a triphasic OC pill could alter anaerobic performances in female athletes. In this study, rowing performances were improved at TDL, that is, during the ingestion of the placebo or inert OC pill. Although both anaerobic power and capacity were significantly increased at TDL, the most significant finding lies with a 3-s improvement in the 1000-m row time trial. Given that 2000 m is the national and international distance for most rowing events, this performance outcome could translate to a 3- to 5-s difference in rowing performance at different times of the OC cycle and the difference between first and eighth place in a National or International final. Given that OC pills are already being used by athletes to manipulate the timing of menstrual bleeding in relation to competition (3), the OC could be used by female athletes engaged in anaerobic activities to ensure that the hormonal milieu on the day of competition would foster an improvement in performance and, in this case, would in some cases correspond with menstrual bleeding.

In summary, this study found that anaerobic performance is altered in cyclic patterns throughout the OC cycle with the greatest performances observed under conditions of low E2 and P4 equivalents. Enhanced anaerobic performance could be the result of the secondary cellular effects of sex steroids on substrate choice and kidney function. The OC standardized all performance and metabolic variables across each OC cycle, shown by the lack of significant differences in measured variables observed between OC cycles. Orally administered sex steroids act to indirectly reduce the natural production of estrogens and progestogens, allowing the menstrual cycle to be manipulated by daily exogenous hormone concentrations. The ability of the low-dose OC pill to control the natural fluctuations in endogenous hormone concentration allows testing days to be more accurately predicted and decreases the inter- and intra-individual variability of endogenous hormone concentrations within and between menstrual cycles, providing more homogenous subject group conditions. Provided the OC pill is taken as directed, it will provide a consistent hormonal milieu from cycle to cycle, therefore creating a more controlled environment in which the acute effects of female sex steroids on exercise performance can be studied.

The use of the Queensland Academy of Sport’s Performance Enhancing Laboratory for the exercise tests and the assistance of their staff are gratefully acknowledged.

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Keywords:

ESTRADIOL; PROGESTERONE; METABOLISM; ANAEROBIC PERFORMANCE; ROWING

©2004The American College of Sports Medicine