© Springer Science+Business Media New York 2016
Diana Vaamonde, Stefan S du Plessis and Ashok Agarwal (eds.)Exercise and Human Reproduction10.1007/978-1-4939-3402-7_1717. Impact of Combined Oral Contraceptive Use on Exercise and Health in Female Athletes
(1)
Department of Kinesiology, Pennsylvania State University, 16802 University Park, PA, USA
(2)
Department of Kinesiology, Pennsylvania State University, 104 Noll Laboratory, 16802 University Park, PA, USA
Keywords
Aerobic capacityAnaerobic capacityBone healthBody compositionMetabolismOral contraceptionIntroduction
In a recent report from the Centers for Disease Control and Prevention, 44 % of women between the ages of 15 and 24 years use hormonal contraceptives [1]. Of these women, 49 % use oral contraceptive (OC) preparations and approximately another 14 % use other forms of hormonal contraceptives [1]. The prevalence of OC use among athletic women is increasing and is now estimated to match the prevalence of use in the general population [2, 3]. The increased use of combined hormonal contraceptives (containing both estrogen and progestin) in the athletic community is likely due to the reduction in cycle-length variability and the consistent 28-day “cycle” that is achieved with administration of exogenous sex steroids and subsequent systematic control of endogenous sex hormone concentrations [3]. The effect of OC preparations on athletic performance and health has not been conclusively answered as early investigators utilized a diverse number of OC preparations, range of participant fitness levels, and timing of experiments within the OC pill cycle. Most research studies have been cross sectional and were not carried out using the most commonly prescribed contraceptives today (low and ultra-low OC preparations and the vaginal ring).
Modern combined hormonal contraceptives can be provided in a variety of types and formulations. These include, but are not limited to, OC pills (monophasic, biphasic, and triphasic), the contraceptive transdermal patch, and the contraceptive vaginal ring. Current combined hormonal contraceptives typically contain one of the two types of synthetic estrogen, ethinyl estradiol (EE) or manestrol, while the progestin component is typically one of eight different forms. The biological properties of the progestogen derivatives used in contraceptives relate to the potency and the relative binding affinity [4]. Progestins with a higher relative binding affinity for the progesterone receptor exert their progestational effects with smaller doses. Progestins with high relative binding affinity for the androgen receptor cause undesirable side effects that counteract the positive effects produced by the synthetic estrogen [4].
Since the introduction of the OC pill, the dose of the estrogen component has been decreased. The high-dose OC preparations contain 50 µg/d or more of the estrogen component, low-dose OC preparations contain less than 50 µg/d (generally 20–35 µg/d of the estrogen component), and ultra-low-dose OC preparations contain 15 µg/d of the estrogen component [5, 6]. Low and ultra-low OC pills are the most commonly prescribed pills today [6].
The progestin component has undergone four developmental generations since the introduction of the OC pill. The first generation was the first to demonstrate the desired activity on the reproductive axis and include norethindrone and medroxyprogesterone acetate [7–9]. The successive generations were designed to minimize the side effects and increase the safety profile (i.e., to decrease the risk of blood clots). The second generation includes levonorgestrel and norgestrel, the third generation includes norgestimate and etonogestrel, and the fourth generation includes drospirenone and nomegestrol acetate [7–9]. All conventional 28-day hormonal contraceptive regimens provide exogenous steroids for 21 days (active pill phase) followed by a 7-day hormone-free phase. The hormone-free phase, which allows for breakthrough bleeding, can consist of either inactive/placebo pills or the cessation of taking any pills for 7 days. Monophasic OC formulations contain the same dose of estrogen and progestin in all active pills, whereas biphasic OC formulations contain a constant estrogen dose, but the progestin dose increases in the second half of the active pill phase [10]. Triphasic OC formulations have two doses of estrogen (higher in the second week of the active pill phase than the first or third week) and the progestin increases in three steps during the active pill phase [10]. The contraceptive transdermal patch delivers 20 µg of EE daily with a third-generation progestin, norelgestromin [11], and the contraceptive vaginal ring delivers 15 µg EE daily with a third-generation progestin, etonogestrel [12].
The purpose of this brief review is to elucidate the current understanding of the effects of OC preparations on factors that influence exercise performance and the health of athletes. Information regarding the impact of the contraceptive transdermal patch and vaginal ring on exercise performance is not currently available. This review will focus on the effects of OC preparations on the factors that affect exercise performance, including substrate utilization, aerobic and anaerobic capacity , ventilatory capacity, and anaerobic strength, and factors that affect the health of female athletes, including body composition, bone health, and menstrual function.
Factors That Affect Exercise Performance
Substrate Utilization
The new low- and ultra-low-dose and third-generation progestin OC preparations minimize the side effects of early preparations on insulin resistance, decreased glucose tolerance, and high plasma cholesterol and triglyceride levels [3]. However, current formulations of ultra-low-dose OC preparations have not been extensively evaluated to determine the differences in substrate utilization throughout the active pill phase or hormone-free phase and during exercise. It has been hypothesized that during the active pill phase, with high EE, there would be a glycogen sparing effect and increased lipid use during exercise [13–16]. However, progestins are hypothesized to oppose the lipolytic effects induced by EE [13–16]. These hypotheses were based on observations of increased lipolysis and reduced carbohydrate oxidation during exercise with high estrogen in the follicular phase of the natural menstrual cycle and an opposition to lipolysis in the luteal phase with high progesterone and estrogen [17–19]. However, in rats and humans, the use of exogenous sex steroids has been shown to interfere with substrate utilization [13, 20]. A few researchers have utilized various modes (e.g., cycle ergometer and treadmill) and intensities of exercise to investigate changes of fuel utilization during exercise when combined with OC use.
Inconsistent results have been reported with regard to substrate availability following endurance exercise between monophasic OC users and nonusers. For example, Bonen et al. [14] investigated substrate utilization during exercise in monophasic OC users compared to non-OC users. During 30 min of exercise at 40 % of maximal oxygen consumption (VO2max), a shift to high plasma free fatty acids and decreased plasma glucose concentrations was observed in monophasic OC users compared to non-OC users; however, no differences between the active pill and hormone-free phases were observed [14]. Similarly, Bemben et al. [13] reported a decrease in carbohydrate utilization and lower plasma glucose in the 40th and 50th minutes of a 90 min treadmill test at 50 % VO2max in monophasic (35 µg EE) and multiphasic (35 µg EE) OC users during the active pill phase compared to the luteal phase in non-OC users. However, no difference in fat oxidation was observed between OC users and non-OC users [13]. In contrast, a cross-sectional analysis by Sunderland et al. [21] investigated the impact of long-term (12 months) monophasic (20–30 µg EE) OC use on substrate utilization in regularly active women and failed to observe any changes in blood glucose concentrations between the active pill and hormone-free phases, and between OC users and non-OC users following a 30-s sprint treadmill exercise [21].
In support of the proposed hypothesis that high EE would spare glycogen and encourage lipid utilization but high progestin concentrations would oppose lipolysis, Redman et al. [22] investigated the impact of the third week of the active pill phase and the hormone-free phase of triphasic OC use on metabolism in five elite female rowers. During three consecutive triphasic OC cycles increased glucose (37 and 8 %) and reduced plasma triglyceride (8 and 31 %) concentrations were reported at rest and post-anaerobic exercise in the hormone-free phase compared to the active pill phase [22]. Similarly, in the menstrual cycle before and following 4 months of triphasic OC use, Casazza et al. [15] evaluated lipid utilization during exercise in eight active, eumenorrheic women and reported greater triglyceride mobilization during 60 min of leg ergometer exercise at 45 % of VO2max in the third week of the active pill phase (21.6 %) and at 65 % of VO2max during the active pill (20.4 %) and hormone-free phase (21.6 %) compared to pre-OC use [15]. However, no differences were observed in free fatty acid oxidation, but re-esterification of free fatty acids was not investigated [15]. In addition, Jacobs et al. [23] observed increased free fatty acid appearance, disappearance, and oxidation during exercise at 45 and 65 % of peak oxygen uptake (VO2peak) compared to during rest in eight active women during the natural menstrual cycle prior to OC initiation and following 4 months of triphasic OC use. They also observed an increase in free fatty acid re-esterification and a decrease in the proportion of plasma free fatty acid rate of disappearance at rest and during exercise with triphasic OC use compared to pre-OC use. Suh et al. [24] investigated the effects of 4 months of triphasic OC use on substrate metabolism in eight active women; no difference in the rate of glucose appearance was observed between the third week of the active pill phase and the hormone-free phase at rest or during exercise at 45 and 65 % VO2peak. Further, Suh et al. [24] did not observe any differences in glucose or lipid oxidation rates between the natural menstrual cycle and triphasic OC use or between the third week of the active pill phase and the hormone-free phase of the OC cycle. A decrease in the rate of glucose appearance was observed at rest and during exercise at 45 and 65 % VO2peak during triphasic OC use compared to the menstrual cycle prior to OC use [24]. The investigations of the effect of triphasic OC use on substrate metabolism to date used differing modes and durations of exercise [15, 22–24], and there appears to be a shift toward increased free fatty acid mobilization, while substrate oxidation does not show a clear difference between phases of the OC cycle (active pill and hormone-free phases) or cross-sectionally between triphasic OC users and non-OC users.
Limited research has been conducted to evaluate differences in substrate utilization with OC use in various prandial states. Tremblay et al. [25] investigated measures of substrate oxidation during 120 min of cycle ergometer exercise at 57 % of VO2max following ingestion of 2 g/kg of glucose between triphasic OC users and non-OC users and reported no differences in exogenous and endogenous carbohydrate oxidation rates. However, Issaco et al. [26] reported a greater reliance on lipids during 45 min of exercise at 65 % of VO2max in the fasting state compared to the post-prandial state in 21 active women using a monophasic (20–30 µg EE) OC preparation. However, a greater reliance on lipids during exercise in the fasted state compared to the post-prandial state was also observed in non-OC users, and there were no differences in lipid reliance between OC users and non-OC users [26].
In summary, current research findings do not indicate consistent changes in substrate utilization during exercise in women using monophasic and triphasic OC preparations. It is possible that during shorter exercise tests, where there is less reliance on glycogen and lipid utilization, less opportunity is available for the exogenous steroids to exert an influence over substrate flux [10, 27]. An insufficient number of studies have been conducted at various durations and intensities of exercise and durations of OC use to definitively determine whether OCs have an impact on substrate utilization during aerobic and anaerobic exercise. Further research is also needed to determine the effects of transdermal and vaginal hormonal contraceptives on substrate utilization during exercise in athletic women.
Exercise Capacity
The availability and use of glucose and lipids as a fuel source during exercise are known to impact aerobic and anaerobic capacity [28]. With the use of OC pills, early reports indicate an alteration of (1) fat and carbohydrate metabolism, as previously described, (2) movement of glucose into muscle (glucose flux), and (3) insulin sensitivity [13–16, 20]. These early observations of alterations in the proportion of fuel availability, glucose flux, and insulin sensitivity led to theories of the impact of OC use and phase of OC cycle on both aerobic and anaerobic capacity.
During the active pill phase, EE may exert a glycogen sparing effect, which is proposed to enhance sustained aerobic capacity [29]. During the hormone-free phase, when sex steroid hormone concentrations are lowest, the carbohydrate metabolism is upregulated and may enhance anaerobic capacity [29, 30]. It is noteworthy that this review will focus on the effects of OC use on factors influencing exercise capacity, which are commonly assumed to correlate with athletic performance [31, 32], and will not focus on athletic performance per se, which is defined as the complex interaction between physiological, tactical, technical, and psychological factors [32].
Aerobic Capacity
The large majority of investigators have evaluated the effects of monophasic OC preparations on aerobic capacity in both trained [33–37] and untrained [36, 38, 39] women. Few investigators have evaluated the influence of biphasic or triphasic OC preparations on aerobic capacity [23, 24, 40, 41]. Differences in aerobic capacity have been evaluated in both cross-sectional [33, 35–37] and longitudinal studies [23, 24, 40, 41]. Researchers have also focused on evaluating the variation in aerobic capacity between the active pill phase and the hormone-free phase [33, 38].
In untrained women using monophasic OC pills, study results have been inconsistent [36, 38, 39]. Early work indicated a significant increase in VO2peak for a standardized workload in untrained women following two cycles of high-dose (50 µg EE) monophasic OC use when compared with baseline measures (assessed during the menstrual cycle prior to OC initiation) [39]. However, more recently, investigators have suggested that low-dose (20–35 µg EE) monophasic OC use for 12 months had no effect on oxygen consumption (VO2) in untrained women during the hormone-free phase or the final week of the active pill phase [38]. Similarly, Rebelo et al. [36] reported no difference in VO2peak in sedentary women who had been using a low-dose (20 µg EE) monophasic OC for 18 months compared to non-OC users.
Varying results have also been reported in studies evaluating the impact of monophasic OC pill use in trained women, both in recreational and in elite athletes [33, 34, 36, 37, 42]. In recreationally active women who had been using a low-dose (20–30 µg EE) monophasic OC for a minimum of 18 months, submaximal VO2 was reduced by 3–6 % during the active pill phase compared to the hormone-free phase [33]. Other reports in recreationally active women using OCs for a minimum of 12 months demonstrate a reduced VO2peak and a lower VO2 at the anaerobic threshold, with no differences reported in time to exhaustion during a submaximal aerobic test compared to non-OC users [35]. Likewise, Notelovitz et al. [43] described a 7–8 % decrease in VO2peak following 6 months of low-dose (35 µg EE) OC use in trained women, while the non-OC user control group had a 7.5 % increase in VO2peak during the same timeframe. However, in 2010, Rebelo et al. [36] did not observe a significant difference in VO2peak nor VO2 at the anaerobic threshold of trained women who had been using a low-dose (20 µg EE) monophasic OC for a minimum of 18 months compared to non-OC users. Similarly, in trained rowers, who had been on a low-dose (20 µg EE) OC preparation for a minimum of 3 months, no significant differences in measurements of VO2max and VO2 at the aerobic-anaerobic transition were observed between day 8 or day 20 of a monophasic OC pill cycle [37]. Studies of monophasic OC use in highly trained women also indicate that there is no significant effect of OC preparations on aerobic capacity [34].
Due to the relatively equal proportion of studies that have shown a decrease or no significant change in VO2 with OC use in untrained and trained women, the impact of monophasic OC use on aerobic capacity remains unclear. In summary, the conflicting results in VO2 among untrained and trained women using a monophasic OC have been reported to be secondary to a wide variety of causes, that include (1) differences in progestational and androgenic activity of the progestins within the contraceptives in the studies, (2) duration of OC use (no use, 3, 6, 12 or 18 months of use prior to study initiation), (3) varying training status of the study participants, (4) small sample sizes that may not power studies adequately for primary study outcomes, and (5) variation in the exercise protocols utilized in each research study [10, 29]. Further research will be necessary to resolve this issue.
Very few investigators have evaluated the impact of triphasic OC preparations on aerobic capacity in trained women [23, 24, 40, 41]. In a study of trained athletes over two triphasic OC pill cycles, a 4.7 % decrease in VO2peak but no change in endurance performance, as measured by time to exhaustion, was observed [41]. Similarly, a study of triphasic OC pills over four [24] and six [23] cycles demonstrated a decrease in VO2peak of 11–15 % in active women, agreeing with results from a study conducted by Casazza et al. [40] which demonstrated an 11 % decrease in VO2peak among recreationally active women using a triphasic OC for 4 months. In summary, triphasic OC preparations appear to decrease VO2 in trained athletes during use of up to 6 months. No studies to date have investigated the impact of triphasic OC use (1) of long duration (> 6 months) or (2) in untrained women.
Ventilatory Capacity
Early research on the menstrual cycle has shown that the natural rise in progesterone during the luteal phase increases chemosensitivity of the lungs to hypoxia and hypercapnia [44]; thus, monophasic OC preparations with high doses of progestin and the third phase of the triphasic OC may also increase the chemosensitivity of the lungs and increase ventilatory capacity. In agreement with this hypothesis, a few studies conducted in trained women using monophasic OC preparations demonstrated an increase in ventilation (VE) [37, 45]; however, these results have not been consistent, particularly when considering the ventilation response to OC preparations in untrained women or with a different OC formulation, i.e. triphasic OC preparations.
For example, during a 1-h endurance test performed by trained cyclists taking monophasic OC preparation mean VE and mean ventilation per oxygen consumption (VE/VO2) were 7 and 5 % higher, respectively, during the active pill phase compared to measures taken early and late in the hormone-free phase [45]. However, in trained rowers administered a monophasic OC, there was a tendency toward an increased ventilatory response during the hormone-free phase demonstrated by increased VE and VE/VO2 at VO2max (1.6 and 6.3 %, respectively) compared to the active pill phase [37].
On the other hand, no significant differences in VE were observed in untrained women, who had been using a monophasic OC preparation (20–30 µg EE) for a minimum of 18 months and performed submaximal treadmill exercise during any phase (active pill and hormone-free) of a monophasic OC pill cycle [33]. Further, Redman et al. [46] evaluated ventilatory measures in untrained women who had been using contraceptives for a minimum of 6 months. Participants changed their OC use in a cross-over design to a 35 µg EE monophasic OC with high (1000 µg norethisterone) or low (500 µg norethisterone) progesterone concentration. VE and VE/VO2 measured at VO2peak were not different between progesterone concentrations; however, VE and VE/VO2 were 10.5 and 8.5 % higher, respectively, at rest during use of the high progesterone OC compared to the low progesterone OC [46]. Rebelo et al. [36] evaluated VE in active and sedentary OC users (20 µg EE; minimum use 18 months) and non-OC users and did not observe any differences among the four groups. Further, Joyce et al. [35, 42] did not observe any differences in VE between untrained or trained OC users (minimum use 12 months) and non-users.
Contrary to the hypothesis that the third week of the triphasic OC preparation may increase ventilatory capacity due to the high progestational content, the majority of studies have demonstrated no effect of triphasic OC preparation use on VE in both untrained and trained women. For example, in untrained women who utilized a triphasic OC for 4 months, VE measured in the active pill and hormone-free phases were not significantly different compared to measurements taken during the follicular or luteal phase of the menstrual cycle prior to OC initiation [40]. Similarly, in trained women, VE did not differ between measurements taken following 4 months of triphasic OC use during the active pill and hormone-free phases nor compared to the follicular and luteal phases in the menstrual cycle prior to OC initiation [24]. Further, VE did not change with a single cycle of a triphasic OC in trained women compared to the menstrual cycle prior to OC initiation [41].
Overall, OC use (monophasic or triphasic formulations) does not appear to have an effect on ventilatory capacity. The conflicting results observed with monophasic OC use can be attributed to the initial fitness level of the participants, the intensity and duration of the exercise, and/or the androgenic activity of the progestin in the OC preparations used. Further research is necessary to confirm the results from studies evaluating triphasic OC use.
Anaerobic Exercise
Few studies have been conducted on the influence of OC preparations on anaerobic performance, including capacity and strength. Variation in anaerobic performance during an OC cycle could be caused by the impact of EE and progestins on substrate utilization, buffering capacity, and neuromuscular function [47].
Anaerobic Capacity
During short-duration maximal exercise , the maximal amount of ATP resynthesized via anaerobic metabolism (phosphocreatine and glycolysis) is considered anaerobic capacity [29]. During a menstrual cycle, a low estrogen environment and increased circulating aldosterone following a drop in progesterone (as observed at the end of the luteal phase) upregulate carbohydrate metabolism , which is necessary for the anaerobic production of ATP [29]. Further, increases in circulating aldosterone may increase body fluid and electrolyte retention, thus increasing buffering and anaerobic capacity [48]. As such, in accordance with the influence of endogenous steroid hormones on substrate utilization and buffering capacity, it is plausible that anaerobic capacity would be greatest during the hormone-free phase of the OC cycle [29]. Currently, available literature does not provide hypotheses surrounding the impact of OC use on anaerobic capacity when compared to non-OC users. Production of lactate, a by-product of anaerobic metabolism, has been evaluated in women using monophasic and triphasic OC preparations, thus serving as an indicator of the impact of OC use on anaerobic capacity.
The majority of studies, however, do not support the proposed hypothesis that anaerobic capacity is enhanced during the hormone-free phase. For example, Bonen et al. [14] evaluated lactate concentrations following a walk to elicit 85 % VO2max in monophasic OC users (30–50 µg EE) and observed no differences between phases of the OC cycle [14]. Similarly, Bernardes et al. [18] investigated anaerobic capacity in a mixed group of women using monophasic and triphasic OC preparations of a similar EE dose (30–40 µg EE) and found no variation in blood lactate between the active pill phase and the hormone-free phase following intermittent exercise (80 % VO2max) tests. Likewise, Lynch et al. [49] investigated the impact of low-dose (30–35 µg EE) monophasic OC administration on intermittent exercise (five by 20-s treadmill runs at increasing speed on a 10.5 % incline) in recreationally active women and observed no significant difference in peak blood lactate between OC users and non-OC users; however, peak blood lactate was significantly higher during week 1 compared to week 2 (11.2 vs. 9.6 mmol/L) of the active pill phase of OC users. Notably, in contrast to their previous report, Lynch et al. [38] observed no difference in blood lactate concentrations between week 1 and week 3 of the active pill phase after repeating the intermittent exercise test on untrained women who had been using a low-dose OC preparation (20–35 µg EE) for at least 12 months. Interestingly, neither of the studies by Lynch et al. [38, 49] evaluated differences between the active pill and hormone-free phases. Following high intensity, intermittent exercise in the heat, no differences in blood lactate concentrations were observed between OC cycle phases in trained women using monophasic (20–35 µg EE) OC preparations [50]. In a study by Redman et al. [22], conducted among 6 highly trained rowers who had been taking a triphasic OC for a minimum of 12 months, a significant increase in glucose and decrease in plasma triglyceride with no difference in lactate concentrations were observed in the hormone-free phase compared to the third week of the active pill phase following a 1000 m simulated row test. The participants were tested in three consecutive OC cycles and the results were consistent between the active pill phase and hormone-free phase [22]. In 2011, Sunderland et al. [21] also evaluated the impact of monophasic OC use (20-30ug EE) on blood lactate concentration following an all-out 30-s sprint. Blood lactate concentrations were not different between the active pill phase and hormone-free phase in OC users [21]. These results indicate that there is no difference in carbohydrate metabolism between OC cycle phases during tests of short, intense activity when compared with studies of aerobic and submaximal endurance exercise [27].
Recently, however, findings by Rechichi et al. [27] do support the hypothesis that anaerobic capacity is greater during the hormone-free phase of the OC cycle. For example, Rechichi et al. [27] measured a 23 % decrease in peak blood lactate following a 200 m time trial in the hormone-free phase compared to late in the active pill phase in competitive swimmers using a monophasic OC (30 µg EE) preparation, indicating an increase in buffering capacity.
In addition to comparing differences between phases of the OC cycle, three teams of investigators also reported comparisons between OC users and non-OC users. In 1991, Bonen et al. [14] evaluated lactate concentrations following a walk to elicit 85 % VO2max in monophasic OC users (30–50 µg EE) and non-OC users. No differences in blood lactate concentrations were observed between OC users and non-OC users [14]. Sunderland et al. [50] investigated high-intensity intermittent exercise in the heat in trained women on monophasic (20–35 µg EE) OC preparations and non-OC users and found no differences in blood lactate concentrations. However, in 2011, Sunderland et al. [21] found significantly higher blood lactate concentrations following an all-out 30-s sprint in OC users compared to non-OC users. Of three available reports comparing OC users and non-OC users, this was the first report demonstrating a difference in blood lactate concentrations between OC users and non-OC users. The differences in the available data are due to the large variability in EE concentrations as well as the variation in the exercise evaluated and the conditions of the test.
The majority of available data to date do not support the hypothesis that anaerobic capacity would be greater during the hormone-free phase compared to the active pill phase in OC users. The limited comparisons of anaerobic capacity between OC users and non-OC users indicate the exogenous hormones in OC preparations have a diminished capacity to alter aldosterone and lactate buffering capacity; however, further research is needed to conclude if there is a general impact of OC use (compared to non-OC users) on anaerobic capacity. Data regarding blood lactate accumulation following anaerobic exercise during use of monophasic or triphasic OC preparations suggest the impact of aldosterone on buffering capacity is not consistent with changes observed in the natural menstrual cycle. Thus, future studies should investigate the alterations in the progesterone/aldosterone ratio at more than one time point during the active pill and hormone-free phases. It is also possible that duration of activity is a significant factor in the ability of exogenous hormones from OC preparations to impact anaerobic capacity . With shorter duration tests the influence of sex steroids on lipid and glycogen utilization is limited; thus there may be less opportunity for the exogenous steroids to exert their influence [10, 29].
Anaerobic Strength
It has been proposed that endogenous estrogens, when highest, may have a positive impact on muscle strength, while progesterone inhibits the effects of estrogen [51]. However, EE may not influence the estrogen receptors within the skeletal muscle in the same way as endogenous estrogen or the progestin consumed with OC use may influence the interaction of EE with the neuromuscular pathway [47]. Rechichi et al. [47] evaluated reactive strength with a 45 cm drop height test in team sport athletes taking a monophasic OC preparation and found reactive strength was approximately 11 % lower during the hormone-free phase (early or late) compared to the active pill phase. In contrast, Sarwar et al. [52] observed no differences with handgrip strength or isometric quadriceps strength between active pill and hormone-free phases of a monophasic OC cycle. Similarly, Elliot et al. [53] found no differences in dynamic or isometric leg strength between the active pill and hormone-free phases in 14 women using a monophasic OC (30–35 µg EE) preparation. In addition, Elliot et al. [53] did not observe differences in dynamic or isometric leg strength between the 14 monophasic OC users and 7 eumenorrheic women. A study by Peters et al. [54] on trained athletes using monophasic OC (30 µg EE) preparation showed no difference in maximal leg isokinetic strength through extension or flexion between the active pill and hormone-free phases of the OC cycle. Similarly, Ekenros et al. [55] did not observe any differences in isokinetic knee extensor strength or isometric handgrip strength between monophasic (20–35 µg EE) OC users and non-OC users nor between phases of an OC cycle. Further testing is required to determine if there is an effect of OC use on reactive strength; however, the available data indicates that isometric strength is not influenced by monophasic OC use or phase of the monophasic OC cycle. Further research is needed to determine the impact of triphasic OC use on all aspects of muscular strength.
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