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Published in final edited form as: Respir Physiol Neurobiol. 2021 Jun 9;292:103712. doi: 10.1016/j.resp.2021.103712

Menstrual Phase Does Not Influence Ventilatory Responses to Group III/IV Afferent Signaling in Eumenorrheic Young Females

Emma Lee a, Kathryn Vera b,c, Ninitha Asirvatham-Jeyaraj d, Daniel Chantigian a, Mia Larson e, Manda Keller-Ross a,b
PMCID: PMC8319100  NIHMSID: NIHMS1715763  PMID: 34118436

Abstract

Estrogen can reduce sympathetic activity, but its effects on minute ventilation (VE) with group III/IV afferent activation remain unclear. This study examined the influence of estrogen on VE during lower-extremity exercise with group III/IV activation. Females completed two identical visits in follicular and ovulatory menstrual phases. Nine participants (age 25±4 years) performed three minutes of baseline steady-state cycle ergometry and then group III/IV afferents were further activated with proximal thigh cuffs inflated to 20, 60, and 100mmHg (randomized) for two minutes with five minutes of cycling between each occlusion. Metaboreflex was isolated by post-exercise circulatory occlusion. Ventilation was measured continuously and rating of perceived exertion (RPE) was recorded for each stage. During rest and exercise, VE (p<0.001) and tidal volume (VT) (p=0.033) were higher in the follicular than ovulatory phase. Minute ventilation, VT, and respiratory rate (RR) with ergoreflex and metaboreflex activation were similar across phases. With cuff occlusion of 100mmHg, VE increased from baseline by 26.3±7.0 L/min in the follicular phase (p<0.001) and by 25.3±7.7 L/min in the ovulatory phase (p<0.001), with no difference between phases (p>0.05); RR and VT increased similarly with occlusion, also with no phase differences. In eumenorrheic females, menstrual phase influences ventilation but not ventilatory responses to group III/IV isolation.

Keywords: estrogen, ergoreflex, metaboreflex, ventilation, exercise

1. INTRODUCTION

Estrogen, an essential female sex hormone, has been shown to inhibit the activity of the sympathetic nervous system (Charkoudian, 2001). Estrogen can blunt group III/IV afferent activity, which transmits sensory feedback from skeletal muscle to multiple brainstem compartments, including the ventral respiratory column, dorsal respiratory group, and pontine respiratory group (Smith et al., 2009). Group III/IV afferents are key to ventilatory control during exercise (Amann et al., 2010; Amann et al., 2015). These afferents mediate the ergoreflex, in which mechanical activation (mechanoreflex) and metabolite accumulation (metaboreflex) in active skeletal muscles provoke cardiorespiratory responses to support the metabolic demands of exercise (Amann et al., 2020; Rowell and O’Leary, 1990). Estrogen’s blunting effects on these fibers thereby elicits an attenuated metaboreflex in females (Ettinger et al., 1996; Koba et al., 2012; Minahan et al., 2018; Schmitt et al., 2006; Schmitt and Kaufman, 2003a, b, 2005). Estrogen may play a role in ventilation, potentially mediating resistance of the upper airways and acting synergistically with progesterone to promote ventilation (Behan and Wenninger, 2008). However, its impact on ventilation has not been well defined.

Circulating levels of sex hormones during the menstrual cycle have been shown to alter the ventilatory response to group III/IV afferent stimulation. Exercise immediately activates group III/IV afferents; the physiological response to exercise stimulation of these afferents is termed the ergoreflex. The luteal phase, where progesterone is highest with moderately high estrogen levels (Reed and Carr, 2018), has been associated with a greater ventilatory response with dynamic exercise (England and Farhi, 1976; Jurkowski et al., 1981; Saperova and Dimitriev, 2013; Schoene et al., 1981; Slatkovska et al., 2006; Williams and Krahenbuhl, 1997). However, some studies have reported no influence of menstrual phase on resting or exercise ventilation (Beidleman et al., 1999; De Souza et al., 1990; Dombovy et al., 1987; Lamont, 1986; Lebrun et al., 1995; Matsuo et al., 2003; Packard et al., 2011). Thus, more clarity is needed on the role of estrogen in the ventilatory response to group III/IV afferent activation during dynamic exercise.

Group III/IV afferent activation is enhanced through sub-systolic vascular occlusion (Keller-Ross et al., 2019; Keller-Ross et al., 2016; McClain et al., 1993). While partial circulatory occlusion activates both group III and group IV afferents, supra-systolic vascular occlusion after exercise produces an increase in metabolic buildup in the active tissue with minimal to no stimulation of group III stretch receptors, thereby allowing for isolation of the activity of group IV muscle afferents (i.e., metaboreflex) (Haouzi et al., 2004). The role of estrogen levels in ventilatory responses to metaboreflex activation in human females, however, remains unclear.

Although prior work has investigated the effects of female sex hormones on ventilatory responses to exercise, how these factors specifically influence ergoreflex and metaboreflex activation are unclear. Furthermore, most studies have compared the luteal phase, when high levels of progesterone may obscure effects of estrogen, to the low-hormone follicular phase. Therefore, the purpose of this study was to determine how fluctuations of estrogen between the early follicular (low estrogen) and ovulatory (high estrogen) phases influence the ventilatory responses to group III/IV afferent activation during sub-systolic occlusion (activating the entire ergoreflex) and supra-systolic occlusion (isolating the metaboreflex) of the lower-extremity muscles. Based on previous research findings, we hypothesized that the ventilatory responses to ergoreflex and metaboreflex activation would be greater during the follicular phase than the ovulatory phase.

2. METHODS

2.1. Participants

Participants reported natural menstrual cycles and no current use of hormonal contraceptives. Inclusion criteria consisted of being between ages 18–35 years and having no prior history of cardiovascular, pulmonary, neuromuscular, or orthopedic disorders. Participants were excluded if they were currently pregnant or breastfeeding.

All participants gave written informed consent. The study was approved by the University of Minnesota Institutional Review Board (study authorization number: 1608M92482) and was conducted in accordance with the Declaration of Helsinki.

2.2. Experimental Protocol

Participants completed one introductory session and two experimental sessions. The experimental sessions were randomized and included one visit during the early follicular phase (days 1–4) of the menstrual cycle and one visit during the ovulatory phase (days 10–14) of the menstrual cycle. During the introductory session, participants were consented and were provided with ovulation kits (One-Step Ovulation Test Kit, Wondfo, Guangzhou, China) with take-home instructions. The participants were instructed to 1) begin using the kit 10 days after the onset of menses and 2) contact the study team when the ovulation test was positive, allowing testing for the OV experimental session to proceed. They also took urine pregnancy tests (Consult Diagnostics hCG Urine Tests Dipstick, McKesson Medical-Surgical Inc., Richmond, VA) to confirm that they were not pregnant.

Prior to beginning the experimental session, pressure tourniquets (D.E. Hokanson, Inc., Bellevue, WA) were placed bilaterally on the proximal thighs and were held up by a custom-designed belt around the waist to ensure that no undue pressure to the legs by the tourniquets occurred during deflation. Participants then completed a steady-state exercise test on an upright stationary bicycle at a constant load of 60 watts and cadence of 65–70 RPM; cuffs were intermittently inflated and deflated (Figure 1).

Figure 1.

Figure 1.

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Timeline of experimental procedures. Occlusion pressures are given in mmHg. Baseline exercise was preceded by three minutes of seated rest. Exercise was constant-load cycling at 60 W. The occlusion pressure for full occlusion was determined by blood pressure in the final exercise stage. PECO: post-exercise circulatory occlusion.

The exercise protocol included three minutes of seated rest, followed by three minutes of baseline exercise (BL); cuffs were then inflated intermittently to sub-systolic pressures (20, 60, and 100 mmHg) in a randomized manner (Keller-Ross et al., 2019; Keller-Ross et al., 2016). Numbers between one and six were generated using a random number generator (random.org) to represent the six potential occlusion orders for each experimental visit. Participant identification numbers were assigned to the occlusion scenarios in the corresponding sequences (e.g., participant number five was assigned to the occlusion orders that were generated fifth in the first and second series of random numbers). Intermittent cuff inflation was used to enhance group III/IV afferent activity (ergoreflex) (Keller-Ross et al., 2019; Keller-Ross et al., 2016; McClain et al., 1993). Inflations lasted for two minutes and were followed by five minutes of deflation between each circulatory occlusion pressure. Participants exercised continuously at 60 watts and 65–70 RPM throughout cuff inflation and deflation. Rating of perceived exertion (RPE) on a 6–20 scale (Borg and Noble, 1974) was recorded at the end of each exercise stage. During the end of the last stage of steady-state cycling, the cuffs were inflated to 20–50 mmHg above systolic pressure (post-exercise circulatory occlusion, PECO). Once the cuffs were fully inflated, participants ceased exercise but remained seated on the bicycle, isolating the metaboreflex (Haouzi et al., 2004). The cuffs remained inflated for two minutes, then were deflated for two minutes of seated recovery.

2.3. Physiologic Monitoring and Data Collection

Breath-by-breath ventilation and gas exchange were continuously measured with a metabolic cart for the duration of the protocol (Ultima CardiO2 gas exchange analysis system, MGC Diagnostics, St. Paul, MN). The gas analyzer was calibrated per manufacturer specifications before each testing session using calibration gases of 12% oxygen, 5% carbon dioxide, and balance nitrogen. A three-liter calibration syringe and Prevent® flow sensor were used to measure gas volumes, and ambient conditions were accounted for in the gas-analysis software. Ventilatory and gas-exchange outcome measures included oxygen consumption (VO2), carbon dioxide production (VCO2), minute ventilation (VE), tidal volume (VT), respiratory rate (RR), and partial pressure of end-tidal carbon dioxide (PETCO2). Blood pressure was measured manually during the final minute of each stage of the protocol. Systolic blood pressure in the final exercise stage was used to determine the supra-systolic pressure applied during PECO (approximately 20–50 mmHg > systolic blood pressure).

2.4. Statistical Analysis

We based our a priori sample size calculation on minute ventilation data from a study comparing exercise VE between the early follicular phase and mid-luteal phase (Williams and Krahenbuhl, 1997). Based on their means and standard deviations, VE in females during exercise in the early follicular phase was 63.6 ± 6.3 L/min and during the mid-luteal phase was 68.8 ± 8.5 L/min (p < 0.05). A large effect size (d = 0.7) was obtained and thus for such a result, only eight females would have been needed to have sufficient power of 80%. Demographic characteristics of the participants were analyzed using mean and standard deviation. Breath-by-breath data were averaged through the final 30 seconds of each stage of the exercise test protocol and subsequent PECO.

For the ergoreflex, a two-way repeated measures analysis of variance (RM ANOVA) was used to compare the effects of condition and menstrual phase on absolute values of VE, RR, VT, PETCO2, and respiratory exchange ratio (VCO2/VO2; RER) between rest, BL, and the randomized cuff-inflation periods. A RM ANOVA was also used to compare condition and phase effects on RPE during BL and exercise with cuff inflation.

Similarly, for the metaboreflex, a two-way RM ANOVA was used to determine the effects of condition and phase on absolute VE, RR, VT, PETCO2, and RER at rest, during the exercise stage immediately preceding PECO (pre-PECO exercise), and during PECO.

Bonferroni adjustment was conducted for multiple comparisons, and post-hoc t-tests were used to investigate significant interactions. Statistical analyses were performed using SPSS v26.0 (IBM, Armonk, NY) with the α-level for significance set at p < 0.05. Data are presented as mean ± standard deviation.

3. RESULTS

Nine participants completed the study; their anthropometric and demographic characteristics are presented in Table 1. The participants were 25±4 years old with a body mass index of 21.6 ± 1.5 kg/m2, and they reported no medical conditions that would influence their physiological responses to study procedures.

Table 1.

Demographic characteristics of the participants (n = 9).

Mean ± standard deviation
Age (years) 25 ± 4
Height (m) 1.67 ± 0.08
Mass (kg) 60.1 ± 7.9
BMI (kg/m2) 21.6 ± 1.5
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BMI: body mass index.

3.1. Effects of menstrual phase and ergoreflex activation on ventilatory responses to exercise

Table 2 displays values of respiratory variables as measured at rest. Responses of VE, RR, and VT to steady-state exercise and ergoreflex activation are shown in Figure 2. Minute ventilation increased from rest throughout BL and with ergoreflex activation (main effect of condition, p < 0.001; Figure 2A). During BL and at each occlusion pressure, VE was higher than at rest (p < 0.001); VE values at 60 and 100mmHg were elevated compared to 20mmHg (p = 0.007, p = 0.025, respectively). Additionally, there was a main effect of phase on VE in which VE was higher during the follicular phase than the ovulatory phase (p < 0.001). There was no interaction of condition and phase on VE (p > 0.05).

Table 2.

Resting values of respiratory measures.

Follicular phase Ovulatory phase
VE (L/min) 9.5 ± 2.5 8.9 ± 1.5 *
RR (breaths/min) 14 ± 5 15 ± 5
VT (mL) 732 ± 273 619 ± 202 *
PETCO2 (mmHg) 36.8 ± 3.9 36.8 ± 3.2
RER 0.89 ± 0.11 0.89 ± 0.07
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VE: minute ventilation; RR: respiratory rate; VT: tidal volume; PETCO2: partial pressure of end-tidal carbon dioxide; RER; respiratory exchange ratio.

*

main effect of phase, p < 0.05. Data are presented as mean ± SD.

Figure 2.

Figure 2.

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Effects of ergoreflex activation on A) minute ventilation (VE), respiratory rate (RR), and C) tidal volume (VT). Ventilation increased with exercise; VE and VT were higher during FOL than OV. FOL: early follicular phase; OV; ovulatory phase; BL: baseline exercise. Data are presented as mean ± standard deviation. There were main effects of phase on VE and VT (p < 0.05) but not on RR (p > 0.05). ** significantly different from rest, p < 0.01; *** significantly different from rest, p < 0.001; + significantly different from rest, p < 0.05; ++ significantly different from BL, p < 0.01; # significantly different from 60mmHg, p < 0.05. Stages labeled with the same symbols are not significantly different (p > 0.05).

Similarly, RR during BL and at each occlusion pressure was elevated from rest (main effect of condition, p < 0.001; Figure 2B). Respiratory rate at 100mmHg was higher than at BL and 60mmHg (p = 0.019, p = 0.035, respectively), but RR did not differ between 20mmHg and BL, 60mmHg, or 100mmHg (p > 0.05 for all comparisons). There was neither a main effect of phase nor an interaction of condition and phase on RR (p > 0.05 for both).

Tidal volume during BL and ergoreflex activation increased from rest (main effect of condition, p < 0.001; Figure 2C). However, VT values during BL and at each occlusion pressure were not different (p > 0.05 for all comparisons). There was a main effect of phase on VT whereby VT during the follicular phase was higher than during the ovulatory phase (p = 0.033). There was no interaction of condition and phase on VT (p = 0.81), indicating that ergoreflex activation did not alter VT differentially by menstrual phase.

We observed a main effect of condition on PETCO2 (p = 0.001). During BL and with 20 and 60mmHg of occlusion, PETCO2 was higher than at rest (+ 4.8mmHg, p < 0.001; + 4.3mmHg, p = 0.003; and + 3.4mmHg p = 0.013, respectively). However, there was no main effect of phase on PETCO2, nor was there an interaction of condition and time (p > 0.05 for both comparisons).

Furthermore, there was a main effect of condition on RER (p = 0.007). Respiratory exchange ratio with 100mmHg of occlusion was higher than at rest (+ 0.16, p = 0.032); during BL (+ 0.10, p = 0.044); and with occlusion pressures of 20 and 60mmHg (+ 0.07, p = 0.044, and + 0.07, p = 0.025, respectively). We did not observe a main effect of phase or an interaction of condition and phase on RER (p > 0.05 for both tests).

Rating of perceived exertion (Figure 3) increased with ergoreflex activation (main effect of condition, p < 0.001). However, while RPE at each occlusion pressure was higher than during BL (20mmHg: p = 0.047; 60mmHg: p = 0.002; 100mmHg: p = 0.002), RPE was not significantly different between occlusion pressures (p > 0.05 for all tests). Menstrual phase did not alter RPE, and there was no interaction of condition and phase (p > 0.05 for both comparisons).

Figure 3.

Figure 3.

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Rating of perceived exertion (RPE) during baseline exercise and exercise with sub-systolic vascular occlusion. Rating of perceived exertion increased with occlusion but was not different between occlusion pressures nor between menstrual phases. FOL: early follicular phase; OV: ovulatory phase; BL: baseline exercise. Data are presented as mean ± standard deviation. * significantly different from BL, p < 0.05; ** significantly different from BL, p < 0.01.

3.2. Effects of menstrual phase on ventilatory responses to metaboreflex activation

Effects of metaboreflex activation on VE, RR, and VT are presented in Figure 4. We observed a main effect of condition on VE (p < 0.001 Figure 4A); VE during pre-PECO exercise was higher than during rest (p < 0.001) and PECO (p < 0.001), and PECO VE was elevated compared to rest (p = 0.008). There was no main effect of phase or interaction of condition and phase on VE (p > 0.05 for both comparisons).

Figure 4.

Figure 4.

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Effects of metaboreflex isolation on A) minute ventilation (VE), B) respiratory rate (RR), and C) tidal volume (VT). Ventilation, RR, and VT were elevated during pre-PECO exercise compared to rest and PECO. Menstrual phase did not affect ventilatory responses to the metaboreflex. FOL: early follicular phase; OV: ovulatory phase; BL: baseline exercise; PECO: post-exercise circulatory occlusion. Data are presented as mean ± standard deviation. ** significantly different from rest, p < 0.01; *** significantly different from rest, p < 0.001; + significantly different from pre-PECO, p < 0.05; +++ significantly different from pre-PECO, p < 0.001.

There was a main effect of condition on RR (p < 0.001; Figure 4B). While RR during pre-PECO exercise was higher than at rest and with PECO (p = 0.002, p = 0.012, respectively), values of RR during rest and PECO were similar (p > 0.05). We did not find a main effect of phase or an interaction of condition and phase on RR (p > 0.05 for both tests).

Similarly, there was a main effect of condition on VT (p < 0.001; Figure 4C). As with RR, VT was elevated with pre-PECO exercise compared to rest and PECO (p < 0.001, p = 0.016, respectively). There was not a main effect of phase or interaction of condition and phase on VT (p > 0.05 for both tests).

We found a main effect of condition on PETCO2 (p = 0.001). While PETCO2 during PECO was reduced from pre-PECO exercise (− 4.8mmHg, p < 0.001), there was no significant difference between rest and pre-PECO exercise values (p > 0.05 for both comparisons). There was neither a main effect of phase nor an interaction of condition and phase on PETCO2 (p > 0.05 for both tests).

Respiratory exchange ratio increased from rest with pre-PECO exercise and PECO (main effect of condition, p < 0.001). During PECO, RER was elevated compared to rest (+ 0.34, p = 0.001) and to pre-PECO exercise (+ 0.28, p = 0.003), whereas RER was similar at rest and in the exercise stage (p > 0.05). Although there was not a main effect of phase on RER (p = 0.05), we found an interaction of condition and phase (p = 0.021); however, the individual between-phase comparisons of RER at rest, in pre-PECO exercise, and during PECO were not significant (p > 0.05).

4. DISCUSSION

The main findings of the present study were that menstrual phase influenced the ventilatory response to ergoreflex activation during upright cycling exercise, whereby VE was higher during the follicular phase than the ovulatory phase in healthy eumenorrheic females. The observed increase in VE during FOL was driven by an elevated VT, which was similarly elevated in the follicular phase compared to the ovulatory phase. However, menstrual phase did not affect ventilatory responses to metaboreflex isolation. Thus, the results of this study suggest that estrogen levels may mediate ventilatory drive during exercise but may not alter the influence of group IV afferent activation on VE during exercise of the lower extremity.

4.1. Estrogen modulation of ventilatory control during exercise

Estrogen is known to reduce sympathetic outflow in human (Ettinger et al., 1996; Ettinger et al., 1998) and animal (Schmitt et al., 2006; Schmitt and Kaufman, 2003a, b) models, suggesting that it may influence sympathetic responses, including VE, during exercise. Indeed, Schmitt and colleagues have demonstrated direct inhibitory effects of estrogen on group III/IV afferent activity in rats (Schmitt et al., 2006) and in cats (Schmitt and Kaufman, 2003a, b). Because of the contribution of group III/IV afferent activity to exercise ventilation (Amann et al., 2010), we might then expect greater VE during periods associated with lower estrogen levels than when estrogen is high. We demonstrate that in naturally cycling females, VE and VT are higher during the follicular phase, when estrogen is low, than during the ovulatory phase, when estrogen levels are high. However, these findings are limited to rest and submaximal exercise, with no effect of menstrual phase on metaboreflex responses.

During exercise, group III/IV afferents and central command adjust cardiorespiratory function to meet the demands of working muscles (Ichinose et al., 2014). Group III/IV afferents and central command both send input to the nucleus tractus solitarius (NTS) (Degtyarenko and Kaufman, 2006), which contains estrogen receptors and contributes to ventilatory function (Behan and Wenninger, 2008). Circulatory occlusion after exercise (i.e., PECO) isolates the contribution of group III/IV activity to cardiorespiratory alterations from the influence of central command (Keller-Ross et al., 2019; Parmar et al., 2018). Despite the presence of estrogen receptors in the NTS, prior work does not support a role of estrogen as a strong influence on VE (Regensteiner et al., 1989), and we did not observe an effect of estrogen on ventilatory responses during PECO, when group III/IV afferent activity was the main driver of ventilatory function. However, we have shown that VE differs between low- and high-estrogen menstrual phases during rest and exercise with and without partial circulatory occlusion. Thus, our findings suggest that estrogen may act to influence ventilation via a central mechanism as opposed to through skeletal muscle group III/IV afferent activity. Prior work in rodent models has established that estrogen receptors in the central nervous system regulate ventilatory responses in female rats (Dougherty et al., 2017; Inamdar et al., 2001). Furthermore, plasticity of respiratory responses varies by estrogen level in female rats (McIntosh and Dougherty, 2019) and is mediated centrally, rather than genomically (Dougherty et al., 2017). Our results extend these findings and imply that estrogen is implicated in central ventilatory signaling in human females.

Although estrogen is a major modulator of blood pressure at rest (Harvey et al., 2015) and during activation of the ergoreflex (Ettinger et al., 1996; Ettinger et al., 1998), it appears that progesterone may be a stronger modulator of ventilatory function in females and can influence both resting and exercise VE. For example, during the luteal phase, when levels of progesterone are high while estrogen levels are lower (but elevated relative to the follicular phase) (Reed and Carr, 2018), regularly menstruating females may demonstrate higher resting (England and Farhi, 1976; Saperova and Dimitriev, 2013; Schoene et al., 1981; Slatkovska et al., 2006; Williams and Krahenbuhl, 1997) and/or exercise (Jurkowski et al., 1981; Williams and Krahenbuhl, 1997) VE compared to phases when progesterone levels are lower.

Progesterone may modulate VE by augmenting respiratory neural drive (Schoene et al., 1981), and estrogen may act in synergy with progesterone to enhance ventilation (Behan and Wenninger, 2008). In contrast, others have shown that VE does not change with menstrual phase; however, these authors did not examine the components of VE—i.e.., respiratory rate and tidal volume—separately and therefore may not have accounted for alterations in these components (Beidleman et al., 1999; De Souza et al., 1990; Dombovy et al., 1987; Lamont, 1986; Lebrun et al., 1995; Matsuo et al., 2003; Packard et al., 2011). Notably, the present study examined ventilatory responses during two periods of low progesterone levels, and our observation of differential responses by estrogen level imply that estrogen does play a role in modulating breathing at rest and during submaximal exercise.

To this note, we observed a greater VE at rest and during BL exercise in the follicular phase, when estrogen is lowest, compared to the ovulatory phase, when estrogen is highest. This difference appeared to be driven by a larger VT during the follicular phase and not by alterations in RR. This finding suggests that when estrogen levels are lower, females take deeper breaths at rest and during exercise while breathing at a rate that does not fluctuate with estrogen levels. Minute ventilation and RR were within normal ranges across both menstrual phases in the present study, despite being relatively elevated in the follicular phase. Thus, it would be unexpected for PETCO2 to differ between phases.

Participants in the present study reported a higher perceived exertion with sub-systolic cuff occlusion than during baseline exercise. This finding is consistent with prior work showing that RPE during exercise with sub-systolic occlusion is higher than free-flow exercise (Keller-Ross et al., 2016; McClain et al., 1993), indicating contribution from the skeletal muscle afferents to perceived exertion. In addition, we did not observe a difference in RPE between menstrual phases, which also aligns with earlier research (Beidleman et al., 1999; De Souza et al., 1990; Dombovy et al., 1987). Thus, estrogen levels in naturally cycling females do not appear to influence perceived effort level during exercise with partial circulatory occlusion.

With ergoreflex activation, we did not find a main effect of menstrual phase on PETCO2 or RER. Similarly, phase did not affect PETCO2 during metaboreflex activation. In contrast, we observed an interaction of condition and phase on RER with metaboreflex isolation; however, post-hoc tests did not reveal significant between-phase differences during rest, pre-PECO exercise, or PECO. Therefore, we cannot speculate as to the physiological significance of this finding.

4.2. Limitations

This study has several limitations to be considered when interpreting the data. First, we relied on self-reports of menses and ovulation with ovulation kits and did not measure hormone levels directly. Therefore, we cannot confirm that estrogen was indeed high when testing occurred during the presumed ovulation phase or low during the follicular phase. Furthermore, we did not screen for prior use of oral contraceptives. If any of our participants had taken oral contraceptives in the past, their endogenous ovarian hormone production may have remained suppressed, as such effects can last for years after ending oral contraceptive use (Chan et al., 2008; Packard et al., 2011). Finally, while all participants exercised at the same absolute intensity, relative intensity may have differed between individuals.

4.3. Conclusions

Results from the present study suggest that eumenorrheic females may experience a higher VE, due to a greater VT, in the follicular phase than the ovulatory phase at rest and during lower-extremity exercise. However, menstrual phase does not alter ventilatory responses to metaboreflex activation, indicating that estrogen may be mediating the ventilatory response via central mechanisms. Additionally, perceived exertion during submaximal cycling exercise is not influenced by menstrual phase. Therefore, although menstrual phase may alter ventilatory responses in young, eumenorrheic females, phase does not affect submaximal exercise tolerance or signaling from skeletal muscle afferents in this population.

Highlights.

  • Estrogen levels fluctuate during a woman’s menstrual cycle

  • Group III/IV afferents in skeletal muscles influence ventilatory control

  • Rest and exercise ventilation in naturally cycling young women differs in menstrual phases with high and low estrogen levels

  • Menstrual phase does not affect group III/IV skeletal muscle afferent ventilatory signaling

Acknowledgements

We would like to thank all of the individuals who participated in this study.

Funding

The authors were supported by the National Institutes of Health (NIH) K01 (AG064038-01A1) (MKR) and a University of Minnesota Grant-in Aid (MKR & EL).

Footnotes

Declarations of interest

None.

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