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Sex Differences in Muscle Metaboreflex Activation after Static Handgrip Exercise

LEE, JORDAN B.1; NOTAY, KARAMBIR1; SEED, JEREMY D.1; NARDONE, MASSIMO1; OMAZIC, LUCAS J.1; MILLAR, PHILIP J.1,2

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Medicine & Science in Sports & Exercise: December 2021 - Volume 53 - Issue 12 - p 2596-2604
doi: 10.1249/MSS.0000000000002747
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Abstract

Metabolite production and accumulation in contracting skeletal muscle stimulates group III/IV afferents, leading to reflex increases in sympathetic activity directed toward vascular smooth muscle and the heart, and withdrawal of cardiac vagal activity (1,2). This pathway, called the muscle metaboreflex, increases blood pressure (BP) and perfusion pressure to active muscle while increasing cardiac output (2). The metaboreflex has been studied using several methods, such as graded limb ischemia or manipulation of limb perfusion to accentuate or reduce metabolite accumulation during rhythmic or static exercise, with hemodynamic and autonomic responses exaggerated during flow restriction and reduced when perfusion is increased (3–6). The most common method uses postexercise circulatory occlusion (PECO) to trap metabolites in the exercising limb to partition the hemodynamic and autonomic effects of the metaboreflex from those of central command and mechanical stimulation of group III/IV afferents (2,7–10).

Premenopausal women demonstrate attenuated changes in BP, pH, and metabolites during PECO compared with age-matched men, suggesting a smaller activation of the metaboreflex (7–10). Two main hypotheses have been proposed to explain the blunted metaboreflex response in women (8,11). The first hypothesis proposes that because women typically have a greater proportion of type 1 skeletal muscle fibers and greater rates of oxidative metabolism (12), women should have lower metabolite production and accumulation, resulting in less group III/IV afferent stimulation and subsequent sympathetic activation (7,8,11). Proponents of this hypothesis assert that sex differences in muscle mass and strength contribute minimally to smaller metaboreflex responses in women, as BP and muscle sympathetic nerve activity (MSNA) responses to static thumb adductor contractions and PECO are blunted in women despite similar maximal voluntary contraction (MVC) and muscle mass (7).

The second hypothesis proposes that smaller BP responses during exercise and PECO may be the result of lower sympathetic neurovascular transduction in women (11). Support is derived from pharmacologic studies showing attenuated vascular resistance responses in women during norepinephrine infusion (11,13). Smaller changes in limb vascular resistance and larger changes in conductance in women during stressors, such as mental stress, handgrip exercise, and the cold pressor test, despite similar MSNA responses, may also indicate lower neurovascular transduction (14,15). To our knowledge, no studies have examined whether differences in resting sympathetic transduction relate to sex differences in muscle metaboreflex-mediated BP responses.

An underappreciated consideration in most investigations of sex differences in muscle metaboreflex activation is that female participants have, on average, a lower MVC, leading to a lower absolute contraction force at the same relative intensity during the exercise preceding PECO (7–10). As intramuscular flow is interrupted at an absolute contraction intensity (16–18), it may be that blunted metaboreflex responses in women are due to less metabolite accumulation during exercise at the same relative intensity due to lower intramuscular pressure and vascular occlusion, as opposed to reduced metabolite production and sympathetic transduction accounting for smaller BP responses (11). Unlike earlier work on thumb adduction and pressor response (7), strength matching and statistically adjustment for MVC abolishes sex differences in BP responses during static forearm, elbow flexor, and knee extensor exercise (19–21). We recently found that smaller BP responses in women during static leg exercise are correlated to smaller increases in deoxygenated hemoglobin and explained by accounting for differences in MVC (21), providing indirect evidence of a blunted metaboreflex stimulus. Unfortunately, of the studies which have investigated whether controlling for muscle strength accounts for sex differences in BP responses to exercise (7,19,20), only one study reported whether sex differences persisted during isolated metaboreflex activation with PECO (7), leaving it unclear as to whether sex differences in the muscle metaboreflex are accounted for by MVC.

The present study sought to investigate sex differences in BP responses to static exercise and PECO from a large sample of healthy young men and women. The primary aim of the study was to reconcile whether sex differences in BP responses and muscle metaboreflex activation after fixed duration exercise bouts are related to an intrinsic sex difference or related to differences in muscle strength and absolute workload intensity. We hypothesized that, in accordance with prior work, men would display larger BP responses to PECO, but that adjusting for MVC would abolish these differences. Secondary aims sought to investigate whether 1) the combined influence of central command and the muscle mechanoreflex would be greater in men versus women; 2) sex differences in MSNA responses to static exercise are related to MVC (absolute load intensity); and 3) resting sympathetic transduction differed between men and women, and was associated with the magnitude of the BP response to static handgrip exercise and PECO.

METHODS

Study participants

Data were retrospectively analyzed from 200 healthy men and women (age 18–30 yr) who participated in previous studies (22,23) on unrelated hypotheses. A portion of these data (n = 132) has been used previously to examine the effects of MVC on BP responses to static handgrip exercise in men and women (20). Exclusion criteria for this study included a history of cardiovascular, metabolic, or neuromuscular disease, smoking, or the use of any chronic medication aside from oral contraception (n = 47). A total of 15 participants (eight women) self-reported to be left-handed. All women (n = 109) were studied during days 1 to 5 of the early follicular phase, or during the first 5 d of placebo use if taking oral contraceptives. All procedures were approved by the University of Guelph’s Research Ethics Board. Informed written consent was provided before data collection.

Experimental protocol

Participants were instructed to abstain from alcohol, caffeine, and vigorous exercise for 24 h before data collection. Data were collected in a single visit, in which the participants were studied in a light- and temperature-controlled laboratory. After voiding bladder and collection of anthropometrics, participants were seated upright with their feet on an ottoman and instrumented. Maximal voluntary contraction was measured by having the participant exert a minimum of two maximal squeezes on a handgrip dynamometer (Lafayette Instrument, Lafayette, LA) on the left side. Each contraction was performed for ~3 s, and at least 30 s of rest was provided between attempts. The highest value was taken as MVC.

Participants were provided 10 min of rest, after which resting hemodynamics were measured over a 5- to 10-min period. In a subset of the participants, resting MSNA was also measured. After this, the participants completed a 2-min resting baseline, 2 min of static handgrip at 30% MVC, 2 min of PECO, and 2 min of recovery. Postexercise circulatory occlusion was accomplished by inflating a manual sphygmomanometer (DS400 Aneroid Sphygmomanometer; D.E. Hokanson Inc., Bellevue, WA) to 220 mm Hg on the left upper arm ~5 s before exercise cessation.

Measures

During the resting period, discrete BP measures were taken every minute using an automated sphygmomanometer placed on the left arm (BPTru Medical Devices, Coquitlam, BC, Canada). Beat-to-beat measures of BP were recorded during rest and exercise using finger photoplethysmography (Finometer, MIDI; Finapress Medical Systems). Continuous measures of heart rate were collected using single lead electrocardiography (Lead II). Respiration was monitored using a piezo-electric belt placed on the abdomen (Pneumotrace II; UFA, Morro Bay, CA). Muscle sympathetic nerve activity was measured in a subset of participants (n = 39, 21 women). A 2-mΩ was inserted percutaneously into a motor fascicle of the right peroneal nerve and adjusted until spontaneous bursts of muscle sympathetic activity were observed. Muscle sympathetic nerve activity was confirmed by increased activity from end expiratory apnea and lack of responsiveness to unexpected auditory stimuli (loud clap) and stroking of the skin. The raw neural signal was amplified (×75,000), band pass filtered (0.7–2.0 kHz), rectified, and integrated (model 662C-4; Nerve Traffic Analyzer, Absolute Design and Manufacturing Services, Salon, IA). The nerve signal was monitored throughout to ensure no changes in signal quality during the visit. Continuous measures of hemodynamics and respiration were collected at 1000 Hz, whereas the MSNA was collected at 10,000 Hz.

Data analysis

Resting BP, heart rate, and MSNA were calculated as the average of the 5- to 10-min baseline measures. Muscle sympathetic nerve activity was analyzed using a custom semiautomated LabVIEW program, in which sympathetic bursts were counted using a 3:1 signal to noise ratio and a 1.1 to 1.5 s time shift of the cardiac cycle. Muscle sympathetic nerve activity was quantified as both burst frequency (bursts per minute) and burst incidence (bursts per 100 heart beats) to account for interindividual differences in resting heart rate. Total MSNA was also calculated as burst area normalized to the largest resting burst multiplied by burst frequency. During static handgrip exercise, continuous BP, heart rate, and MSNA were averaged during the 2-min rest preceding exercise and during the second minute of handgrip and PECO. The changes from baseline for minute 2 of handgrip and PECO were used for statistical analysis. The differences in BP and MSNA from the second minute of handgrip to PECO were also calculated for each participant to be used as an index of the combined influence of central command and the muscle mechanoreflex on exercise responses. Spontaneous sympathetic baroreflex sensitivity was calculated by assessing the relationship between diastolic BP and MSNA burst occurrence during the 10-min baseline (23). A weighted regression line was fit between the likelihood of a MSNA burst in 2 mm Hg diastolic BP bins in each participant, and the slope was taken as baroreflex sensitivity. The regression was deemed acceptable if the regression coefficient was >0.5; three participants did not meet this criterion. Sympathetic transduction was assessed according to previously used methods (24,25). Briefly, the changes in diastolic BP and total vascular conductance (TVC) (mL·min−1·mm Hg−1) were calculated at each cardiac cycle after a burst, and the changes were assessed over 15 subsequent cardiac cycles, during the 10-min baseline. Technical issues in the continuous BP signal prevented transduction analysis in six participants.

Statistical analysis

Baseline anthropometrics and hemodynamics were compared between men and women using unpaired t tests. Two-way repeated-measures ANOVAs were used to compare changes in hemodynamics and MSNA during handgrip and PECO between men and women. Bonferroni post hoc testing was used to assess differences between groups. An ANCOVA was used to statistically adjust for the effects of MVC during PECO responses. In addition, a strength matched cohort of the 20 weakest men (MVC range, 28–41 kg) and 20 strongest women (MVC range, 32–41 kg) participants were compared. The difference scores between the second minute of handgrip and PECO responses were compared between men and women using unpaired t tests. Pearson correlations were used to assess the relationships between MVC, MSNA, and hemodynamic responses to handgrip and PECO. Simple and multiple linear regression analyses were performed to investigate the predictors of the BP response to handgrip and PECO, with MVC, resting MSNA, ΔMSNA, resting sympathetic transduction (peak rise in diastolic BP and nadir in TVC, respectively), sympathetic baroreflex sensitivity, and resting systolic or diastolic BP included as predictor variables. Significant predictor variables with simple linear regressions were selected for multiple regression analyses. The relative importance of each predictor (individual contribution of a predictor to the total R2) in the multiple regression was calculated using RStudio. Statistical significance was defined as P < 0.05. All data are presented as mean ± SD, unless otherwise stated.

RESULTS

Baseline characteristics are presented in Table 1. Body weight, height, BMI, MVC, and systolic BP were all higher in the men (all P < 0.001), whereas resting heart rate was lower in men (P < 0.001). Diastolic and mean arterial BP were similar between men and women (both P > 0.08).

TABLE 1 - Baseline anthropometric and hemodynamic characteristics.
Characteristics Male (n = 91) Female (n = 109) P
Age, yr 22 ± 2 21 ± 2 0.02
Height, cm 178 ± 7 165 ± 7 <0.0001
Weight, kg 76 ± 13 62 ± 9 <0.0001
Body mass index, kg·m−2 24 ± 3 23 ± 3 0.003
Heart rate, bpm 63 ± 6 70 ± 11 <0.0001
Systolic BP, mm Hg 108 ± 7 102 ± 8 <0.0001
Diastolic BP, mm Hg 65 ± 6 66 ± 7 0.09
Mean arterial pressure, mm Hg 79 ± 6 78 ± 7 0.37
MVC, kg 47 ± 10 27 ± 6 <0.0001
Values are presented as mean ± SD.

As shown in Figure 1, systolic and diastolic BP increased during static handgrip and PECO (time effect, P < 0.0001 for all). The changes in systolic and diastolic BP were larger in men during static handgrip (both P < 0.0001) and PECO (P < 0.0001 for systolic BP, P < 0.05 for diastolic BP). Heart rate showed a significant interaction (P = 0.02), with a trend for higher heart rate during static handgrip in men (15 ± 9 vs 13 ± 8 bpm (men vs women, respectively), P = 0.07) but not during PECO (P = 0.9). Maximal voluntary contraction was correlated to the systolic BP during static handgrip (r = 0.43, P < 0.0001) and PECO (r = 0.29, P < 0.0001) (Fig. 2). Similar results were found for diastolic BP (r = 0.28, P < 0.0001 and r = 0.22, P = 0.0012, respectively). Adjusting for MVC abolished the sex differences in systolic and diastolic BP during PECO (both P > 0.7). Exclusion of women using oral contraceptives did not alter these results (data not shown). Comparison of a strength-matched cohort (n = 40, 20 women; men vs women MVC: 35 ± 4 vs 35 ± 2 kg, P = 0.8) showed no sex differences in either systolic or diastolic BP, or heart rate responses during static handgrip exercise or PECO (all P > 0.2 for sex and interaction effects).

FIGURE 1
FIGURE 1:
Systolic BP (panel A), diastolic BP (panel B), and heart rate (panel C) responses to static handgrip exercise and PECO. Data obtained from 200 participants and presented as mean ± SEM. *P < 0.05, ****P < 0.0001 men vs women.
FIGURE 2
FIGURE 2:
Relationships between MVC and systolic BP responses during static handgrip exercise and PECO.

Men had larger changes in stroke volume and stroke volume index during PECO (both P < 0.01) (Table 2). Men also had larger changes in cardiac output and cardiac index during static handgrip (both P < 0.0001), but only cardiac output (not cardiac index) was larger in men during PECO (P < 0.05). There were no sex or interaction effects for changes in total peripheral resistance or total peripheral resistance index (all P > 0.6).

TABLE 2 - Hemodynamic responses to static HG exercise and post-PECO.
P
Variables Sex ∆HG ∆PECO Sex Time Sex × Time
SV, mL Male 2.4 ± 1.0 10.6 ± 0.9*** 0.007 <0.0001 0.003
Female 0.6 ± 0.6 6.7 ± 0.7
SVI, mL·m−2 Male 1.2 ± 0.5 5.4 ± 0.4** 0.02 <0.0001 0.02
Female 0.3 ± 1.2 3.8 ± 0.4
CO, L·min−1 Male 1.5 ± 0.1*** 0.7 ± 0.1* 0.002 <0.0001 0.002
Female 1.1 ± 0.1 0.4 ± 0.1
CI, L·min−1·m−2 Male 0.8 ± 0.1**** 0.3 ± 0.1 0.01 <0.0001 0.0005
Female 0.6 ± 0.1 0.3 ± 0.1
TPR, mm Hg·L−1·min−1 Male 1 ± 19 107 ± 22 0.82 <0.0001 0.66
Female 7 ± 19 90 ± 18
TPRI, mm Hg·L−1·min−1·m−2 Male 2 ± 10 56 ± 12 0.9 <0.0001 0.9
Female 6 ± 12 54 ± 11
Mean ± SEM.
*P < 0.05 vs women.
**P < 0.01 vs women.
***P < 0.001 vs women.
****P < 0.0001 vs women.
HG, handgrip; SV, stroke volume; SVI, stroke volume index; CO, cardiac output; CI, cardiac index; TPR, total peripheral resistance; TPRI, total peripheral resistance index.

Change in BP from static handgrip to PECO

Men had a greater change in systolic BP from static handgrip to PECO (P = 0.01) (Fig. 3A), whereas the difference in diastolic BP was trending toward being larger in men (P = 0.06). Adjusting for MVC removed the sex difference in systolic BP (P = 0.5) (Fig. 3B) or diastolic BP (P = 0.3) changes from exercise to PECO. There were no sex differences in the change in either systolic or diastolic BP from handgrip to PECO in the strength matched cohort (both P > 0.6).

FIGURE 3
FIGURE 3:
Change in systolic BP from static handgrip exercise to PECO in men and women. Panel A shows unadjusted comparison, whereas panel B adjusts for differences in MVC using an ANCOVA. Data obtained from 200 participants and presented as mean ± SEM.

MSNA responses to static handgrip and PECO

Similar to the larger cohort, in the MSNA subset, MVC was higher in men (49 ± 10 vs 27 ± 5 kg, P < 0.0001). Systolic and diastolic BP increased during handgrip and PECO in the MSNA subset (time effect, P < 0.0001 for all). No sex or interaction effects were detected for either systolic or diastolic BP responses (P > 0.09 for all). Although these effects did not reach statistical significance, the mean systolic BP (23 ± 10 vs 17 ± 10 mm Hg, men vs women, respectively, P = 0.08) and diastolic BP (17 ± 5 vs 14 ± 7 mm Hg, P = 0.08) static handgrip responses in the subset showed similar absolute differences as the full cohort (systolic BP: 24 ± 10 vs 17 ± 9 mm Hg; diastolic BP: 18 ± 7 vs 15 ± 7 mm Hg). Men and women had similar resting MSNA burst frequency (24 ± 8 vs 21 ± 7 bursts per minute, P = 0.2), whereas burst incidence was higher in men (37 ± 11 vs 30 ± 8 bursts per 100 heart beats, P = 0.01). Resting total MSNA trended toward being greater in men (17.0 ± 8.2 vs 12.5 ± 6.3 au, P = 0.06). Sympathetic baroreflex sensitivity did not differ between men and women (4.3 ± 0.4 vs 4.1 ± 0.3 bursts per 100 heart beats·mm Hg−1, P = 0.6). The peak diastolic BP change after an MSNA burst (resting sympathetic transduction) did not differ between men and women (2.2 ± 1.4 vs 2.4 ± 1.1 mm Hg, all P > 0.6). The nadir change in TVC after an MSNA burst trended toward being larger in men (−1.9 ± 0.3 vs −1.0 ± 0.1 L·min−1·mm Hg−1, P = 0.051). Men and women had similar changes in MSNA burst frequency, burst incidence, and total MSNA during both handgrip and PECO (interaction and sex effect, all P > 0.2) (Fig. 4). The change in MSNA burst frequency during static handgrip was correlated to MVC (r = 0.43, P = 0.005) and systolic (r = 0.46, P = 0.003) and diastolic (r = 0.49, P = 0.001) BP responses. Muscle sympathetic nerve activity burst frequency during PECO was unrelated to MVC (r = 0.2, P = 0.2) or systolic (r = 0.1, P = 0.5) and diastolic (r = 0.26, P = 0.1) BP responses.

FIGURE 4
FIGURE 4:
Changes in MSNA burst frequency (panel A), burst incidence (panel B), and total MSNA (panel C) during static handgrip exercise and PECO. Data obtained from 39 participants and presented as mean ± SEM.

Regression analysis

Simple and multiple linear regression analyses from data in the MSNA subset are presented in Table 3. Multiple linear regression analyses showed that MVC and the change in MSNA burst frequency during static handgrip exercise were independently associated with the systolic BP response to static handgrip exercise and explained 31% of the variability in BP responses. The relative importance of MVC accounted for 17% of the total R2 (i.e., ~55% of the explained variability). Maximal voluntary contraction and the change in MSNA during static handgrip exercise explained 30% of the variability in diastolic BP, with MVC contributing 12% of the total R2. Maximal voluntary contraction and the change in MSNA explained 19% and 21% of the variability in systolic and diastolic BP response during PECO, respectively. Maximal voluntary contraction had a relative importance of 18% and 16% for the systolic and diastolic BP response during PECO, respectively.

TABLE 3 - Simple and multiple linear regression coefficients for predictors of BP responses during static handgrip exercise and PECO.
Variables Simple Linear Regression β (95% CI), P Multiple Linear Regression β (95% CI), P
Static handgrip exercise
 Systolic BP
  MVC 0.49 (0.152 to 0.595), 0.002 0.35 (0.03 to 0.5), 0.026
  Resting sympathetic transduction-TVC a −0.04 (−3.4 to 2.7), 0.81
  Resting sympathetic transduction-BP −0.005 (−4.54 to 4.41), 0.98
  Resting Systolic BP 0.21 (−0.12 to 0.62), 0.18
  Resting MSNA burst frequency −0.12 (−0.583 to 0.266), 0.46
  Resting sympathetic baroreflex sensitivity b −0.03 (−2.52 to 2.0), 0.84
  Δ MSNA burst frequency 0.46 (0.172 to 0.778), 0.003 0.3 (−0.003 to 0.63), 0.052
 Diastolic BP
  MVC 0.43 (0.06 to 0.34), 0.007 0.26 (−0.02 to 0.27), 0.1
  Resting sympathetic transduction-TVC a −0.03 (−1.8 to 2.2), 0.84
  Resting sympathetic transduction BP −0.02 (−3.15 to 2.7),0.9
  Resting diastolic BP 0.14 (−0.19 to 0.50), 0.36
  Resting MSNA burst frequency −0.04 (−0.3 to 0.23), 0.79
  Resting sympathetic baroreflex sensitivity b 0.15 (−0.78 to 2.0), 0.37
  Δ MSNA burst frequency 0.50 (0.13 to 0.50), 0.001 0.38 (0.04 to 0.44),0.01
PECO
 Systolic BP
  MVC 0.43 (0.10 to 0.57), 0.006 0.43 (0.09 to 0.57), 0.009
  Resting sympathetic transduction-TVC a −0.04 (−3.5 to 2.7), 0.8
  Resting sympathetic transduction-BP 0.03 (−4.14 to 5.1), 0.83
  Resting systolic BP 0.16 (−0.19 to 0.57), 0.33
  Resting MSNA burst frequency −0.01 (−0.45 to 0.42), 0.95
  Resting sympathetic baroreflex sensitivity b −0.18 (−3.4 to 1.0), 0.28
  Δ MSNA burst frequency 0.11 (−0.27 to 0.52), 0.53 0.01 (−0.36 to 0.39), 0.9
 Diastolic BP
  MVC 0.42 (0.05 to 0.31), 0.007 0.38 (0.03 to 0.3), 0.02
  Resting sympathetic transduction TVC a −0.07 (−1.4 to 2.1), 0.69
  Resting sympathetic transduction BP −0.01 (−2.8 to 2.6), 0.94
  Resting diastolic BP 0.07 (−0.25 to 0.39), 0.65
  Resting MSNA burst frequency −0.11 (−0.15 to 0.32), 0.46
  Resting sympathetic baroreflex sensitivity b −0.11 (−1.6 to 0.8), 0.51
  Δ MSNA burst frequency 0.27 (−0.03 to 0.39), 0.1 0.36 (−0.08 to 0.32), 0.23
Data from the subset of 39 participants with measurements of MSNA.
Bold data indicates significant predictors (P < 0.05).
an = 33.
bn = 36.
β, standardized regression coefficient.

DISCUSSION

The present study sought to determine the role of absolute contraction load on BP responses during muscle metaboreflex activation using PECO. The novel findings of this study were as follows. First, men had larger BP responses to isolated metaboreflex activation after static handgrip exercise, which were correlated with MVC. Statistical adjustment for MVC or comparison of a strength-matched subset abolished these sex differences. Second, the change in BP from handgrip to PECO was larger in men, and correlated to MVC, indicating a strength-related difference in the effects of central command and/or the muscle mechanoreflex. Third, MVC was the only significant predictor of the BP responses during PECO in regression analyses of the MSNA subset, explaining 16% to 18% of the variability. These findings add to a growing body of work, which suggests that hemodynamic and autonomic differences observed during fixed duration, relative intensity exercise between sexes are, in part, related to differences in absolute contraction load.

Women have a blunted BP response to static exercise compared with men (7–9,19–21,26,27). As this difference persists during PECO, it is thought that these attenuated BP responses are caused by less activation of the muscle metaboreflex during exercise (7,8). Yet it is unclear whether this is an inherent sex difference or is related to lower muscle strength. Because women had smaller BP responses to static thumb adductor contractions and PECO, despite similar MVC, sex differences in muscle metaboreflex activation were thought to be independent of muscle mass and strength (7,11). Instead, the smaller responses have been attributed to two hypotheses. The first hypothesis is that women have lower metabolite production at the same absolute exercise intensity because of the differences in skeletal muscle fiber type distributions and/or oxidative metabolism, the latter of which may be influenced by the effects of sex hormones (7,11,12,28). The work with thumb adduction exercise did not control for menstrual cycle phase or estrogen supplementation (7), whereas the present work only studied women during the early follicular (low hormone) phase. Because the pressor response to handgrip exercise is greater during oral contraceptive use and potentially increased during the low hormone phase of the menstrual cycle (8,29–31), it is possible that the discrepancy between the thumb adductor work and the present study is explained by variations in sex hormones (7). The present study, with a much larger sample size, supports the hypothesis that MVC can play a role in determining sex differences in BP (when the menstrual cycle is controlled) during muscle metaboreflex activation using PECO, as well as differences in central command/muscle mechanoreflex activation. It is important to consider that MVC is not the sole determinant of BP responses to muscle metaboreflex activation, as MVC alone explained only ~18% of the variance in systolic BP to handgrip and PECO, respectively (data from whole sample and subset multiple linear regression analyses). Evidence supports that other factors, such as relative exercise intensity, sex hormones, oral contraceptives, and genetic variations of receptor populations found on group III/IV skeletal muscle afferents, can modulate the pressor response (22,29–32).

The second hypothesis proposed to explain the smaller metaboreflex responses in women is blunted sympathetic transduction, based on sex differences in regional measures of vascular resistance or conductance relative to the changes in sympathetic outflow during various stressors and norepinephrine infusion (11,13–15). One mechanism for reduced transduction in women involves greater contributions of β2-adrenergic receptor-mediated vasodilation offsetting α-adrenergic–mediated constriction in women (33). However, these regional differences may not translate to systemic differences in BP, as the ratio of BP to MSNA responsiveness during handgrip did not differ between sexes, providing evidence against blunted transduction (8). In agreement with previous literature (34), our results showed that resting signal-averaged sympathetic transduction of BP was not different between men and women. In contrast, resting sympathetic transduction of TVC tended to be greater in men, in agreement with prior work using regional vascular conductance (35). However, in regression analyses, resting sympathetic transduction of BP or TVC was not a significant predictor of the BP response to handgrip exercise or PECO. The muscle metaboreflex was originally suggested to act as a blood flow restorative mechanism meant to correct mismatches between oxygen delivery and metabolic demand, by increasing perfusion pressure to the active muscle (2). The ability of the muscle metaboreflex in restoring blood flow in humans has been debated, as some contend that vasoconstriction occurs in the active skeletal muscle, which originally evoked the muscle metaboreflex, offsetting any flow raising the effect of systemic pressure changes (36). This has led to the hypothesis that muscle metaboreflex-mediated increases in sympathetic outflow are intended to restrain BP from declining because of vasodilation in active muscle. In contrast, isolated metaboreflex activation from distant muscle groups increases blood flow to the active muscle, supporting the original hypothesis that the metaboreflex is acting to raise flow (37). Although large muscle mass, high-intensity, dynamic exercise may require sympathetic restraint of BP, given that the muscle metaboreflex can increase cardiac output, it would appear that, at least in some scenarios, it functions to attempt to restore blood flow (2). If the intended purpose of the sympathetically mediated increase in BP is to overcome mechanical compression of the vasculature, this could explain why our data indicate a strength-related influence on the muscle metaboreflex. Vascular occlusion of the forearm was shown to occur at an absolute workload of 34 kg, regardless of relative intensity (16), thus in higher-strength individuals, there would be an earlier perfusion impairment when matched for relative intensity. This would lead to greater metabolite accumulation, which would stimulate group III/IV afferents to a larger extent and lead to greater increases in sympathetic outflow (1). The greater buildup of metabolites would continue to stimulate these afferents during PECO, as suggested by the relationship between MVC and BP during PECO. Interesting, the change in MSNA was associated independently with the diastolic BP response during static handgrip exercise but not PECO. Prior work has shown the capacity for leg vascular resistance to be dissociated from MSNA during PECO, and instead related to changes in BP (38). These results highlight the complexity in delineating the neural contribution to BP regulation.

A novel finding was that the decrease in BP from static handgrip to PECO was larger in men than in women. This indicates a greater influence of central command and/or the muscle mechanoreflex in men, in addition to the muscle metaboreflex in regulating BP during static handgrip exercise. In the decerebrate rat, a model in which central command is absent, the pressor responses to passive stretch or stimulated contractions are larger when these stimuli involve two limbs compared with one, indicating that the hemodynamic effects of the muscle mechanoreflex are influenced by the size of the active muscle mass (39). These findings have also been shown in humans (40), suggesting that a larger muscle mass in stronger individuals leads to a greater muscle mechanoreflex contribution to BP responses. Notably, women display smaller cardiovascular responses to muscle mechanoreflex activation via passive limb movement in prior work (41). These results demonstrate that differences in muscle mass contribute to attenuated BP responses during exercise in women, as observed in our study. Alternatively, because the muscle mechanoreflex can be sensitized by metabolites (42), if stronger individuals had a greater accumulation of metabolites during exercise, this could increase the gain of the mechanoreflex and cause a larger BP response during exercise. It is unclear why central command would be influenced, as the percentage of MVC was matched between men and women. Heart rate and BP responses are smaller in women during the first 30 s of handgrip exercise (43), a time course that is thought to primarily reflect the influence of central command and the mechanoreflex (44). The smaller cardiovascular responses in women during this time were associated with weaker activation of cortical regions involved in cardiovascular regulation, providing indirect evidence of a blunted contribution of central command (43). Estrogen attenuates the effects of central command in animal models (45). However, this does not explain why adjusting for MVC abolished the BP differences between static handgrip exercise and PECO. Time-to-failure is negatively correlated with MVC (19), such that stronger individuals may be closer to failure, thereby requiring a larger increase in central motor drive. This could be explained by greater metabolite accumulation and group III/IV afferent motoneuron inhibition, as men have greater increases in central motor drive during sustained contractions but not intermittent static contractions, which would promote greater perfusion and metabolite clearance (46). Finally, it is possible that sex differences in perceived effort could influence hemodynamic responses. However, men and women have similar perceived exertion during static contractions with the knee flexors and forearm muscles during short to moderate duration (5–200 s) contractions (43,47), whereas men have higher perceived exertion during longer duration (≥200 s) contractions (46). Future work should seek to measure ratings of perceived exertion or forearm electromyography to offer insight into the mechanisms of this difference.

We acknowledge several methodological and interpretation considerations. First, because of technical and time constraints, MSNA was measured only in a subset of the participants (n = 39). Comparison of the hemodynamic responses in the subset between men and women did not reach statistical significance, unlike the larger cohort, although the absolute trends were similar between the smaller and larger sample. This was likely due to the variability in the hemodynamic responses and lower statistical power in the subset. Second, the influence of MVC on the BP responses during muscle metaboreflex activation is based on correlational and linear covariate analyses. Future studies should focus on experimentally manipulating muscle strength or the muscle metaboreflex in men and women to directly investigate potential mechanisms. Third, unlike prior work that has investigated sex differences during static exercise (7,8), changes in MSNA were similar between sexes. This likely reflects between-study differences in exercise protocols. Static handgrip exercise at 40% MVC observed sex differences in total MSNA responses only after >2 min of contraction (8), whereas 60% MVC adductor pollicus exercise found sex differences in total MSNA during the second minute of contraction (11). We cannot exclude the possibility that MSNA may contribute to BP differences between sexes to other exercise intensities or modes. Fourth, resting sympathetic transduction analyses are based on beat-to-beat changes in BP and TVC after a burst. This analysis does not account for potential interactive effects of burst area or patterns of firing nor the potential for nonneural mechanisms to influence beat-to-beat hemodynamics, such as myogenic autoregulation (48). Measures of sympathetic transduction and sympathetic baroreflex sensitivity were only assessed at rest and may not be representative of measures during exercise or PECO. Finally, we did not assess exercise BP responses to failure, in line with prior work in this area (7,9,10,21). Because differences in time-to-failure are associated with MVC (19), normalizing BP responses to a percentage of time to failure is thought to abolish sex differences in BP responses (49). However, static contractions to failure may be confounded by uncontrolled factors, such as subjective differences in pain, perception of effort, and motivation (50), limiting the ability to accurately normalize time-to-failure and adding uncertainty as to whether true physiological failure was reached. Moreover, sex differences in BP responses can persist even during contractions to volitional failure (8,19), and the magnitude of this response has been correlated to MVC (19).

In conclusion, we demonstrate that statistically controlling for MVC using an ANCOVA, eliminates sex differences in BP responses to PECO, providing evidence that assessments of muscle metaboreflex activation are influenced by absolute contraction load during the prior exercise. The larger BP responses to fixed-duration, 30% MVC static handgrip exercise in men may also involve a greater contribution of central command and/or the muscle mechanoreflex. Our results provide evidence against recent hypotheses that a blunted exercise pressor reflex in women is related to innate sex differences in metabolite production or sympathetic transduction (11), and instead suggest an influence of absolute contraction load (i.e., muscle strength) as an important determinant of the pressor response to static handgrip exercise and PECO. Future work investigating the mechanisms responsible for potential sex differences in exercise BP should account for interindividual differences in MVC.

The authors wish to thank Drs. David Mutch and Shannon Klingel for their assistance with participant recruitment.

Disclosures: The authors declare no conflicts of interest to report. The results of the present study do not constitute endorsement by ACSM. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.

Author Contributions: J. B. L., K. N., and P. J. M. conceived and designed the research. J. B. L., K. N., J. D. S., and P. J. M. performed experiments. J. B. L., K. N., L. J. O., M. N., and P. J. M. analyzed the data. J. B. L. and P. J. M. interpreted the results. J. B. L. drafted the figures and prepared the article. J. B. L. and P. J. M. edited and revised article. All authors have read and approved the final article.

Funding: This research was supported by a Natural Science and Engineering Research Council of Canada (NSERC) Discovery Grant (P. J. M. 06019), the Canada Foundation for Innovation (P. J. M. 34379), the Ontario Ministry of Research, Innovation, and Science (P. J. M. 34379), and an Early Researcher Award by the Ontario Ministry of Economic Development, Job Creation and Trade. An NSERC Doctoral award supported J. B. L. None of the funding sources had a role in designing or conducting the study; analysis or interpretation of data; or preparation or review of the manuscript before submission.

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Keywords:

MAXIMAL VOLUNTARY CONTRACTION; SEX; MUSCLE SYMPATHETIC NERVE ACTIVITY; METABOREFLEX

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