Effects of dynamic arm and leg exercise on muscle sympathetic nerve activity and vascular conductance in the inactive leg

Abstract

The influence of muscle sympathetic nerve activity (MSNA) responses on local vascular conductance during exercise are not well established. Variations in exercise mode and active muscle mass can produce divergent MSNA responses. Therefore, we sought to examine the effects of small- versus large-muscle mass dynamic exercise on vascular conductance and MSNA responses in the inactive limb. Thirty-five participants completed two study visits in a randomized order. During visit 1, superficial femoral artery (SFA) blood flow (Doppler ultrasound) was assessed at rest and during steady-state rhythmic handgrip (RHG; 1:1 duty cycle, 40% maximal voluntary contraction), one-leg cycling (17 ± 3% peak power output), and concurrent exercise at the same intensities. During visit 2, MSNA (contralateral fibular nerve microneurography) was acquired successfully in 12/35 participants during the same exercise modes. SFA blood flow increased during RHG (P < 0.0001) and concurrent exercise (P = 0.03) but not cycling (P = 0.91). SFA vascular conductance was unchanged during RHG (P = 0.88) but reduced similarly during concurrent and cycling exercise (both P < 0.003). RHG increased MSNA burst frequency (P = 0.04) without altering burst amplitude (P = 0.69) or total MSNA (P = 0.26). In contrast, cycling and concurrent exercise had no effects on MSNA burst frequency (both P ≥ 0.10) but increased burst amplitude (both P ≤ 0.001) and total MSNA (both P ≤ 0.007). Across all exercise modes, the changes in MSNA burst amplitude and SFA vascular conductance were correlated negatively (r = −0.43, P = 0.02). In summary, the functional vascular consequences of alterations in sympathetic outflow to skeletal muscle are most closely associated with changes in MSNA burst amplitude, but not frequency, during low-intensity dynamic exercise.

NEW & NOTEWORTHY Low-intensity small- versus large-muscle mass exercise can elicit divergent effects on muscle sympathetic nerve activity (MSNA). We examined the relationships between changes in MSNA (burst frequency and amplitude) and superficial femoral artery (SFA) vascular conductance during rhythmic handgrip, one-leg cycling, and concurrent exercise in the inactive leg. Only changes in MSNA burst amplitude were inversely associated with SFA vascular conductance responses. This result highlights the functional importance of measuring MSNA burst amplitude during exercise.

INTRODUCTION

The functional consequences of altering muscle sympathetic nerve activity (MSNA) during dynamic exercise remain poorly studied, although increased peripheral vasoconstrictor drive is considered important for maintaining perfusion pressure and redistributing blood flow (14). Early microneurography studies reported that static handgrip exercise increased fibular nerve MSNA (i.e., inactive limb) coincident with reductions in calf blood flow and increases in vascular resistance (38, 40). However, similar work was not replicated during dynamic exercise; low-intensity rhythmic plantar flexion increased active limb calf blood flow without a change in contralateral leg MSNA (15, 37). Interpreting these results during dynamic exercise is difficult given that MSNA can be differentially regulated to active and inactive legs during exercise (1) and that neural vasoconstriction is blunted in the active limb by local vasodilatory signals and the presence of functional sympatholysis (11). Whether similar uncoupling of neural and vascular responses exists in the resting limb during dynamic exercise, particularly during low to moderate intensities in which cardiac output is not limited, has not been studied.

An important consideration of prior work investigating MSNA and blood flow responses during exercise (15, 37, 38, 40) has been the reliance on low- to moderate-intensity small-muscle mass exercise (handgrip or plantar flexion), which increases MSNA (1, 15, 24, 38, 40). In contrast, similar-intensity large-muscle mass exercise (cycling or knee extension) decreases MSNA secondary to elevations in central venous pressure and loading of the cardiopulmonary baroreflex (3, 16, 35). Our group recently demonstrated that one-leg cycling can reduce MSNA burst frequency but also simultaneously increase MSNA burst amplitude (i.e., larger but less frequent multiunit bursts) (3). Variations in resting MSNA burst amplitude (or area) have been associated with graded reductions in leg vascular conductance (5) and transduction into diastolic blood pressure (2), but whether changes in multiunit MSNA burst frequency versus amplitude have different functional significance during exercise is unclear.

Given the capacity for small- versus large-muscle mass exercise to evoke divergent MSNA responses, one unresolved question is whether performing these exercise modes concurrently, such as encountered commonly in everyday life and sport, produces additive neural and vascular responses. Past work has demonstrated that the addition of static handgrip exercise to dynamic knee extension exercise had no effect on plasma norepinephrine or leg norepinephrine spillover, despite causing a reduction in leg vascular conductance (42). More recently, reductions in MSNA during lower body positive pressure (i.e., cardiopulmonary loading) were blunted by high-, but not low- or moderate-, intensity muscle metaboreflex activation using postexercise circulatory occlusion (17). These results suggest nonadditive sympathetic responses (i.e., threshold effect) during stimulation of opposing peripheral afferent reflexes. To our knowledge, a comprehensive examination of neural and vascular responses during rhythmic handgrip (RHG), leg cycling, and concurrent exercise has not been reported.

Therefore, the primary objective of the present study was to investigate the changes in resting leg superficial femoral artery (SFA) vascular conductance and MSNA during 1) small-muscle mass (RHG), 2) large-muscle mass (1-leg cycling), and 3) concurrent RHG and cycling exercise in young healthy men and women. In contrast to prior work that assessed calf blood flow with venous occlusion plethysmography (15, 37, 38, 40), we collected Doppler ultrasound measures of SFA blood flow to align closely with the neural target of the recorded changes in MSNA. It was hypothesized that 1) RHG would increase total MSNA and decrease SFA vascular conductance; 2) cycling would reduce total MSNA and increase SFA vascular conductance; 3) concurrent RHG and cycling exercise would not change total MSNA or SFA vascular conductance (i.e., additive response); and 4) across all exercise modes the relationships between MSNA and SFA vascular conductance would differ based on use of burst frequency or burst amplitude. Secondary objectives were to investigate the effects of each exercise mode on cardiac baroreflex sensitivity (BRS) and heart rate variability.

METHODS

Participants.

Thirty-five healthy young men (n = 20) and women (n = 15) were recruited by local advertisement to participate in the study after providing written informed consent. All were nonsmoking, in sinus rhythm, normotensive, and free of known cardiovascular or metabolic diseases. Participants were free of acute or chronic medications, with the exception of oral contraception (n = 8) and hormone-releasing intrauterine devices (n = 3). All women were tested during the first 5 days of the early follicular period or low-hormone contraception phase. All procedures were approved by the University of Guelph Research Ethics Board.

Procedures and protocol.

All participants completed an initial familiarization visit to discuss the study protocol in detail and collect anthropometric data. After this, participants completed a maximal exercise test to determine peak power output with a 20 W/min ramped cycle ergometer (Velotron; Racermate, Seattle, WA) protocol (1 W every 3 s) to exhaustion, determined as the inability to maintain a pedaling frequency of at least 50 rpm. The starting resistance was individualized based on exercise training status and physical activity levels so that each participant reached maximum effort between 8 and 14 min. Peak oxygen consumption was measured via indirect calorimetry (Quark CPET; COSMED, Rome, Italy).

During each of the experimental testing visits, participants entered the laboratory having abstained from intense exercise, alcohol, and caffeine for 24 h and were positioned in a semirecumbent posture with the torso elevated at an angle of 30° to the bed. Participants were secured with a belt over the hips to reduce excessive movement of the nonexercising lower limb and had their right leg strapped to a modified cycle ergometer (Monark Rehab Trainer 881E; Monark Exercise, Vansbro, Sweden) positioned at the end of the bed on a custom-built platform. Next, participants completed three maximal voluntary contractions (MVCs) on a handgrip dynamometer (MLT004/ST; ADInstruments, Sydney, NSW, Australia) and underwent instrumentation for hemodynamic monitoring. After a 5-min rest period, 10 min of resting baseline measurements was recorded. Next, continuous measures of hemodynamic, neural, and/or leg blood flow data were collected during a 3-min resting baseline followed by 3 min of RHG exercise (right hand) with a 1:1 duty cycle at 40% MVC. The force output was displayed on a screen so that each participant could maintain the required MVC target. After at least 8 min of rest to permit hemodynamic and neural variables to return to baseline, participants completed another 3-min resting baseline followed by 3 min of one-leg cycling (right leg) at 50–60 rpm at an intensity of 25 W for male participants and 20 W for female participants (both 17 ± 3% of peak power output). This intensity was selected on the basis of pilot testing, which identified the maximum work rate that could be performed while maintaining a stable MSNA site. After a second recovery period, participants completed a 3-min baseline followed by 3 min of concurrent RHG and one-leg cycling at the same intensities and duty cycles as described above. This protocol was replicated twice in all participants with 2–7 days between sessions, measuring resting leg SFA blood flow during visit 1 and contralateral leg MSNA during visit 2. Importantly, measures of MSNA, lower leg vascular conductance, and SFA blood flow have each been shown to exhibit high day-to-day repeatability (12, 19, 29, 48). The order of these two experimental visits was randomized.

Measurements.

Discrete blood pressure was measured from the right arm by an automated oscillometric device (BPTru Medical Devices, Coquitlam, BC, Canada), and continuous beat-to-beat blood pressure was acquired by finger photoplethysmography from the left middle finger (Finometer MIDI; Finapres Medical Systems). The model flow method (49) was used to estimate stroke volume and permit the calculation of cardiac output and total vascular conductance. Continuous measures of respiratory rate and depth (% change from baseline) were collected with a piezoelectric transducer belt (Pneumotrace II; UFA, Morro Bay, CA), and heart rate was recorded from lead II of the electrocardiogram (ADInstruments, Bella Vista, NSW, Australia).

Blood flow measurements were determined by duplex-mode ultrasound (Vivid q; General Electric, Boston, MA) via a probe operating at 13 MHz for B mode and 5 MHz for Doppler, to permit collection of concurrent SFA diameter and blood velocity profiles, the latter collected at the lowest possible insonation angle (≤60°). Images of the artery diameter and associated blood velocity profiles were captured and stored on a computer at 60 Hz with the DVI2USB 3.0 video grabber (Epiphan System, Ottawa, ON, Canada). During each baseline measurement a foam guide was taped to the surface of the participant’s inner thigh to maintain consistency of probe placement during subsequent exercise trials. Semiautomated off-line analysis of recorded images determined artery diameter and tracing of the Doppler peak velocity envelope (Cardiovascular Suite; Quipu, Pisa, Italy) (47). The program determines vessel diameter by continuously measuring the median distance from the lumen-intima interface of the near wall to the far wall within a user-defined region of interest. To determine blood velocity, the program traced peak antegrade and retrograde blood velocity envelopes above and below the zero point, and each value was halved to estimate mean antegrade and retrograde velocity (4, 23). Mean blood velocity was calculated subsequently by adding mean antegrade and retrograde blood velocities together. Mean SFA blood flow was calculated in 1-s time bins based on the formula (mean blood velocity) × (cross-sectional area) × 60. The cross-sectional area of the artery was calculated with the following equation: π × r², where r is the radius of the artery found by dividing the mean baseline diameter by 2. Because resting leg SFA diameters were unchanged after exercise (all P > 0.6), and to avoid errors related to participant movement during leg exercise, mean baseline diameters were used to calculate blood flow in the inactive limb. In the present study, resting SFA blood flow measurements had an intraobserver coefficient of variation of 10.4%.

Multiunit microneurographic recordings of postganglionic MSNA were obtained with a tungsten microelectrode (Frederick Haer, Brunswick, ME) inserted percutaneously into the left fibular nerve, as described previously (3, 28, 30). Adjustments of the microelectrode were made until spontaneous multiunit sympathetic activity were detected from background activity. The MSNA signal was amplified (75,000×), band-pass filtered (0.7–2.0 kHz), rectified, and integrated using a 0.1-s time constant to obtain a mean voltage neurogram (Nerve Traffic Analyzer, model 662C-4; Absolute Design and Manufacturing Services, Solon, IA). Confirmation of muscle sympathetic activity was completed by observing increases in activity in response to an end-expiration breath hold and lack of responsiveness to unanticipated clapping. Visual and audible monitoring throughout each recording and exercise period ensured that there was no movement in microelectrode placement. If a shift in the neurogram was detected, the protocol was stopped and restarted after a sufficient recovery period to return all measures to baseline. Our group has demonstrated high interday repeatability of resting MSNA (intraclass correlation coefficient > 0.76) (29). All continuous signals were collected at 1 kHz, with the exception of the raw neurogram, which was sampled at 10 kHz (PowerLab; AD Instruments, Sydney, NSW, Australia).

Data analysis.

Hemodynamic measures are calculated as the mean over the 3-min baseline and the last minute of each exercise trial. Spontaneous cardiac BRS was calculated from the full 3-min baseline and exercise epochs with the sequence technique (LabChart v8; ADInstruments, Sydney, NSW, Australia) as described previously (3). Briefly, we identified three consecutive and simultaneous increases or decreases in systolic blood pressure and R-R interval with minimum threshold changes of 1 mmHg and 6 ms, respectively (34). All sequences were reviewed, and cardiac BRS was quantified by plotting R-R interval over systolic blood pressure for each identified series and averaging the slope of all up sequences and all down sequences, and the average of all sequences. Sequences were deemed acceptable to use if the regression coefficient (r) was ≥0.8 and a minimum of three up and three down sequences were identified for each baseline and exercise period. In total 18 participants did not meet these criteria for each exercise protocol, and data on cardiac BRS are presented for the remaining 17 participants. Time-domain heart rate variability was assessed as a noninvasive marker of cardiac parasympathetic modulation (41) and determined with Kubios Heart Rate Variability Analysis Software 3.1 (Biosignal Analysis and Medical Imaging Group, Department of Applied Physics, University of Eastern Finland, Kuopio, Finland). Heart rate variability variables included the standard deviation of normal R-R intervals (SDNN) and the root mean square of successive R-R interval differences (RMSSD).

Vascular conductance was calculated systemically and regionally from the division of cardiac output or leg blood flow by mean arterial blood pressure. MSNA was analyzed with a custom LabVIEW program (National Instruments, Austin, TX) (3, 28, 30). Determination of a sympathetic burst was based on a minimum signal-to-noise ratio of 3:1 and alignment with the time-shifted R-wave of a cardiac cycle. The integrated neurogram signal was analyzed to collect MSNA burst frequency (bursts/min), normalized burst amplitude (% of tallest burst from the preceding baseline), and total MSNA. Total MSNA was calculated as the product of burst frequency and normalized burst amplitude.

Statistical analysis.

Changes in hemodynamic and neural variables during the last minute of RHG, one-leg cycling, and concurrent exercise were compared by two-way (time × exercise modality) repeated-measures ANOVA (GraphPad Software, La Jolla, CA). Significant interactions and main effects of exercise mode were probed by Tukey’s post hoc tests; main effects for time were further examined by Holm-Sidak post hoc tests. Paired t-tests were used to ensure that variables were similar between visits, and unpaired t-tests were used to confirm that variables were comparable between the larger cohort and the subset with complete MSNA data and to explore potential sex differences. Bivariate Pearson’s correlation analyses were performed to determine the relationships between changes in MSNA (burst frequency or amplitude, total MSNA) and SFA vascular conductance. Potential outliers in correlational analyses were identified as those with residuals ≥ 2 SD from the regression. One participant was identified as an outlier in each of the exercise conditions, and correlational data are reported with and without the identified outlier present. P < 0.05 was considered statistically significant. All data are presented as means ± SD, unless otherwise stated.

RESULTS

Thirty-five participants were recruited and completed hemodynamic and blood flow measures during exercise (participant characteristics, Table 1). Given the high degree of technical difficulty, we were unsuccessful in locating (n = 3) or maintaining (n = 20) a microneurographic recording site in 23 participants. Furthermore, in two participants motor activation during exercise precluded the analysis of MSNA burst amplitude. Thus final MSNA analysis was completed on 12 participants (6 women, 6 men) for burst frequency and 10 participants (5 women, 5 men) for burst amplitude. Representative 1-min MSNA recordings from one participant at rest and during RHG, one-leg cycling, and concurrent RHG and cycling exercise are shown in Fig. 1.

Table 1.

Participant characteristics

Variable	Mean	Range
Sex, male/female	20/15
Age, yr	23 ± 3	18–29
Height, cm	172 ± 8	157–184
Weight, kg	70 ± 14	47–101
Body mass index, kg/m2	23.4 ± 3	18.6–29.7
Peak V̇o2, ml·kg−1·min−1	47 ± 8	30–63
Peak cycling power, W	278 ± 59	173–400

Values are means ± SD. V̇o₂, oxygen consumption.

Fig. 1.

Representative microneurography tracings at rest (Baseline) and during the last minute (Exercise) of rhythmic handgrip, 1-leg cycling, and concurrent exercise.

Hemodynamic and respiratory responses.

The effects of RHG, one-leg cycling, and concurrent exercise on hemodynamic and respiratory responses are shown in Table 2. Heart rate, blood pressure, cardiac output, and respiration frequency all increased from rest during each of the three exercise protocols (all P < 0.0001). In addition, stroke volume, total vascular conductance, and respiration depth increased during cycling and concurrent exercise (all P < 0.0001). Both cycling and concurrent exercise increased heart rate, cardiac output, stroke volume, total vascular conductance, and respiration frequency and depth to a greater extent than RHG; increases in diastolic blood pressure were smaller with cycling, but greater during concurrent exercise, than RHG (all P < 0.0001). Concurrent exercise had greater increases in heart rate, diastolic blood pressure, cardiac output, and respiration frequency than cycling (all P < 0.006). The MSNA subgroup (n = 12) had a smaller increase in respiratory rate during RHG (P = 0.02) and cycling (P = 0.03) than the remaining cohort (n = 23) and a smaller increase in stroke volume (P = 0.03) and total vascular conductance (P = 0.04) during concurrent exercise. All other hemodynamic and respiratory responses were not statistically different between the MSNA subset and the remaining cohort (all P > 0.1). When potential sex differences were explored, men had a larger increase in systolic (P < 0.001) and diastolic (P = 0.02) blood pressure during RHG and a larger increase in systolic blood pressure (P = 0.002) and a smaller increase in total vascular conductance (P = 0.02) during concurrent exercise.

Table 2.

Hemodynamic and respiratory variables at rest and during exercise

	RHG		Cycling		Concurrent
Variable	Rest	Exercise	Rest	Exercise	Rest	Exercise
Heart rate, beats/min	64 ± 12	79 ± 14*	64 ± 12	87 ± 12*†	65 ± 13	96 ± 14*†#
Systolic BP, mmHg	122 ± 10	141 ± 15*	124 ± 10	142 ± 12*	126 ± 10	154 ± 15*†#
Diastolic BP, mmHg	67 ± 6	82 ± 8*	68 ± 7	77 ± 8*†	69 ± 7	85 ± 7*†#
Stroke volume, ml	97 ± 21	100 ± 24	98 ± 21	105 ± 24*†	98 ± 21	106 ± 25*†
Cardiac output, l/min	6.2 ± 1.7	7.9 ± 2.3*	6.2 ± 1.7	9.1 ± 2.3*†	6.3 ± 1.7	10.1 ± 2.5*†#
TVC, ml·min−1·mmHg−1	72 ± 21	74 ± 22	71 ± 21	90 ± 24*†	71 ± 22	90 ± 22*†
Resp. frequency, breaths/min	17 ± 4	23 ± 5*	18 ± 4	26 ± 5*†	17 ± 3	27 ± 4*†#
Resp. depth, %	100	120 ± 35	100	200 ± 66*†	100	202 ± 84*†

Values are means ± SD. BP, blood pressure; Resp, respiration; RHG, rhythmic handgrip; TVC, total vascular conductance.

^*P < 0.05 compared with rest;

^†P < 0.05 compared with RHG;

^#P < 0.05 compared with cycling.

SFA blood flow and vascular conductance responses.

SFA blood flow and vascular conductance data are presented in Fig. 2. SFA antegrade blood velocity increased in each of the exercise modes (all P < 0.0001), with larger increases during concurrent exercise compared with RHG (Δ 2.2 ± 2.1 vs. 1.3 ± 1.5 cm/s, P < 0.0001) and cycling (Δ 2.2 ± 2.1 vs. 1.5 ± 1.8 cm/s, P = 0.001). SFA retrograde blood velocity increased during cycling (Δ 1.3 ± 0.8 cm/s) and concurrent (Δ 1.4 ± 0.9 cm/s) exercise; these responses were both greater than RHG (Δ −0.1 ± 0.6 cm/s, both P < 0.0001). Overall, mean SFA blood flow increased during RHG (Δ 24 ± 30 ml/min, P < 0.0001) and concurrent (Δ 10 ± 38 ml/min, P = 0.03) but not cycling (Δ 2.3 ± 30 ml/min, P = 0.91) exercise; responses during exercise were larger during RHG compared with cycling (P = 0.0009). SFA vascular conductance was reduced in both cycling (Δ −0.14 ± 0.33 ml·min⁻¹·mmHg⁻¹, P = 0.001) and concurrent (Δ −0.14 ± 0.38 ml·min⁻¹·mmHg⁻¹, P = 0.002) exercise and lower compared with RHG (Δ 0.02 ±0.29 ml·min⁻¹·mmHg⁻¹, both P < 0.02). There was a smaller increase in retrograde flow during RHG in the MSNA subset (P = 0.02), but there were no other statistical differences in responses between the subset of participants with complete MSNA and the remaining cohort (all P > 0.3). There were no statistically significant sex differences in relative SFA blood flow or vascular conductance responses (% change in retrograde flow trended greater in women: P = 0.08; all other comparisons P > 0.2).

Fig. 2.

Superficial femoral artery (SFA) antegrade and retrograde blood velocity, mean blood flow, and vascular conductance at rest (Baseline) and during the last minute (Exercise) of rhythmic handgrip (RHG), 1-leg cycling, and concurrent exercise. Data obtained from 35 participants and analyzed by 2-way repeated-measures ANOVA. Values presented as means ± SE. *P < 0.05, **P < 0.001 compared with within-group baseline.

MSNA responses.

MSNA data are displayed in Fig. 3. All measures of MSNA were not statistically different at baseline before each exercise mode (all P > 0.12). RHG increased MSNA burst frequency (Δ 4 ± 8 burst/min, P = 0.04) but not total MSNA [Δ 488 ± 570 arbitrary units (a.u.), P = 0.26]; MSNA burst amplitude was unchanged (Δ 7 ± 14%, P = 0.7). In contrast, cycling (Δ −3 ± 6 burst/min) and concurrent (Δ 4 ± 6 burst/min) exercise did not alter MSNA burst frequency (P > 0.1), but both increased burst amplitude (Δ 34 ± 30%, Δ 31 ± 31%) and total MSNA (Δ 791 ± 1,274 a.u., Δ 1,145 ± 1,507 a.u., all P ≤ 0.04). During exercise, MSNA burst frequency was lower during cycling than both RHG (P = 0.0006) and concurrent (P = 0.03) exercise; burst amplitude was greater during both cycling (P = 0.007) and concurrent (P = 0.01) exercise compared with RHG; and total MSNA was not different during any exercise condition (P = 0.5). When potential sex differences were explored, men demonstrated larger increases in MSNA burst amplitude during RHG (P < 0.001).

Fig. 3.

Muscle sympathetic nerve activity (MSNA) at rest (Baseline) and during the last minute (Exercise) of rhythmic handgrip (RHG), 1-leg cycling, and concurrent exercise. Data obtained from 12 participants for MSNA burst frequency and 10 participants for MSNA burst amplitude and total MSNA. Data analyzed by 2-way repeated measures ANOVA. Values presented as means ± SE. a.u., Arbitrary units. *P < 0.05 compared with within-group baseline.

Relationships between changes in MSNA and SFA vascular conductance.

No significant relationships were observed between changes in MSNA burst frequency or total MSNA and SFA vascular conductance during exercise. In contrast, changes in MSNA burst amplitude during cycling were correlated negatively with changes in SFA vascular conductance [r = −0.68, P = 0.04 (with outlier included P = 0.4)]. As shown in Fig. 4, pooled data across all exercise modes demonstrated a significant negative correlation between changes in MSNA burst amplitude and SFA vascular conductance [r = −0.43, P = 0.02 (with outlier included P = 0.7)].

Fig. 4.

Relationship between changes in muscle sympathetic nerve activity (MSNA) burst amplitude and superficial femoral artery (SFA) vascular conductance during exercise. Pearson’s correlation data obtained from 9 participants per exercise, excluding 1 individual identified as an outlier. RHG, rhythmic handgrip.

Cardiac baroreflex sensitivity and heart rate variability responses.

Cardiac BRS was unchanged during RHG (23 ± 13 vs. 20 ± 16 ms/mmHg, P = 0.38) but decreased during both cycling (23 ± 12 vs. 8 ± 9 ms/mmHg, P < 0.0001) and concurrent (21 ± 10 vs. 5 ± 3 ms/mmHg, P = 0.0001) exercise modes. The reductions in cardiac BRS were similar between cycling and concurrent exercise (P = 0.20) and greater than RHG (both P = 0.0001). SDNN and RMSSD both decreased during RHG (56 ± 31 vs. 40 ± 32 ms, P < 0.0001; 63 ± 45 vs. 50 ± 49 ms, P = 0.03), cycling (59 ± 30 vs. 22 ± 12 ms, P < 0.0001; 67 ± 46 vs. 21 ± 17 ms, P < 0.0001), and concurrent (60 ± 28 vs. 16 ± 7 ms, P < 0.0001; 64 ± 41 vs. 15 ± 11 ms, P < 0.0001) exercise. The decreases in SDNN and RMSSD were comparable during cycling and concurrent exercise (P = 0.19 and P = 0.48, respectively) but greater than RHG (both P < 0.0001).

DISCUSSION

The present study examined the effects of RHG, one-leg cycling, and concurrent dynamic arm and leg exercise on SFA vascular conductance and MSNA in the resting leg in young healthy men and women. SFA vascular conductance was unchanged during RHG but decreased during cycling and concurrent exercise. In contrast, RHG increased MSNA burst frequency without altering burst amplitude or total MSNA, whereas cycling and concurrent exercise increased both MSNA burst amplitude and total MSNA without impacting burst frequency. Across all exercise modes, SFA vascular conductance responses were correlated with changes in MSNA burst amplitude but not burst frequency or total MSNA. These results highlight the functional importance of measuring MSNA burst strength during exercise.

To our knowledge, this is the first study to describe how independent and concurrent RHG and one-leg cycling impact blood flow velocity profiles. In contrast to prior work using static handgrip exercise and calf blood flow (38, 40), RHG demonstrated an increase in SFA blood flow, whereas cycling and concurrent exercise modes evoked no change or small increases, respectively. Increases in mean SFA blood flow during RHG were mediated by an increase in antegrade blood velocity without a change in retrograde blood velocity, whereas cycling had offsetting increases in both antegrade and retrograde blood velocities. This finding is consistent with work demonstrating increases in contralateral antegrade, but not retrograde, brachial blood flow during RHG exercise but increases in both antegrade and retrograde blood flow during cycling (9, 46). Variability in blood flow patterns has been attributed to differences in pressure gradients (i.e., upstream arterial pressure vs. downstream pressure from resistance vessels) produced by individual exercise modalities (9). Neural vasoconstriction is thought to play a role in mediating these responses, as acute sympathetic activation increases retrograde blood velocity and flow (27, 33, 45). Our findings may support this hypothesis, as blood pressure responses were comparable between RHG and cycling but only the latter increased burst amplitude and total MSNA from baseline.

The sympathetic nervous system facilitates blood flow redistribution and the maintenance of perfusion pressure during exercise (25, 36). Characterizing the changes in SFA vascular conductance as a measure of vasoconstrictor tone, we observed reductions in SFA vascular conductance during one-leg cycling and concurrent exercise. These changes occurred despite increases in total vascular conductance (Table 2), demonstrating vasoconstriction of the resting limb. The alterations in SFA vascular conductance were correlated negatively to changes in MSNA burst amplitude during cycling (r = −0.68, P = 0.04) and across all exercise modes (r = −0.43, P = 0.03). Our results in the resting limb differ from concurrent increases in leg vascular conductance and norepinephrine spillover during high-intensity (≥80% maximum oxygen consumption) double-arm cranking (39). However, these measures were taken after ≥9 min of exercise, and the dissociation between sympathetic activity and vascular conductance may be explained by augmented cutaneous blood flow to aid systemic thermoregulation during prolonged exercise (32) or the effects of increased metabolic vasodilation (43). Interestingly, prior work has reported that the addition of ischemic static handgrip exercise to mild- and moderate-intensity dynamic leg kicking can reduce leg vascular conductance, without a change in leg norepinephrine spillover (42). Whether this reflects poor sensitivity of discrete norepinephrine spillover measures during exercise, high interindividual variability in responses, or the small sample size studied is unclear. In the present study changes in MSNA and SFA vascular conductance were not different during cycling versus concurrent exercise.

The functional characteristics of multiunit MSNA discharge patterns during exercise have not been thoroughly explored. In line with the concept that MSNA burst frequency and amplitude can be differentially regulated (18), we found that RHG increased burst frequency without a change in burst amplitude, whereas both cycling and concurrent exercise had no effect on burst frequency but increased burst amplitude. Furthermore, we observed relationships between MSNA burst amplitude and SFA vascular conductance. This aligns with prior work using single-unit MSNA analysis that reported greater within-burst multiple spike firing (which relates to multiunit MSNA burst amplitude) was associated with increased cardiac norepinephrine spillover without a difference in multiunit MSNA burst occurrence (21). It has also been shown that increases in MSNA burst amplitude (or area) are related to graded reductions in leg vascular conductance at rest (5). Collectively, these results demonstrate that quantification of MSNA burst strength (e.g., amplitude) yields important information regarding neurovascular coupling at rest and during exercise.

With respect to the reflex control of MSNA, the observed differences in burst frequency responses between exercise modes likely reflect variation in the degree of cardiopulmonary baroreflex loading (3, 16, 35). The mechanisms responsible for controlling MSNA burst amplitude during cycling and concurrent exercise are less clear. We reported a role for central command in mediating changes in MSNA burst amplitude during low-intensity cycling (3). Although RHG and cycling would both engage central command and group III/IV skeletal muscle afferent feedback, the contributions of these mechanisms may be impacted by the amount of muscle mass involved (6, 7). The interactive effects of peripheral and central neural pathways also cannot be excluded, as cardiopulmonary baroreflex loading during muscle metaboreflex stimulation can augment whether sympathetic outflow is directed toward the heart or vasculature (44). Interestingly, the present results exhibit largely additive responses in MSNA during concurrent exercise compared with RHG and cycling alone, which differs from recent work reporting that cardiopulmonary baroreflex-mediated reductions in MSNA burst frequency and total MSNA via lower body positive pressure are blunted only by high, not low or moderate, muscle metaboreflex activation (17). However, this study did not assess responses during a voluntary challenge, such as exercise.

Limited work has investigated the effects of combined arm and leg dynamic exercise on reflex and tonic measures of cardiac parasympathetic modulation. It has been reported that both SDNN and RMSSD remain largely unchanged during static exercise (8, 26). In contrast, dynamic handgrip (20), arm cranking (22), and cycling (8, 22) each decrease measures of time-domain heart rate variability. In the present study, heart rate variability was reduced during each exercise mode but to a larger extent during cycling and concurrent exercise. This difference can likely be attributed to the greater increases in heart rate evoked by larger-muscle mass exercise (Table 2). Alterations in cardiac BRS, representing reflex cardiovagal modulation (31), may also be related to the amount of active muscle mass. Static or dynamic handgrip exercise has no effect on cardiac BRS, whereas similar-intensity leg exercise can cause a reduction in cardiac BRS (10, 13). These results are consistent with the present findings, in which cardiac BRS was unchanged during RHG but reduced similarly during one-leg cycling and concurrent exercise, suggesting important muscle mass or limb differences in the regulation of neural responses during exercise.

We acknowledge several methodological considerations. First, because of the high degree of technical difficulty in maintaining microelectrode placement during one-leg cycling exercise we were only able to collect MSNA in a subset of participants. Our success rate of holding a microneurographic site was much lower than we previously reported during recumbent one-leg cycling (3), suggesting that this may have been impacted by posture. Nevertheless, our sample is larger than prior studies, which each contained six or fewer participants (15, 37), and we confirmed that our hemodynamic and blood flow measures were consistent between the subset and the remaining sample. Second, to reduce the risk of microelectrode movement during data collection, MSNA and blood flow measurements were not assessed concurrently but during separate study visits. MSNA and lower leg vascular conductance have both been shown to be highly reproducible at rest and during stress (12, 19, 48). In particular, the high validity and reproducibility of MSNA reduces the sample size needed to detect significant differences (30). Again, we demonstrated no differences in hemodynamic responses to exercise between testing days, in the subset with MSNA or the remaining cohort. Third, our measurements of MSNA were confined to a resting limb at low intensities based on the need to limit microelectrode movement and may not be generalizable to the neural responses in the active leg (1) or higher exercise intensities. Fourth, our protocol was completed in a semisupine posture to facilitate data collection. Prior work has shown that the supine position can abolish the sympathoinhibitory effects of dynamic leg exercise (35), providing a rationale for the lack of reductions in MSNA burst frequency during one-leg cycling exercise.

Conclusions.

This is the first study to comprehensively study the effects of dynamic arm, leg, and concurrent exercise on MSNA, vascular conductance, and blood flow responses in the resting limb. The changes in SFA vascular conductance during cycling and across all exercise modes were associated negatively with alterations in MSNA burst amplitude but not burst frequency or total MSNA. The independent regulation of MSNA burst frequency and amplitude may have functional consequences for the control of peripheral vasoconstriction during stress.

GRANTS

This research was supported by a Natural Science and Engineering Research Council of Canada Discovery Grant (no. 06019, P. J. Millar), the Canada Foundation for Innovation (no. 34379, P .J. Millar), and the Ontario Ministry of Research, Innovation, and Science (no. 34379, P. J. Millar).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

P.J.M. conceived and designed research; C.J.D., T.J.K., A.V.I., J.B.L., A.D.S., J.A.C., J.T.S., J.F.B., and P.J.M. performed experiments; C.J.D., T.J.K., A.D.S., and J.A.C. analyzed data; C.J.D., T.J.K., and P.J.M. interpreted results of experiments; C.J.D. prepared figures; C.J.D., T.J.K., and P.J.M. drafted manuscript; C.J.D., T.J.K., and P.J.M. edited and revised manuscript; C.J.D., T.J.K., A.V.I., J.B.L., A.D.S., J.A.C., J.T.S., J.F.B., and P.J.M. approved final version of manuscript.