Effects of passive and active leg movements to interrupt sitting in mild hypercapnia on cardiovascular function in healthy adults

Abstract

Prolonged sitting in a mild hypercapnic environment impairs peripheral vascular function. The effects of sitting interruptions using passive or active skeletal muscle contractions are still unclear. Therefore, we sought to examine the vascular effects of brief periods (2 min every half hour) of passive and active lower limb movement to interrupt prolonged sitting with mild hypercapnia in adults. Fourteen healthy adults (24 ± 2 yr) participated in three experimental visits sitting for 2.5 h in a mild hypercapnic environment (CO₂ = 1,500 ppm): control (CON, no limb movement), passive lower limb movement (PASS), and active lower limb movement (ACT) during sitting. At all visits, brachial and popliteal artery flow-mediated dilation (FMD), microvascular function, plasmatic levels of nitrate/nitrite and endothelin-1, and heart rate variability were assessed before and after sitting. Brachial and popliteal artery FMDs were reduced in CON and PASS (P < 0.05) but were preserved (P > 0.05) in ACT. Microvascular function was blunted in CON (P < 0.05) but was preserved in PASS and ACT (P > 0.05). In addition, total plasma nitrate/nitrite was preserved in ACT (P > 0.05) but was reduced in CON and PASS (P < 0.05), and endothelin-1 levels were decreased in ACT (P < 0.05). Both passive and active movement induced a greater ratio between the low-frequency and high-frequency bands for heart rate variability (P < 0.05). For the first time, to our knowledge, we found that brief periods of passive leg movement can preserve microvascular function, but that an intervention that elicits larger increases in shear rate, such as low-intensity exercise, is required to fully protect both macrovascular and microvascular function and circulating vasoactive substance balance.NEW & NOTEWORTHY Passive leg movement could not preserve macrovascular endothelial function, whereas active leg movement could protect endothelial function. Attenuated microvascular function can be salvaged by passive movement and active movement. Preservation of macrovascular hemodynamics and plasma total nitrate/nitrite and endothelin-1 during prolonged sitting requires active movement. These findings dissociate the impacts induced by mechanical stress (passive movement) from the change in metabolism (active movement) on the vasculature during prolonged sitting in a mild hypercapnic environment.

Keywords: autonomic function; endothelial function; mechanoreflex; metaboreflex; microvascular function.

Graphical abstract

Figure 1.

Participant allocation and study flow. A: randomized crossover study design and participant allocation to the control (CON, n = 14), passive (PASS, n = 14), and active (ACT, n = 14) groups. B: representation of the study flow for the CON, PASS, and ACT groups. Measurements were taken before sitting (baseline) and after sitting (post), which included resting heart rate, blood pressure, venous blood sampling, mean femoral arterial shear rate and blood flow, autonomic nervous system activity by heart rate variability, brachial and popliteal artery endothelial function (flow-mediated dilation, FMD), arterial stiffness (pulse-wave velocity, PWV), calf girth, and microvascular circulatory function. During sitting, measurements were taken at rest [2 min after the movement (MVT) bout in PASS and ACT if applicable] and during movement every 15 min and consisted of heart rate, blood pressure, and atmospheric CO₂ monitoring.

Figure 2.

Flow-mediated dilation (FMD, %) and change (pre to post) in FMD (%) in the brachial and popliteal arteries pre- and post-prolonged sitting in control (CON, n = 14), passive movement (PASS, n = 14), and active (ACT, n = 14) groups. A: popliteal artery FMD was significantly reduced after prolonged sitting in the CON and PASS groups, whereas FMD was maintained after prolonged sitting in the ACT group. Post-CON and post-PASS FMDs were significantly lower than post-ACT. B: the reduction in popliteal artery FMD was significantly greater in both CON and PASS groups compared with the ACT group. C: brachial artery FMD was significantly reduced after prolonged sitting in the CON and PASS groups, whereas FMD was maintained in the ACT group. Post-CON and post-PASS FMDs were significantly lower than post-ACT. D: the reduction in brachial artery FMD was significantly greater in both CON and PASS groups compared with the ACT group. Values are represented as means ± SE. A and C: two-way ANOVA with repeated measures and Tukey’s post hoc test. B and D: one-way ANOVA and Tukey’s post hoc test. *P < 0.05 vs. Pre. †P < 0.05 vs. ACT.

Figure 3.

Measurements of microvascular circulatory function before and after prolonged sitting in control (CON, n = 14), passive movement (PASS, n = 14), and active movement (ACT, n = 14) groups. A: tissue oxygenation index (TOI) recovery rate (%·min⁻¹) was significantly reduced after sitting in the CON group, and post-CON was significantly lower than both post-PASS and post-ACT. Post-ACT was significantly greater than both post-CON and post-PASS. B: reduction in TOI recovery rate (%·min⁻¹) was significantly greater in CON compared with PASS and ACT, and the reduction in PASS was significantly greater than ACT. Values are represented as means ± SE. A: two-way ANOVA with repeated measures and Tukey’s post hoc test. B: one-way ANOVA and Tukey’s post hoc test. *P < 0.05 vs. Pre. †P < 0.05 vs. PASS. ‡P < 0.05 vs. ACT.

Figure 4.

Total plasma nitrate/nitrite levels (nM), endothlin-1 levels (pg/mL), and total plasma nitrate/nitrite to endothelin-1 ratios pre- and post-prolonged sitting in control (CON, n = 10), passive (PASS, n = 10), and active (ACT, n = 10) groups. A: total plasma nitrate/nitrite was significantly reduced after prolonged sitting in the CON and PASS groups, whereas it was maintained in the ACT group. Post-CON and post-PASS were significantly lower than post-ACT. B: endothelin-1 was significantly reduced after prolonged sitting in the ACT condition, and post-ACT was significantly greater than post-CON and post-PASS. C: the total plasma nitrate/nitrite to endothelin-1 ratio in post-CON and post-PASS was significantly lower than post-ACT. Values are represented as means ± SE. Two-way ANOVA with repeated measures and Tukey’s post hoc test. *P < 0.05 vs. Pre. †P < 0.05 vs. ACT

Figure 5.

Fraction of end tidal carbon dioxide (FECO₂) and tidal volume (V_t) during sitting time in ambient (CO₂ = 400 ppm) and mild hypercapnic (CO₂ = 1,500 ppm) conditions in a subset of participants. A: FECO₂ is significantly higher in a mild hypercapnic condition (n = 8) at all time points compared with FECO₂ in an ambient condition (n = 8). B: V_t is significantly higher in a mild hypercapnic condition (n = 8) at 4 min, 12 min, 14 min, 16 min, and 150 min compared with V_t in an ambient condition (n = 8). Values are represented as means ± SE. Two-way ANOVA with repeated measures and Tukey’s post hoc test. *P < 0.05 vs. ambient.