Absence of compensatory vasodilation with perfusion pressure challenge in exercise: evidence for and implications of the noncompensator phenotype

Individual variability in lactate response to cycling prescribed using physiological thresholds and peak work rate: a crossover within-participant repeated measures study

PURPOSE

(1) To determine if the blood lactate concentration ([BLa]) response is a repeatable individual trait, and (2) To examine whether threshold-based prescription (THR) reduces interindividual variability in [BLa] response compared to traditional (maximally anchored) exercise prescription (TRAD).

METHOD

A crossover within-participant repeated measures design was used to assess [BLa] during the TRAD and THR exercise in 17 participants (9 M/8F). Participants initially undertook an incremental test to exhaustion to determine peak work rate (WRpeak), a lactate threshold (LT) test and a critical power (CP) test. All baseline tests were repeated twice. Participants then completed 6 15-min bouts of continuous cycling at 65%WRpeak (TRAD; 3 bouts) and 80% of the difference (Δ80) between LT and CP (THR; 3 bouts). [BLa] response was measured at 10 and 15 min of exercise.

RESULTS

Across individuals, there was a wide range in [BLa] response, but within individual responses were repeatable. [BLa] ranges and mean individual 90% confidence interval width (CIw) were as follows: TRAD@10 min = 2.1-9.7 mmol, CIw = 0.5 mmol, THR@10 min = 3.4-9.3 mmol, CIw = 0.6 mmol, TRAD@15 min = 2.2-9.9 mmol, CIw = 0.6 mmol, THR@15 min = 3.6-12.3 mmol, CIw = 0.7 mmol. Levene's tests revealed no significant differences in the variability of [BLa] response between TRAD and THR at 10 min (F = 0.523, p = 0.475) or 15 min (F = 0.351, p = 0.558) of exercise.

CONCLUSION

Our results demonstrate that true interindividual variability in the [BLa] response to exercise exists, but failed to confirm that variability in [BLa] response is reduced with the use of THR.

Leg blood flow during exercise with blood flow restriction: evidence for and implications of compensatory cardiovascular mechanisms

Proximal limb cuff inflation to 40% arterial occlusion pressure (AOP) is assumed to reduce exercising leg perfusion, creating "blood flow restriction" (BFR). However, no study has validated this assumption. Eighteen healthy young participants (9 F) performed two-legged knee flexion/extension exercise at 25% WRpeak with bilateral cuffs applied to the proximal thigh at 0% AOP (CTL), 20% AOP, and 40% AOP. Leg blood flow (LBF; Doppler and echo ultrasound) and cardiac output (CO; finger photoplethysmography) were measured during rest and exercise. LBF values were doubled to account for both exercising legs. AOP (20% and 40%) reduced exercising LBF in a dose-response manner (P < 0.01). However, the magnitude of the leg blood flow restriction by 40% AOP was progressively attenuated across the exercise bout (5-15 s: 37%, 50-70 s: 20%, 240-300 s: 16%; P < 0.01) due to compensatory increases in leg vascular conductance (LVC) (P < 0.01). Between 5 and 15 s of exercise, 40% AOP significantly reduced CO compared with CTL and 20% AOP (8.0 ± 1.3 vs. 8.4 ± 1.5 L/min, P < 0.001 and 8.5 ± 1.5, P < 0.001). By 240-300 s, there were no significant differences in CO between cuff pressures (all P > 0.13). Pneumatic cuff inflation at 20% and 40% AOP reduces LBF in a dose-response manner, but this impairment was progressively attenuated across the exercise bout by an increase in LVC. Importantly, this compensatory response differed across participants, which may have implications for the degree of adaptations following BFR training. Furthermore, restoration of normal CO during BFR despite compromised limb perfusion suggests that other tissue perfusion is increased as part of the response.NEW & NOTEWORTHY It remained to be determined whether BFR set below 60% AOP impairs leg blood flow during continuous exercise. We showed that BFR at 20% and 40% AOP impairs exercising leg blood flow in a dose-response manner. However, the leg blood flow impairment was progressively attenuated across the exercise bout. Both initial compromise and partial restoration varied across participants, which may have implications for the degree of muscle adaptations following BFR training.

The effect of self-identified arm dominance on exercising forearm hemodynamics and skeletal muscle desaturation

The human forearm model is commonly employed in physiological investigations exploring local vascular function and oxygen delivery; however, the effect of arm dominance on exercising forearm hemodynamics and skeletal muscle oxygen saturation (SmO2) in untrained individuals is poorly understood. Therefore, the purpose of this study was to explore the effect of self-identified arm dominance on forearm hemodynamics and SmO2 in untrained individuals during submaximal, non-ischemic forearm exercise. Twenty healthy individuals (23±4 years, 50% female; 80% right-handed) completed three-minute bouts of supine rhythmic (1 second contraction: 2 second relaxation duty cycle) forearm handgrip exercise at both absolute (10kg; 98N) and relative (30% of maximal voluntary contraction) intensities in each forearm. Beat-by-beat measures of forearm blood flow (FBF; ml/min), mean arterial blood pressure (MAP; mmHg) and flexor digitorum superficialis SmO2 (%) were obtained throughout and averaged during the final 30 seconds of rest, exercise, and recovery while forearm vascular conductance was calculated (FVC; ml/min/100mmHg). Data are Δ from rest (mean±SD). Absolute force production did not differ between non-dominant and dominant arms (97±11 vs. 98±13 N, p = 0.606) whereas relative force production in females did (69±24 vs. 82±25 N, p = 0.001). At both exercise intensities, FBFRELAX, FVCRELAX, MAPRELAX, and the time constant tau for FBF and SmO2 were unaffected by arm dominance (all p>0.05). While arm dominance did not influence SmO2 during absolute intensity exercise (p = 0.506), the non-dominant arm in females experienced an attenuated reduction in SmO2 during relative intensity exercise (-14±10 vs. -19±8%, p = 0.026)-though exercise intensity was also reduced (p = 0.001). The present investigation has demonstrated that arm dominance in untrained individuals does not impact forearm hemodynamics or SmO2 during handgrip exercise.

Passive hallux adduction decreases lateral plantar artery blood flow: a preliminary study of the potential influence of narrow toe box shoes

Background

Blood flow is essential in maintaining tissue health. Thus, compromised blood flow can prevent tissue healing. An adducted hallux, as seen inside a narrow shoe, may put passive tension on the abductor hallucis, compressing the lateral plantar artery into the calcaneus and restricting blood flow. The purposes of this study were to compare lateral plantar artery blood flow before and after passive hallux adduction and to compare blood flow with arch height.

Methods

Forty-five healthy volunteers (20 female, 25 male; age = 24.8 ± 6.8 yr; height = 1.7 ± 0.1 m; weight = 73.4 ± 13.5 kg) participated in this cross-over design study. Arch height index (AHI) was calculated, and blood flow measurements were obtained using ultrasound (L8-18i transducer, GE Logiq S8). The lateral plantar artery was imaged deep to abductor hallucis for 120 s: 60 s at rest, then 60 s of passive hallux adduction. Maximal passive hallux adduction was performed by applying pressure to the medial side of the hallux. Blood flow was calculated in mL/min, and pre-passive hallux adduction was compared to blood flow during passive hallux adduction.

Results

Log transformed data was used to run a paired t-test between the preadduction and postadduction blood flow. The volume of blood flow was 22.2% lower after passive hallux adduction compared to before (- 0.250 ± 0.063, p < 0.001). As AHI decreased, there was a greater negative change in blood flow. As baseline blood flow increased, there was also a greater negative change in blood flow.

Conclusions

Our preliminary findings of decreased blood flow through passive hallux adduction indicate conditions that elicit passive hallux adduction (e.g. wearing narrow-toed shoes) may have important effects on foot blood flow. Individuals with lower AHI appear to have a greater risk of decreased blood flow with passive hallux adduction.

Superficial femoral artery blood flow with intermittent pneumatic compression of the lower leg applied during walking exercise and recovery

The purpose of this study was to determine if muscle blood flow during walking exercise and postexercise recovery can be augmented through the application of intermittent compression of the lower legs applied during the diastolic phase of the cardiac cycle. Results from four conditions were assessed: no compression (NoComp), compression during walking (ExComp), compression during postexercise recovery (RecComp), and compression applied throughout (AllComp). Superficial femoral artery (SFA) blood flow was measured (Doppler ultrasound) during rest and postexercise recovery. Mean arterial blood pressure (MAP, finger photoplethysmography) was used to calculate vascular conductance as VC = SFA flow/MAP. Near infrared spectroscopy measured changes in oxygenated (O2Hb) and deoxygenated hemoglobin concentration throughout the test. Compression during exercise increased SFA blood flow measured over the first 15 s of postexercise recovery (AllComp: 532.2 ± 123.1 mL/min; ExComp: 529.8 ± 99.2 mL/min) compared with NoComp (462.3 ± 87.3 mL/min P < 0.05) and corresponded to increased VC (NoComp: 4.7 ± 0.9 mL·min−1·mmHg−1 versus ExComp: 5.5 ± 1.0 mL·min−1·mmHg−1, P < 0.05). Similarly, compression throughout postexercise recovery also resulted in increased SFA flow (AllComp: 190.5 ± 57.1 mL/min; RecComp: 158.7 ± 49.1 mL/min versus NoComp: 108.8 ± 28.5 mL/min, P < 0.05) and vascular conductance. Muscle contractions during exercise reduced total hemoglobin with O2Hb comprising ~57% of the observed reduction. Compression during exercise augmented this reduction ( P < 0.05) with O2HB again comprising ~55% of the reduction. Total hemoglobin was reduced with compression during postexercise recovery ( P < 0.05) with O2Hb accounting for ~40% of this reduction. Results from this study indicate that intermittent compression applied during walking and during postexercise recovery enhanced vascular conductance during exercise and elevated postexercise SFA blood flow and tissue oxygenation during recovery.

NEW & NOTEWORTHY Intermittent compression mimics the mechanical actions of voluntary muscle contraction on venous volume. This study demonstrates that compression applied during the diastolic phase of the cardiac cycle while walking accentuates the actions of the muscle pump resulting in increased immediate postexercise muscle blood flow and vascular conductance. Similarly, compression applied during the recovery period independently increased arterial flow and tissue oxygenation, potentially providing conditions conducive to faster recovery.

Fatigue-independent alterations in muscle activation and effort perception during forearm exercise: role of local oxygen delivery

The oxygen-conforming response (OCR) of skeletal muscle refers to a downregulation of muscle force for a given muscle activation when oxygen delivery (O₂D) is reduced, which is rapidly reversed when O₂D is restored. We tested the hypothesis that the OCR exists in voluntary human exercise and results in compensatory changes in muscle activation to maintain force output, thereby altering perception of effort. In eight men and eight women, electromyography (EMG), oxyhemoglobin (O₂Hb) and deoxyhemoglobin (HHb), forearm blood flow (FBF), and task effort awareness (TEA) were measured. Participants completed two nonfatiguing rhythmic handgrip tests consisting of 5-min steady state (SS) followed by two bouts of 2-min brachial artery compression to reduce FBF by ~50% of SS (C1 and C2), separated by 2 min of no compression (NC1) and ending with 2 min of no compression (NC2). When FBF was compromised during C1, EMG/Force (1.58 ± 0.39) increased compared with SS (1.31 ± 0.33, P = 0.001). However, EMG/Force was not restored upon FBF restoration at NC1 (1.48 ± 0.38, P = 0.479), consistent with C1 evoking skeletal muscle fatigue. When FBF was compromised during C2, EMG/Force increased (1.73 ± 0.50) compared with NC1 (1.48 ± 0.38, P = 0.013). EMG/Force returned to NC1 levels during NC2 (1.50 ± 0.39, P = 0.016), consistent with an OCR in C2. TEA (SS 2.2 ± 2.3, C1 3.9 ± 2.5, NC1 3.4 ± 2.7, C2 4.6 ± 2.7, NC2 3.9 ± 2.8) mirrored changes in EMG. It is noteworthy that during the second compromise and then restoration of muscle oxygenation EMG and TEA were rapidly restored to precompromise levels. We interpreted these findings to support the existence of an OCR and its ability to rapidly modify perception of effort during voluntary exercise. NEW & NOTEWORTHY In healthy individuals, when force output is maintained during rhythmic handgrip exercise, muscle activation and perception of effort rapidly increase with compromised muscle oxygen delivery (O₂D) and then return to precompromised levels when muscle O₂D is restored. These findings suggest that an oxygen-conforming response (OCR) exists and is able to modify perception of effort during voluntary exercise. Therefore, similar to fatigue, an OCR may have implications for exercise tolerance.

Dietary nitrate restores compensatory vasodilation and exercise capacity in response to a compromise in oxygen delivery in the noncompensator phenotype

Recently, dietary nitrate supplementation has been shown to improve exercise capacity in healthy individuals through a potential nitrate-nitrite-nitric oxide pathway. Nitric oxide has been shown to play an important role in compensatory vasodilation during exercise under hypoperfusion. Previously, we established that certain individuals lack a vasodilation response when perfusion pressure reductions compromise exercising muscle blood flow. Whether this lack of compensatory vasodilation in healthy, young individuals can be restored with dietary nitrate supplementation is unknown. Six healthy (21 ± 2 yr), recreationally active men completed a rhythmic forearm exercise. During steady-state exercise, the exercising arm was rapidly transitioned from an uncompromised (below heart) to a compromised (above heart) position, resulting in a reduction in local pressure of -31 ± 1 mmHg. Exercise was completed following 5 days of nitrate-rich (70 ml, 0.4 g nitrate) and nitrate-depleted (70 ml, ~0 g nitrate) beetroot juice consumption. Forearm blood flow (in milliliters per minute; brachial artery Doppler and echo ultrasound), mean arterial blood pressure (in millimeters of mercury; finger photoplethysmography), exercising forearm venous effluent (ante-cubital vein catheter), and plasma nitrite concentrations (chemiluminescence) revealed two distinct vasodilatory responses: nitrate supplementation increased (plasma nitrite) compared with placebo (245 ± 60 vs. 39 ± 9 nmol/l; P < 0.001), and compensatory vasodilation was present following nitrate supplementation (568 ± 117 vs. 714 ± 139 ml ⋅ min^-1 ⋅ 100 mmHg^-1; P = 0.005) but not in placebo (687 ± 166 vs. 697 ± 171 min^-1 ⋅ 100 mmHg^-1; P = 0.42). As such, peak exercise capacity was reduced to a lesser degree (-4 ± 39 vs. -39 ± 27 N; P = 0.01). In conclusion, dietary nitrate supplementation during a perfusion pressure challenge is an effective means of restoring exercise capacity and enabling compensatory vasodilation.NEW & NOTEWORTHY Previously, we identified young, healthy persons who suffer compromised exercise tolerance when exercising muscle perfusion pressure is reduced as a result of a lack of compensatory vasodilation. The ability of nitrate supplementation to restore compensatory vasodilation in such noncompensators is unknown. We demonstrated that beetroot juice supplementation led to compensatory vasodilation and restored perfusion and exercise capacity. Elevated plasma nitrite is an effective intervention for correcting the absence of compensatory vasodilation in the noncompensator phenotype.