Peak oxygen uptake during exercise (˙VO2peak) is dependent on the integrated activities of each step in the oxygen pathway, from the environment to the terminal substrate oxidases located on the inner membranes of mitochondria (27). The determinants of˙VO2 are cardiovascular oxygen transport and oxygen extraction from capillaries to mitochondria in the periphery(1,3,22,27). Oxygen extraction approaches a plateau at submaximal ˙VO2 in healthy humans under conditions of normoxia (27); therefore, ˙VO2peak is highly dependent on maximum cardiac output.
During cycle exercise, leg muscles are the major tissues receiving blood flow and consuming oxygen (21). Therefore, the characteristics of blood flow distribution to the leg muscles will have an important effect on ˙VO2peak. ˙VO2peak has been shown to be significantly correlated with peak vascular conductance of the calf after brief periods of arterial occlusion, with or without local exercise(18,25). These ischemic stimuli result in activation of various processes regulating local vascular resistance, including myogenic relaxation of resistance vessel walls and the local release of vasoactive substances. Peak vascular conductance, measured immediately after the release of arterial occlusion, reflects primarily the local myogenic relaxation of vessel wall (16). However, the duration of vasodilation has been hypothesized to be partly dependent on the local myogenic component but mainly on the local release of vasoactive substances(2,16,26).
We postulated that aerobic training should result in the ability of leg muscles to perform a greater amount of ischemic work, resulting in a greater metabolic debt produced by the intense muscle exercise under anaerobic conditions (ischemia). This would result in a prolonged duration of post-ischemic vasodilation to restore a normal metabolic environment. We hypothesized, therefore, that whole body peak exercise capacity would be more closely related to the duration of calf vasodilation after local ischemia than to peak calf conductance, since duration of vasodilation better reflects both myogenic and metabolic effects on vascular smooth muscle. In this case, the duration of vasodilation may be a better index of peak exercise capacity than peak calf conductance.
Subjects. Twenty-one male subjects aged 19-39 yr participated in the study. The study population included 14 sedentary and 7 trained subjects. The untrained subjects did not regularly participate in athletic activities. Three subjects in the trained group were national class speed skaters, and the remaining four were long distance runners or cross-country skiers who had been engaged in regular endurance exercise training for at least 2 yr. All subjects were in good health and had normal physical examinations. The study protocol was approved by the Institutional Ethics Committee. Written informed consent was obtained from all subjects.
Calf blood flow protocol. Three initial determinations of calf blood flow and conductance were made at rest, and the averages were used as the baseline values. New baseline values were ascertained before all interventions. Calf blood flow, calf conductance (blood flow/mean blood pressure), vasodilatory capacity (peak/baseline conductance), and duration of vasodilation were then determined under two conditions in set order: first, after release of thigh tourniquet occlusion alone (reactive hyperemia); second, after release of thigh tourniquet occlusion with calf exercise(ischemic exercise).
Calf blood flow measurements. Calf blood flow was determined noninvasively after rapid inflation of a thigh cuff to subdiastolic pressure(≈50 mm Hg) with the technique of femoral venous occlusive plethysmography(12). The accuracy and reproducibility of this technique have been discussed elsewhere (11,12). Calf blood flow measurements were made in the supine position with the leg raised to 30° to facilitate venous drainage between flow measurements. The foot was isolated hemodynamically from the calf circulation by inflation of an ankle cuff to 250 mm Hg. Serial measurements of calf arterial inflow were determined using a self-balancing and electrically calibrated mercury strain gauge plethysmograph (EC-4 Plethysmograph, D.E. Hokanson Inc., Bellevue, WA). Calf volume changes produced proportional elongations of a mercury-in-silastic transducer, which was positioned around the widest part of the calf. Length alterations in the transducer produced a signal on a strip chart recorder(Hewlett-Packard 7758B System, Boston, MA) from which calf blood flow was calculated by drawing a tangent line to the initial part of the slope of the volume vs time plot on the recording paper. Blood pressure measurements were obtained indirectly with a sphygmomanometer from a brachial artery.
Measurements after thigh tourniquet occlusion alone. Vascular parameters (calf blood flow, calf conductance, vasodilatory capacity, and duration of vasodilation) were determined following thigh tourniquet occlusion with the thigh cuff inflated to 250 mm Hg for 10 min. Serial measurements were made at 30 s, 60 s, and every minute thereafter following the release of the thigh cuff until the conductance recovered to baseline values (conductance within 10% of baseline conductance and followed by equal or smaller values), which was defined as the duration of vasodilation (duration of hyperemic blood flow).
Measurements after thigh tourniquet occlusion with calf exercise. The time interval between thigh tourniquet occlusion and maximum ischemic calf exercise protocols was at least 30 min. Subjects performed maximum symptom-limited calf exercise with alternate heel raising-resting (frequency of 12/min, with 3 s raising and 2 s resting) to the point of exhaustion with the femoral thigh cuff inflated to 250 mm Hg. Calf blood flow, conductance, vasodilatory capacity, and duration of vasodilation were determined following release of the thigh cuff at 30 s, 60 s, and every minute thereafter until the conductance recovered to baseline values, as defined for femoral artery occlusion alone.
Reproducibility of calf blood flow measurements. To assess the intrasubject reproducibility of the calf blood flow measurements, eight of the subjects underwent a second study of thigh tourniquet occlusion without calf exercise on a separate day. There was an intrasubject variability of 8.2± 2.7% in peak conductance and 11.9 ± 13.5% in duration of vasodilation.
Furthermore, to confirm that ischemic calf exercise maneuvers were maximal, seven subjects repeated maximum calf exercise with thigh tourniquet occlusion on a separate visit to the laboratory 2 months after the first measurements. The maximum calf exercise duration with thigh tourniquet occlusion was highly reproducible for a given subject, with a between test variability of 5.2± 1.8%. Peak conductance varied 6.5 ± 2.9% and duration of vasodilation by 6.4 ± 5.6%.
Effort dependence of maximum vasodilation with ischemic calf exercise. To determine the effects of varying submaximal efforts on the vascular measurements with ischemic calf exercise, seven subjects repeated the ischemic calf exercise studies on a separate day with submaximal efforts of 30, 60, and 90 s duration, as well as to volitional exhaustion.
Calf conductance reached a plateau after approximately 90-120 s of ischemic calf exercise (Fig. 1). However, duration of vasodilation was linearly related to ischemic exercise time (Fig. 1).
Cycle exercise studies. Exercise studies were performed a minimum of 1 h after the peripheral vascular studies. Subjects underwent maximum cycle exercise using a 1-min incremental protocol (14) until they were unable to continue. Subjects were seated on an electrically braked cycle ergometer (Mijnhardt, Holland), and expired gas was collected and analyzed using a computerized cardiopulmonary exercise system (model 2001, Medical Graphics Corp., Minneapolis, MN). After 3 min of no pedaling, subjects cycled at a speed of 60-70 rpm, with the workload increasing by 30 W·min-1 to the maximum. All subjects stopped pedaling owing to leg fatigue (with or without dry mouth), and none stopped because of dyspnea. The subjects were verbally encouraged during the exercise test to reach their maximum, which was defined using a value of the respiratory exchange ratio being higher than 1.15 and peak heart rate within 10% of age-predicted maximum. Subjects breathed through a low-resistance two-way valve (model 2700, Hans Rudolph Co., Kansas City, MO) connected on the expiratory limb to a heated linear pneumotachograph (Fleisch no. 3, Lausanne, Switzerland). Expired end-tidal oxygen and carbon dioxide tensions were monitored using a zirconium fuel cell O2 analyzer (Datex Co., Helsinki, Finland) and an infrared absorption CO2 analyzer (Normocap, Helsinki, Finland), respectively, with response times of less than 100 ms. The pneumotachograph and gas analyzers were calibrated prior to each exercise study. Predicted˙VO2peak and heart rate were calculated according to Jones(14).
Body composition. To normalize ˙VO2 for lean body mass, body composition was estimated from skinfold thickness measured with a Lange Caliper (Cambridge Scientific, Inc., Cambridge, MA) at triceps, biceps, iliac crest, and subscapular sites. Percent body fat was derived from the sum of skinfold thicknesses with gender-specific, age-corrected equations(20). Lean body mass (LBM) was calculated by subtracting body fat from the total body weight.
Data analysis. Student's t-test for paired observations was used to evaluate the significance of differences in vascular variables for individual subjects following thigh tourniquet occlusion, with and without calf exercise. Significance was set at the 0.05 level of confidence. The relationships between exercise capacity and vascular variables were established using least squares linear regression. Data are reported as mean± standard deviation.
Calf blood flow. Baseline measurements of calf blood flow, mean blood pressure, and baseline conductance are shown in Table 1. Peak conductance, vascular capacity, and duration of vasodilation after ischemia alone and after ischemic exercise are also presented inTable 1. Maximum calf exercise with thigh tourniquet occlusion resulted in significantly higher values for peak conductance, vasodilatory capacity, and duration of vasodilation compared with thigh tourniquet occlusion alone (Table 1).
Cycle exercise studies. Peak ˙VO2 for the group of study subjects was 3.5 ± 1.1 l·min-1 and˙VO2peak/LBM 56 ± 17 ml·min·kg-1 (120± 35% predicted). The subjects reached peak heart rates of 97 ± 5% of the age-adjusted predicted values (range, 165-190 beats·min-1) and respiratory exchange ratios of 1.32 ± 0.12.
Relationships between calf blood flow responses and cycle exercise capacity. Ischemic calf exercise time was linearly related to˙VO2peak/LBM (r = 0.806; P < 0.01). Peak calf conductance after thigh tourniquet occlusion alone and with maximum calf exercise were significantly related to ˙VO2peak/LBM (r = 0.496,P < 0.05 and r = 0.556, P < 0.01, respectively;Fig. 2). However, ˙VO2peak/LBM was more closely correlated with the duration of vasodilation after maximum ischemic calf exercise (r = 0.861; P < 0.001; Fig. 2).
The main finding of this study is that parameters reflecting local vasodilation after calf ischemia correlate significantly with˙VO2peak determined on the cycle ergometer in healthy humans. Furthermore, ˙VO2peak was most highly correlated with the duration of vasodilation after maximum ischemic calf exercise.
Essential to the interpretation of our findings is the assumption that cycle and ischemic calf exercise were maximal, since both of these activities are highly dependent on the degree of subject effort. During the cycle exercise tests, we verbally coached the subjects to reach their maximum capacity. The elevation of respiratory exchange ratio above 1.15 and peak heart rates within 10% of the age-predicted maximums in all subjects suggest that the efforts were at least close to maximal on the cycle ergometer test.
With respect to the calf exercise protocol, increasing ischemic calf exercise duration resulted in peak conductance values, which reached a plateau after 90-120 s (Fig. 1), a finding that suggests that the potential of vascular conductance to increase is limited. In contrast, duration of vasodilation was linearly related to the ischemic exercise duration and, therefore, was effort-dependent (Fig. 1). Surprisingly, little attention has been paid to the important effects of exercise time on peak local blood flow in previous studies using this type of exercise (17,18,25). In our study, subjects received strong verbal encouragement to produce maximum effort. Furthermore, on repeating ischemic exercise maneuvers in seven subjects, maximum exercise times were found to be highly reproducible, as were the values for peak conductance and duration of vasodilation. This reproducibility suggests that the ischemic calf exercise studies represented relatively maximal efforts. However, since calf blood flow responses to ischemic calf exercise had an effort-dependent component, we also studied a second stimulus, thigh tourniquet occlusion without exercise, which is effort-independent.
We selected ischemic calf exercise as a stimulus to study local vascular responses, since it has been widely used in studies of a similar nature(17,18,25) and represents a major metabolic stress. Of note, we observed peak conductance values following exercise that were lower than those reported in several of these studies, which is likely related to different calf exercise protocols. We used alternating heel raising-resting, which would be expected to activate predominantly the musculature of the posterior calf compartment, since it most closely resembles the type of calf exercise encountered during cycle ergometry. In comparison, other investigators have used alternate heel-toe raising, which probably recruits both posterior as well as anterior calf muscles and likely represents a greater overall stimulus to total calf vasodilation.
Immediately after the release of an ischemic stimulus, the vascular conductance of a resting or exercising muscle increases markedly in situ (2,13). This increase in conductance is thought to represent primarily the local myogenic relaxation of the vessel wall (2,8,16). Despite the fact that ischemic exercise is a much more intense stimulus than ischemia alone, peak calf conductance after maximum ischemic calf exercise was only modestly greater than after thigh tourniquet occlusion alone (Table 1). This may be attributable to one (or a combination) of several factors, including a limited potential of conductance to increase(Fig. 1), unequal durations of ischemic stimulus (10 min for thigh tourniquet occlusion alone, 1.6-4.4 min for thigh tourniquet occlusion with calf exercise), or limited anterior calf muscle activation with the calf exercise protocol used.
In contrast to what was observed with peak conductance, the duration of skeletal muscle vasodilation in response to ischemia measured in situ has been found to be less dependent on myogenic relaxation characteristics than on the local metabolic environment(2,16,26). Depolarization of the exercising skeletal muscle contributes to vasodilation as a result of local release of vasoactive substances such as potassium, endothelium-derived nitric oxide, magnesium, inorganic phosphate, and prostaglandins, as well as by changes in osmolality of the venous effluent(6,10,19,23). Furthermore, changes secondary to tissue hypoxia are thought to be important in the maintenance of vasodilation. When oxygen demands exceed oxygen delivery locally, tissue hypoxia leads to increased lactic acid production, intracellular acidosis, and decreased ATP stores. The hydrogen ion concentration and adenosine levels increase in the extracellular fluid and have local vasodilatory actions(4,5,7). Low tissue oxygen tension is thought to act only to trigger these metabolic alterations and is probably not a direct vasodilator itself, since recovery of tissue PO2 is far more rapid than the recovery of tissue acidosis and of vascular tone(16).
It is not known which of the factors causing the initiation and maintenance of vasodilation are critical in an exercising muscle during cycle exercise. However, it can be postulated that both myogenic and metabolic influences are present in view of the intermittent ischemia from arteriolar occlusion occurring during muscle contraction, as well as of the accumulation of metabolic by-products of exercising muscle. Our observation that maximum ischemic calf exercise results in a much longer duration of vasodilation than does thigh tourniquet occlusion alone suggests that, when conductance reaches a maximum, the increased demands of the periphery for oxygen during recovery after an intense metabolic stress can only be met by increased duration of vasodilation.
Correlations between ˙VO2peak/LBM with peak calf conductance and duration of vasodilation following ischemia alone, although present, were not as significant as those seen after ischemic calf exercise, probably because arterial occlusion with local exercise results in a more intense metabolic stimulus. There are several potential mechanisms by which local vascular characteristics may be correlated with ˙VO2peak. The determinants of ˙VO2 are cardiac output, its distribution, and oxygen extraction from capillaries to mitochondria in the periphery(1,3,22,27). Since oxygen extraction in muscles is known to plateau at submaximal oxygen consumption in healthy humans(27), ˙VO2peak is critically dependent on maximum cardiac output. During high-intensity cycle exercise, the blood flow to the leg muscles has been estimated to represent more than 65-70% of the cardiac output, and the leg ˙VO2 is greater than 75% of total˙VO2 (21). Hence, the factors determining blood flow to the exercising muscles (mean blood pressure and local vascular resistance) would be expected to be important determinants of˙VO2peak. In fact, we observed significant relationships between peak calf vascular conductance after thigh tourniquet occlusion, with or without local exercise, and ˙VO2peak. These findings are consistent with those previously reported by Martin and colleagues(18) and Snell et al. (25).
The correlations between ˙VO2peak and peak calf conductance after thigh tourniquet occlusion with and without exercise most likely reflect differing myogenic responses to ischemia in subjects of varying levels of fitness. It has been proposed that a “training effect” on the precapillary arterioles, which have been dilated repeatedly during training, may induce a greater ability of these arterioles to vasodilate in response to an ischemic stimulus (15). Furthermore, a trained subject may have a relatively greater volume of muscle in proportion to other tissues of the calf, and, since trained muscles contain larger vascular beds than untrained muscles (15,24), the vasodilatory response to ischemia will be further augmented.
Trained subjects have higher aerobic exercise capacity and skeletal muscles that are more fatigue-resistant owing to preponderance of fibers of Type I and IIa (28). Training also results in more conditioned skeletal muscles with higher oxidative capacity through numerous metabolic, structural, and functional adaptations at many levels, including increased myoglobin oxygen stores owing to preponderance of fibers of Type I, increase in leg blood flow, increase in capillary density that facilitates oxygen diffusion from blood to fibers, increase in mitochondrial volume, increase in the concentration of mitochondrial enzymes, and a greater oxidative enzyme capacity (24). Furthermore, glycogen stores can increase slightly in physically conditioned muscle fibers, and, more importantly, the rate of glycogen use is markedly depressed (9). The functional consequence is that the highly oxidative muscles of trained subjects have the capability of doing more anaerobic work compared with the untrained muscles of sedentary subjects. In the presence of ischemic calf exercise, the conditioned muscles of a trained subject are able to accumulate greater concentrations of vasoactive substances owing to tissue hypoxia and acidosis, and they can achieve a lower pH. In other words, there is a longer duration of ischemic calf exercise, which leads to a greater disturbance of the metabolic environment. This greater metabolic debt requires a prolonged post-ischemic duration of vasodilation for clearance of metabolites and restoration of the normal resting metabolic environment. Hence this would account for the close relationship we observed between duration of vasodilation following ischemic exercise and ˙VO2peak/LBM.
In summary, we found that the duration of vasodilation following maximum ischemic calf exercise is highly correlated with peak cycle exercise capacity in healthy young males. Furthermore, the duration of calf vasodilation following this stimulus appears to be a better index of peak exercise capacity than peak calf conductance in healthy subjects.
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BLOOD FLOW; VASCULAR CONDUCTANCE; REACTIVE HYPEREMIA; VASODILATION; MAXIMAL OXYGEN UPTAKE; EXERCISE