Aging results in lower exercise tolerance, manifested as decreased Critical Power (PCRIT). Aging is also associated with reduced physical activity, decreased muscle mass, and altered muscle blood flow, all of which may contribute to the age-related decrease in PCRIT. Purpose: The purpose of this study was to determine if the age-related decrease in PCRIT occurs independently of changes in physical activity and muscle mass and if it is related to impaired muscle blood flow. Methods: 10 Old (63.1 ± 2.5 years, 5 female and 5 male) and 10 Young (24.4 ± 4.0 years, 5 female and 5 male) physically active volunteers enrolled in this study. Physical activity was measured with accelerometry. Leg muscle mass was quantified with dual x-ray absorptiometry (DEXA). PCRIT and the maximum power achieved during a graded exercise test (PGXT) during single-leg knee extension exercise were determined over the course of 4 visits. On the fifth visit, vascular function of the leg was assessed with the passive leg movement (PLM) hyperemia. Subsequently, subjects performed knee extension exercise at 10 watts (W), 20 W, 90% PCRIT, and 100% PGXT while blood flow and blood pressure were measured at the femoral artery for each intensity. Results: Young and Old subjects did not differ in daily step count (Old = 13001.1  2464.0 vs Young = 13527.0  3213.8 steps, P = 0.735) or in leg lean mass (9.06  0.62 g/kg, P = 0.901). The Old subjects had a lower mass-specific PCRIT (Old = 3.20  0.94 vs Young = 4.60  0.87 W/kg, P = 0.004), vascular function (mass-specific Passive Leg Movement (PLM): Old = 79.4  38.3 vs Young = 128.8  34.9 ml/min/kg, P = 0.010) and leg blood flow at 90% PCRIT (mass-specific: Old = 378  122 vs Young = 522  124 ml/min/kg, P = 0.014) and 100% PGXT (mass-specific: Old = 391  109 vs Young = 544  136 ml/min/kg, P = 0.013). When normalized for leg muscle mass, PCRIT was strongly correlated to peak leg blood flow in response to PLM (R2 = 0.53; P < 0.001) and leg blood flow during knee extension exercise at 90% PCRIT (R2 = 0.36; P = 0.007). Conclusion: The age-related decline in PCRIT is associated with major decreases in muscle endurance and is correlated with concomitant reductions in vascular function in healthy active adults. Future research should determine if interventions known to improve vascular function can ameliorate exercise tolerance in Old adults.

Recent evidence suggests that heat, a major byproduct of exercise, may be the mediator for many vascular adaptations that come from exercise. Thus, heat therapy that increases muscle temperature in a comparable way to exercise may be an advantageous alternative for enhancing cardiovascular health in individuals where treatment with exercise is either not possible or undesired. PURPOSE: Compare the effects of exercise and heat training on resistance artery function at rest and during exercise. METHODS: Thirty-five (18 female) healthy, untrained subjects completed a 6-week training program utilizing either high intensity knee extension (KE) exercise (40 min), localized heat therapy (pulsed shortwave diathermy; 120 min), or a sham heat therapy protocol (120 min). We randomly selected 8 subjects from each group to have a temperature probe inserted into their vastus lateralis muscle during one of their training sessions to evaluate the effect of the interventions on muscle temperature. We assessed resistance artery function at rest with the passive leg movement technique (PLM) prior to and after completion of the training protocols. We assessed peak exercise blood flow (KE peak flow) and peak power output (KE peak power) during the KE graded exercise test and prior to and after completion of the training protocols. RESULTS: Peak muscle treatment temperature was significantly different between all groups with those assigned to the diathermy heat training exhibiting a higher peak temperature (~40.80°C) than those in the exercise (~37.75°C, P < 0.001) and sham training groups (~36.10°C, P < 0.001). KE peak flow during PLM increased to the same extent (P = 0.625) in both the exercise (~10.5% increase, P = 0.009) and heating groups (~8.5% increase, P = 0.044); but tended to decrease in the sham group (P = 0.087). KE peak flow increased in the exercise group (~19%, P = 0.005), but did not change in the heat group (P = 0.523) and decreased in the sham group (~7%, P = 0.020). Peak vascular conductance during KE significantly increased by ~25% in the exercise (P = 0.030) and heat (P = 0.012) groups. KE peak power increased in the exercise group by ~27% (P = 0.001) but did not significantly change in the heat (P = 0.175) and sham groups (P = 0.111). The change in vascular function, assessed via PLM, showed a correlation with both ∆KE peak flow (R = 0.55, P = 0.01) and ∆KE peak power (R = 0.56, P = .010). Likewise, ∆KE peak flow showed a strong association with ∆KE peak power (R = 0.64, P < 0.001). CONCLUSION: Localized diathermy heat treatment increased resistance artery function at rest and during exercise to a similar extent as single-leg KE exercise training but did not yield significant improvements in performance. Thus, heat training mimics some but not all of the benefits associated with exercise and may be used to replace exercise treatment to some extent.