Purpose: The health benefits of a training program are largely influenced by the exercise dose and intensity. We sought to determine whether during a training bout of continuous versus interval exercise the workload needs to be reduced to maintain the prescribed target heart rate (HR).
Methods: Fourteen obese (31 ± 4 kg·m) middle-age (57 ± 8 yr) individuals with metabolic syndrome, underwent two exercise training bouts matched by energy expenditure (i.e., 70 ± 5 min of continuous exercise [CE] or 45 min of interval exercise, high-intensity interval training [HIIT]). All subjects completed both trials in a randomized order. HR, power output (W), percent dehydration, intestinal and skin temperature (TINT and TSK), mean arterial pressure, cardiac output (CO), stroke volume (SV), and blood lactate concentration (La) were measured at the initial and latter stages of each trial to assess time-dependent drift.
Results: During the HIIT trial, power output was lowered by 30 ± 16 W to maintain the target HR, whereas a 10 ± 11 W reduction was needed in the CE trial (P < 0.05). Energy expenditure, CO, and SV declined with exercise time only in the HIIT trial (15%, 10%, and 13%, respectively). During HIIT, percent dehydration, TINT, and TSK increased more than during the CE trial (all P = 0.001). Mean arterial pressure and La were higher in HIIT without time drift in any trial.
Conclusion: Our findings suggests that while CE results in mild power output reductions to maintain target HR, the increasingly popular HIIT results in marked reductions in power output, energy expenditure, and CO (21%, 15%, and 10%, respectively). HIIT based on target HR may result in lower than expected training adaptations because of workload adjustments to avoid HR drift.