Acute Physiological Responses to Different Load Components During Interval Training

Interval training is a popular method for enhancing athletic performance, adjustable through speed, distance, and rest. The acute physiological and subjective responses to isolated manipulation of these components remain unclear. This study examined acute physiological and perceptual responses to systematic manipulation of speed, distance, or rest during interval training. Twenty-two young male physical-education students (average age 25.2 ± 3.5) with high baseline physical activity but no prior structured interval-training experience completed three randomized interval protocols. In all protocols, the first interval was identical (92% vVO2max, 250 m, 180 s rest). In the remaining five intervals, either speed, distance, or recovery duration was progressively changed in 8% steps (speed, distance, and rest protocols, respectively), while the other two components were kept constant. Blood lactate (La), oxygen consumption (VO2), ventilation (VE), respiratory exchange ratio (RER), heart rate (HR), and rating of perceived exertion (RPE) were measured throughout. Overall, the speed protocol elicited the highest average peak values across all measured parameters (p < 0.001). However, the percentage change in VO2 from Interval 1 to Interval 6 did not differ significantly between the speed and distance protocols. Between-protocol differences varied across outcomes: the speed protocol elicited greater changes in Lactate from Interval 2 onward and in VE and HR from Interval 3 onward (p < 0.05). Rate of perceived exertion was highest during the speed protocol (p < 0.001) but did not differ significantly between distance and recovery protocols. Applying the same relative 8% progression across interval components elicited distinct physiological responses. Progressive increases in running speed produced greater acute peak cardiorespiratory and metabolic responses than progressions in distance or reductions in recovery duration.