Establishing VO2peak-based exercise intensity classification and physiological validation in ICR mice

This study aimed to develop a VO2peak-based framework for classifying exercise intensity in ICR mice and to validate the associated physiological responses.

Male ICR mice (8 weeks) completed four maximal graded exercise test (GXT) protocols (P1-P4) using a treadmill-equipped metabolic chamber. P1 and P2 began at 10 m/min with a fixed 15° incline, increasing speed by 3 m/min every 3 min (P1) or 1.5 m/min every 1.5 min (P2). P3 and P4 also started at 10 m/min but from a 0° incline; P3 increased both speed (3 m/min every 3 min) and incline (3° every 3 min), while P4 increased speed (1.5 m/min every 1.5 min) and incline (1.5° every 1.5 min) until 15°, after which only speed increments continued. VO2peak values were then used to calculate running speeds at 40%, 60%, and 80% VO2reserve to represent low, moderate, and high intensity, respectively. Constant-load tests (CXT) at these speeds measured metabolic responses and blood lactate concentrations.

VO2peak did not differ significantly among protocols, but P4 showed greater variability. Target speeds were therefore determined from P1-P3. During CXT, oxygen consumption, energy expenditure, and VO2plateau rose with intensity, and blood lactate at 60% and 80% VO2reserve exceeded that at 40%, with 60% surpassing the 4 mM aerobic-anaerobic threshold.

These findings provide standardized VO2peak-based reference values for classifying exercise intensity in ICR mice. Fixed-incline GXT protocols reduce variability, and the derived speed thresholds can guide future exercise interventions.