# Core body temperature correlates of transition from aerobic to anaerobic metabolism in running

## TL;DR

This study explores how core body temperature changes during running and its link to shifts in metabolism from aerobic to anaerobic.

## Contribution

The study introduces a novel method to identify temperature thresholds corresponding to metabolic transitions during exercise.

## Key findings

- Core body temperature increases in three distinct phases during graded running.
- The second temperature threshold (TT2) strongly correlates with the second gas exchange threshold (VT2).
- Test-retest reliability of the method was found to be low.

## Abstract

We investigated core body temperature (CBT) during a graded exercise test (GXT) in comparison with gas exchange dynamics.

Thirty-two active males performed a treadmill GXT (0.5 km/h increments every 30 seconds, 1.5% incline) until exhaustion. Gas exchange data and rectal temperature (Tre) were continuously registered. Ten participants repeated the test for reliability assessment. The first and second gas exchange thresholds (VT1 and VT2) were determined by the simplified V-slope method, while CBT dynamics and eventual temperature thresholds (TT1 and TT2) were assessed according to the criteria defined in this study. Three independent evaluators determined gas exchange and temperature thresholds.

In 29 subjects, Tre increase was best fitted with a 3-phase segmented model of successively steeper slopes, with a linear relationship in all three segments (17 subjects), or in two segments, with a quadratic relationship for the remaining segment (12 subjects). The between-segment intersection points were considered as TT1 and TT2. In three participants, Tre was best fitted with a two-segment, single-breakpoint (TT1 or TT2) model. The evaluators’ objectivity was satisfactory for VT1 (α = 0.786), very high for TT2 (α = 0.941) and VT2 (α = 0.948). TT1 and VT1 were moderately correlated (ρ = 0.41, p = 0.021) while VT2 and TT2 were highly correlated (r = 0.78, p < 0.001) showing a small, yet statistically significant difference (12.95 ± 1.9 vs 13.43 ± 1.7 km/h, p = 0.039). However, test-retest reliability was low.

The breakpoints in CBT increase observed during graded running may represent transitions between the three intensity domains of physical activity.