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

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 (T_re) 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, T_re 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, T_re 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.