# Physiological adaptations to 6 weeks of high‐intensity interval and moderate‐intensity continuous training in horses: A randomized crossover study

## TL;DR

This study found that high-intensity interval training (HIIT) improved performance and cardiovascular function more than moderate-intensity training in horses.

## Contribution

The study shows that HIIT leads to greater physiological improvements than MICT in horses, despite equal total running distance.

## Key findings

- HIIT improved run distance to exhaustion by 29% compared to 4.5% with MICT.
- HIIT increased speed at V˙O2max by 9.8% versus 2.4% with MICT.
- Both training methods similarly improved V˙O2max and maximal cardiac output by about 7-13%.

## Abstract

This study tested the hypothesis that 6 weeks of high‐intensity interval training (HIIT) would induce greater physiological adaptations than moderate‐intensity continuous training (MICT) in Thoroughbred horses. Seven untrained horses completed two distance‐matched treadmill training protocols (three sessions per week) in a randomized crossover design, separated by a three‐month washout: MICT (6 min at 70% V˙O2max) and HIIT (6 × [30 s at 100% V˙O2max with 30 s at 30% V˙O2max]). Incremental exercise tests were conducted at weeks 0, 3, and 6 to assess exercise performance and physiological responses. Mixed‐effects models were used to analyze the effects of time, protocol, and their interaction (p < 0.05). After 6 weeks, HIIT elicited greater improvements than MICT in run distance to exhaustion (+29% vs. +4.5%), speed at V˙O2max (+9.8% vs. +2.4%), and speed at maximal heart rate (+15% vs. +4.3%). Speed at 10 mmol/L of plasma lactate increased only after HIIT (+13% vs. +6.0%). Both protocols similarly improved V˙O2max (+13%) and maximal cardiac output (+7%–8%). In conclusion, despite being matched for total running distance, HIIT induced superior improvements in performance, cardiovascular function, and lactate kinetics compared with MICT, highlighting training intensity as a key determinant of training adaptations in horses.