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

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% <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mrow><mml:mn>2</mml:mn> <mml:mi>max</mml:mi></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ \dot{\mathrm{V}}{\mathrm{O}}_{2\max } $$</mml:annotation></mml:semantics> </mml:math> ) and HIIT (6 × [30 s at 100% <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mrow><mml:mn>2</mml:mn> <mml:mi>max</mml:mi></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ \dot{\mathrm{V}}{\mathrm{O}}_{2\max } $$</mml:annotation></mml:semantics> </mml:math> with 30 s at 30% <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mrow><mml:mn>2</mml:mn> <mml:mi>max</mml:mi></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ \dot{\mathrm{V}}{\mathrm{O}}_{2\max } $$</mml:annotation></mml:semantics> </mml:math> ]). 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mrow><mml:mn>2</mml:mn> <mml:mi>max</mml:mi></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ \dot{\mathrm{V}}{\mathrm{O}}_{2\max } $$</mml:annotation></mml:semantics> </mml:math> (+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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mrow><mml:mn>2</mml:mn> <mml:mi>max</mml:mi></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ \dot{\mathrm{V}}{\mathrm{O}}_{2\max } $$</mml:annotation></mml:semantics> </mml:math> (+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.