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Physiological reports2026; 14(4); e70785; doi: 10.14814/phy2.70785

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

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% ) and HIIT (6 × [30 s at 100% with 30 s at 30% ]). 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 (+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 (+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.
© 2026 The Author(s). Physiological Reports published by Wiley Periodicals LLC on behalf of The Physiological Society and the American Physiological Society.
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Publication Date: 2026-02-18 PubMed ID: 41703755PubMed Central: PMC12913712DOI: 10.14814/phy2.70785Google Scholar: Lookup
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  • Journal Article
  • Randomized Controlled Trial

Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study compared the effects of 6 weeks of high-intensity interval training (HIIT) versus moderate-intensity continuous training (MICT) on physiological and performance adaptations in Thoroughbred horses.
  • It found that HIIT produced greater improvements in running performance, cardiovascular function, and lactate metabolism than MICT, despite both protocols matching total running distances.

Study Design

  • Subjects: Seven untrained Thoroughbred horses participated.
  • Training protocols:
    • MICT: Continuous treadmill running for 6 minutes at 70% of maximal oxygen consumption (VO2max).
    • HIIT: Six intervals consisting of 30 seconds at 100% VO2max alternated with 30 seconds at 30% VO2max (total time approximately 6 minutes at high intensity with recovery periods).
  • The protocols were distance-matched to ensure equal total running volume.
  • Design: Randomized crossover with each horse performing both protocols with a 3-month washout period between them.
  • Training frequency: Three sessions per week for 6 weeks.

Testing and Measurements

  • Incremental treadmill exercise tests were performed at baseline (week 0), week 3, and week 6.
  • Measured parameters included:
    • Run distance to exhaustion – how far horses ran before fatigue.
    • Speed at specific physiological thresholds like VO2max, maximal heart rate, and at a plasma lactate concentration of 10 mmol/L.
    • VO2max – maximal oxygen consumption, an indicator of aerobic capacity.
    • Maximal cardiac output – the greatest volume of blood the heart can pump per minute.
    • Lactate kinetics – how the horses’ bodies metabolize lactate during exercise.
  • Data analyzed using mixed-effects models to evaluate effects of time, training protocol, and their interaction with significance set at p < 0.05.

Key Findings

  • Performance improvements:
    • Run distance to exhaustion increased significantly more after HIIT (+29%) compared to MICT (+4.5%).
    • Speed at VO2max improved by +9.8% with HIIT versus +2.4% with MICT.
    • Speed at maximal heart rate improved by +15% for HIIT vs. +4.3% for MICT.
    • Speed at 10 mmol/L plasma lactate increased only in HIIT (+13%), showing better tolerance to high lactate levels.
  • Physiological adaptations:
    • Both HIIT and MICT similarly enhanced VO2max by approximately 13%, indicating increased aerobic capacity under both conditions.
    • Maximal cardiac output increased by 7–8% in both training types, showing improvements in cardiovascular efficiency.

Interpretation and Implications

  • Training intensity has a substantial effect on physiological adaptations in horses, with HIIT eliciting superior benefits compared to continuous moderate-intensity training.
  • The superior improvements in speed at various physiological thresholds and lactate tolerance suggest that HIIT enhances both aerobic and anaerobic systems more effectively.
  • Since total running distance was matched, the intensity of effort rather than volume was the key factor influencing adaptation.
  • This supports the use of HIIT in equine training programs to optimize performance gains.
  • The study’s randomized crossover design strengthens the confidence in the findings by reducing inter-animal variability.

Summary

  • Six weeks of high-intensity interval training in untrained Thoroughbred horses led to greater enhancements in endurance performance, cardiovascular function, and lactate metabolism than an equivalent volume of moderate-intensity continuous training.
  • These findings highlight the importance of training intensity in designing equine conditioning programs for improved athletic performance.

Cite This Article

APA
Mukai K, Takahashi Y, Ebisuda Y, Sugiyama F, Yoshida T, Miyata H. (2026). Physiological adaptations to 6 weeks of high-intensity interval and moderate-intensity continuous training in horses: A randomized crossover study. Physiol Rep, 14(4), e70785. https://doi.org/10.14814/phy2.70785

Publication

Physiological reports
ISSN: 2051-817X
NlmUniqueID: 101607800
Country: United States
Language: English
Volume: 14
Issue: 4
Pages: e70785
PII: e70785

Researcher Affiliations

Mukai, Kazutaka
  • Sports Science Division, Equine Research Institute, Japan Racing Association, Shimotsuke-shi, Tochigi, Japan.
Takahashi, Yuji
  • Sports Science Division, Equine Research Institute, Japan Racing Association, Shimotsuke-shi, Tochigi, Japan.
Ebisuda, Yusaku
  • Sports Science Division, Equine Research Institute, Japan Racing Association, Shimotsuke-shi, Tochigi, Japan.
Sugiyama, Fumi
  • Sports Science Division, Equine Research Institute, Japan Racing Association, Shimotsuke-shi, Tochigi, Japan.
Yoshida, Toshinobu
  • Sports Science Division, Equine Research Institute, Japan Racing Association, Shimotsuke-shi, Tochigi, Japan.
Miyata, Hirofumi
  • Biological Sciences, Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Yamaguchi-shi, Yamaguchi, Japan.

MeSH Terms

  • Animals
  • Horses / physiology
  • High-Intensity Interval Training / methods
  • Physical Conditioning, Animal / methods
  • Physical Conditioning, Animal / physiology
  • Adaptation, Physiological
  • Cross-Over Studies
  • Male
  • Oxygen Consumption
  • Heart Rate
  • Female

Grant Funding

  • 2019-2844-01 / Japan Racing Association (JRA)

Conflict of Interest Statement

K.M., Y.T., Y.E., F.S., and T.Y. were the employees of the JRA. The JRA had no role in the design of the study; in the collection, analysis, or interpretation of data; or in the writing of the manuscript.

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