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Updated: Dec 22, 2025
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Short Session High Intensity Interval Training and Treadmill Assessment in Aged Mice
Published on: February 2, 2019
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Exercise Time and Intensity: How Much Is Too Much?
Summary
High-intensity interval training requires careful intensity management. Training over 9% of total time above 90% maximum heart rate (HR) may lead to overtraining symptoms, with 4-9% being optimal.
Area of Science:
- Exercise Physiology
- Sports Science
- Endocrinology
Background:
- Interval training is increasingly popular, demanding clear guidelines on maximal time and intensity.
- Overtraining is a concern for athletes, impacting hormonal balance and performance.
Purpose of the Study:
- To investigate the relationship between time spent in high heart-rate (HR) zones and salivary hormone responses.
- To determine if exceeding 9% of total exercise time above 90% maximum HR impacts cortisol variation post-exercise.
Main Methods:
- 35 healthy, active adults recorded HR for 3 weeks.
- Participants completed two 30-minute high-intensity interval sessions with a 4-hour recovery.
- Saliva samples were collected pre- and post-exercise to measure cortisol levels.
Main Results:
- Increased time (>9% weekly) spent above 90% maximum HR correlated with overtraining indicators.
- Reduced salivary cortisol fluctuation was observed in individuals training >40 minutes weekly in the >90% maximum HR zone (P < .001).
Conclusions:
- For regular exercisers, 4% to 9% of total training time above 90% maximum HR is recommended.
- This optimal duration aims to maximize fitness gains while minimizing overreaching symptoms.
Keywords:
cortisolexercise prescriptionheart ratehigh-intensity interval trainingoverreachingovertrainingMore Related Videos
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