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Too Much Interval Training

Last week, the New York Times featured an article that discussed a new research study published in the scientific journal Cell Metabolism. The study focused on one of my favorite topics (and it should be one of yours as well): What is the appropriate dose response relationship of high intensity interval training (also known as HIIT; intense cardio for brief periods followed by a brief recovery period)? Just like with medicine, exercise researchers are constantly trying to determine what dose of exercise elicits the desired response (improvements in cellular measures of health and fitness, fat loss, muscle gain, etc.).
To be clear, most exercisers have mistakenly assumed that if some exercise is good for us, more must be better. Of course, the research continues to demonstrate that this isn’t true when it comes to strength training and based on this new study, it isn’t true when it comes to HIIT either.
In this 4-week study, each week, the subjects performed more frequent HIIT workouts. Here is what the participants did:
Week 1: 2 HIIT workouts on a stationary bike
Week 2: 3 HIIT workouts on a stationary bike
Week 3: 5 HIIT workouts on a stationary bike
Week 4: They cut the amount of exercise they did in half in an attempt to recover
The results?
Subjects improved metabolic fitness during week 1 and 2. However, during week 3 (5 HIIT workouts), the subjects experienced a diminished ability to produce power and perhaps more importantly, important health biomarkers started to move in the wrong direction. Increased HIIT frequency was only valuable up to 3 weekly sessions. Beyond 3 sessions, subjects’ metabolic health declined. The lead researcher, Mikael Flockart concludes, “HIIT exercise should not be excessive if increased health is a desired outcome.”
A major strength of this study is that the laboratory assessments of health indices were incredibly well rounded (and invasive) and included biopsies of muscle to examine mitochondrial function and metabolic health. A potential weakness of the study is a small sample size (11 participants) and short duration (what would happen if this was a 12-week study and they increased frequency more gradually?).
Take home message:
For best results, you don’t need to use your Peloton very often (and you shouldn’t). If a little bit of HIIT is great for you (and it is), more isn’t better (and in fact, it’s worse). 
Important note: This only applies to HIIT. It doesn’t apply to all of your activity or exercise. During a week, you could do 1-2 HIIT sessions on a bike or treadmill; strength train 1-2 times and go on a long run or walk. 

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