HIIT adaptations can occur as fast as 2 weeks. It’s clear that from the very first session your body will begin to adapt to this training style, with significant results being seen after a 2 week period.
High intensity interval training is where very intense bouts of exercises are performed with periods of rest or easy work. This can be done with mono-structural exercises like running, cycling, rowing etc or mixed model where multiple exercises are used (CrossFit). HIIT is not strength training. Please never do HIIT for your glutes (as seen in the Nike training app) or HIIT for any other muscle group. HIIT is cardiovascular training, that can have other effects on the body, but it is predominantly cardiovascular training.
Often when we think about results or adaptations from HIIT they fall into two main categories, body composition (weight loss) and cardiovascular adaptation ( VO2 max ). Both of these categories involve a large variety of physiological adaptations that I have discussed in depth in my article on this topic specifically. So when giving a time frame for results, these will be the physiological changes to that we will be discussing, not strength etc. HIIT as even been shown to improve brain plasticity and learning , but I will not be talking about the speed of adaptations in this article.
By results, I think it is important to distinguish that we are discussing significant, notable results. Technically the adaptation process will begin from your first session. When talking about results that are significant the time scale becomes 2 weeks to see the results in terms of both body composition and cardiovascular adaptations. That is a very fast time frame, this can be a very motivating aspect of HIIT that the results can be very fast.
Frequency is the key factor that will effect the speed of the results. Unfortunately I did not find any studies which compared the frequency of HIIT to the speed of adaptation. I think it makes a great deal of sense to assume that the speed of results from performing HIIT three times a week and once a week will be notably different.
HIIT effects the body in a very potent way within a six week period when performed for three times a week. If you can find a way to consistently add it into your schedule then you are putting yourself in a great position to make a variety of cardiovascular and body composition adaptations.
4 x 3’ @90% Bike Intervals w/ 2’ Rest
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This resource was written by Sean Klein. Sean Richard Klein has thousands of hours of coaching experience and a BSc in Sports Science with Management from Loughborough University. He owns a gym in Bayonne France, CrossFit Essor, which runs group classes and a Personal training studio.