With the rise of programs such as Les Mills GRIT Series over the past half decade or so it is interesting to see when happens to participants. I have seen it myself – people flood into these varied, high energy classes, and you can see the appeal; it’s quick and the science has told us that it does reap benefits, such as fat loss and improvements in strength, power and aerobic fitness.
I am the same, I jumped into Crossfit about 7 years ago and have been trying it off and on ever since. But after a week or so I kept on getting ill (true story), so I chilled out a little bit. This is the same when I decide one week that I’m on a mission and exercise 10 hours more than a usual. I get a cold. Maybe some of these examples are familiar to you. Now some science specifically targeted at high intensity training may give us (and me!) a reason why.
A recent study published in the Frontiers of Physiology has found that high intensity interval training (HIIT) for more than two consecutive days can have negative effects on the immune system, making the participant more susceptible to infections and colds.
The study took a group of regular Crossfit participants and took them through two high intensity Workout of the Days (WODs) on consecutive days and measured for metabolic changes and immune system changes. They found that there were large metabolic changes, such as increased lactate and blood glucose (which is to be expected from sessions of high intensity exercise), and there was a general decrease in immune system function after the two sessions.
Although these decreases were not high enough to be immediately threatening to health, the recommendations from this study were to add a rest day (or low intensity) in to the workout routine after two days straight of high intensity exercise, to allow the immune system to ‘catch up’ and be back to normal levels ready for the next session.
This general advice isn’t anything radically new; rest days have been recommended by fitness professionals for decades, but it does add weight to an increasing body of evidence that suggests that too much HIIT style training with not enough rest can have serious implications on health.
The bottom line
The advice from this research paper is worth following.
There is a massive body of scientific evidence spanning decades about the virtues of rest days, and this study adds HIIT style training into the mix. Just because the workout doesn’t last as long, it doesn’t mean that you shouldn’t still take rest days. Rest days, aside from improving the immune system as suggested by this new study, also allows the body to recover from the trauma that it has been put through during training.
Rest days build and recover muscles, removes waste products and gives you an important psychological recovery.