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Serious athletes are obsessives. If they thought training eight days a week would give them a performance or physique edge over the competition, they would find a way to do it.

But medical professionals and certified trainers have long known that overtraining is counterproductive. Overtraining -- literally the maximization of work output to the point where the body cannot sufficiently recover from it -- can lead to a whole host of negative outcomes including higher than normal resting heart rate, lingering soreness, impaired immune function, irregular sleep or appetite, and loss of focus and concentration.

By definition, overtraining also undermines muscle growth and recovery, and has a negative impact on performance. Until recently, the consensus among experts has been that the work output threshold necessary to put one at risk for overtraining was relatively high. It was not something your average gym-goer need worry about. But now those same experts are coming to believe that overtraining may be more common than was previously believed.

What exactly is the "sweet spot" for productive training? Three days per week? Four? Five? What exactly is happening at the cellular level in overtrained muscles and how much exercise is necessary to trigger it? Earlier this year, researchers at Stockholm's Swedish School of Sport and Health Sciences and the Karolinska Institute mounted a clinical study to get some answers. They arrived at some surprising conclusions.

Finding the Sweet Spot of Maximal Benefit

In the study, which has been published online in the journal Cell Metabolism, the researchers set out to compare different amounts of work output by measuring associated markers of muscle cell efficiency for each. They recruited 11 healthy men and women and assessed their current level of fitness and metabolic health, including resting blood sugar levels.

Then they put them to work. During the first week, test subjects performed two sessions of high impact interval training (HIIT) in the form of four-minute intervals on a stationary bicycle five times, with three minutes of rest between each bout. Subjects had their leg muscles biopsied and their blood-sugar levels measured.

In week two, a third weekly session was added and high-intensity intervals were increased to a mix of four- and eight-minute bouts of maximum pedaling. In week 3, the workout regimen was expanded to five days, again with an emphasis on longer bouts of high-speed pedaling. Finally, in week 4, the amount of work output and its intensity were cut in half. At the end of each week, the researchers repeated the muscle biopsies and measurements.

Existing studies have shown that short, intense bursts of exercise (such as HIIT) can enhance aerobic fitness by increasing the number of (and energy-producing efficiency of) muscle cell mitochondria. And indeed, this is what the researchers in the Stockholm study discovered when reviewing the Week 1 and Week 2 cell biopsies. Test subjects registered increased total cell mitochondria in muscle tissue and those mitochondria were producing greater amounts of energy than they were at baseline (the study's beginning). Subjects also exhibited better daily blood sugar control.

After Week 3, however, muscle cell efficiency nosedived. Test subjects' ability to generate power and velocity plateaued as cellular mitochondria produced only 60 percent of the energy generated in the two previous weeks. Blood sugar control also deteriorated, with test subjects experiencing erratic spikes and dips throughout the day. Reduced work output in Week 4 facilitated some bounce back in cellular function, but energy output was still down 25 percent, and blood sugar control was still impaired.

The researchers concluded that adding multiple, intense interval workouts per week with little rest between them was likely to lead to a tipping point, after which the added work output serves little or no constructive purpose, and may even be counter-productive. Clearly, not everyone's tipping point is the same, and more study with larger sampling sizes is necessary.

Arriving at a Work / Rest Ratio That Works For You

What do the results of this study tell us? Well, first of all, moderation -- in the form of proper work / rest balance -- is essential to consistent, ongoing productivity, growth and gains. Secondly, overtraining is not exclusively the concern of elite athletes. The line between training and overtraining is well within the reach of even recreational fitness enthusiasts.

How can you find your own tipping point? First of all, listen to your body. If you have to literally drag yourself to the gym or you're still experiencing soreness from your last visit, consider taking a day off. Or just go for a walk. Make sure you get the daily nutrition, sleep and supplementation you need to fuel your exercise regimen.

Consider varying the intensity of your workouts. Two to three high-intensity, heavy-volume days per week is plenty. Add lower-intensity days (lighter weights, more reps) to make long-term progress possible. Experienced athletes also know the value of simply taking a week off to reset and recharge. A week off every 8 to 10 weeks or so can do wonders for productivity and motivation once you return to the gym.

Athletes will always be tempted to drive themselves to (and beyond) their limits, and regard ongoing fatigue as the price paid for productive training. There's a lot to be said, however, for respecting those limits and recognizing the price of overdoing it.




Mikael Flockhart, Lina C. Nilsson, Senna Tais, Björn Ekblom, William Apró, Filip J. Larsen. Excessive exercise training causes mitochondrial functional impairment and decreases glucose tolerance in healthy volunteers. Cell Metabolism, March 18, 2021, DOI:https://doi.org/10.1016/j.cmet.2021.02.017.


The articles featured herein are for informational purposes only and should not be construed as medical advice. Specific medical advice should only be obtained from a licensed health care professional. No liability is assumed by ProSource for any information herein.