Exercising regularly helps to prevent disease and prolong lives, and exercising intensely can prevent disease and prolong lives even more (PLoS Medicine, January 12, 2021). However, a study from Karolinska Institute suggests that exercising intensely too often can harm your health, and perhaps even shorten your life (Cell Metab, Mar 13, 2021;S1550-4131(21):00102-9). This study showed that people who tried to do intense workouts on an almost daily basis developed severe damage to their mitochondria that markedly:
• reduced their ability to exercise by reducing their maximum ability to take and use oxygen, and
• increased their risk for suffering high blood sugar levels and diabetes by making their cells less responsive to insulin.

Mitochondria are chemical structures inside your cells that help to turn food into the energy needed to drive biochemical reactions. When participants in this study reduced the frequency of their intense workouts, their mitochondria recovered and their health and training problems disappeared.

Stress and Recover — The Basic Rule for Exercisers
Every healthy person should exercise by stressing and recovering because you gain more health benefits and a higher level of fitness by exercising more intensely on one day, feeling a little sore on the next day and then going at a slow pace for as many days as it takes for your muscles to feel fresh again.

Stress Days: On your stress day, warm up by starting out at a very slow pace, and then alternate a series of going a little harder and faster until you feel a slight burning or tightness in your muscles. Immediately slow down and go at a slow pace until you feel fresh again. In the beginning, you may want to pick up the pace for only 5 to 10 seconds (“intervals”). As you improve, you can work up to 30-second “intervals” at your harder pace. For non-competitive exercisers, you don’t ever need to stay in an intense “interval” for more than 30 seconds. You then go at a slow pace for as long as it takes for you to breathe normally and for your muscles to feel fresh again. When your muscles feel fresh, you can pick up the pace and slow down immediately when you feel a burning or tightness in your muscles again. Novice cyclists can do 5 to 10 pedal strokes, and then go slow for as long as they need to recover. Novice runners can start out by taking only 5 to 10 running steps. Swimmers can do short bursts of their favorite stroke, and so forth for any activity you choose. Continue to alternate these slight pickups of intensity until your muscles stop feeling better as soon as you slow down. Eventually you may want to work up to 30-second intervals. At your peak, your stress workout can include a 10 minute warmup, a maximum of 30 minutes of intervals, and a 5 to 10 minute cool-down.

Recovery Days: Your recovery days (“easy days”) will always be governed by how you feel. On the day after a stress day, your muscles will probably feel slightly sore and you are supposed to recover by going at a very slow pace. The muscle discomfort is called Delayed Onset Muscle Soreness (DOMS). You warm up by going very slowly, and if your muscles don’t feel more comfortable after a 5-10 minute warm up, take the day off altogether.
• If your muscles feel better after your warmup, go at an easy pace for as long as you like and stop your workout when your muscles feel any protracted discomfort. You are never supposed to work through pain. Always stop if you feel fatigue or muscle tightness or discomfort.
• Continue to take slow recovery days for as many days as it takes for your muscles to feel fresh after you warm up for 5-10 minutes. Only then should you take your next hard stress day workout. Most exercisers follow each stress day with one to three or more recovery days before they take their next intense workout.

How Often Should You Do A Stress Day?
Non-competitive exercisers can gain significant health benefits, such as lowered high blood pressure and body fat, with just one intense workout a week followed by six days for slow recovery workouts. You can harm yourself by doing stress workouts too often. In one study, exercisers who worked out intensely three times a week for six weeks did not lower their high blood pressures or body fat as much as those who exercised more casually five times a week. (Med Sci Sports Exerc, Dec 11, 2021).

You will recover faster from a hard workout by doing nothing at all, but going slowly on your recovery days causes more fibrous tissue to form in your muscles so that they are more resistant to injury.

How Can You Tell If You Are Exercising Too Much?
Most competitive athletes learn sooner or later that when they exercise intensely too often, they may suffer an overtraining syndrome in which they can’t train or race at all. There are no laboratory tests that will tell you that you are overtraining. You should take days off, or markedly reduce your workouts, if you suffer:
• any injury
• muscle soreness that does not go away after you warm up
• continued difficulty falling asleep or staying asleep
• continued loss of appetite
• continued fatigue or loss of energy
• recurrent or prolonged infections
• any illness (check with your doctor)
• a persistent increase in resting heart rate of more than 10 beats per minute
• decreased muscle strength, endurance, or ability to perform and recover from your regular workouts

If you are a competitive athlete who is suffering from overtraining, you need to go back to background work. These principles apply to any sport. For a runner, jog on the days that you can, and take days off when you feel sore. After several weeks, you will be able to start regular jogging and your muscles will feel fresh again. When this happens, you are ready to start training, but first you must promise yourself that you will never try to run fast when you feel soreness in your muscles and tendons. Set up a schedule in which you take a hard-fast workout, feel sore on the next day, and then go at an easy pace in your workouts until the soreness has completely disappeared. You may try to take a hard workout every third to seventh day, but skip a hard workout on any day that you still feel sore after you warm up.

My Recommendations
Exercise helps to strengthen your heart, prevent disease and prolong lives, and intense exercise one or more times a week increases these benefits. However, you should know the signs and symptoms of exercising too much and the warning signs of damage from excessive exercise. Non-competitive exercisers can do very well on one day a week of intense intervals followed by six days of easy exercise.

Caution: Check with your doctor before you start or increase the intensity of an exercise program, particularly if you have chest pain, heart attack risk factors or heart problems.