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Harvard Commentaries
35320
Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School


Focus on Fitness Focus on Fitness
 

Reaching the Limits of Exercise


July 09, 2013

By Howard LeWine M.D.

Brigham and Women's Hospital

When your mind and body tell you, "I can't take another step or complete another rep," do you listen to your body or push beyond it, thinking, no pain, no gain? There is no right answer, but learning why you feel you've reached your limit will help move you to a higher fitness level.

Improving Oxygen Delivery

Cells break down sugars and fats to create energy, and they do that most efficiently when oxygen is present. This is called aerobic metabolism, a process that goes on perpetually throughout our bodies. Without oxygen, cells can still burn calories for energy — called anaerobic metabolism. But parts of the body can only function for short bursts of time under anaerobic conditions before energy production declines.

Oxygen delivery has long been considered the most important factor that limits exercise.

Oxygen in the air must make its way to individual muscle cells. After the lungs deliver the oxygen to the blood, the heart and the arteries move the oxygen-rich blood to the contracting muscles.

The lungs, heart, and arteries have maximal capacities. Once the maximal capacity of any one of these three components is reached, the muscles won't get any incremental increase in oxygen no matter how much they crave it. To keep the muscles contracting, the cells switch to anaerobic metabolism. In this state, the cells produce lactic acid, which causes muscle pain and fatigue.

The more athletically fit you are, the more efficiently your body delivers oxygen to your muscles. To increase your exercise capacity you want to maximize the oxygen delivery. This starts with getting more oxygen deep into the lungs. Be aware of your breathing as you exercise. Inhale through your nose, reaching a natural pause, then take another short inhale, and slowly let the air exhale through your mouth. As exertion increases, this becomes a challenge, because the increased oxygen demand will cause you to breathe faster. Try to stick with the technique even at the quicker breathing rate.

The heart can increase the amount of blood pumped to the arteries in two ways: by increasing the rate of heart contractions and by expanding the volume of blood that is pushed out with each heartbeat. With fitness training, the heart will enlarge over time so that it can push more out, but cranking up your heart rate is the predominant way that your heart does its part to improve oxygen delivery.

When an inactive person tries to exercise vigorously (NOT recommended), heart rate rises very quickly and hits the maximal rate within several minutes. Unlike your breathing, you can't voluntarily tell your heart rate to slow down. To achieve a more gradual heart rate rise during exercise, you need to improve your overall fitness. With improved physical conditioning, you increase both exertional effort and exercise time without a racing pulse.

During exercise, the walls of normal arteries relax to allow the heart to pump blood with less resistance, increasing blood flow and oxygen delivery to the working muscles. There are no specific techniques to improve artery function, but you can harm it. Smoking makes arteries stiffer by impairing their ability to relax. In addition, studies show that lower LDL cholesterol and triglycerides and higher HDL cholesterol are associated with healthier arteries.

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Dealing With the Sense of Effort

When you are exercising and you reach the point where you feel you cannot move another muscle, your brain is perceiving "exercise effort." This sense of exercise effort is related to both the duration of time you have exercised and the level of power output. This sense of effort is an unpleasant sensation that causes you to decide to stop exercising when it becomes intolerable. Like any painful or unpleasant sensation, your willingness to put up with the sense of effort is subjective and influenced by cognitive and emotional factors. People vary widely in how much "exercise effort" they can stand.

The nerve pathways involved in perception of effort are connected to both the skeletal muscles and the muscles that control breathing. Our brain will sense that the effort is coming from one or the other, or both at the same time. Without stopping your exercise activity, you can quickly lessen the unpleasant sense of effort by temporarily slowing down. The reason is that power output has a much more dramatic affect on sense of effort than does length of time doing the exercise. Small decreases in power output rapidly diminish sense of effort and allow you to continue exercising. This relationship is true for both breathing and skeletal muscle performance. So if you are running or pedaling on a bike, slow down a little to relieve the sense of effort. And then increase your pace again after a few minutes.

Stronger, conditioned muscles are much more able to use oxygen efficiently (aerobic metabolism). It is when muscles run out of oxygen and are operating without it (anaerobic metabolism) that lactic acid is created, which reinforces the sense of effort. By strengthening your muscles, you will decrease the point at which your brain perceives the effort sensation.

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Putting It Together

A progressive exercise session, where you start at a lower power output and gradually increase your effort, enables you to think about the limiting factors I described. During your walk, pick up your pace a little and think about your breathing — in through your nose twice and out through your mouth. If you can, check your heart rate, either by wearing a heart rate monitor or feeling your pulse. Stay at the faster pace until the sense of effort kicks in. The sense of effort is not chest pain or some other pain — that kind of pain should make you stop immediately. But the sense of effort is a sensation you will need to get beyond to reach a higher level of fitness.

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Howard LeWine, M.D., is chief editor of Internet publishing, Harvard Health Publications. He is a clinical instructor of medicine at Harvard Medical School and Brigham and Women's Hospital. Dr. LeWine has been a primary care internist and teacher of internal medicine since 1978.

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