Physiological Changes During Exercise

Why Early Fatigue Happens During a Warm Up

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The transition from a sedentary lifestyle to an active one is a physiological marvel. Whether you are new to exercise or are a longtime athlete, understanding what happens to your body during those first few minutes of exercise will help you stay safe, comfortable, and motivated. You'll also be able to get the most out of your warm-up and entire workout.

Early Fatigue During Exercise

If the first five minutes of your workout are the hardest, you aren't alone. That feeling of early fatigue during exercise is actually quite normal. It happens to even highly trained athletes. But rather than feeling demoralized by your heavy breathing and burning legs, use this knowledge as a reminder of the importance of the warm-up.

Sensations such as heavy breathing and burning muscles are normal in the first few minutes of exercise. It is caused by the body's lag time in delivering adequate fuel to the working muscles.

The physiological changes that allow you to maintain steady-state exercise occur during almost any exercise, but that heavy sensation of fatigue and breathlessness is most commonly noticed during running, cycling, stair climbing, swimming, and other high-intensity, full-body endurance exercise.

Causes of Early Fatigue

In order for muscles to contract, they require energy in the form of adenosine triphosphate or ATP. But you only have enough available ATP to last for about 10 seconds. Then you need to start manufacturing it. The body uses several energy pathways, including anaerobic glycolysis and aerobic glycolysis, to provide fuel to the working muscles.

The anaerobic system offers about two to three minutes worth of ATP, but soon the body will need to switch to aerobic glycolysis to continue making fuel. "Aerobic" means "with oxygen." This process uses lots of oxygen to convert stored glycogen, fat, and even lactic acid into energy. Once you tap into aerobic glycolysis, you can essentially keep the muscles contracting for hours on end.

Switching from one energy system to the other can create an oxygen debt until you are able to move oxygen to the working muscles fast enough to facilitate aerobic glycolysis. You may feel a burning sensation and discomfort in the muscles, and start breathing heavily as you increase the delivery of oxygen to the muscles.

Your heart rate will also increase to pump more blood, along with oxygen and nutrients, throughout the body. Once the transition occurs, you can move along comfortably at a steady pace and your breathing rate and heart rate will settle down as well.

Some people may find this initial burning and fatigue a major deterrent to exercise. But keep in mind that if you make it through this challenging phase, your body will adapt to a steady state in which the aerobic energy system is able to keep up with the oxygen demand more readily.

To manage the fatigue, you may need to adjust your pace to an appropriate level based upon your physical conditioning. But whether you are running a marathon or out for a brisk walk, you will find a steady pace where you will be able to continue moving for quite some time before you run out of energy.

How to Prevent Early Fatigue

If those first five or 10 minutes are simply too uncomfortable for you to push through, or if you are new to exercise, you can avoid this discomfort by engaging in a longer and more gradual warm-up. A warmup is smart for a variety of reasons—particularly for injury prevention—but easing into any intense cardio exercise is far more comfortable and enjoyable if you don't feel like you are wiped out before you've begun.

By starting your workout at a comfortable pace and gradually adding speed or intensity during the first five to 10 minutes, you will avoid the early discomfort of oxygen debt and the rest of your workout may feel much more effective.

Although these temporary physiological changes occur in anyone switching from inactivity to activity, if you exercise regularly, this transition will happen faster and more seamlessly. But that's not the only good thing that will happen: Long-term physical changes, or adaptations, also occur in the bodies of regular exercisers.

The heart muscle becomes stronger and able to pump more blood with each contraction, which results in a lower heart rate. Lung capacity and oxygen transfer also increase. Blood vessels become wider and more elastic, blood pressure decreases and new capillaries form. All of these changes lead to many long-term health benefits from regular exercise.

A Word From Verywell

When you start exercising and feel the discomfort of going out too hard or too fast, visualize what's occurring in the body. Then breathe, ease up a bit—and keep going.

For a gentler approach to a workout, use the first five minutes as an easy warm-up, the next five minutes to ramp up the pace, and then settle into your workout as usual. With a newfound understanding of warm-up physiology, you may find that you start looking forward to your workout rather than dreading it.

Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

By Elizabeth Quinn, MS
Elizabeth Quinn is an exercise physiologist, sports medicine writer, and fitness consultant for corporate wellness and rehabilitation clinics.