Updated December 04, 2014.
The transition from being sedentary to being active is a physiological marvel. Knowing what's happening your body during those first few minutes of exercise can help you make that transition safely and comfortably. You will also be better able to get the most out of your warm up, and your entire workout.
Why Do I Feel Out of Breath in the First Few Minutes of Exercise?
If the first five minutes of your workout feels 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 and psyched out by the heavy breathing and burning legs, use this as a reminder of the importance of the warm up, and remember what's happening in your body in the first few minutes of exercise.
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 or other high-intensity, full-body endurance exercise. The sensation is caused by the body's lag time in delivering adequate fuel to the working muscles.
For muscles to contract, they require energy in the form of ATP (adenosine triphosphate). The problem is that you only have enough available ATP to last for about ten seconds and 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 or three minutes worth of ATP, but soon, you'll need to switch to aerobic glycolysis to continue making fuel that the muscles can use. Aerobic, means 'with oxygen' and this system 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.
But 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 glycolosis. 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, through 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.
While some may find this initial burning and fatigue a huge deterrent to continuing exercise, keep in mind that if you make it through this phase, your body will adapt to a steady state where the aerobic energy system is able to keep up with the oxygen demand more readily. Yes, you may need to adjust your pace to the 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.
Benefits of a Gradual Warm Up
If those first five or ten minutes are simply too uncomfortable for you to push through, or if you are new to exercise, you can easily avoid this discomfort by engaging in a longer and more gradual warm up. A warm up is smart for a variety of reasons--particularly for preventing injuries--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 adding speed, or intensity, gradually during the first five or ten minutes, you will avoid the early discomfort of "oxygen debt" and the rest of your workout may feel far more effective.Although these temporary physiological changes occur in anyone switching from stillness 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 body of regular exercisers. The heart muscle becomes stronger and able to pump more blood with each contraction (increased stroke volume) which results in a lower heart rate. Lung capacity and oxygen transfer also increases. 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.
The next time you start exercising and feel the discomfort of going out too hard or too fast, visualize what's occurring in your body, ease up a bit, and keep on going. If you want a gentler approach when you head out for a workout, use the first five minutes as an easy warm up, then next five minutes to ramp up the pace, and then settle into your workout as usual.
With these tips, and an understanding of the physiology of the warm up, you may find that you start to look forward to your workout rather than dreading it.
Source
Kenney, Wilmore, and Costill. Physiology of Sport and Exercise: 5th Edition. 2011. Human Kinetics Publishing.