If you're not warming up before you work out, this could be the cause of your fatigue. Gradually warming up instead of jumping right into an intense workout will increase your metabolic rate, help prevent injury to your muscles, help your muscles contract faster and let you work out longer with less lactic acid build-up. All of these advantages will also result in your experiencing less fatigue after a workout. A warm-up doesn't have to be complicated. Do some slower aerobic exercises first, such as jogging slowly or cycling in a lower gear, and then gently stretch your muscles.
Watch Your Intensity
Exercising more intensely than your body is prepared for can also lead to fatigue. If you are just beginning an exercise program or if you are out of shape, you should start with a lighter intensity that is just 40 to 50 percent of your maximum heart rate. To calculate your maximum heart rate, subtract your age from 220. This number is the maximum number of times that your heart should be beating each minute that you are working out. If you are in shape already or have been working out for awhile, you will likely want to work out within your target heart rate zone. The lower range of this zone is your maximum heart rate multiplied by 0.7, and the higher range is your maximum heart rate multiplied by 0.85. To avoid overexerting yourself and causing fatigue, you should try to stay within this range.
Mix It Up
If you are just doing one type of exercise each time you work out, this can also cause fatigue. By only working out the same muscle group over and over, you will be overexerting these muscles at the expense of not working out other muscle groups at all, which can cause your body to feel tired and fatigued. Try to change up your workout routine instead. If you are swimming all the time, add in some jogging or elliptical routines. If you're only doing aerobic exercises, substitute strength training for a couple days out of the week.
Sometimes, working out can cause fatigue if you have an underlying medical disorder. Of course, many other factors can be contributing, such as your weight, your diet, or not getting enough sleep. But if you've exhausted your options and you're still tired all the time, consider visiting a doctor. Anemia, vitamin deficiencies, thyroid problems, diabetes and even depression are just a few of the medical disorders that can cause you to feel even more fatigue when you exert yourself physically.