When you're motivated to exercise and determined to get results, spending extra time at the gym may seem like a good idea -- but more exercise is not necessarily better. Finding a balance between duration, intensity and recovery will get you the best results while sparing you from adverse effects of overtraining.
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Energy Pathways and Training
To the lay person, exercise is a function of calories being burned to generate energy. While this is fundamentally true, energy recruitment during exercise is a bit more complex -- your body draws upon different fuels, called substrates, for activities of different intensity and length. For very high intensity exercises lasting for less than two minutes, such as a heavy weightlifting or an all-out sprint, your body generates energy without oxygen, drawing on muscle stores of creatine phosphate and glycogen, the storage form of glucose. For lower-intensity exercise of longer duration like walking, running or cycling, your body uses oxygen to burn glycogen and fat.
Your exercise goals for cardiovascular training should determine the length of your cardiovascular workout. If your interest is simply cardiovascular fitness without concern for weight loss, the American College of Sports Medicine recommends 30 minutes of moderate-intensity aerobic exercise five days per week. However, according to Sahand Rahnama of the University of Michigan Medical School, it is not until after 40 minutes of aerobic exercise that your body shifts to recruiting mostly fat for fuel. If fat metabolism and weight loss is your goal, the ACSM advises that 60 to 90 minutes of daily exercise may be necessary. Lifting weights before doing your cardio will deplete glycogen stores and force your body to recruit fat earlier in your cardio session.
For weight training your muscles rely on limited stores of creatine phosphate, good for only about 10 seconds, and glycogen. Available stores of both substrates vary from one individual to the next depending on nutrition status and storage capacity, which is enhanced through exercise. Because of these limitations, high-intensity exercise sessions lasting longer than 30 minutes deplete stored glycogen and force the body to break down muscle protein for energy, sub-optimizing workout benefits, notes Rahnama. In other words, weight training sessions lasting longer that 30 minutes could be breaking down rather than building up muscle.
Spending too much time exercising and not allowing enough time between sessions for muscles to recover can result in overtraining syndrome marked by irritability, apathy, sleep disturbances, loss of appetite and moodiness. Exercise scientist and professional bodybuilder David Knowles cautions that overtraining can lead to an increased risk of injury, staleness and lack of motivation to continue working out. To prevent overtraining, allow 48 to 72 hours between training sessions for the same muscle group. If you need to add more workout time to reach a daily goal of 60 to 90 minutes, add a session of cardio rather than working another muscle group. No matter how determined you are to stick to your training regimen, listening to your body and taking a break when you feel sore or exhausted can help you achieve better results in the long run.