For a long time people have believed that exercising for extended periods of time at a lower intensity means you will burn more fat compared to a shorter period of higher intensity training. It is true that if you exercise at a lower intensity that you will be able to endure the exercise for longer, but the reason for this is that you are fatiguing your muscles and accessing your energy stores at a slower rate. So yes you will be burning more fat but it is not necessarily more effective for weight-loss and fitness gains. An alternative to this form of exercise is interval training, which is a short period of high intensity exercise followed by a bout of low intensity activity (rest period). This interchange is then repeated a number of times. This method of training will enable you to burn more calories in a shorter period of time which suggests you can achieve even better results in less time.
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