Getting the most from weight lifting requires proper form, controlled movements and the right amount of weight. Lifting weights that are too heavy for you is a set-up for injury. If you can't maintain proper form while lifting, the weights are too heavy for you.
Exercising when you are fatigued is another easy way to get sloppy with your form. When you are tired, it's much harder to maintain proper technique and stay focused. Mental fatigue can put you at risk, particularly if you are cycling or running, as you may be more likely to ignore surrounding traffic and road conditions.
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