Do you feel sore in training, even though you didn't train the day before? Be aware that this pain or difficulty lifting a heavier weight or running an extra mile may be due to dehydration, not a lack of strength. Research shows that a drop in hydration of just 2% can take away your strength and motivation while increasing your feeling of fatigue. Dr Navarro says that when we get the right amount of water for our body, our muscles relax, which increases our energy and performance.