Good morning, many people now a day do not recognize signs of thirst and the amount of water we need varies depending on which organizational website you are looking at. RDNs do a quick calc for people who do not have draining wounds or other medical Nutritional issues. The quick calculation we use is 1 cc per calorie consumed. For individuals who work out, that amount is even higher. Remember 1 cup equals 240 cc, so for an average 5 11 man that is active the minimum amount of water you want to shoot for is 11 cups per day considering you are probably consuming 2700 kcal/day with your activity. Use a sports bottle, I have found that carrying my hot pink sports bottle makes me drink more water. Another trick - weigh yourself before and after workout and drink 1 cup of water for every pound lost - more if you don t replace your wt. by the end of the day. Have you addressed a possible imbalance in nutrients or electrolytes? If electrolyes are off, choose something that will give you back your electrolytes. In America we use a drink called Gatorade. Potassium, Magnesium, Sodium - all may be low with the description of the muscle fatigue. If increasing your water intake as suggested and adding an electrolyte drink doesn t work, you should definitely see a doctor as many chronic illness can also start out this way. Talk to your doctor. It was nice working with you. Respectfully, Ms. Kathryn J. Shattler, MS,RDN