Take a break, ingest sugars, travel at a slower pace, and train up. Sometimes the sleeping position, an injury, or mild arthritis can be involved. Clothing, shoes, and parts of the bike can also be responsible. Very dry weather can cause skin cracking. Some of the areas that are most likely to become sore are the neck, the back, the hands, the arms, the shoulders, the feet, the crotch, and the bottom.
Muscles can also cramp.
Some creams and ointments might provide a little relief. A lack of fluids under these circumstances is very dangerous. To avoid the problem in the future, ride only in the morning and in the evening during very hot weather. Cold tiredness is caused by riding in low temperatures and getting chilled. A headache can be a side effect.