MUSCLE TIGHTNESS
If you’re not up there with Sir Bradley Wiggins yet and (like me) simply cycle to keep fit and for fun, you might find that you have quite tight hamstrings and calves. The tightness doesn't always manifest itself when you're on the bike because the body adapts to the rhythm of the pedal stroke, but try a non-cycling movement pattern like running and you soon notice that your legs aren't right. The problem also gets exacerbated by cycling longer distances than you're used to, when tightness in a calf or hamstring can lead to muscular tearing. Not a fun experience.
The simple answer is to make sure that you warm up before exercise and cool down afterwards. Stretch your calves out so the muscle fibres retain their elasticity. A foam roller used at the end of a ride can also do a huge dollop of help: it's painful, but using them to smooth out your muscles will reduce any lasting tightness.