Muscle contraction is the tightening, shortening, or lengthening of muscles when you do some activity. It can happen when you hold or pick up something, or when you stretch or exercise with weights.
Institute of Human Anatomy - IOHA on MSN

The different types of muscle in your body

Discover the three main types of muscle in the human body and their distinct functions.
Repeated exercise, or wasting, can change the way key genes work.