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.

The back consists of large, complex muscles that support your trunk and spine. There are many types of back muscles, and they are all essential, helping you walk, bend, twist, and hold your body ...

Discover the three main types of muscle in the human body and their distinct functions.

Here’s why some muscle groups respond faster (or slower) to strength training—and what you can do about it.