What is a Muscle Pull?

Before jumping in to what a strained muscle is, it's important to cover a few basics on muscles.   The human body contains three kinds of muscle fibers - cardiac (heart muscle), smooth (involuntary muscles such as the ones in the stomach lining), and voluntary skeletal muscles.  The last category - skeletal muscles - makes up the majority of the muscles in your body.  It is these muscles that cause motion, and it is these muscles that athletes and others strain or pull.

Skeletal muscles are connected to bones via tendons.  These muscles are made of bundles of fibers that are designed to contract.  When the fibers contract, the muscle shortens, pulling on the attached bone and rotating the bone about the joint.

Often times, during this contraction, the muscle fibers encounter resistance.  This can be because the muscle is being used to lift something (like a weight in the case of a bicep curl).  Resistance can also come from the ground, which is what creates the ability to run, jump, cut to switch directions, etc.

The problem arises when the resistance is strong or sudden enough to be too much for the muscle fibers to handle.  In such a case, in the attempt to contract against this resistance, the muscle fibers tear.   If enough of the fibers tear, you end up with a muscle pull.