If you have ever played sports, chances are you have experienced a muscle strain injury. Even if you aren't an athlete, muscle strains are some of the most common injuries; however, this type of injury is often confused with a sprain, so let's start by clarifying the difference.

A strain is an injury that affects the muscle or the tendon. The tendon is the tissue that connects muscle to bone.
A sprain is an injury that affects a ligament. A ligament is connective tissue joining bones together.
In this lesson, we're specifically concerned with muscle strains, which can come in different types: the muscle itself can partially or completely tear, or the tendons can partially or completely tear. In any case, it's painful. You may also hear a strain referred to as a pulled muscle.

You'll usually feel a muscle strain as soon as it happen. They can hurt. In addition to sudden pain, you may experience lingering soreness in the area, bruising, swelling, weakness, stiffness, spasms, limited movement of the area, and maybe even a 'knotted' feeling. If the strain is mild, you will likely just experience stiffness and soreness; however, if the strain is major, you may have more pain and more difficulty using the muscle before it heals.

What exactly happens that causes a muscle strain? The most common places you may experience a strain are the neck, back, shoulder, and hamstring. Strains can happen if you begin to exercise without warming up properly (this is why we stretch) or if you lack flexibility or stamina. Sometimes, all it takes is one wrong move and you may accidentally strain a muscle. A sudden injury to the muscle causing a tear is called acute muscle strain.

https://study.com