The hamstring muscles flex your knee and extend your hip. A muscle pull or strain is an injury to the fibers of the muscle. The most common way to strain the hamstring muscle is doing something that contracts the muscle suddenly, like sprinting. However, a strain can also occur gradually as a result of inadequate stretching and flexibility. A knot that forms after a strain is an indication of a more severe injury.
The hamstring is actually a term used to describe a group of three muscles in the back of the thigh: the semimembranosus, the semitendinosus and the biceps femoris. These muscles work together to bend your knee and extend your hip, but it is when you straighten your leg suddenly, as in a sprint or leap, that you stretch the muscle too quickly or beyond its flexibility limit and cause it to tear.
An immediate, sharp pain in the back of the thigh accompanies a hamstring tear. The area of the tear will be tender to the touch and there may be heat or swelling. The muscle will be resistant to stretching and will feel stiff. Depending on how bad the tear is, you may have bruising or discoloration. If enough of the muscle fibers have torn, the torn ends will contract and ball up, producing a hardened, knot-like area in the muscle.
Muscle strains are graded according to severity using a scale of 1 to 3 according to the Sports Injury Clinic. A Grade 1, or mild, strain has a tear of less than 10 percent of the fibers and pain is minimal. A Grade 2 has a tear of 10 to 50 percent of the fibers and makes it difficult to walk without a limp. A Grade 3 tear affects more than half the muscle fibers, and you will likely need crutches or a cane while it heals. The greater percentage of fibers that tear, the larger the knot you will feel where the muscle has bunched up.
If you have an immediate hamstring tear, the best thing you can do is ice it and protect it with a compression wrap. You will need to introduce some mild stretching and strengthening exercises as soon as possible to help with rehabilitation of the injury. A sports chiropractor or physical therapist can help with ultrasound therapy, stretching and exercises. A sports massage therapist can also help reduce the knot and minimize scar tissue.