A muscle strain is a stretch or tear of muscle fibers. In the leg, muscle strains happen when a muscle is either stretched beyond its limits or forced into extreme contraction. Because the leg has many different muscles, it is vulnerable to several different types of muscle strains. Some of the more common ones are:
Calf muscle strain (gastrocnemius strain). The calf muscle typically gets strained when the foot suddenly bends upward, stretching the calf muscle beyond its limits. At the time of injury, you may hear or feel a pop inside your calf -- the sound of the muscle tearing or shearing away from the Achilles tendon. Calf muscle strains are common in athletes, especially tennis players and joggers. However, they also can happen during a simple stroll, if your foot flexes upward when you step into a hole in the sidewalk or if your heel slips off the edge of a curb.
Plantaris strain. The plantaris is a thin muscle that begins at the lower end of the femur (the large bone of the upper leg), stretches across the knee joint and attaches to the back of the heel along with the Achilles tendon. Because the plantaris doesn't contribute much force in bending the knee, a tear in this muscle may not seriously affect your knee function. However, a severe plantaris strain can cause significant pain, usually at the back of your calf rather than near the knee. A plantaris strain can occur alone or accompany a gastrocnemius strain or a tear of the anterior cruciate ligament (ACL), a major, stabilizing ligament in the knee.
Hamstring strain (pulled hamstring). Hamstrings are long muscles that extend down the back of the thigh. Because hamstrings work to pull back the leg and bend the knee, they can be injured during running, kicking or jumping. You may feel a pop, usually at the back of the thigh, when the muscle tears.
Quadriceps strain. The quadriceps is a large group of muscles in the front of the thigh that straighten out the knee, the opposite action from the hamstrings. Quadriceps strain is a common injury in runners. However, it also may occur during a strenuous leg press at the gym. The pain of a quadriceps strain is felt in the front of the thigh. The strain may be described as a groin pull if the tear is fairly high in the muscle.
To help simplify diagnosis and treatment, doctors often classify muscle strains into three different grades, depending on the severity of muscle fiber damage.
Grade I. Only a few muscle fibers are stretched or torn, so the muscle is mildly tender and painful, but muscle strength is normal.
Grade II. A greater number of muscle fibers are torn, so there is more severe muscle pain and tenderness, together with mild swelling, noticeable loss of strength and sometimes bruising (called ecchymosis).
Grade III. The muscle tears all the way through. Either it rips into two separate pieces or the fleshy part of the muscle breaks away from the tendon. Grade III muscle strains are serious injuries that cause complete loss of muscle function, as well as considerable pain, swelling, tenderness and discoloration. A Grade III strain also causes a break in the normal outline of the muscle, often producing an obvious dent or gap under the skin where the ripped pieces of muscle have come apart.