Those muscles include the hamstrings, quadriceps, and calf muscles. Each target the lower back, glutes and the hamstrings differently. The quadriceps, hamstrings, adductor magnus, and soleus serve as synergists during the exercise. In the lower extremities, muscle tone is increased at the hamstrings, quadriceps and ankles. Complimenting these muscles are the hamstrings which aid in short bursts of action. Those muscles which fulfill all of the four criteria are called true hamstrings. The two most common sources for tissue are the patellar ligament and the hamstrings tendon. A portion of the adductor magnus is sometimes considered a part of the hamstrings. One popular exercise used to strengthen the hamstrings is the leg curl. It serves as the origin for the inferior gemellus muscle and the hamstrings. This is an isolation exercise for the hamstrings. The hamstrings are quite susceptible to injury. This requires great strength in the hamstrings. Byrnes underwent an MRI that showed he had tears in both of his hamstrings. Working on hamstrings is also important.