Going fftopic: because I love to learn.I just hit a quick google search earlier in effort to be more of an ass. http://www.innerbody.com/anatomy/skeletal/semitendinosus-tendon
Something doesn't make sense when you say that it's a tendon. Having studied this more recently due to muscle issues that I am experiencing after a major ice skating accident to build strength for ice hockey, this particular muscle was affected and is still problematic...
How can a tendon be a muscle? Tendons do not contract like muscles do; therefore, how would it be possible for a tendon to actively move various body parts? Surely, a tendon isn't creating a leverage point since a tendon is unlike a stiff broom stick.
I have reviewed your link and did a search of my own. Here are my findings. The link that you provided is very contradictory.
"The semitendinosus tendon is one of the hamstring muscles. It is a long, band like muscle on the back of the thigh toward the inside, connecting the ischium to the proximal end of the tibia. It is so named because it becomes tendinous in the middle of the thigh, continuing to its movable end as a long, cordlike tendon. It functions to flex and rotate the leg medially and to extend the thigh." (http://www.innerbody.com/anatomy/skeletal/semitendinosus-tendon)
By definition, "A tendon (or sinew) is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae; all three are made of collagen. Ligaments join one bone to another bone; fasciae connect muscles to other muscles. Tendons and muscles work together to move bones." (http://en.wikipedia.org/wiki/Tendon)
Going back to our disagreement.
Based on this picture of the semitendinosus, it is predominantly a muscle, but the tendon portion is larger than most tendons, thus, the common confusion as a tendon. (http://upload.wikimedia.org/wikipedia/commons/3/3f/Semitendinosus_muscle.PNG)
"The semitendinosus helps to extend (straighten) the hip joint and flex (bend) the knee joint. It also helps medially rotate the tibia on the femur when the knee is flexed and medially rotate the femur when the hip is extended. It counteracts forward bending at the hips as well." (http://en.wikipedia.org/wiki/Semitendinosus_muscle)