Semitendinosus

What it is

One of the two medial hamstrings, lying on the inner side of the back of the thigh. Its name describes it exactly: semi-tendinosus, half tendon. The muscle belly finishes around the middle of the thigh and a long, round, cord-like tendon carries on to the tibia.

You can feel that tendon. Sit down, bend your knee, and press behind the inner side of it — the prominent cord standing out is this one, with semimembranosus as a broader, deeper mass beside it.

It joins sartorius and gracilis to form the pes anserinus, the "goose's foot", a fan of three tendons converging on the upper inner tibia.

What it does

Bends the knee and extends the hip, crossing both joints.

With the knee bent it rotates the lower leg inward — the opposite of what biceps femoris does, which is how the two sides of the hamstring group steer the knee between them.

What loads it

Hip hinging and knee flexion under load, along with the other hamstrings. Running, and anything requiring rapid deceleration of the swinging leg.

Its long tendon is commonly harvested as a graft in anterior cruciate ligament reconstruction, which is a surgical fact rather than a training one, but it is the reason many people have heard of the muscle at all.

What is commonly got wrong about it

That the hamstrings are three copies of the same muscle. They differ in where they insert, which way they rotate the knee, and whether they cross the hip. The short head of biceps femoris does not cross the hip at all.

Pes anserinus pain as "a hamstring problem". Pain at the inner knee where those three tendons converge is a recognised complaint, but two of the three tendons there are not hamstrings — sartorius and gracilis come from elsewhere entirely.

That you can feel which hamstring is working. You cannot separate them by sensation.