These two muscles make up the inner hamstrings. The semimembranosus has a flattened wide belly. The semitendonosus is fusiform in shape (tapered at both ends) with the distal end forming a long tendon. Both muscles originate from the ischial tuberosity. They have separate insertions on the proximal tibia, one on the inside of the back of the tibia (semimembranosus) and one on the inside of the front of the tibia (semitendonosus). The semitendonosus insertion combines with the sartorius and gracilis muscles to form a broad duckfoot-like insertion on the anterior tibia called the pes anserinus.
The semimembranosus and semitendonosus flex the straight knee, inwardly rotating the lower leg in the bent knee. This rotary component accentuates seated twists, but in the opposite direction of the biceps femoris. Contraction of these muscles also assists the gluteus maximus in extension of the thigh at the hip, as in virabhadrasana III. Tightness in this muscle limits forward bends and certain standing poses, especially those involving external rotation of the leg.
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