Which muscles are active during knee flexion?

• A. Biceps Femoris; Semimembranosus; Semitendinosus; Gracilis; Sartorius; Popliteus; Gastrocnemius; Plantaris; Tensor fasciae latae (in 45-145 degrees flex)
• B. Rectus Femoris; Vastus Medialis; Vastus Intermedius; Vastus Lateralis
• C. Tibialis Anterior; Fibularis Longus
• D. Gluteus Maximus; Adductor Longus

Answer: (A)

Knee flexion is produced by muscles that cross the knee joint and shorten to pull the tibia toward the thigh. The primary knee flexors are the hamstrings: biceps femoris, semimembranosus, and semitendinosus, which cross the knee on the posterior side and drive bending of the knee. In addition, several other muscles can assist knee flexion: the gastrocnemius crosses the knee and can aid flexion when the ankle is relaxed; the plantaris also contributes; the popliteus helps initiate flexion from near extension (unlocking the knee); and the medial muscles gracilis and sartorius cross the knee and assist in flexion during multi-joint movements. The tensor fasciae latae can contribute to knee flexion in mid-range (about 45 to 145 degrees) through the iliotibial tract. Muscles that don’t flex the knee, like the quadriceps (knee extensors), and those primarily acting on the hip or ankle, aren’t involved in knee flexion. So the group listed best matches the muscles actively involved in knee flexion.