The knee is the largest, most complex synovial joint in the body. It is primarily limited to flexion and extension, but allows for some minimal rotation. It is composed of the distal femur, proximal tibia, and patella. The knee is stabilized by the posterior cruciate ligament (PCL), anterior cruciate ligament (ACL), the medial collateral ligament (MCL), the lateral collateral ligament (LCL), and meniscus. The patella is held into place by the patellar ligament. 

In weight-bearing, as is the case during walking, ankle dorsiflexion and plantarflexion cause the tibia to move anteriorly and posteriorly, respectively. This results in flexion or extension of the knee, respectively. Keeping this in mind, although the gastrocnemius is a plantarflexor, it is also a knee flexor, so weakness in this muscle would not contribute to knee buckling. Because of the effect of dorsiflexion during weight bearing, the tibialis anterior would actually promote knee buckling, not resist it. Although the tibialis posterior is a plantarflexor, its primary role in gait involves maintaining the shape of the medial longitudinal arch. The soleus is ankle plantarflexor that eccentrically resists knee flexion (anterior movement of the tibia during weight-bearing). Accordingly, soleus weakness would explain this patient's knee buckling.

The long head of the biceps femoris attaches to the ischial tuberosity, and has attachments to the sacrotuberous ligament. Through this attachment, it is thought that the long head of the biceps femoris helps stabilize the sacroiliac joint. Although the gluteus maximus attaches to the sacrotuberous ligament, the gluteus medius does not, and neither do the gemellus superior nor the adductor magnus.

The biceps femoris muscle forms part of the hamstrings, and consists of a long and short head. Both heads perform knee flexion, and the long head is involved in hip extension as it originates in the pelvis.

Quadratus femoris is a strong external rotator and adductor of the thigh. Piriformis is a lateral rotator, and laterally rotates the femur, abducting it, during hip extension. The quadriceps consist of rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. The quadriceps flex the hip and extend the knee.

The hamstring muscles consist of three posterior muscles and their tendons (semitendinosus, semimembranosus, and biceps femoris). The hamstring muscles are responsible for knee flexion and hip extension. The semitendinosus, semimembranosus, and the long head of biceps femoris all originate at the ischial tuberosity (having these muscles cross the hip joint), while the short head of biceps femoris originate at the linea aspera and the lateral supracondylar line of the femur. 

The psaos major and the iliacus muscles are distinct in the abdomen and unite at the level of the inguinal ligament to form the iliopsoas muscle in the thigh. The iliopsoas muscle crosses the hip joint and inserts on to the lesser trochanter of the femur. The iliopsoas is involved in flexion and lateral rotation of the thigh. It is the strongest of the hip flexors, and is important in standing, walking, and running.

The gluteus medius, gluteus minimus, and piriformis insert onto the greater trochanter. 

The flexor digitorum brevis lies in the middle of the sole of the foot above the plantar aponeurosis. It arises from a tendon from the medial process of the calcaneus, it continues towards the toes and divides into four tendons, one for each of the four lesser toes. It is innervated by the medial plantar nerve. The tibialis posterior, extensor digitorum longus, and soleus all cross the ankle joint thus having leverage above it. 

The biceps femoris inserts on the lateral surface of the fibular head. The other muscles of the hamstring insert of the medial surface of the tibia.

The only muscle listed that is not innervated by the obturator nerve is flexor hallucis longus, as this is innervated by the tibial nerve, as well as the S2 and S3 nerve roots. Flexor hallucis longus runs from the fibula distally to the plantar surface of the foot, and functions to flex the joints of the big toe and plantar flex the ankle joint. 

The other muscles listed (adductor brevis, adductor minimus, adductor longus, and gracilis) are thigh muscles that are all innervated by the obturator nerve, and they collectively function to adduct the hip.

Adductor magnus is a large muscle, consisting of two parts, located at the medial side of the thigh. One portion of the muscle originates at the ischiopubic ramus (called the pubofemoral portion) while the other portion originates from the ichial tuberosity (called the ischiocondylar portion). The adductor magnus inserts onto the linea aspera above the knee. The biceps femoris (long and short heads), semitendinosus, and semimembranosus all insert below the knee thus having leverage about the knee.