The gastrocnemius is one of the muscles that makes up the calf muscles in the leg. The other calf muscle is the soleus, which lies under the gastrocnemius. The gastrocnemius also has two heads which give it a "diamond" shape. There are several layers of different muscles on the soles of the feet. The hamstrings are the group of muscles found on the back of the thigh, with the biceps femoris being the largest. The quadriceps are the muscle group on the front of the thigh, the vastus lateralis is the largest of these. 

Sartorius, gracilis, semitendinosus make up the pes anserine group whose conjoined tendons insert onto the anteromedial surface of the proximal extremity of the tibia. They flex and medially rotate the leg at the knee joint. 

The tensor fascia latae attaches from the anterior superior iliac spine to the iliotibial band. 

The adductor magnus is innervated by both the sciatic nerve and the obturator nerve. 

The largest muscle in the entire body is the Gluteus Maximus which is found in the buttocks. It has the function of keeping the entire body erect by inserting itself in the pelvic bone.

Gluteus maximus is an thigh extensor, additionally it laterally stabilizes the hip and knee, and laterally rotates and adducts the thigh. It's insertion is the gluteal tuberosity. It is innervated by the inferior gluteal nerve.

The superior gluteal nerve innervates the tensor fascia lata, gluteus medius, and gluteus minimus. It does not innervate gluteus maximus, which is innervated by the inferior gluteal nerve. The obturator nerve provides sensation to skin on the medial aspect of the thigh and innervates adductor muscles of the lower extremity. The sciatic nerve provides sensation to most of the skin on the leg, and it innervates muscles of the posterior thigh, leg, and foot.

The gracillis muscle is thin and flat muscle at the medial side of the thigh; it is the most superficial muscle on the medial side of the thigh. It originates at the anterior margin of the symphysis pubis and the pubic arch. The muscle runs downward, ending in a tendon that passes behind the medial condyle of the femur and onto the medial condyle of the tibia where it inserts into the tibia below the condyle (as part of the pes anserinus). 

The gracillis adducts, medially rotates, and flexes the hip, it also aids in knee flexion.

The superior gluteal nerve innervates the tensor fascia lata, gluteus medius, and gluteus minimus. It does not innervate gluteus maximus, which is innervated by the inferior gluteal nerve. 

Tensor fascia lata helps stabilize the knee, especially in extension, while gluteus medius and minimus abduct the thigh and help to medially rotate the thigh. Paralysis of these gluteal abductors leads to a characteristic limp called the Trendelenburg gait. The Trendelenburg can be described when a person shifts their body weight tot he affected side to keep from tripping over the other side.

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.