Knee pain is commonly caused by shortening in muscles that activate the joint. These muscles are innervated by segmental nerves from the lumbar spine; therefore, examination and treatment of the knee always includes the back, where L2–3, 3–4 and 4–5 are usually found to be involved.

Knee flexors
Two on the lateral side:

Biceps femoris (long head from the medial part of the ischial tuberosity, short head from the linea aspera to the head of the fibula).
Popliteus (lateral condyle of the femur to the tibia above the soleal line).
Four on the medial side (pes anserinus):
Sartorius (the anterior superior iliac spine and outer edge of the iliac crest for 2 inches to the medial side of the upper end of the tibia).
Gracilis (the outer surface of the inferior pubic ramus to the medial side of upper end of tibia).
Semitendinosus.
Semimembranosus.

With knee flexed and leg at a right angle to thigh, sartorius insertion is placed anteriorly, gracilis intermediate, and semitendinosus posteriorly.

Medial knee joint pain

Shortening of the above muscles is a common cause of pain in the medial aspect of the knee; the knee usually cannot fully extend. Releasing the shortened muscles relieves medial joint pain, and improves range, sometimes within minutes, even when there is a minor tear of the medial meniscus (without locking). With knee flexed, the muscles are needled at their musculotendinous junctions, about two inches above the medial joint line.

Knee extensors

Pain in the anterior aspect of the knee is commonly caused by shortening of the knee extensors. The quadriceps femoris consists of four muscles:
Rectus femoris has two heads (the anterior inferior iliac spine, the groove above the acetabulum).
Vastus lateralis (the base of the greater trochanter, the line to the linea aspera, the lateral lip of the linea aspera).
Vastus medialis (the lower half of the anterior intertrochanteric line, the spiral line, the medial lip of the linea aspera).
Vastus intermedius (the proximal 2/3 of the anterior and lateral surfaces of the shaft of the femur).
These four muscles join the common extensor tendon which is inserted into the patella. From the lower margin of the patella, the insertion is continued by the ligamentum patellae to the tubercle of the tibia.
Treating the pes anserinus, the tendinous expansion and attachment of the sartorius, gracilis, and semitendinosus.

Patellofemoral pain

Shortening of the knee extensors increases patellofemoral loading, and can cause knee pain, e.g. the patellofemoral pain syndrome. The patient is often aware of crepitus (a creaking feeling behind the patella, especially when squatting) and a feeling of stiffness.
Chondromalacia patellae
Shortening also causes misalignment and pathologic lesions in the articular surface of the patella. These usually begin in the medial facet. Softening and swelling of the cartilage is followed by fragmentation and fissuring and, eventually, erosion of the cartilage to the bone.
This common, but much misunderstood condition responds to needling of the quadriceps femoris, but the upper lumbar paraspinal muscles are always involved and must be treated to free the nerve roots.