Are there any muscles in the lower leg that are responsible for bending and straightening the knee?

Mostly. upper leg muscles are responsible flexion and extension around the knee. The gastrocnemius(calf muscles), popliteus, and plantaris can contribute to knee flexion though, and are lower leg muscles.

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After blow to kneecap, cannot straighten knee or bear weight? X-rays normal, how to proceed? Other symptoms: blue lower leg, pain on both sides of knee, muscle spasms when aggravating knee

Tear quadricepes . The quadriceps tendon works with the muscles in the front of your thigh — the quadriceps — to straighten your leg. Read more...
MRI. Firstly, make sure you are evaluated by an orthopedic surgeon. Mri would probably be the best diagnostic test to evaluate your extensor mechanism and surrounding structures if the symptoms persist. Read more...