The vastus intermedius muscle can cause and contribute pain down the front of the thigh and knees. Pain increases when walking up an incline or stairs.
The vastus medialis can cause and contribute to pain on the inside of the knee and front of the thigh. Dysfunction in this muscle will cause your knee to buckle. It is one of the four quadriceps muscles.
The rectus femoris muscle contributes to knee and thigh pain. It also contributes to restless leg syndrome and "knee bugs".