Muscle beneath right knee is very soft & weak as compare to left, it pains more if i drive for an hour or more, i am 30 years old. never had injury?

Knee pain: Could be patellar tendonitis, referred sciatic pain, meniscal tear, synovitis, or just tight quads. Start with a good quad stretching program.

Meniscal injury?: Posterior knee pain with prolonged sitting can come from a meniscal injury. The best thing to do is to do a trial of physical therapy to help reduce inflammation and then strengthen the muscles that protect the knee.

Muscle weakness: The muscle you a are discribing in the anterior tibial muscle . It is used to lift your foot up esp. Used during driving. There is a possibility that you have a nerve injury to that muscle or a chronic compartment syndrome. See an orthopedic surgeon firat then possibly need to be seen by a neurologist. Depending on the ultimate diagnosis pt is in your future to strengthen the weakened muscle.