The meat on the side of the knee appears to throb, this place is the quadriceps muscle place, if the quadriceps muscle appears involuntary throbbing, may be caused by the following reasons: First, there may be fatigue strain injury of the quadriceps muscle, or local inflammation caused. The most common are people who often do lower limb physical exercise, or heavy physical labor is more common. Second, it may be caused by the brain, the brain brain cells appear abnormal discharge, especially the nerves of the dominant quadriceps muscle, muscle movement cells appear abnormal discharge, will also appear this place of muscle throbbing. Third, it may be caused by lumbar spine diseases that compress the nerve roots of the lower limbs, commonly caused by lumbar disc herniation, lumbar spinal stenosis or lumbar spine slippage.