The leg extension is an isolation exercise that focuses on the quadriceps muscles at the front of the thigh. It is performed using a leg extension machine, where the individual sits down and places their lower shins behind padded bars. The movement involves extending the knees and raising the weighted bar until the legs are straight, all while ensuring the rest of the body remains stationary. This exercise is particularly effective for isolating and building the quadriceps, providing a targeted way to enhance leg muscle definition.