Leg extension machine: Sit on the padded seat of the machine. Hook the feet under the padded bar. Adjust the bar so it rests comfortably at the lower end of the leg just above the feet. Adjust the machine so your legs can bend at a 90-degree angle. Don't perform the exercise with less than a 90-degree angle, because this places the knees beyond the toes, which stresses the knee. Choose a weight that enables you to extend the legs straight with effort but not too much strain. Grip the bars firmly at your sides. Hold your head steady and brace your core. Extend your legs, engaging your quadriceps to lift the weighted bar. Pause at the top of the extension. Slowly lower the weight back to the starting position.