One of the most effective machines for working out the quads is the smith machine. The smith machine consists of a metal track with a barbell that is suspended in the track and can only move straight up and down. The barbell can be loaded with standard weight plates and can be used to perform squats, but no spotter is required since the lifter can lock the weight at any time by rotating the bar.
Another useful quad machine is the leg press machine. Leg press machines come in several different forms, but at its heart it consists of a seat with handles on either side and a platform for the feet to rest on and push against. The platform can be weighted down by a weight stack or in some cases, standard weight plates must be loaded onto the machine to provide resistance.
The quad extension machine is another popular fitness machine. This machine consists of a seat with a couple of pads by the ankles that you hook your feet behind, and then you simply extend your legs until your knees are straight against the resistance that the pads provide.
The smith machine doubles as an effective hamstring workout, since squats engage the hamstrings and glutes in addition to the quads.
Perhaps the most effective hamstring machine is the leg curl machine. A leg curl machine is essentially the opposite of a quad extension machine. The legs begin straight at the knee and the heels are placed on pad with resistance, and then the knee joint is closed against the resistance. Some hamstring curl machines allow the lifter to lie down during the exercise while others are performed while sitting or standing. Regardless of the body's position, the motion of the leg itself will be the same.
Aside from machines designed specifically to target the muscles of the upper legs to build muscle tone or raw strength, there are several popular cardio workout machines that can improve overall leg fitness while working the heart and burning calories. Treadmills and elliptical machines are great machines for burning calories while working the legs. Stationary bikes are also useful for focusing on leg endurance which typically does not raise the heart rate as much as running.