Tricep pushdowns isolate the triceps using a cable machine.
How-to: Stand in front of a cable machine with a straight or V-shaped bar attachment. Grasp the bar with an overhand grip, hands shoulder-width apart. Keep your elbows close to your body, and push the bar down until your arms are fully extended. Slowly return to the starting position.
Focus: Keep your back straight and avoid using momentum to push the weight down.