The triceps dip is a compound exercise that involves movement at more than one joint: The elbows and shoulders. The amount of involvement of each joint changes the emphasis of the primary movers. Technique is important because you can increase your risk of joint injury if you use improper form.
The majority of movement during the dip exercise occurs at the elbow joints, making the triceps the primary movers, but because there is some movement at the shoulder joints, other muscles assist in the exercise, including the front shoulders and chest muscles. The more you lean forward during the triceps dip exercise, the more movement occurs at the shoulders, which increases the involvement of the chest and front shoulders.
To perform a dip, press your body weight up by straightening your arms as you hang suspended between two parallel bars. Elbow extension, or straightening of the joint, is the primary movement during the dip exercise. When performing the dip, do not fully extend, or lock out, your elbows, as this can cause wear and tear on the joints. Always keep a slight bend in your elbows to protect them from excessive pressure.
During the triceps dip movement, the shoulders go through shoulder flexion -- the angle of the joint is decreased. Lifting the arm forward is an example of shoulder flexion. The position of your body during the triceps dip exercise places the shoulders in a biomechanically disadvantageous angle; they support a lot of weight in an extremely flexed position. The lower you dip, the more stress you place on the shoulder joints. Limit the range of motion and stop the downward movement when your elbows are at a 90 degree angle, or the upper arms are parallel to the floor, to reduce your risk of injury.
Dips are a challenging exercise because you press your entire body weight with mostly the strength of your triceps, which are relatively small muscles. Variations of the dip exercise are less challenging, but involve the same technique and joint movements. For a self-assisted dip, position a bench under the dip bars and rest your feet on the bench. Press through your feet to help push yourself up. For a bench dip, sit on a bench and grab the edge with your hands just outside your thighs. Slide your buttocks off the bench, bend your elbows and lower yourself until your upper arms are parallel to the floor.