In this brief exercise analysis (Quicktime ipod compatible, 6MB, Google streaming flash video), Bryce Dettloff demonstrates the weighted triceps dip on a Life Fitness dip and chin machine at Gold's Gym, Ann Arbor. As its name implies, this exercise targets the triceps brachii highlighted in the first photo at left. Additionally, muscles in the upper back, chest, and shoulders play a synergistic role stabilizing and controlling the movement. With 90 lbs. of plates in addition to his own 175, Bryce notes that the upper back plays a significant role in helping him control the exercise.
As shown at the start of the video, Bryce prepares for the exercise by putting on a dip belt and then attaching two forty five lbs. plates to it. He then mounts the machine and starts with his triceps fully flexed. He then descends to the point where his triceps is fully stretched as shown in the second picture at left. In our discussion afterward, it's at this point that Bryce most notices the role of his back muscles in controlling the movement.
By comparing Bryce's posture with the grid in photos two through four, you can see that he keeps his torso fairly erect. This posture keeps the main focus of the exercise on the triceps. Were Bryce to incline his body forward, emphasis would be moved to the pectorals.
Once in the fully descended position, Bryce begins the positive movement by flexing his triceps. The third picture on the right shows Bryce about half way through the movement. Careful examination indicates that Bryces' shoulders have begun to roll forward. This roll indicates that the anterior deltoid is potentially playing a role in the movement.
Finally, in the fourth photo, we see Bryce complete the movement. His triceps are fully flexed and his arms are straight. He has kept his back straight throughout the movement to support the 90 lbs. of weight hanging from his waist. This extra weight places extra emphasis on the role of the back muscles in controlling the movement.
Note also in the fourth photo, that Bryce's shoulders are now fully forward. The shoulder movement is natural and just indicates the important role of the shoulders in helping control the movement. If you have trick shoulders like I do, this exercise may not be for you. You can minimize strain on the shoulders by limiting the depth of your descent so that your upper arms do not move past parallel with the floor.