your not doing it correctly if you need to strap your body down. first, one arm at a time. this will help with symetry. second. keep your upper arm perfectly still throughout the entire motion. absolutely no momentum or sway or help from the shoulder. third, SLOW during the negative. nobody does this. 5 second count , steady paced, from fully extended all the way back up to fully stretched. third, POWERFUL during the concentric. fourth , hold and SQUUUUEEEZZZZZEEEE the tricep at the bottom. squeeze as hard as you can for a 3 second count. repeat. you will not be able to use any where near the amount of weight you normally could, nor near the amount of reps or sets.. but it will dig deeper into the fibers and result in much better gains over time.