(Adapt from: Jim Smith, C. S. C. S.Muscle&Fitness.com,3-4-2014,(http://www.muscleandfitness.com/workouts/workout-tips/unlock-gains-shorter-range-motion)
Go to YouTube and watch a few videos of pro bodybuilders training (as if you haven’t already). What do you notice, apart from the massive weights they use and the faces they make from exertion? Bodybuilders rarely lock out their joints, preferring instead to use a slightly reduced range of motion to keep the target muscles fully engaged. Called ‘constant tension, this trick can end a plateau and save your joints.
Try it on a variety of compound and isolation moves. For example, on dumbbell overhead presses, lower the weights until you feel a mild stretch in your shoulders, but don’t go all the way down; then press them back up three-quarters of the way. Because the weights aren’t supported by the joints or connective tissues on either end of the range of motion, the muscles (in this instance, the deltoids) work at their max throughout the entire set.
This also means that the joints aren’t stressed by heavy loads. So if you have an injury, constant tension sets will prevent you from aggravating it.
Constant tension isn’t always appropriate if your goal is strength. To lift the heaviest weights, you need to lock out your joints. Do your main lifts normally, and use partial reps on assistance lifts.