The half rep is a technique of doing a push or pulling exercise using only partial range of motion for a particular muscle group. While they can be beneficial, most are done for the wrong reasons.
First, to clear up any misinformation, lets go over what exactly a half rep does that is beneficial, and what it is actually used for. These lifts are a training technique used by more advanced lifters to get them past sticking points in their lift, basically, to target areas of weakness in each lift.
Most of the time half reps are done incorrectly simply so that some over egoistic person in the gym can convince themselves that they are lifting more weight than they can actually handle. For example: Have you ever seen someone in the gym load way to much weight on the bench press and then maybe bring the weight down 25% before pushing it back up (usually with a lot of noise to make sure everyone sees them lifting heavy)? They call this a lift but unfortunately this does nothing for the development of chest size or strength. Another example would be this same egoistic person putting hundreds of pounds too much on a squat rack and squatting down only a few inches. This defeats their entire purpose and unfortunately most people, including personal trainers that I see perform these incorrectly and for the wrong reasons.
Over time, performing these lifts incorrectly does two things. First, it decreases the range of motion in which the muscle is viable. Second, the weaker parts of the muscle are not developed, they are left to atrophy and get weaker still. This inevitably leads to injury when a part of the body suddenly finds itself beyond the half rep training motion.
The correct way to do a half rep is to work the hardest part of the movement only, usually the lower part of the movement. (sorry gym rats but going 25% of the way down is pointless) Then stop 1/2 way up and go back down and do another 1/2 rep in the hard part of the movement range. DO NOT BOUNCE THE WEIGHT! Pause at the end of the movement for a split second and then press the weight back up, pausing at the top as well. Using momentum by bouncing the weight is just another way glorify the ego, and does not help in building real strength.
For example: A bench press lift would begin with the bar resting on your chest, go 40%-60% of the way up, pause, and return the bar to your chest. Pause again before performing another rep and do not bounce the bar off your chest. A squat would require you to go all the way into a deep squat, legs bent more than 90 degrees. Move from that position upward until the legs are parallel to the floor, then pause before returning to the bottom. A pull-up can be done in whatever area you are weaker, either from hanging with straight arms up to 40%-60% range of motion, or from half way up to chin above the bar. The idea again is to get you past your weak points.
So the question is; Should I do half reps? I would say no. Focus on developing strength throughout your full range of motion. If you must use them, use the correct technique and use it for the right reasons, to get past a weak point. Do not make it part of your every day lifting and do not use it to add more weight than you can safely handle.
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