What are partial repetitions?
Partials are a fraction of the range of motion of a traditional exercise: Quarter and half squats or dead lifts, bench press lockouts, etc. You stay in an abbreviated range of motion where you are very strong, i.e. the top quarter of the squat, not the bottom.
How does this prepare my body to lift heavier weights?
Since you are staying in a small range of motion where you have a lot of mechanical advantage you can use more weight than you can in the full range exercise. When you use more weight your body will be forced to adapt.
How do I do them?
My advice is to use a solid power cage. Set the safety pins on the cage to the desired height and move the bar from the pins to the end of your range of motion. For a squat this means that you will start at the bottom of the motion and stand up.
Since this is primarily for strength, you should stick to five repetitions or less. You can do full range and partials on the same day or you can alternate doing partials one day and the full range exercise the next workout.
Your abs and low back will definitely be challenged. Because of this you should start slowly and follow an intelligent progression.
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