In simplest terms, partial reps are any reps you complete where you do not use a full range of motion (or very close to it).
Generally the range of motion used is much smaller, typically in the quarter to half range.
When partial reps are used as a set extending intensity method, the goal is to complete all the full range reps with good form until another good full rep cannot be finished then switch to partial reps.
Even though your muscles cannot complete another full range of motion rep with the weight you’re using because of in-set fatigue, that doesn’t mean they have been fully stimulated throughout the whole range of motion.
You cannot complete another full rep simply because you can’t keep the weight moving through the sticking (weakest) point. However, if the range of motion was shortened to remove that sticking point, you could complete more reps in the stronger position.
For example, in the bench press, once you’ve completed all the full reps you can do you could continue doing shortened reps from the chest up to the sticking point or from above the sticking point (usually mid-range) to full extension. Any reps completed using the shortened range of motion are called partial reps because you will no longer be completing full range of motion reps.
Since you’re only as strong as your weakest link, partial reps can also be used as an overload method. The range of motion used would either be the area you are the strongest or the area you would like to strengthen by removing the typical sticking point in a full rep.
The goal is to overload the muscle to help stimulate and strengthen the muscle fibers in that particular range of motion.
Both muscles and connective tissues (tendons and ligaments) are strengthened when stressed with heavy weights.
Therefore, the idea with overload is to start with a weight that is too heavy to complete any good full reps with. In this case you will complete partial reps only. A common example of this would be doing rack pulls, or partial deadlifts.
Most people are weakest deadlifting off the floor but typically can complete the rep if they get the bar near the knees.
Rack pulls (partial deadlifts) above this sticking point allow heavier weights to be used and expose the muscles to a greater stimulus where they are strongest.
This overload can help to stimulate new muscle fibers since the weights used will be heavier than normal.
The assistance provided by supporting muscles can also be reduced by using partial reps (depending on the range of motion used) so that more load is transferred to the target muscle. For example, partial squats at the top of the movement (quarter or half squats) are generally more quad dominant whereas deeper squats involve the glutes and hamstrings to a greater degree.
Partial reps can be used with all exercises where you normally use full range of motion reps.
Unfortunately, too many people use partial reps because of ego by shortening the range of motion just so they can use heavier weights where they are strongest. The obvious goal being to impress rather than using the partial reps as intended.
As with all intensity methods, partial reps are a way to push beyond normal failure but can be overused resulting in overtraining and injury. It’s best to cycle their use and also be careful when combining with other intensity methods.
It’s important to note that always using partial reps in the strongest range only can create a strength imbalance by continuing to further strengthening the already strongest part of the range of motion.
Using the quarter/half rep squat example, the quad muscles would continue to get stronger than the glutes/hamstrings possibly causing future knee problems when doing full range squats.
Therefore, partial reps (especially in the strongest range) should be used infrequently with full reps used the majority of the time to avoid these problems.
How many partial reps you should complete is up to you and how far you want to push the muscles. Since you’re dealing with fatigue and overload, your ability to maintain reasonable form needs to be considered for your safety. The general rule would be to continue the partial reps as long as you maintain a good safe form.