What is the optimal range of motion to enhance muscle hypertrophy? There are several considerations and beliefs surrounding this topic.
One viewpoint is a full range of motion produces greater force output due to the increased demands as a muscle lengthens (length-tension relationship). Another consideration is time under tension. If the speed of contraction is equivalent, time under tension will be greater for the full range of motion. Conversely, could you obtain the same benefits from slow partials or rapid full range of motion?
A final consideration is recovery and potential harm. We tend to see greater levels of DOMS when training in lengthened positions. It is common for trainers and physios to instruct clients to restrict motion and limit the lengthened position (e.g., shoulder protraction during rows or squatting to full depth). There are many variables (speed of contraction, resistance type, etc.) and outcomes (strength, power, recovery, etc.) to consider. For now, I am going to stick with hypertrophy. This recent systematic review explored the effects of range of motion on muscle hypertrophy.
The trials included in this review were at least four weeks long and they either compared full range of motion training with partial range of motion or they compared two different partial ranges of motion (e.g., lengthened vs. shortened partials). The review yielded 11 trials (n=297) ranging from 5 to 15 weeks long. Eight of the studies compared full versus partial range of motion. Overall, the review found lengthened partials and full range of motion training to be superior for hypertrophy compared to shortened partials. When comparing lengthened partials and full range of motion, the results were inconclusive.
If we add strength to the mix, you need to train the motion you want to be strong in. If you only need strength to parallel squat depth and you want to hypertrophy the quads, there is no need to go past parallel. If you want the full depth strength, however, you need to train full depth.
If muscle size is all you care about, you have many options as partials can be equally effective. This is very valuable for patients in pain. Often, the patient is pain-free in the lengthened position, allowing you to train at a moderate to high intensity without pain interfering. This is a strategy I often employ in the clinic and with personal rehab.