Doing only the bottom part of an exercise (where the muscle is stretched) can build as much muscle in your arms and legs as doing the full movement, as long as you put in the same amount of effort and work.
Evidence from Studies
Supporting (3)
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The effects of lengthened-partial range of motion resistance training of the limbs on arm and thigh muscle cross-sectional area
The study found that doing shorter lifts that focus on the stretched part of the muscle works about as well as full lifts for building arm and leg muscle in trained people, as long as effort and volume are the same.
The study found that doing partial reps that focus on the stretched part of the movement builds arm muscle just as well as full reps, as long as everything else is equal. This supports the idea that both ways work about the same.
The study found that doing bicep curls through a shorter range of motion, but where the muscle is stretched, builds similar muscle size compared to full-range curls in trained people.
Contradicting (1)
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From full to partials: Investigating the impact of range of motion training on maximum isometric action, and muscle hypertrophy in young women
The study looked at women who were not experienced in training, and found that certain partial reps didn’t build as much muscle as full reps—especially when the partial was at the shorter range of motion. This doesn’t back up the idea that lengthened partial reps are just as good as full reps for muscle growth in trained people.
Gold Standard Evidence Needed
According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.