When a muscle crosses two joints, stretching it at one joint makes it longer overall, which can make it grow more when you exercise it.
Evidence from Studies
Supporting (1)
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Joint angle-specific neuromuscular time course of recovery after isometric resistance exercise at shorter and longer muscle lengths
This study found that when muscles are stretched more during exercise (like bending your knee more), they work harder — and working harder like this is how muscles grow bigger over time. So even though it didn’t measure muscle growth directly, it shows the right conditions for it.
Contradicting (1)
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Does Muscle Length Influence Regional Hypertrophy? A Systematic Review and Meta-Analysis
The study found that doing exercises that stretch muscles more doesn’t make those muscles grow bigger than exercises that keep them more relaxed — so stretching muscles more during workouts doesn’t give you extra muscle growth.