Collagen supplementation provides no additional benefit for muscle hypertrophy when total daily protein and vitamin C intake are sufficient, due to its low biological value and absence of a plausible mechanistic pathway for direct muscle growth.

Study: Partly Substituting Whey for Collagen Peptide Supplementation Improves Neither Indices of Muscle Damage Nor Recovery of Functional Capacity During Eccentric Exercise Training in Fit Males.

When people took collagen along with their usual protein shake, they didn’t get stronger or recover better than people who just took the protein shake alone. So collagen didn’t help extra, even though they were both getting the same total protein.

Study: Nutritional Supplements for Muscle Hypertrophy: Mechanisms and Morphology—Focused Evidence

Taking collagen pills won’t make your muscles bigger if you’re already eating enough protein and vitamin C—it just helps your tendons and joints recover better.

Study: Effects of 12 Weeks of Hypertrophy Resistance Exercise Training Combined with Collagen Peptide Supplementation on the Skeletal Muscle Proteome in Recreationally Active Men

This study found that people who took collagen pills after working out gained more muscle and got stronger than those who didn’t, even though they were already eating enough protein. This contradicts the claim that collagen doesn’t help.