Different variations of the biceps curl exercise produce small but measurable differences in regional muscle hypertrophy and movement-specific strength gains in humans.

Study: Distinct muscle growth and strength adaptations after preacher and incline biceps curls

Doing biceps curls in different positions (like preacher vs. incline) makes different parts of your biceps grow bigger and makes you stronger in that specific movement—so the way you do the exercise matters a little bit.

Study: Training in the Initial Range of Motion Promotes Greater Muscle Adaptations Than at Final in the Arm Curl

This study found that doing biceps curls with a shorter range of motion (starting from the bottom) made the lower part of the biceps muscle grow more and made people stronger than doing curls from the top. This proves that small changes in how you do the exercise can lead to different results.

Study: Small muscle mass exercise enhances muscular adaptations? Effects of unilateral and bilateral biceps curl on maximum strength and muscle size changes.

This study found that doing biceps curls with both arms at once versus one arm at a time didn’t make your biceps grow differently, but it did help the trained arm get a little stronger at that specific movement. So, not all variations make your muscles grow differently — which goes against part of the claim.

Study: Comparison Between Shoulder Flexed and Extended Positions in Elbow Flexion Resistance Training on Regional Hypertrophy and Maximum Strength: Preacher versus Bayesian Cable Curls

This study found that two slightly different ways of doing biceps curls didn’t make a noticeable difference in muscle growth or strength. So, it doesn’t support the idea that small changes in how you curl lead to measurable differences.