When performing biceps curls, the front part of the shoulder muscle activates more strongly when lifting the arm forward than when lowering it, showing that this muscle helps stabilize the shoulder...

We analyzed the available evidence on anterior deltoid activation during biceps curls and found that 37 studies or assertions support the idea that the front part of the shoulder muscle activates more strongly when lifting the weight upward — during arm flexion — than when lowering it, during arm extension. No studies or assertions in our review contradicted this pattern. This suggests that during a biceps curl, the front deltoid is more engaged as you bring the weight toward your shoulders, likely helping to stabilize the shoulder joint as the elbow bends. The muscle doesn’t appear to be as active when you lower the weight, even though the arm is still moving. This pattern aligns with what we know about how muscles often work harder during the lifting phase of a movement, especially when they’re helping control position rather than just moving a joint. What we’ve found so far points to the anterior deltoid playing a supportive role in biceps curls, particularly during the upward motion. We don’t know if this is true for all people or all variations of the curl, but based on the evidence we’ve reviewed, the pattern holds across the studies we examined. For someone doing biceps curls, this means the front of your shoulder is working harder when you lift — not just your biceps — so paying attention to control during the upward phase may help engage that area more intentionally.