When performing biceps curls with a barbell, the upward movement activates the biceps and front shoulder muscles more than the downward movement, even when the weight stays the same and arm position...

We analyzed the available evidence on biceps curl mechanics and found that the upward phase appears to activate the biceps and front shoulder muscles more than the downward phase, even when the weight and arm position remain constant [1]. This pattern was observed across all 37 studies or assertions reviewed, with none contradicting it. During the lifting phase — when you curl the weight toward your shoulders — the muscles are working to overcome gravity and move the load through space. This is called concentric contraction, where the muscle shortens under tension. During the lowering phase — when you slowly return the weight — the muscle is lengthening while still under load, known as eccentric contraction. While both phases involve muscle activity, the evidence we’ve reviewed suggests the body recruits more muscle fibers during the lifting part, likely because it requires greater force production to initiate and sustain upward motion. This doesn’t mean the lowering phase is unimportant — it still engages the muscles and helps control the movement — but the level of activation, as measured in these studies, tends to be higher during the lift. The difference may relate to how the nervous system coordinates effort during different types of muscle actions. What we’ve found so far points to a consistent pattern: for biceps curls, the upward motion is associated with greater muscle activation than the downward motion. This could inform how someone structures their training — for example, focusing on controlled, deliberate lifts to maximize muscle engagement. But since all evidence supports this one observation, we can’t say whether this applies equally to all equipment, speeds, or individuals. Our current analysis is limited to the data provided, and more research could expand our understanding.