Bicep Curls: Know The Difference ⚠️ . Here are 3 main variations of Bicep Curls that you want to be aware of. Variation #1: When performing Preacher Curls this will make the biceps work the hardest at the stretched/lengthened position of the repetition. With the focus on the Inner Head of the Biceps. ✅ Variation #2: When performing Spider Curls, this will make the biceps work the hardest in the shortened/contracted position of the repetition. With the focus on the Inner Head of the Biceps. ✅ Var