Forced negative reverse grip pull ups. Chain some plates to your waist. Place a chair next to a pull up station. Stand on the chair gripping the bar in the up position...lift your legs and lower yourself slowwwww. Focus on the bicep as you lower yourself. The burn was way more intense than slowly releasing the weight on pull downs. The key is to belt more weight than you can pull up. It destroyed my arms. Give it a shot and see if your arms aren't sore