Standing, grasp your left wrist with your right hand.

Apply pressure with the hand as you slowly and deliberately curl your left arm. Once your elbow is fully bent, give your biceps an extra squeeze and hold for a second before slowly lowering your arm. Complete all reps for one side before switching arms. Since you don't have the benefits of a weight, you need to do this exercise in a controlled fashion - focus on keeping your working biceps fully engaged throughout each rep and apply a challenging pressure with the non-working hand.