Fig. 2-6. Muscular actions. When the arm is abducted against the examiner's resistance, the deltoid becomes tense. On adduction against resistance, the deltoid relaxes and the weight sinks into it. On adduction produced by lowering a pail from a horizontal position, the pectoralis major is relaxed. The contracted deltoid controls the descent by lengthening. The deltoid is now an antagonist to gravity, which is the prime mover, and is doing negative work (paradoxical action). Click for high resolution image.