An elbow orthosis with pneumatic artificial muscles has been developed to assist and enhance upper limbs movements and has been examined for effectiveness. The effectiveness of the elbow orthosis was examined by comparing muscular activities during alternate dumbbell curl motion wearing and without wearing the orthosis. The subjects participating in the experiment were young adults in their twenties. The subjects were instructed to perform a dumbbell curl motion in a sitting position with and without wearing an orthosis in turn, and a dynamometer was used to measure elbow joint torque in isokinetic mode. The measurements were done with four various dumbbell loads: 0, 1, 3, and 5 kg. We examined the effectiveness of the elbow orthosis in two control methods. First, the orthosis was pneumatically actuated and controlled in the passive control mode. Then, it was controlled in the active control mode using the muscular stiffness force of the muscle that is measured from a force sensor through a cDAQ-9172 board. For the analysis of muscular power, the muscular activities of the subject were measured during alternate dumbbell curl motion using MP150 (BIOPAC Systems, Inc.). The muscles of interest were biceps brachii muscle, triceps brachii muscle, brachioradialis muscle, and flexor carpi ulnaris muscle in the upper limbs of the right side. The elbow joint torque was measured during elbow flexion motion using a dynamometer at $60^{circ}$ per second for isokinetic strength. The experimental result showed that the muscular activities wearing the elbow orthosis were reduced and elbow joint torque wearing the elbow orthosis was higher because of the assist of the orthosis. As a result of this experiment, the effectiveness of the developed elbow orthosis was confirmed and the level of assistance was quantified. With this, we confirmed the effectiveness of the developed elbow orthosis.