Reciprocal arm swing during human gait is known to reduce energy expenditure by minimizing vertical ground reaction moments. Arm swing may be a passive phenomenon resulting from the trunk and shoulder movements during walking, with eccentric muscle contractions merely dampening passive pendular motions of the upper limbs. Alternatively, arm swing could be driven actively by rhythmical concentric shortenings of arm and shoulder muscles. Neural commands to the respective motor neurons may be part of a central motor pattern, similar to the spinal locomotor pattern known from experiments with quadrupedal animals. Also a motor cortical influence on arm swing during human gait has been proposed. We used surface electromyography in twenty healthy volunteers to study the activity of arm and shoulder muscles during four different gait conditions: normal walking, walking while holding the arms still (parallel to the trunk), walking with anti-normal phasing of arm swing, and with physical restriction of the arms. Concentric and eccentric contractions occurred regularly as the swinging arms approached their anterior and posterior turning points. Holding the arms still during walking involved more muscular activity than natural gait, as did anti-normal phasing of arm swing. The results indicate that natural arm swing involves both active and passive components. Since some rhythmical muscle activity of the deltoid muscle persisted even when the arms were immobilized, arm and leg movements during human gait may be coupled by a central locomotor program.