Introduction: 

During normal gait, both upper limbs move reciprocally, each arm swinging in phase with the contralateral leg. To find out whether arm swing is an active or a passive movement, the activity of arm and shoulder muscles was examined during different conditions of gait. If rhythmical activity persisted even when the arms are immobilized during walking, this might imply that the neural commands to arm and shoulder muscles are part of a centrally determined locomotor pattern, similar as in quadrupeds.

Methods: 

The activity of seven arm and shoulder muscles was studied with electromyography (EMG) in twenty volunteers during four conditions of treadmill walking: A) Normal gait with unrestrained natural arm swing; B) Held = volitionally holding the arms still during gait; C) Bound = walking with immobilized arms; D) Anti-Normal = walking with the arms swinging in phase with the ipsilateral legs.

Results: 

Arm swing was not an entirely passive pendular motion, but involved rhythmical activity of shoulder muscles. Co-contractions of shoulder extensor and flexor muscles with predominant extensor EMG activity were found in the Normal condition. Increased triceps activity prevented arm swing in the Held condition. Flexor and latissimus dorsi EMG signals were conspicuous in the Anti-Normal condition. Deliberate suppression (Held) and opposite-to-normal phasing of arm swing (Anti-Normal) involved stronger muscle activity than normal walking. Step-related rhythmical EMG signals of the ipsilateral deltoid, trapezius and latissimus dorsi muscles persisted when an arm was immobilized with a brace, suggesting the influence of a central motor pattern.