BACKGROUND: Peripheral nerves straighten and stretch in order to accommodate increases in bed length during joint movements. The ulnar nerve is predicted to show large bed length changes, particularly on elbow flexion. The present study examines sliding of the ulnar nerve during limb movements, to determine how far these changes are accommodated by straightening and stretch. METHODS: Ultrasound imaging was used to measure longitudinal nerve sliding in the forearm and upper arm during 40 degrees wrist extension, 90 degrees elbow flexion and 50 degrees shoulder abduction. Nerve trunk folding in the upper arm was measured from still ultrasound images taken in a series of limb positions from 40 degrees shoulder abduction, elbow extended and wrist neutral to full elbow flexion, 90 degrees shoulder abduction and wrist extension, a position designed to stretch the ulnar nerve. FINDINGS: Wrist extension led to clear nerve sliding in the forearm with movements of up to 4 mm. However, shoulder abduction and elbow flexion caused remarkably little nerve movement. Images of the ulnar nerve showed considerable curvature with 40 degrees shoulder abduction and elbow extension but a much straighter path with the elbow flexed. INTERPRETATION: The ulnar nerve appears unloaded and follows a wavy path in most functional upper limb positions. During elbow and shoulder movements, changes in bed length appear to be accommodated largely by straightening of the nerve path, with only modest stretch of the nerve itself when the elbow flexes. The ulnar nerve is thus well adapted for the large changes in bed length that occur during limb movements.
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