Mechanosensitivity of the median nerve and mechanically produced motor responses during Upper Limb Neurodynamic Test 1

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Objectives In spite of the widespread use and recognised importance of the Upper Limb Neurodynamic Test 1 (ULNT1) in clinical practice, controversy remains about the neurophysiological basis for sensory and motor responses to the test. The aims of this study were to determine the effects of two limb positions (neutral and ULNT1) on mechanosensitivity of the median nerve, and to investigate the mechanisms underlying muscle stiffness and loss of range during the elbow extension component of the ULNT1. Design In both limb positions, a KIN-COM ® dynamometer controlled passive elbow extension at 3°/second, and recorded both elbow extension range of movement and elbow flexor resistive torque. Setting Electrophysiology laboratory, School of Physiotherapy, University of South Australia. Participants Twenty-six asymptomatic subjects. Main outcome measures Range of elbow extension and elbow flexor resistive torque. Surface electrodes placed over 10 upper limb muscles recorded electromyographic (EMG) activity. Using a hand-held micro-switch, participants indicated occurrence of pain onset and pain that limited further movement. Results The median nerve was more sensitive to mechanical longitudinal stresses during passive elbow extension in the ULNT1 position than in the neutral position, as demonstrated by increased EMG activity and increased mean elbow flexor resistive torque (neutral position, 0.8 Nm, 95% confidence interval 0.7–0.9 Nm; ULNT1 position, 3.9 Nm, 95% confidence interval 3.8–4.0 Nm). Pain onset and pain limit occurred earlier in range in the ULNT1 position (pain onset, 45°, 95% confidence interval 40–50°) than in the neutral position (3°, 95% confidence interval 0–6°). In the ULNT1 position, EMG activity was greater in muscles responsible for an antalgic posture in the upper limb, although some EMG activity was evident prior to pain onset. Conclusions The mechanosensitivity of the median nerve was greater during elbow extension in the ULNT1 position than in the neutral position. The increased EMG activity that occurred primarily in muscles contributing to an antalgic posture with concurrent changes in resistive torque and range of movement may be explained by a flexor withdrawal response to pain that acts to reduce the stretch on the median nerve. Muscle activity prior to pain onset is probably due to mechanoreceptor activation following preferential stretch of the median nerve.