The supinator reflex is testing the reflex motor arc associated with the brachioradialis muscle.
The supinator reflex is tested by striking the lower end of the radius just above the wrist with a tendon hammer. This normally causes contraction of the brachioradialis and hence flexion of the elbow.
If the only response is finger flexion, then this reflex is said to be inverted. An inverted brachioradialis reflex is associated with an absent biceps jerk and an exaggerated triceps jerk. It is indicative of a spinal cord lesion at C5 or C6, e.g. due to trauma, syringomyelia, or disc prolapse. It occurs because a lower motor neurone lesion of C5 is combined with an upper motor neurone lesion affecting reflexes below C5.
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