A phasic tendon stretch reflex is said to be inverted when the movement elicited is opposite to that normally seen, e.g., extension of the elbow rather than flexion when eliciting the supinator (brachioradialis) jerk; flexion of the forearm when tapping the triceps tendon (paradoxical triceps reflex); and flexion (hamstring contraction) rather than extension of the knee when tapping the patellar tendon.
The finding of inverted reflexes may reflect dual pathology, but more usually reflects a single lesion which simultaneously affects a root or roots, interrupting the local reflex arc, and the spinal cord, damaging corticospinal (pyramidal tract) pathways which supply segments below the reflex arc. Hence, an inverted supinator jerk is indicative of a lesion at C5/6, paradoxical triceps reflex occurs with C7 lesions; and an inverted knee jerk indicates interruption of the L2/3/4 reflex arcs, with concurrent damage to pathways descending to levels below these segments.
Boyle RS, Shakir RA, Weir AI, McInnes A. Inverted knee jerk: a neglected localizing sign in spinal cord disease. Journal of Neurology,Neurosurgery and Psychiatry 1979; 42: 1005-1007