The "Extensor habitus" phenomenon occurs in finger flexor tendon injuries and consists of a paradoxical extension of the interphalangeal joints after an attempt to flex the finger. The mechanism of extension is considered to be a contraction of the flexor digitorum profundus that is then transmitted via the lumbrical muscle structure to the extensor expansion. Using electromyography, we recorded the lumbrical muscle activity during the paradoxical extension phenomenon to determine whether the lumbrical muscle contributed to this event. Two patterns of electromyographical activity of the lumbrical muscle were observed. Group I (6 fingers) displayed electrical activities in the lumbrical muscle during flexion tasks, while group II (12 fingers) did not. In group I, the lesions were mainly located in zone V, and the response to range of motion exercises was satisfactory. In group II, nearly all of the lesion were located in zone II, and half of the cases required additional surgical interventions. Group II appeared to exhibit the "Extensor habitus" phenomenon, while group I exhibited an "Extensor habitus-like phenomenon." To distinguish between these two phenomena, an electromyographical examination of the lumbrical muscle must be performed.
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