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SUMMARY A simple electrophysiological method is described for detecting weakness of psychiatric origin. The method involves the interpolation of a single indirect stimulus to a muscle during the course of a supposedly maximal voluntary contraction. Incomplete motor activation is revealed by the presence of a twitch superimposed on the recording of voluntary torque.One of the most difficult problems that may confront the clinical neurologist is the decision as to whether muscle weakness is "organic" or "functional". During the past two years we have employed a technique originally devised by DennyBrown1 to examine motor unit recruitment. The technique was used by Merton2 in his classic study of muscle strength and fatigue in the human adductor pollicis but, so far as we are aware, has never been exploited in a clinical situation. The principle of the method is to request the patient to make as strong a contraction as possible and, during this period, to deliver a single maximal stimulus to the appropriate motor nerve. If some motor units have not been recruited, or are firing at suboptimal rates for tension development, they will generate a twitch response which will be superimposed on the voluntary torque recording. On the contrary, if all the motor units are fully activated in the voluntary effort, the interpolated stimulus will not be able to produce any further torque. MethodsThe studies are performed on the dorsiflexors and plantarflexors of the ankle using a leg-holding device described elsewhere;3 this apparatus enables isometric twitch and voluntary torques to be measured for either of the two muscle groups, and at any chosen position of the ankle joint. During plantarflexion the tibial nerve is stimulated in the popliteal fossa through two surface electrodes constructed of aluminium foil; for evaluating dorsiflexion, similar electrodes are placed either over the motor point of

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