The technique of twitch interpolation is being used more commonly to assess whether human muscles are activated fully during attempted maximal voluntary efforts. A circuit which allows the measurement of small twitches superimposed on the maximal voluntary contractions is described. Use of the circuit in simulated and actual maximal voluntary contractions are presented. The sensitivity of the technique is enhanced when more than one stimulus is interpolated and responses are averaged. If rigorously applied, twitch interpolation should allow failure to activate a small fraction of the force output from a muscle (less than 2%) to be detected. The device can be applied to measure any small evoked signal which is superimposed on a large DC offset.