The functioning of the lumbrical muscle in the human finger is difficult to visualise. This is mainly due to the fact that the origin and insertion of the lumbrical is on tendons of other motors (the deep flexor and the extensor assembly, respectively), instead of on bone. In this paper the functioning of the lumbrical in the human finger is kinematically investigated by explicitly considering the slackness or tautness of tendon parts which are in parallel or in series with the lumbrical, and by deriving from a standard finger model equivalent representations from which the functioning of the lumbrical is visually more clear. These models are used to review and interpret the results of previous studies. Further, it is indicated that the lumbrical is in an ideal position to contribute to the control of certain fast movements, as may be of importance for the musician, and that its role in other fast movements may be limited because of its large displacements (contraction speed).