This study investigated the activity of the extrinsic finger flexor muscles during active extension in the Kleinert splint. Electromyographic data on the activity of the profundus and superficialis flexor muscles in 10 healthy subjects were recorded with use of fine needle electrodes. The subjects exercised in the original Kleinert splint as well as in several modifications of the splint, which varied with respect to (1) wrist position, (2) position in which extension of the metacarpophalangeal joint was blocked, (3) number of fingers dynamically splinted, (4) nature of the spring mechanism, (5) amount of resistance, and (6) use of a palmar pulley. Persistent flexor muscle activity during active extension was observed in the majority of subjects. This coactivity was more often observed for the superficialis muscle than for the profundus muscle. The least amount of coactivity was found when extension was least resisted. This study does not support the concept that the flexor muscles relax during resisted extension in the Kleinert splint.