Spike-triggered averaging (STA) has been used to extract twitch profiles of single motor units (SMU) within the human masseter muscle. However, the reported twitch tensions may have been biased by the voluntary firing frequency of the SMUs, the complex architecture of the muscle, and by the biomechanical linkage of the jaw. In this study, a rigid STA paradigm was used to record spike-triggered "measured tensions" (STMTs) for 32 SMUs in the masseter muscles of four subjects. STMTs were recorded at two different orientations of a force transducer placed between the incisor teeth. The STMTs produced by each unit were used to calculate jaw torque. STMTs were also recorded in 11 units with differing degrees of muscle coactivation. STMTs for each unit varied according to the orientation of the force transducer. However, no systematic changes in STMTs occurred with reciprocal changes in the jaw moment arm. STMTs could be altered significantly by different degrees of muscle co-activation. The use of STA as a method for determining SMU tension in the human masseter muscle appears to be highly task-dependent and in the presence of co-activation may be inappropriate.