It has been observed consistently, and is well accepted, that the steady-state isometric force following active shortening or stretch of a muscle is smaller or greater, respectively, than the corresponding isometric force for electrically stimulated muscle and maximal voluntary contractions. Force depression after shortening and force enhancement after stretch can reach considerable values and might affect normal movements. However, force depression and force enhancement have not been studied for sub-maximal voluntary contractions;​ therefore, it is not known whether force depression or force enhancement may affect everyday movements, which are typically contractions at submaximal levels. The purpose of this research was to determine if there is force depression and force enhancement at sub-maximal voluntary contractions, and through these observations, possibly gain further understanding of the possible mechanisms underlying these properties. Human adductor pollicis muscle was studied using a custom-built dynamometer, and both, force and activation, were measured while activation and force were controlled at sub-maximal voluntary levels, respectively. The steady-state isometric force and activation following active shortening or stretch were compared with the corresponding values obtained for purely isometric reference contractions. The average steady-state force was decreased by 20% and activation was increased by 18% following shortening compared with the corresponding isometric reference contractions, and force depression was observed consistently in all subjects. Unlike force depression, only about one half of the tested subjects showed consistent and repeatable force enhancement (10%) and activation deficit (16%), suggesting that force enhancement for sub-maximal voluntary contractions occurs in a sub-set of subjects, and that force enhancement may depend on the level of voluntary contraction with a subject-specific threshold. In order to address these issues, isometric reference contractions were performed at a constant sub-maximal level of activation. The corresponding "active stretch test contractions" were performed at the same sub-maximal levels as the isometric reference contractions, except for the stretch phase in which activation was systematically changed. The results showed that force enhancement in voluntary contractions increases with increasing level of activation during stretch with a threshold that varies among subjects, suggesting that force enhancement is likely activation-dependent, and that force enhancement in voluntary contractions may be associated with cross-bridge function.