It has been observed consistently and is well accepted that the steady-state isometric force following active muscle stretch is greater than the corresponding isometric force for electrically stimulated muscles and maximal voluntary contractions. However, this so called force enhancement has not been studied for sub-maximal voluntary efforts, therefore it is not known if this property affects everyday movements. The purpose of this study was to determine if there was force enhancement during sub-maximal voluntary contractions. Human adductor pollicis muscles (n=17) were studied using a custom built dynamometer and both force and activation were measured while muscle activation and force were controlled at a level of 30% of maximal voluntary contraction, respectively. The steady-state isometric force and activation following active stretch were compared to the corresponding values obtained during isometric reference contractions. There was consistent and reliable force enhancement in eight of the seventeen subjects, while there was no force enhancement in the remaining subjects. Subjects with force enhancement had greater post-activation potentiation and a smaller resistance to fatigue in the adductor pollicis. We conclude from these results that force enhancement exists during sub-maximal voluntary contractions in a sub-set of the populations, and suggest that it may affect everyday voluntary movements in this subset. Based on follow up testing, it appears that force enhancement during voluntary contractions is linked to potentiation and fatigue resistance, and therefore possibly to the fiber type distribution in the adductor pollicis muscle.