Static changes of the shoulder joint from 30 degrees adduction (ANT) to 30 degrees abduction (POST) in the horizontal plane reduce the gain of the input--output relationship of the corticospinal pathway to the abductor digiti minimi (ADM) muscle [F. Ginanneschi et al. (2005)Exp. Brain Res., 161, 374--382]. The present study examined force estimation under conditions in which the input--output relationship of the corticospinal innervation to ADM was modified by changing shoulder position as above. The input--output relationship was studied using transcranial magnetic stimulation. Estimates of force were assessed using a matching procedure; subjects first matched a target level (10--40% of maximum) on a screen by applying a reference (Ref) isometric contraction of ADM and then they reproduced the same level of force without visual feedback by a test contraction (Test). When Ref and Test contractions were performed at either ANT or POST (i.e. the same input--output), the respective force levels were closely matched. In contrast, when the Test and Ref were performed in POST and ANT, respectively (i.e. different input--output), subjects exerted more force than required. Errors were in the opposite direction when the Test and Ref were in ANT and POST, respectively. The present results suggest that the process of force estimation is based on the effort : tension ratio which is a direct function of the corticomotoneuronal input--output relationship. This notion may contribute to explaining the pathophysiology of central fatigue.