We assessed the ability of healthy subjects (n = 7) and a patient deprived of proprioception (GL) to produce and assess different levels of isometric forces. They first produced a target force with one hand (the reference control hand) and then, after a delay of 3 s, they attempted to match it with the other hand (the experimental matching hand). Despite abnormal variations in motor outputs, we found that GL could, as could the control subjects, maintain a constant relationship between the force exerted by the control hand and the force exerted by the experimental hand. As GL was deprived of proprioceptive cues, these results suggest that she indirectly perceived muscular force through central effort. Interestingly, when carrying out the task the patient reported neither feelings of fatigue nor awareness of how hard she tried to perform the matches. Hence, under certain circumstances (such as in our motor task), it seems possible to assess and scale muscular force on the basis of endogenous signals only. However, internally generated signals related to the size of the motor command may need to interact with afferent input to gain full access to consciousness.