Subjects are able to judge the strength of muscle contraction. In theory, the force of muscular exertion could be perceived either from mechanoreceptor afferents or from knowledge of central motor command (corollary discharge). Sensations of great effort or exerted force have been described by subjects when their limbs were weakened by fatigue or partial paralysis. This has been taken as evidence that effort sensations arise from central motor commands rather than from mechanoreceptor afferent signals produced by muscle contraction. To differentiate between these possibilities, we used neuromuscular block to completely paralyze four waking subjects and required them to attempt maximal contraction of inspiratory muscles and of hand muscles. They were questioned after recovery about what their sensations were when attempting these contractions. None described the sensations of exerted force, great effort, or heaviness, which would have been expected if motor commands alone were the source of these sensations. The contradiction between our findings and those previously reported suggests that the specific neural mechanisms for effort sensations must be reexamined.