The purpose of this study was to demonstrate the usefulness of a small inertia sensor for quantitative classification of movement disorders based on the change in mechanical energy in patients following a stroke. We measured the sit-to-stand motion in acute stroke patients using inertial sensors in a small clinic. Three acute stroke patients and three healthy adults performed the sit-to-stand paradigm. The three-dimensional angle in the global coordinate system of the inertial sensor attached to the participant's body was then calculated. The movements of healthy adults were measured using inertial sensors and a camera motion capture system simultaneously, and only sagittal plane angles were used for the analysis, which were similar in the two devices. Subsequently, link segment models were created, and the mechanical work until seat-off was calculated. In stroke patients, the thoracic potential energy was not converted to kinetic energy, and deceleration of the thorax was greater in stroke patients than in healthy adults. Furthermore, the mean pelvic kinetic energy in stroke patients was approximately one tenth of that in healthy adults. In healthy adults, the waveforms of the angular velocities of the thorax and pelvis were synchronized. Such synchronization was not observed in the waveforms of stroke patients. A reason for the low pelvic kinetic energy in stroke patients is the fact that deceleration of the thorax by lumbar muscles does not lead to acceleration of the pelvis. The lack of synchronization of thoracic and pelvic angular velocities reduced the energy transfer efficiency. The usefulness of a small inertial sensor was demonstrated based on the evaluation of energy change efficiency during the sit-to-stand motion performed by an individual following a stroke.