Changes in the Behavior of the Ankle Plantar Flexor Motor Unit Due to Knee Assistance for Heel-Raise

The heel-rise is frequently performed to train patients who are unable to plantar flex their ankle joints. Patients are often assisted by therapists because their wholebody muscle strength is impaired. However, it is unclear how the muscle activity of the ankle plantar flexors changes with the assistance. Furthermore, since patients often suffer from neurological disorders, it is important to observe not only the amplitude of the compound action potential but also the behavior of the motor units. In this study, we examined the number and frequency of motor unit firings in the ankle planter flexors during the heel-raise in 14 healthy adults with and without a knee contracture orthosis. In addition, the Peason's correlation coefficient was used to verify whether the same motor units were detected across conditions; r > 0.8 was considered identical. The high-density surface electromyogram used to detect the motor units. Correspondence t-test was used for comparison between conditions. As a result, the number of motor unit firings of the soleus was higher under the condition with orthosis (p = 0.03). On the other hand, there was no significant difference in the frequency of firing of motor units between the conditions. In addition, the number of the same motor units detected between the conditions was only 16.84%. We cannot directly conclude the efficacy of using the orthosis during training, and further verification is necessary because of the possible limitation of detecting motor units from four electrodes of the measurement device.