Effect of ankle-foot orthosis with a built-in spring on muscle activity during the sit-to-stand movement in healthy individuals

[Purpose] This study aimed to evaluate the influence of a newly developed ankle-foot orthosis with a built-in spring on the activity of lower limb muscles during the sit-to-stand movement. [Participants and Methods] This cross-sectional study recruited 20 male volunteers. The sit-to-stand movement (rising from a chair) was performed under three conditions: no ankle-foot orthosis (NA), ankle-foot orthosis with no spring (NS), and ankle-foot orthosis with a built-in spring (SP). Muscle activity during the sit-to-stand movement was measured using surface electrodes placed on the vastus medialis, tibialis anterior, medial gastrocnemius, and soleus muscles. Root mean square and integral value were calculated from the raw data, and statistical analysis was performed using SPSS version 24.0. [Results] The electromyography data of the vastus medialis, medial gastrocnemius, and soleus muscles showed a significant decrease in muscle activity in the SP condition, whereas the activity of the tibialis anterior muscle increased significantly in the SP condition compared to that in the NA and NS conditions. [Conclusion] Our data showed that the use of an ankle-foot orthosis with a built-in spring affected not only the muscle activity at the ankle joint but also the activity of the knee joint extensor muscle. It is possible that the control of the ankle joint motion affects movement above the knee joint; this finding may help development new physical therapy techniques. Further research is warranted in this regard.