Force in the Achilles Tendon During Walking With Ankle Foot Orthosis

Abstract

Background Ankle foot orthoses are used for postoperative treatment of Achilles tendon ruptures and decrease calf muscle electromyography activity during walking. Hypothesis Achilles tendon load decreases with increased restriction of dorsiflexion and is associated with decreased triceps surae activity. Study Design Controlled laboratory study. Methods In 8 subjects, the maximum force and rate of force development in the Achilles tendon were measured with an optic fiber technique, and the activity of the gastrocnemius, soleus, and tibialis anterior muscles was recorded using electromyography. Trial conditions were walking barefoot and wearing an ankle-foot orthoses set in 3 different positions: (1) locked at 20° of plantar flexion and with free plantar flexion but restricted dorsiflexion to (2) 10° plantar flexion and (3) 10° dorsiflexion, respectively. The design of the ankle foot orthoses did not provide heel support when fixed in a plantarflexed position. Results Maximum Achilles tendon force was highest at the ankle-foot orthoses setting of 20° plantar flexion (3.1 times body weight) and decreased to 2.1 times body weight during barefoot walking (P < .01). The rate of Achilles tendon force showed an increasing trend with less-restricted dorsiflexion. Soleus activity was 52% of mean barefoot walking activity at 3 20° plantar flexion (P < .001) and then increased as dorsiflexion was less restricted. Conclusion Weightbearing in ankle-foot orthoses when dorsiflexion is restricted beyond neutral may result in increased forces in the Achilles tendon compared with barefoot walking, despite reduced electromyography activity in the triceps surae and decreased rate of force development. Clinical Relevance If patients bear full weight in an ankle-foot orthoses locked at 20° plantar flexion without heel support, the maximum force in the tendon may exceed that encountered during barefoot walking.