Biomechanical response to ankle–foot orthosis stiffness during running

• Sound and salvaged limb biomechanics were compared during running.
• Participants wore custom ankle–foot orthoses of three stiffnesses.
• Stiffness did not affect ankle and knee joint angles, moments and powers.
• Decreased orthosis stiffness increased the compliancy of the ankle joint.
• Ankle stiffness increased and knee stiffness decreased compared to the sound limb.

BackgroundThe Intrepid Dynamic Exoskeletal Orthosis (IDEO) is an ankle–foot orthosis developed to address the high rates of delayed amputation in the military. Its use has enabled many wounded Service Members to run again. During running, stiffness is thought to influence an orthosis' energy storage and return mechanical properties. This study examined the effect of orthosis stiffness on running biomechanics in patients with lower limb impairments who had undergone unilateral limb salvage.MethodsTen patients with lower limb impairments underwent gait analysis at a self-selected running velocity. 1. Nominal (clinically-prescribed), 2. Stiff (20% stiffer than nominal), and 3. Compliant (20% less stiff than nominal) ankle–foot orthosis stiffnesses were tested.FindingsAnkle joint stiffness was greatest in the stiffest strut and lowest in the compliant strut, however ankle mechanical work remained unchanged. Speed, stride length, cycle time, joint angles, moments, powers, and ground reaction forces were not significantly different among stiffness conditions. Ankle joint kinematics and ankle, knee and hip kinetics were different between limbs. Ankle power, in particular, was lower in the injured limb.InterpretationAnkle–foot orthosis stiffness affected ankle joint stiffness but did not influence other biomechanical parameters of running in individuals with unilateral limb salvage. Foot strike asymmetries may have influenced the kinetics of running. Therefore, a range of stiffness may be clinically appropriate when prescribing ankle–foot orthoses for active individuals with limb salvage.