Individual plantar flexor compensation during walking with a unilateral passive dynamic ankle-foot orthosis : a simulation study
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Ankle foot orthoses (AFOs) are designed to improve walking ability in persons with a variety of musculoskeletal and neuromuscular disorders. Passive dynamic AFOs (PD-AFOs) are a special class of orthoses that function like a torsional spring, which store elastic energy in early stance and release that energy in late stance to supplement impaired muscle (e.g. plantar flexor) force output. Although PD-AFOs have been beneficial to a variety of patient populations, a mismatch between the design characteristics (e.g. stiffness) and a patient's specific requirements can be detrimental. Therefore, the overall objective of this study was to begin to understand the interactions between design characteristics of PD-AFOs and the musculoskeletal system by using forward dynamic simulations of walking with and without a unilateral PD-AFO. Our specific goal was to investigate the influence of PD-AFO stiffness on individual plantar flexor activity while maintaining normal walking mechanics. To achieve this goal, both the soleus group (SOL) and the gastrocnemius group (GAS) were allowed to compensate independently for the PD-AFO while all other muscles were held constant and the resulting changes in activity were quantified. The simulation analyses revealed a systematic relationship between SOL activity and PD-AFO stiffness, but not with GAS. As the PD-AFO stiffness increased, SOL activity decreased. In contrast, GAS activity remained relatively constant with increased stiffness. These results suggest the PD-AFO is more ideally suited for SOL force substitution than GAS force substitution, and therefore patients with SOL weakness may benefit from the prescription of a PD-AFO while patients with GAS weakness may benefit less. To explore other compensatory strategies, a post-hoc analysis where all muscles were allowed to compensate revealed other relationships between PD-AFO stiffness and muscle activity. In addition to the plantar flexors, the iliopsoas, rectus femoris, tibialis anterior, biceps femoris short head and the vastii all compensated for the PD-AFO to provide swing initiation and facilitate knee flexion during swing.