Modifying ankle foot orthosis stiffness in patients with calf muscle weakness: gait responses on group and individual level

<jats:title>Abstract</jats:title> <jats:sec> <jats:title>Background</jats:title> <jats:p>To improve gait, persons with calf muscle weakness can be provided with a dorsal leaf spring ankle foot orthosis (DLS-AFO). These AFOs can store energy during stance and return this energy during push-off, which, in turn, reduces walking energy cost. Simulations indicate that the effect of the DLS-AFO on walking energy cost and gait biomechanics depends on its stiffness and on patient characteristics. We therefore studied the effect of varying DLS-AFO stiffness on reducing walking energy cost, and improving gait biomechanics and AFO generated power in persons with non-spastic calf muscle weakness, and whether the optimal AFO stiffness for maximally reducing walking energy cost varies between persons.</jats:p> </jats:sec> <jats:sec> <jats:title>Methods</jats:title> <jats:p>Thirty-seven individuals with neuromuscular disorders and non-spastic calf muscle weakness were included. Participants were provided with a DLS-AFO of which the stiffness could be varied. For 5 stiffness configurations (ranging from 2.8 to 6.6 Nm/degree), walking energy cost (J/kg/m) was assessed using a 6-min comfortable walk test. Selected gait parameters, e.g. maximal dorsiflexion angle, ankle power, knee angle, knee moment and AFO generated power, were derived from 3D gait analysis.</jats:p> </jats:sec> <jats:sec> <jats:title>Results</jats:title> <jats:p>On group level, no significant effect of DLS-AFO stiffness on reducing walking energy cost was found (<jats:italic>p</jats:italic> = 0.059, largest difference: 0.14 J/kg/m). The AFO stiffness that reduced energy cost the most varied between persons. The difference in energy cost between the least and most efficient AFO stiffness was on average 10.7%. Regarding gait biomechanics, increasing AFO stiffness significantly decreased maximal ankle dorsiflexion angle (− 1.1 ± 0.1 degrees per 1 Nm/degree, <jats:italic>p</jats:italic> < 0.001) and peak ankle power (− 0.09 ± 0.01 W/kg, <jats:italic>p</jats:italic> < 0.001). The reduction in minimal knee angle (− 0.3 ± 0.1 degrees, <jats:italic>p</jats:italic> = 0.034), and increment in external knee extension moment in stance (− 0.01 ± 0.01 Nm/kg, <jats:italic>p</jats:italic> = 0.016) were small, although all stiffness’ substantially affected knee angle and knee moment compared to shoes only. No effect of stiffness on AFO generated power was found (<jats:italic>p</jats:italic> = 0.900).</jats:p> </jats:sec> <jats:sec> <jats:title>Conclusions</jats:title> <jats:p>The optimal efficient DLS-AFO stiffness varied largely between persons with non-spastic calf muscle weakness. Results indicate this is caused by an individual trade-off between ankle angle and ankle power affected differently by AFO stiffness. We therefore recommend that the AFO stiffness should be individually optimized to best improve gait.</jats:p> </jats:sec> <jats:sec> <jats:title>Trial registration number</jats:title> <jats:p>Nederlands Trial Register 5170. Registration date: May 7th 2015. <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://www.trialregister.nl/trialreg/admin/rctview.asp?TC=5170">http://www.trialregister.nl/trialreg/admin/rctview.asp?TC=5170</jats:ext-link></jats:p> </jats:sec>