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Background: Neuromuscular disease and peripheral neuropathy may cause drop foot with or without evertor weakness. We developed a helical-shaped, non-articulated ankle–foot orthosis (AFO) to provide medial–lateral stability while allowing mobility, to improve gait capacity. Our aim was to evaluate the effect of the helical AFO (hAFO) on functional gait capacity (6-min walk test) in people with peripheral neuropathy or neuromuscular disease (NMD) causing unilateral drop foot and compare with a posterior leaf spring AFO (plsAFO). Secondary aims were to compare functional mobility, 3D kinematic and kinetic gait variables and satisfaction between the AFOs. Methods: Single centre, randomised crossover trial from January to July 2017 in 20 individuals (14 with peripheral neuropathy and 6 with NMD, 12 females, mean age 55.6 years, SD 15.3); 10 wore the hAFO for the first week and 10 wore the plsAFO before switching for the second week. The 6-min walk test (6MWT), Timed Up and Go (TUG) test and 3D gait analysis were evaluated with the hAFO, the plsAFO and shoes only (noAFO) at inclusion and 1 week after wearing each orthosis. Satisfaction was evaluated with the Quebec user evaluation of satisfaction with assistive technology (QUEST). Results: Median [interquartile range] 6MWT distance was greater with the hAFO (444 m [79]) than the plsAFO (389 m [135], P < 0.001, Hedge's g = 0.6) and noAFO (337 m [91], P < 0.001, g = 0.88). TUG time was shorter with the hAFO (8.1 s [2.8]) than the plsAFO (9.5 s [2.6], P < 0.001, g = − 0.5) and noAFO (10.0 s [2.6]), P < 0.001, g = − 0.6). The plsAFO limited plantarflexion during the loading response (plsAFO − 7.5 deg [6.0] vs. noAFO -13.0 deg [10.0], P = 0.0007, g = − 1.0) but the hAFO did not (− 11.0 deg [5.1] vs. noAFO, P = 0.05, g = − 0.5). Quasi-stiffness was lower for the hAFO than plsAFO (P = 0.009, g = − 0.7). The dimensionless eversion moment was higher (though not significantly) with the hAFO than noAFO. Neither orthosis reduced ankle power (P = 0.34). Median total QUEST score was higher for the hAFO (4.7 [0.7]) than the plsAFO (3.6 [0.8]) (P < 0.001, g = 1.9). Conclusions: The helical orthosis significantly and considerably improved functional gait performance, did not limit ankle mobility, increased lateral stability, though not significantly, and was associated with greater patient satisfaction than the posterior leaf spring orthosis. Trial registration The trial began before registration was mandatory [ABSTRACT FROM AUTHOR]