The goal of this pilot study with a pre-post design is to investigate the effects of separate individualization of the AFO stiffness towards plantar- and dorsiflexion in a spring-hinged AFO on walking compared to a spring-like AFO (3 types) having the same stiffness in both directions.

People with a neuromuscular disease or nerve injury causing at least plantarflexor weakness (determined as the inability to perform 3 single heel rises), with an indication for or using an AFO, will be fitted with a new, custom-made spring-hinged AFO with the NEURO SWING® system ankle joint (Fior& Gentz, Lüneburg, Germany), of which the stiffness of ventral and dorsal compartment of this spring-hinged AFO will be individualized. For comparison, measurements will be performed with three different prefab spring-like AFOs with different stiffness levels (but which have a similar stiffness towards plantar and dorsiflexion), and the participants' current AFO if applicable, and shoes-only at baseline.

The main outcome parameters will be the maximal ankle plantarflexion angle, ankle angular velocity and knee flexion angle during the loading response, which will be measured using a 3D gait analysis. Secondary outcomes include other gait biomechanics, walking energy cost, walking speed, standing balance, perceived physical functioning and perceived walking ability.

Many neuromuscular diseases cause weakness of the ankle dorsiflexors and plantarflexors, resulting in an altered gait pattern. In particular, weakness of the plantar flexors leads to a reduced walking ability as it hampers safety in both standing and walking. The primary treatment to improve walking ability and safety during standing and walking in dorsiflexor and/or plantarflexor weakness is the provision of ankle-foot orthoses (AFOs). To maximize treatment outcomes in case of lower leg weakness, the optimal AFO stiffness needs to be individually determined. Individual optimization of the stiffness can be performed with a spring-like AFO or with a spring-hinged AFO. With a spring-like AFO, the stiffness towards plantar- and dorsiflexion is similar, oftentimes resulting in a higher than necessary stiffness towards plantarflexion. An advantage of spring-hinged AFOs is that, unlike spring-like AFOs, the stiffness can be separately optimized in the directions of dorsiflexion and plantarflexion.

The objective of this pilot study is to evaluate the effects of separate individualization of the AFO stiffness towards plantar- and dorsiflexion in a spring-hinged AFO compared to three types of spring-like AFO having the same stiffness in both directions on gait biomechanics, walking energy cost, walking speed, and standing balance. Additionally, effects will be evaluated of the optimal spring-hinged AFO 6 weeks after delivery of the AFO on perceived physical functioning, walking ability and satisfaction in daily life compared to the participants' AFO used at baseline if applicable or walking with shoes-only.

In this pilot study with a pre-post design, people with a neuromuscular disease or nerve injury causing at least plantarflexor weakness with an indication for or using an AFO will be fitted with a new, custom-made spring-hinged AFO with the NEURO SWING® system ankle joint (Fior& Gentz, Lüneburg, Germany). The stiffness of ventral and dorsal compartment of this spring-hinged AFO will be individualized using a previously developed optimization algorithm. The spring-hinged AFO with optimal stiffness settings will be used at home for 6-weeks. For comparison, the investigators will test the direct effects of three different prefab spring-like AFOs with different stiffness levels (but which have a similar stiffness towards plantar and dorsiflexion) of 2.8, 1.4 and 0.6 Nm/degrees respectively in a randomized order, and the participants' current AFO if applicable, and shoes-only at baseline.

Inclusion Criteria:

1. Age 18 years or older;
2. Presence of plantar flexor weakness in at least one leg, determined as a score lower than 5 on the manual muscle testing scale (Medical Research Council- MRC) and/or inability to perform three single heel rises, with or without dorsiflexion weakness;
3. Indicated for or using an AFO;
4. Ability to walk 6-minutes consecutively (with assistive device, if necessary).

Exclusion Criteria:

1. When wearing the AFO, not able to walk short bouts of 10m without walking aids, such as a walker;
2. Foot deformities that do not fit in prefab spring-like AFOs;
3. Weakness of the knee extensor muscles, for which a knee-ankle-foot orthosis is indicated.

Anonymized individual participant data (IPD) and meta data will be made available to third parties via Figshare. Other anonymized IPD and documents will be made available on request including data analyses codes such as SPSS syntaxes.