Optimizing gait with bidirectional tuning of the ankle-foot orthosis (AFO) stiffness in people with lower leg muscle weakness*

Many neuromuscular diseases cause weakness of the ankle dorsiflexors and
plantarflexors, resulting in an altered gait pattern. The primary treatment to
improve walking ability in dorsiflexor and/or plantarflexor weakness is the
provision of ankle-foot orthoses (AFOs). In case of dorsiflexion weakness, a
relatively low AFO stiffness towards plantarflexion is required while for
plantarflexor weakness, the required stiffness towards dorsiflexion is much
higher. The required stiffness in both directions maximally normalizing the
gait pattern varies between patients due to differences in characteristics,
like the severity of weakness, body mass and patient activity. Consequently, 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-hinged AFO. An advantage of
spring-hinged AFOs is that the stiffness can be separately optimized in the
directions of dorsiflexion and plantarflexion, which is not possible in
spring-like AFOs without hinge. This separate tuning of the stiffness towards
plantar- and dorsiflexion potentially leads to a more natural gait kinematics
and kinetics in loading response and pre/swing. However, little is known about
the benefits of individualizing the stiffness in plantar- and dorsiflexion
separately (i.e. bidirectional stiffness tuning) in a spring-hinged AFO.

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 NEURO SWING AFO compared to a spring-like AFO without hinge (3
types) having the same stiffness in both directions on walking, as assessed by
gait kinetics and kinematics, walking energy cost, walking speed, standing
balance and posture, satisfaction and perceived walking ability.

A pilot study with a pre-post design

Participants will be fitted with a new, custom-made spring-hinged AFO with the
NEURO SWING® system ankle joint (Fior& Gentz). The stiffness of the 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, we will
test the effect of three different prefab spring-like AFOs without hinge 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.

Participants will visit the hospital four times for 3 hours per visit. During
each visits, patients will undergo a 3D gait analysis and 6MWT. During the
first visit, the spring-hinged NEURO SWING AFO will be casted and tests without
AFO will be performed, during the second visit, tests with the 3 prefab
spring-like AFOs are performed, during the third visit the spring-hinged NEURO
SWING AFO is optimized based on a gait analysis and 6MWT, and the fourth visit
is the follow up visit 6 weeks after AFO provision. For evaluation purposes, we
will contact participants 6 weeks after completion of the study to ask about
their experiences with study participation, and to inform if the NEURO SWING
AFO is still used and if there are any complaints.

Possible burdens are fatigue due to the extensive protocol and physical
discomfort due to walking with new AFOs. A potential benefit of participating
in de study is that if separate individualization of stiffness towards plantar-
and dorsiflexion in spring hinged AFO is successful, patients receive an AFO
better matched to their individual impairments and this may improve their
walking ability more.