Gait
impairments that increase the risk of tripping-related falls are one of
the most serious consequences of ageing, stroke and many neurological
and muscular conditions such as spinal cord injury, multiple sclerosis,
muscular dystrophy or cerebral palsy [1,2,3].
In normal gait, the swing phase is shaped by two events, ‘Mx1’ and
‘Mx2’, representing two vertical foot displacements maxima that frame a
critical moment of Minimum Toe Clearance (MTC) or Minimum Foot Clearance
(MFC) (Fig. 1).
MTC refers to the toe’s clearance above ground, while MFC measures the
lowest part of the forefoot or shoe’s clearance from the ground.
Avoiding contact with walking surface irregularities requires precisely
modulated vertical displacement of the foot, especially at the swing
phase Minimum Foot Clearance (MFC) event [4,5,6].
Ankle
dorsiflexion is crucial for elevating the foot during swing by enabling
substantial adjustments to ground clearance with relatively minor
changes in ankle angles and minimal disruption to overall gait control [6,7,8].
The development of assistive technology for ankle joint dorsiflexion
could play an important role in maintaining safe ground clearance and
preventing tripping-related falls. Rapid progress has been observed in
the development of ankle orthoses, employing advanced actuators to apply
moments that can effectively assist impaired ankle dorsiflexion [9,10,11].
An essential requirement of these devices is to deliver sufficient
mechanical power to ensure the necessary magnitude of ankle assistive
moments. Understanding the kinetics demands of ankle joint dorsiflexion
is, therefore, particularly useful for devices employing energy
harvesting or utilizing lighter but also less power-demanding actuators.
This understanding is the foundation for designing assistive
technologies that harmonize the required ankle moments with required
energy inputs.
Ankle dorsiflexion moments have been determined
experimentally but more commonly in static conditions, rather than when
walking. Takaiwa & Noritsugu (2008) [12]
determined that 2 N.m ankle moment was required to achieve 20 degrees
of ankle dorsiflexion, i.e. from − 15 degrees plantar flexion to + 5
degrees dorsiflexion. A University of Illinois design team adopted Perry
and Burnfield’s (1993) [13] data to calibrate their powered AFO, employing a constant 3 N.m ankle torque throughout swing [11, 14,15,16].
Such time-dependent ankle moment measurements recorded dynamically are
anticipated to be more useful in designing ankle assistive devices to
more closely mimic natural gait. Kao and Ferris, (2009) [17] and Sawicki and Ferris, (2009) [18]
used inverse dynamics to estimate ankle dorsiflexion moments at
1.25 m/s. They found a maximum ankle moment following toe-off of
0.016 N.m/Kg which decreased gradually until end of swing; with an ankle
power range of -0.08 W/Kg to 0.05 W/Kg. Their study did not include
ankle moment and power changes with increasing ankle dorsiflexion but
this control feature may be useful in revealing the kinetics of high
ankle dorsiflexion rotation to determine the required adequate mechanical energy input.
Consistent
with the traditional focus on stance kinetics there are limited data to
show ankle joint energy exchanges during swing, possibly because swing
phase energy requirements are often considered less important components
of lower limb joint kinetics [19, 20].
More recently it has, however, been argued that the energy consumed
during swing is non-trivial, with research by Doke et al. (2005) [21]
concluding that swing phase muscle activity consumes between
one-quarter to one-third of total gait energy. Exploring joint work is
important for understanding the mechanical energy demands of walking
because joint mechanical energy is associated with the ability to
perform work [22,23,24].
Ankle work can, therefore, be calculated to indicate the maximum energy
demands of swing phase ankle dorsiflexion. In the study reported here
we sought to determine the kinetic requirements of increasing swing
phase ankle dorsiflexion by incorporating a treadmill-walking condition
in which foot-ground clearance was manipulated via a continuous foot
trajectory display. Subsequently, we derived the swing-phase profile of
ankle joint moments and the power demands of augmenting ankle
dorsiflexion.
Previous investigators have often described three
swing sub-phases representing approximately 0–35%, 35–65%, and 65–100%
of the swing cycle, corresponding to Initial, Mid, and Terminal swing,
respectively (Fig. 1) [13, 25]. Unusual or pathological gaits may not, however, always be described adequately using these sub-phases [26] and investigation of time-dependent variables such as joint power may also require a more fine-grained analysis [27].
To
explore functional variations in ankle energy demands with greater
specificity, in this study we introduced three new event-dependent swing
sub-phases and also calculated the time and power demands of each
(Fig. 1).
In
addition to determining ankle joint mechanics, foot-ankle computational
modelling has been used to quantify force and power of the Tibialis
Anterior (TA) as the primary dorsiflexor. A systematic review of twelve
studies indicated maximum swing TA forces ranging from 1 to 4 N/kg at
preferred walking speed [28] but there are important variations within sub-phases. Błażkiewicz (2013) [29] found a maximum TA force of 2 N/kg following toe-off and TA power computed by Bogey et al. (2010) [30]
reached an initial negative peak of almost − 2 Watts, followed by a
positive maximum of 12 Watts; those data were, however, time-normalized
to the swing cycle, precluding a post-hoc work calculation. Possibly
because the TA is the primary dorsiflexor, less research attention given
to other ankle dorsiflexor muscles, i.e., Extensor Digitorum Longus
(EDL) and Extensor Hallucis Longus (EHL). In addition to providing a
more complete description of dorsiflexor kinetic contributions to ankle
swing phase control, in this experiment the kinetic contributions of
these three muscles were also derived.
Our objective in this study
was to investigate ankle joint moments, dorsiflexor muscle forces and
mechanical energy requirements of increasing swing phase ankle
dorsiflexion, specifically at the high-risk Minimum Foot Clearance (MFC)
event. By experimentally manipulating foot-ground clearance using a
continuous feedback display, the timing and magnitude of ankle
dorsiflexor moments, forces and work were modelled in response to a
controlled increment in ankle dorsiflexion. It was hypothesized that
relative to an unconstrained-walking control condition, greater ankle
dorsiflexion would require higher ankle moments and power with increased
dorsiflexor muscle forces and work.
Our findings on ankle joint
and dorsiflexor muscle kinetics offer critical insights for enhancing
ankle assistive technologies. Rather than increasing ankle moments, our
study suggests that assistive devices should focus on providing higher
mechanical energy for effective dorsiflexion. This is particularly vital
for bio-inspired devices incorporating artificial muscles, where
accommodating the co-activation of Tibialis Anterior and Soleus at
higher dorsiflexion angles is key. These insights aim to guide the
development of more efficient ankle orthoses and exoskeletons.
More at link.
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