Multi-joint coordination and adaptation of ankle and hip strategy in healthy young and healthy elderly

Both the ankle and the hip joint play an important role in maintaining standing
balance. To compensate for external disturbances (e.g. forces acting on the
body) and to prevent falling, multi-joint coordination must be provided to
generate the appropriate corrective joint torques around the ankles and the
hips to maintain standing balance. In order for the neuromuscular controller to
adjust strategy in various conditions, variability and adaptability of
multi-joint coordination are essential features of standing balance control.
Elderly have altered strategies to maintain standing balance compared to young
adults, which can cause impaired standing balance and possibly falls. We have
developed a new device to study multi-joint coordination by the application of
two continuous and random external disturbances and using closed-loop system
identification techniques (CLSIT). In addition, the device makes is possible to
manipulate ankle and hip strategy, by altering the dynamics of the device (how
the environment is perceived by the subject) and applying extra force fields at
the hip or shoulder level. Adaptation in clinical practice can be tested by
letting subjects stand on a compliant surface, which manipulates the ankle
strategy. These compliant surfaces are implemented using foam-mats, from which
the properties are often unknown and the stiffness is uncontrollable. Our lab
offers the possibility to manipulate the stiffness of the support surface in a
controllable way, using a second device. Using support surface rotations around
the ankle and CLSIT, the underlying mechanisms that cause adaptation can be
identified.

The aim of the study is to investigate the differences in multi-joint
coordination between healthy young and healthy elderly. Secondly, we want to
study adaptation of ankle and hip strategy with the application of whole-body
force fields and investigate how elderly are able to adjust their multi-joint
coordination in comparison to young adults. To relate adaptation more to a
clinical measure of standing on a foam mat, adaptation to compliant surfaces is
studied the young adults.

This study is designed as a non-invasive, cross-sectional intervention study to
assess multi-joint coordination and adaptation in ankle and hip strategy. There
is no randomization in groups, as subjects either belong to healthy young or
healthy elderly. First, The both the young and elderly subjects participate in
one experimental session, in which the test conditions are randomized. The
healthy young subjects are asked to participate in an additional session, to
study the effect of compliant surfaces on balance control. In this experimental
session the test conditions are randomized again.

To study multi-joint coordination, balance disturbances are externally applied
by pushing and pulling at the hips and shoulders or by rotations of the support
surface around the ankles. The disturbances contain multiple frequencies
ranging from 0.05-5Hz, making the disturbance unpredictable. The disturbances
are submaximal, challenging the balance control system, but not intended to
make subjects step or fall. In addition, to study adaptation of the ankle and
hip strategy, external whole-body force fields are applied, by altering the
dynamics of the device and thereby manipulating ankle and hip strategy. To
study the adaptation of the ankle strategy when standing on compliant surfaces,
the stiffness of the support surface is manipulated by altering the dynamics of
the device.

Healthy young and healthy elderly Pparticipants are asked to visit the
VR-laboratory at the University of Twente once. During a two-hour experiment
(including explanation, preparation and measurements), biomechanical responses
will be measured as a result from small push and pull disturbances at the hip
and shoulder level and externally applied force-fields. The healthy young are
asked to participate in an additional experiment, for which they have to visit
the VR-laboratory once more. During this additional two-hour experiment,
biomechanical responses will be measured as a result of small perturbations at
the ankle during stance on firm or compliant surfaces.

Participants can rest between trials according to their own needs. The burden
and the risk for the participants are very low. In this study there is a risk
for the participants to lose their balance. Therefore, participants are secured
by a safety-vest at all times to prevent falls. Participants will have no
direct benefit from taking part in this study. However, they contribute to gain
a better understanding of the (patho)physiological mechanisms in balance
control and impaired balance.