Increased task complexity and unanticipated state pose challenges on dynamic balance regulation
We investigated the time-varying frontal-plane dynamic balance based on whole-body angular momentum (
H)
during transitions of altered anticipation, cut style and complexity in
healthy young adults. Our first hypothesis was that the peak values of
frontal-plane
H would be larger during unanticipated transitions
of increased complexity, such as unanticipated cut/stairs. This
hypothesis was supported. During Correction Phase when participants
transitioned from level ground to stair-walking (Fig.
1a),
positive peak angular momentum P2 was larger in cut/stairs transitions
relative to cuts, regardless of anticipation state (Fig.
3).
Furthermore, within cut/stairs task, P2 was larger in unanticipated
state compared to anticipated state. While it has been shown that
increased range of frontal-plane
H was maintained in steady-state stair ascent compared to straight level walk for healthy subjects [
36], our results further suggest that increased
H was required for unanticipated cut/stairs transitions. The increased peak value of frontal-plane
H
may indicate higher risk of mediolateral fall during unanticipated
complex transitions, which is affected by interrupted neuromuscular task
planning in unanticipated states [
15] and enhanced joint moment requirement for stair walking [
19,
20].
To further understand the regulation of
H, we investigated the average rate of change of
H
that can be considered as the average net external moment about the
body’s COM. Our second hypothesis was that the average rate of change of
frontal-plane
H would also be largest during unanticipated
transitions of increased complexity. This hypothesis was partially
supported. The magnitude of the average rate of change of
H
between N1-P2, during the Correction Phase, was larger in
cut/stair-ascent transitions compared to cuts, while there was no effect
of anticipation state (Fig.
4).
Decreased mediolateral GRFs and vertical moment arms that are main
contributors to the mediolateral net external moment likely explain the
enhanced slope and range of frontal-plane
H during early stance of stair ascent relative to level straight walking [
36].
Similarly, the increased magnitude of N1-P2 slope indicating larger
mediolateral net external moment may explain the increased positive peak
P2 during stair-ascent transitions. Furthermore, the gluteus medius has
been shown to be the major contributor to maintain frontal-plane
angular momentum by rotating the body toward the ipsilateral leg during
stance phase [
26].
It has been also reported that gluteus medius performed similarly in
maintaining mediolateral balance in stair and level walking [
37].
Positive angular momentum relates to rotation toward the leading leg,
consistent with the functional direction of gluteus medius in
maintaining dynamic balance. Thus, the increased frontal-plane
H
may result from reduced gluteus medius activity during a level-stair
transition. Previous study also suggested that increased frontal-plane
angular momentum during steady-state stair walking may be a necessary
strategy to raise body COM while avoiding a trip [
36].
Therefore, complex locomotor transitions from level to stair-ascent
walking may require a different strategy relative to level transitions,
and excessive angular momentum swinging human body toward leading leg
may be needed for dynamic balance. These results may be useful for
assessing risks of balance-challenged populations during complex
locomotor transitions.
Anticipatory changes in dynamic balance are influenced by cut style
Individuals
make cognitive adjustments for their control of dynamic balance before
they approach the transition point, and their strategies depend
exclusively on cut styles according to our results. During Preparation
Phase participants in anticipated states increased the positive peak of
H
(P1) for crossover styles, but reduced the positive peak for sidestep
styles compared to straight walking and unanticipated transitions (Fig.
3). Although we did not analyze
H before P1, style-specific modifications on anticipatory
H were also performed at the first trailing leg heel strike (Time 0) with the same strategies used for P1 (Fig.
2). These adjustments are understandable because the increased
H (rotation toward the leading leg) for crossover styles and reduced
H
(rotation away from the leading leg) for sidestep styles were the same
as the cut styles. These findings may be a generalized strategy in
prepared human walking cuts. A recent study found that during
anticipated 90-degree walk turns, angular momentum was not affected by
the direction of change as long as individuals use sidestep cut style [
38]. Similar conclusions were also made for healthy individuals performing crossover 90-degree cuts to right and left direction [
39].
Although these investigations did not compare different cut styles,
their results support our findings that anticipatory change of dynamic
balance is affected by cut style, but not the direction. Furthermore,
these adjustments of whole-body angular momentum may partially result
from preparatory control of trunk angular momentum that had the same
modification strategy as
H [
40].
Previous study on sidestep cuts also reported that trunk swing is a
strategy assisting in moving body COM toward new walking direction [
41].
They found that the trunk displaced opposite from the cutting direction
before turning, and assisted direction change in an inverted pendulum
manner. However, this different trunk strategy was thought to be used in
a late cue (unanticipated) transition, and contribute less in an early
cue (anticipated) condition. Therefore, individuals in anticipated
walking cuts initiated their control of mediolateral dynamic balance in
advance to prepare for the direction change, and the swing of upper body
segment may contribute to this strategy.
These anticipatory adjustments on
H
may also have implications on the effects of different transition
factors on walking dynamic balance regulation. Our results showed that
individuals modified their dynamic balance at least one half step before
anticipated transitions. This is expected because studies have shown
gaze fixation on the future foot landing area before at least two steps [
42,
43], modifications on gait parameters for two strides before transition to stairs [
24], and high rate of successful direction change when individuals were cued two steps ahead [
44]. Nonetheless, subjects adjusted
H
based on cut styles, not task complexity that requires more
biomechanical changes. Although it could be argued that individuals
prioritize some more closely approaching challenges, they failed to
adjust
H for task complexity at least half step before
transitions. This may suggest a priority of cut style over task
complexity (cut/stairs) in the “internal model” of human nervous system
that regulates locomotion and dynamic balance. This priority may be due
to the fear of knee injury during cuts that are associated with
increased breaking forces (anterior/posterior GRF) and quadriceps
activation [
12].
Therefore, healthy adults pre-rotated their body to prepare for
incoming cut transitions, which may be an effective strategy to maintain
dynamic balance and avoid injuries during these destabilizing tasks.
Our findings on the anticipatory adjustments of healthy dynamic balance
regulation may provide a baseline to evaluate and improve related
routines in rehabilitation training.
Reactive control of dynamic balance is influenced by cut style
Individuals
make reactive changes to dynamic balance in response to unexpected
auditory cue of transition, and the strategies are different for each
cut style. During sidestep-style transitions, they increased the
magnitudes of P1-N1 and N1 for unanticipated states compared to
anticipated states, but during crossover styles the magnitudes remain
for both anticipation states (Fig.
3, Fig.
4).
Furthermore, within unanticipated states the magnitudes of P1-N1 and N1
were greater in sidestep style compared to crossover style, which may
be due to the unique mechanisms that individuals used for each cut
style. There was a delay of the occurrence of the negative peak N1
during crossover-style transitions compared to straight walking and
sidestep-style transitions (Fig.
2).
It was at the trailing leg heel-strike for sidestep style transitions
and straight walking, but at the subsequent leading leg toe-off for
crossover styles. Moreover,
H in crossover-style transitions was
maintained as tightly as straight walking at the trailing leg
heel-strike. With limited response time in unanticipated conditions
(auditory cue at the initiation of leg swing), participants were not
able to change leg swing trajectories rapidly to cross the stance leg,
and an unanticipated gait termination was performed in crossover
transitions. Although angular momentum was tightly regulated,
unanticipated crossover transition with gait termination may still be
challenging for balance-impaired populations [
23].
Furthermore, as direction change continued participants in
unanticipated crossover transitions used the leading leg to turn to the
new direction in a sidestep style. However, this is only an initiation
of direction change because
H was still in the direction away
from the leading leg, i.e., opposite to the new direction of travel.
This mechanism was similar to the previously reported control strategy
of body COM in the initiation of walking direction change where the
trunk was displaced to the opposite of new direction [
41]. However, individuals in unanticipated sidestep transitions may easily swing the trailing leg and trunk [
40]
away from the leading leg direction with increased negative momentum.
Thus, in response to unanticipated walking direction change crossover
style may require rapid gait termination and inverted-pendulum-style
trunk motion to initialize its direction change, while sidestep may take
advantage of the momentum during leg swing to be a more effective
maneuver for quicker changing of locomotion direction [
12].
Nonetheless, the potential balance challenge in the gait termination of
unanticipated crossover transitions and increased momentum in
unanticipated sidestep transitions may still need to be carefully
considered in rehabilitation training.
Limitations and future considerations
One limitation of our study is that we did not include arms in the model used to calculate
H. Although arm swing may contribute to transverse-plane
H during treadmill walking [
45] and anteroposterior fall recovery [
46], evidence have shown that the magnitude of contributions to frontal-plane
H from the arms are very small relative to the trunk and legs during normal walking [
8] and 90-degree turn [
38].
Our statistical results may also be limited by the modest number of
participants. To mitigate this, we collected five trials of each
condition for each subject and analyzed the results objectively. While
we used subject-averaged data for analysis, the effect sizes turned out
to be large (η
p2 > 0.14) for all statistically
significant results. Nonetheless, the results should be interpreted as
initial findings given the modest number of participants. Another
limitation may be that we evaluated
H in the lab (inertial) frame compared to recent research on dynamic balance in the body moving reference frame [
47].
H
in inertial frame can be directly related to ground reaction force
measurements. We also believe that during walking turns, dynamic balance
in the direction of inertial frame is more endangered, evidenced by
significantly larger peak values compared to
H in anatomical medial/lateral direction (not published). Finally,
H
in this study was not normalized by walking velocity. Although previous
studies showed different angular momenta with walking speed [
9,
32], we wanted to incorporate the velocity information in the single metric
H,
which may be part of the strategy that individuals used to maintain
dynamic balance during walking transition, as increased speed of
transition can adversely influence walking stability of both young and
old individuals [
48].
Future work is also needed to understand segmental contributions to
angular momentum in each transition task so that specific strategy of
dynamic balance control in locomotor transitions can be apprehended and
targeted for rehabilitation training. Future experiment and analysis on
patients walking during locomotor transitions may also be useful to
fully understand dynamic balance regulation mechanism and improve
rehabilitation training of these populations.
