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In addition to the effect of crutch support for balance on target stepping, the direction of the required foot placement adaptation is likely to affect balance in different ways. Limiting the BoS (narrowing or shortening) is likely to be more threatening for balance than adaptations that enlarge the BoS (widening and lengthening). Additionally, changes to foot placement in the medio-lateral direction could require more force (Hurt & Grabiner, 2015). Therefore, narrowing and shortening steps were expected to be more challenging with higher errors expected in the unsupported condition and potentially larger reductions in error seen when support is offered than for widening and lengthening steps.

6.1.5.3 Does limiting the BoS affect foot placement accuracy?

BoS limiting steps (narrowing and shortening) did not have increased magnitude of error in comparison to BoS enlarging steps (widening and lengthening). Lengthening (AP ABS 5.2cm, Mean -5.1cm), narrowing (ML ABS 2.3cm, Mean -0.2cm) and widening (ML ABS 2.5cm, Mean -0.6cm) had increased error magnitude (absolute error) and different directions of error (mean error) compared to preferred stepping (AP ABS 4.2cm, Mean -2.4, ML ABS 1.4cm, Mean -0.4cm); while step shortening had similar error magnitudes and direction of error as preferred steps. Surprisingly, despite that narrowing steps is known to be a direct challenge to balance when making a single step from a stationary standing position,

narrowing and widening steps were seen to have similar error magnitudes, which indicate both step adjustments are equally challenging. The higher peak hip abductor moments during widening steps (Hurt & Grabiner, 2015) shows to be as limiting as the reduction of the base of support. The fact that narrowing and shortening steps does not actually increase error more than widening and lengthening steps indicates the balance challenge thought to be different according to different directions of step is not the only factor affecting accurate foot

placement control.

The result that widening has similar errors to narrowing is in contradiction with the results of Nonnekes et al. (2010) that showed medial steps adjustments from standing have higher stepping errors than lateral step adjustments and that of Moraes, Allard, and Patla (2007) who showed shortening and narrowing step adjustments to avoid a sudden obstacle were less successful than widening and lengthening obstacle avoidance steps. So, previous studies demonstrated step narrowing is more challenging that widening both from standing and during walking. The differences between these findings and that of the current study could possibly be explained by differences between studies in the amount of time participants were afforded to adjust their foot placement. In both the study by Nonnekes et al. (2010) and Moraes et al. (2007) the time to adjust foot placement was limited; the target and obstacles were only shown during swing phase of the aiming leg. This could, again, indicate that limited time available to respond (adjustments made during swing phase of the aiming leg) to foot placement adjustments threatens more than foot placement adjustments that can be planned for (targets visible at least two steps ahead), which will be investigated in chapter 7.

6.1.5.4 Are shortening steps more difficult step adjustments than lengthening steps?

Surprisingly older healthy adults and stroke survivors had greater magnitude of error (absolute error) in lengthening than shortening steps. The mean error provides an indication of the direction of error and therefore could be used to explain the difference in absolute magnitude of error between young healthy adults and older healthy adults and stroke

survivors. Mean error indicates young healthy adults overshoot shortening and preferred steps while they undershoot lengthening steps; this is in agreement with the findings of Hoogkamer et al. (2015). The authors give three possible reasons for the difference in success for

shortening and lengthening steps; 1) lengthening steps allow more time to adapt, 2)

lengthening is biomechanically more similar to steady state walking than shortening steps and 3) shortening steps reduces margin of stability by reducing the BoS while the centre of mass has forward momentum. However, older healthy adults, and stroke survivors in this study

were seen to undershoot all targets; including preferred steps. Undershooting is seen as a negative offset (or mean error) and may account for different magnitudes of error with different step adjustments; preferred and shortened steps are undershot to a small extent (i.e. small magnitudes of absolute error), and lengthening steps who would be expected to be undershot is no attention was payed to the targets (preferred footfall location) are now even undershot more, represented by larger error magnitudes of error. This tendency to fall behind the targets could be due to difficulty synchronising to the targets, similar to the lagging of foot falls to auditory beats seen in other studies (Roerdink et al., 2009). Young healthy adults anticipate foot falls with the beats, so timing of foot falls actually occur just before the beat is perceived (Bank et al., 2011; McIntosh et al., 1997; Roerdink et al., 2011; Roerdink et al., 2007; Roerdink et al., 2009). Stroke survivors are known to lag the beat, with timing of foot falls just after the perceived beat (Roerdink et al., 2009). The fact that this phenomenon is present in both stepping to beats and visual target stepping indicates that stroke survivors have difficulty anticipating and synchronising their steps in time as well as space. This might mean that the anticipation and planning of a foot placement is reduced due to stroke and they need more time to synchronise foot placement and have greater difficulty adjusting with less time to respond.

6.1.5.5 Does balance support improve accuracy of step adaptations in some directions more than other directions?

Previously we discussed shortening and narrowing steps were not associated with increased magnitudes of error and support for balance did not decrease error measures more in

shortening and narrowing steps than for lengthening and widening steps. Stroke survivors however, do show a decline in magnitude of error of 1.7cm in narrowing steps and only 0.2cm in widening steps between the supported and unsupported condition, indicating support for balance does help foot placement accuracy when step narrowing is needed. Healthy participants do not show a difference in error with balance support in a specific direction (difference between supported and unsupported: YH AP 0.0cm, ML0.3cm and OH AP 0.8cm, ML 0.3cm), which indicates they are not limited in adjusting their foot placement by balance. It does appear as stroke survivors do benefit from support when narrowing there steps more than stepping laterally. The reduction of foot placement error by 1.7cm is not the difference between a successful and an unsuccessful foot placement on a two-dimensional target. In daily living however, a situation that needs foot placement adaptation is rarely just two dimensional; avoiding a loose tile, reaching a curb etc. It could be imagined in a three-

dimensional situation the difference between an error magnitude of 7.0cm and 4.7cm in anterio-posterior direction and 3.6cm and 3.0cm in medio-lateral direction in stroke survivors could be the difference between keeping and losing balance. These differences in overhang (AP 2.3cm and ML 0.6cm) are effectively the reduction in BoS size within which the CoP can travel before exceeding margins of stability. While, error measures based on CoP and CoF location cannot be used interchangeably to measure foot placement accuracy (see chapter 3.3), the position of CoP under the foot might be more reflective of balance, especially when part of the foot overhangs support (i.e. only part of the foot is placed on a curb). As seen in Chapter 3.3, when adapting foot placement (CoF), the CoP lands more anteriorly when shortening and more posteriorly when lengthening, this indicates naturally the adaptation is kept biomechanically as similar to steady state walking as possible.

However, this means that CoP would be expected to approach the margins of the BoS (which are reduced by the extent of CoF error/overhang), thereby challenging balance. Therefore, although smaller than seen in other studies, the differences between supported and

unsupported target stepping are likely clinically meaningful differences that could reflect the difference between a successful and unsuccessful foot placement adaptation.

6.4.4 Does balance support influence accuracy of foot placement

differently for the paretic and non-paretic legs?

Balance control may be expected to affect the aim of the paretic leg differently than the non- paretic leg; an increased separation between centre of pressure and centre of mass (a

reflection of balance control) in paretic limb stance, compared to the non-paretic has been previously observed (Said et al., 2008). However, foot placement error on the paretic (means: AP 5.6cm ML 3.8cm) and the non-paretic (means: AP 6.1cm ML 2.9cm) legs in stroke survivors were similar across different directions of stepping adjustments and reduced by similar amounts when balance was supported (mean reduction by support, paretic AP 2.4cm, ML 0.9cm and non-paretic AP 2.3cm, ML 0.3cm). Which indicates the paretic and non- paretic leg are affected similarly by balance control.

It was expected that balance support would increase accuracy when aiming with the non-paretic leg more than with the paretic, as maintaining paretic stance has shown to be the most challenging for balance (Said et al., 2008). However, the results seen here (showing no differences according to side of paresis) are in line with the results by Nonnekes et al. (2010) who also show a bilateral increase of stepping accuracy with balance support in stroke

survivors. This indicates control of foot placement relies on bilateral organisation to maintain balance and, adjustments have to be made in both the stance and swing legs when aiming with the lower limb (Reynolds & Day, 2005a)..