John L. Zettel

Determinants and consequences for standing balance of spontaneous weight-bearing on the paretic side among individuals with chronic stroke.

Hemiparetic stroke patients commonly bear more weight on the non-paretic side which seems intuitively linked to unilateral control deficits. However, there is evidence that some post-stroke favour weighting the paretic side, which may be problematic given altered capacity of the paretic limb to contribute to the control of upright posture. This study explores the prevalence and clinical determinants of stance asymmetry, and the relationship between stance asymmetry and postural control among chronic stroke patients. Subjects (n=147; >6 months post-stroke) stood on two force plates in eyes-open and eyes-closed conditions; 59 were symmetric, 18 had paretic asymmetry (PA), and 70 had non-paretic asymmetry (NPA). Root mean square (RMS) of antero-posterior and medio-lateral centre-of-pressure under each limb and both limbs combined were compared. RMS of total medio-lateral centre-of-pressure was greater for both asymmetric groups compared with the symmetric group. PA subjects relied less on the loaded limb for control than NPA subjects and relied more on visual information for postural control than those who were symmetric. There were no differences in the characteristics of individuals between the PA and NPA groups. The loading of the paretic limb was not related to impaired postural control during stationary standing which was attributable, in part, to individuals relying on control from the non-paretic limb, in spite of lower vertical load, and a greater dependence on visual contributions. There was no evidence that greater loading on the paretic limb was related to persisting dyscontrol but may rather reflect a learned strategy.

Reducing fall risk by improving balance control: Development, evaluation and knowledge-translation of new approaches

PROBLEM Falling is a leading cause of serious injury, loss of independence, and nursing-home admission in older adults. Impaired balance control is a major contributing factor. METHODS Results from our balance-control studies have been applied in the development of new and improved interventions and assessment tools. Initiatives to facilitate knowledge-translation of this work include setting up a new network of balance clinics, a research-user network and a research-user advisory board. RESULTS Our findings support the efficacy of the developed balance-training methods, balance-enhancing footwear, neuro-prosthesis, walker design, handrail-cueing system, and handrail-design recommendations in improving specific aspects of balance control. IMPACT ON KNOWLEDGE USERS: A new balance-assessment tool has been implemented in the first new balance clinic, a new balance-enhancing insole is available through pharmacies and other commercial outlets, and handrail design recommendations have been incorporated into 10 Canadian and American building codes. Work in progress is expected to have further impact.

Stepping Responses in Young and Older Adults Following a Perturbation to the Support Surface During Gait

ABSTRACT The objective of this work was to investigate the influence perturbation direction has on postural responses during overground gait, and whether these responses are age related. Differences in stepping patterns following perturbations of the support surface were examined in the frontal and sagittal planes during forward walking. Eleven young and 10 older adults completed Mini BESTest, hip strength tests, and 45 perturbed walking trials, triggered on heel contact. Lateral perturbations were more challenging to postural stability for both groups. Step length measures showed young adults recovered in the step proceeding the perturbation, while older adults needed additional steps to regain balance. Young adults arrested center of mass movement by producing larger step widths than older adults following the support surface perturbation.

Older adults exhibit altered motor coordination during an upper limb object transport task requiring a lateral change in support

Investigating an ecologically relevant upper limb task, such as manually transporting an object with a concurrent lateral change in support (sidestepping alongside a kitchen counter), may provide greater insight into potential deficits in postural stability, variability and motor coordination in older adults. Nine healthy young and eleven older, community dwelling adults executed an upper limb object transport task requiring a lateral change in support in two directions at two self-selected speeds, self-paced and fast-paced. Dynamic postural stability and movement variability was quantified via whole-body center of mass motion. The onset of lead lower limb movement in relation to object movement onset was quantified as a measure of motor coordination. Older adults demonstrated similar levels of stability and variability as their younger counterparts, but at slower peak movement velocity and increased task duration. Furthermore, older adults demonstrated asymmetrical motor coordination between left and right task directions, while younger adults remained consistent regardless of task direction. Thus, older adults significantly modulated movement speed and motor coordination to maintain similar levels of stability and variability compared to their younger counterparts.

Experimentally induced central sensitization in the cervical spine evokes postural stiffening strategies in healthy young adults.

Dysequilibrium of cervicogenic origin can result from pain and injury to cervical paraspinal tissues post-whiplash; however, the specific physiological mechanisms still remain unclear. Central sensitization is a neuradaptive process which has been clinically associated with conditions of chronic pain and hypersensitivity. Strong links have been demonstrated between pain hypersensitivity and postural deficits post-whiplash; however, the precise mechanisms are still poorly understood. The purpose of this study was to explore the mechanisms of cervicogenic disequilibrium by investigating the effect of experimentally induced central sensitization in the cervical spine on postural stability in young healthy adults. Sixteen healthy young adults (7 males (22.6±1.13 years) and 9 females (22±2.69 years)) performed 30-s full-tandem stance trials on an AMTI force plate under normal and centrally sensitized conditions. The primary outcome variables included the standard deviation of the center of pressure (COP) position in medio-lateral (M-L) and antero-posterior (A-P) directions; sway range of the COP in M-L and A-P directions and the mean power frequency (MPF) of the COP and horizontal ground shear forces. Variability and sway range of the COP decreased with experimental induction of central sensitization, accompanied by an increase in MPF of COP displacement in both M-L and A-P directions, suggesting an increase in postural stiffening post-sensitization versus non-sensitized controls. Future studies need to further explore this relationship in clinical (whiplash, chronic pain) populations.

Effects of temporal and spatial cueing on anticipatory postural control in a rapid interceptive task

Balance disruptions induced by voluntary focal arm actions are accommodated via anticipatory postural adjustments, but how this coordinated control is organized by the central nervous system remains unclear: either as combined or separate streams of postural-focal motor commands. For example, a focal arm task that dictates extremely tight temporal constraints may induce a focal response in absence of an anticipatory postural adjustment, providing evidence for separate focal-postural control streams. This study sought to probe the organization of focal-postural control via an interceptive task with very little available response time, and to determine whether focal-postural coordination depends on temporal and/or spatial foreknowledge of the task. Ten healthy young adults (5 males and 5 females; 20-29 years) reacted to catch a ball when standing under four conditions of temporal and spatial foreknowledge. Response onset was characterized by muscle activity from both postural and focal arm muscles. The catching task resulted in rapid muscle responses, but there was no difference between the fastest focal and postural muscle onsets. As expected, temporal cuing resulted in faster focal and postural onsets compared to spatial and control cuing trials. The accompaniment and time-locking of focal and postural muscle onsets, suggests that postural-focal coupling remains intact even under external time constraints and provides evidence for a single combined command stream of postural and focal control under such circumstances.