Marjorie H. Woollacott

Attention and the control of posture and gait: a review of an emerging area of research

Research on the relationship between attention and the control of posture and gait is a new and expanding area with studies on young adults revealing the role of cognitive factors in the control of balance during standing and walking. The use of dual task paradigms to examine the effect of age related changes in attentional requirements of balance control and age-related reductions in stability when performing a secondary task has shown that these are important contributors to instability in both healthy and balance-impaired older adults. The attentional demands of balance control vary depending on the complexity of the task and the type of secondary task being performed. New clinical assessment methods incorporating dual-task paradigms are helpful in revealing the effect of disease (e.g. Parkinsons disease) on the ability to allocate attention to postural tasks and appear to be sensitive measures in both predicting fall risk and in documenting recovery of stability.

Aging and Posture Control: Changes in Sensory Organization and Muscular Coordination

The following study examined two aspects of balance control in the older adult: 1) the coordination of the timing and the amplitude of muscle responses to postural perturbations, and 2) the ability of the participant to reorganize sensory inputs and subsequently modify postural responses as a consequence of changing environmental conditions. Coordination of muscle activity in postural responses of twelve elderly (sixty-one to seventy-eight years) participants were compared to those of young (nineteen to thirty-eight years) adults using a movable platform and recording the electromyographic activity of muscles of the legs. The following changes were noted in the timing and amplitude of muscle activity within a postural response synergy: 1) increases in the absolute latency of distal muscle responses were observed in all older adults; 2) in five of the twelve older adults temporal reversals of proximal and distal muscle response onset were observed; and 3) there was a breakdown in the correlation of the amplitude of responses within a synergy. The ability of the older adult to balance under conditions of reduced or conflicting sensory information was also impaired. When confronted with functionally inappropriate visual and/or somatosensory inputs, half of the older group lost balance. In most instances, however, the older participants were able to maintain stability during subsequent responses to conflicting stimuli.

The Growth of Stability: Postural Control from a Developmental Perspective

This study compared central nervous system organizational processes underlying balance in children of three age groups: 15-31 months, 4-6 years, and 7-10 years, using a movable platform capable of antero-posterior (A-P) displacements or dorsi-plantar flexing rotations of the ankle joint. A servo system capable of linking platform rotations to A-P sway angle allowed disruption of ankle joint inputs, to test the effects of incongruent sensory inputs on response patterns. Surface electromyography was used to quantify latency and response patterns. Surface electromyography was used to quantify latency and amplitude of the gastrocnemius, hamstrings, tibialis anterior, and quadriceps muscle responses. Cinematography provided biomechanical analysis of the sway motion. Results demonstrated that while directionally specific response synergies are present in children under the age of six, structured organization of the synergies is not yet fully developed since variability in timing and amplitude relationships between proximal and distal muscles is high. Transition from immature to mature response patterns was not linear but stage-like with greatest variability in the 4- to 6- year-old children. Results from balance tests under altered sensory conditions (eyes closed and/or ankle joint inputs altered) suggested that: (a) with development a shift in controlling inputs to posture from visual dependence to more adult-like dependence on a combination of ankle joint and visual inputs occurred in the 4- to 6-year-old, and reached adult form in the 7- to 10-year-old age group. It is proposed that the age 4-6 is a transition period in the development of posture control. At this time the nervous system (a) uses visual-vestibular inputs to fine tune ankle-joint proprioception in preparation for its increased importance in posture control and (b) fine tunes the structural organization of the postural synergies themselves.

Effects of Single-Task Versus Dual-Task Training on Balance Performance in Older Adults: A Double-Blind, Randomized Controlled Trial

OBJECTIVE To compare the effect of 3 different approaches to balance training on dual-task balance performance in older adults with balance impairment. DESIGN A double-blind, randomized controlled trial. SETTING University research laboratory. PARTICIPANTS Older adults (N=23) with balance impairment (mean age, 74.8y). They scored 52 or less on the Berg Balance Scale (BBS) and/or walked with a self-selected gait speed of 1.1m/s or less. INTERVENTIONS Participants were randomly assigned to 1 of 3 interventions: single-task training, dual-task training with fixed-priority instructions, and dual-task training with variable-priority instructions. Participants received 45-minute individualized training sessions, 3 times a week for 4 weeks. MAIN OUTCOME MEASURES Gait speed under single-task and dual-task conditions was obtained at baseline, the second week, the end of training, and the twelfth week after the end of training. Other measures, including the BBS and the Activities-specific Balance Confidence (ABC) Scale, were collected at baseline and after training. RESULTS Participants in all groups improved on the BBS (P<.001; effect size [ES]=.72), and walked significantly faster after training (P=.02; ES=.27). When a cognitive task was added, however, only participants who received dual-task training with fixed-priority instructions and dual-task training with variable-priority instructions exhibited significant improvements in gait speed (P<.001, ES=.57; and P<.001, ES=.46, respectively). In addition, only the dual-task training with variable-priority instructions group demonstrated a dual-task training effect at the second week of training and maintained the training effect at the 12-week follow-up. Only the single-task training group showed a significant increase on the ABC after training (P<.001; ES=.61). CONCLUSIONS Dual-task training is effective in improving gait speed under dual-task conditions in elderly participants with balance impairment. Training balance under single-task conditions may not generalize to balance control during dual-task contexts. Explicit instruction regarding attentional focus is an important factor contributing to the rate of learning and the retention of the dual-task training effect.

Control of reactive balance adjustments in perturbed human walking: roles of proximal and distal postural muscle activity

Abstract Studies on the proactive control of gait have shown that proximal (hip/trunk) muscles are the primary contributors to balance control, while studies on reactive balance control during perturbed gait, examining only activity in distal (leg/thigh) muscles, have shown that these muscles are important in compensating for a gait disturbance. This study tested the hypothesis that proximal muscles are also primary contributors to reactive balance control during perturbed gait. Thirty-three young adults participated in a study in which an anterior slip was simulated at heel strike by the forward displacement of a force plate on which they walked. Surface electromyographic data were recorded from bilateral leg, thigh, hip and trunk muscles. Kinematic data were collected on joint angle changes in response to the perturbation. The results did not support the hypothesis that the proximal muscles contribute significantly to balance control during perturbed gait. The proximal muscles did not demonstrate more consistent activation, earlier onset latency, longer burst duration or larger burst magnitude than distal muscles. Moreover, although proximal postural activity was often present in the first slip trial, it tended to adapt away in later trials. By contrast, the typical postural responses exhibited by young adults consisted of an early (90–140 ms), high-magnitude (4–9 times muscle activity during normal walking) and relatively long duration (70–200 ms) activation of bilateral anterior leg muscles as well as the anterior and posterior thigh muscles. Thus, postural activity from bilateral leg and thigh muscles and the coordination between the two lower extremities were the key to reactive balance control and were sufficient for regaining balance within one gait cycle. The adaptive attenuation of proximal postural activity over repeated trials suggests that the nervous system overcompensates for a novel balance threat in the first slip trial and fine-tunes its responses with experience.

Neuromuscular control of posture in the infant and child: is vision dominant?

This study explored the effects of vision and maturation on the characteristics of neuromuscular responses underlying balance control in both seated and standing children of five age groups (3 1/2-5 months, 8-14 months, 2-3 years, 4-6 years, and 7-10 years). A platform was used to unexpectedly disturb the childs balance in the anterior or posterior direction. Responses of the leg, trunk, and neck muscles were recorded using surface electromyograms. Vision was not required for the activation of these responses in any of the age groups tested. However, comparison of the muscle response latencies of standing children to posterior platform translation in the two visual conditions showed a significant reduction in latency for neck flexors in the 2- to 3-year-olds with vision removed and ann increase in the total number of monosynaptic reflexes. No reduction in latency was found in the older age groups. The hypothesis of a shift from an early long latency visual dominance to a shorter latency proprioceptive one during childhood is discussed. Postural control showed a cephalo-caudal developmental gradient with postural responses appearing first in the neck, then trunk, and finally, legs, as children developed from 3 to 14 months of age. A wide variety of response patterns was seen in the 3- to 5-month-olds, indicating that postural responses are not functional prior to experience with stabilizing the center of mass.

The influence of a concurrent cognitive task on the compensatory stepping response to a perturbation in balance-impaired and healthy elders

This study investigated the influence of a concurrent cognitive task on the compensatory stepping response in balance-impaired elders and the attentional demand of the stepping response. Kinetic, kinematic and neuromuscular measures of a forward recovery step were investigated in 15 young adults, 15 healthy elders and 13 balance-impaired elders in a single task (postural recovery only) and dual task (postural recovery and vocal reaction time task) situation. Results revealed that reaction times were longer in all subjects when performed concurrently with a compensatory step, they were longer for a step than an in-place response and longer for balance-impaired older adults compared with young adults. An interesting finding was that the latter group difference may be related to prioritization between the two tasks rather than attentional demand, as the older adults completed the step before the reaction time, whereas the young adults could perform both concurrently. Few differences in step characteristics were found between tasks, with the most notable being a delayed latency and reduced magnitude of the early automatic postural response in healthy and balance-impaired elders with a concurrent task.

Postural Dysfunction During Standing and Walking in Children With Cerebral Palsy: What are the Underlying Problems and What New Therapies Might Improve Balance?

In this review we explore studies related to constraints on balance and walking in children with cerebral palsy (CP) and the efficacy of training reactive balance (recovering from a slip induced by a platform displacement) in children with both spastic hemiplegic and diplegic CP. Children with CP show (a) crouched posture, contributing to decreased ability to recover balance (longer time/increased sway); (b) delayed responses in ankle muscles; (c) inappropriate muscle response sequencing; (d) increased coactivation of agonists/antagonists. Constraints on gait include (a) crouched gait; (b) increased co-activation of agonists/antagonists; (c) decreased muscle activation; (d) spasticity. The efficiency of balance recovery can be improved in children with CP, indicated by both a reduction in the total center of pressure path used during balance recovery and in the time to restabilize balance after training. Changes in muscle response characteristics contributing to improved recovery include reductions in time of contraction onset, improved muscle response organization, and reduced co-contraction of agonists/antagonists. Clinical implications include the suggestion that improvement in the ability to recover balance is possible in school age children with CP.

Timing and force control deficits in clumsy children

This study investigated the link between cognitive processes and neural structures involved in motor control. Children identified as clumsy through clinical assessment procedures were tested on tasks involving movement timing, perceptual timing, and force control. The clumsy children were divided into two groups: those with soft neurological signs associated with cerebellar dysfunction and those with soft neurological signs associated with dysfunction of the basal ganglia. A control group of age-matched children who did not exhibit evidence of clumsiness or soft neurological signs was also tested. The results showed a double dissociation between the two groups of clumsy children and the tests of timing and force. Clumsy children with cerebellar signs were more variable when attempting to tap a series of equal intervals. They were also more variable on the time perception task, indicating a deficit in motor and perceptual timing. The clumsy children with basal ganglia signs were unimpaired on the timing tasks. However, they were more variable in controlling the amplitude of isometric force pulses. These results support the hypothesis that the control of time and force are separate components of coordination and that these computations are dependent on different neural systems.

Postural responses to changing task conditions

SummaryThe experimental goal was to investigate discrepancies in the literature concerning postural adaptation and to determine if the prior presentation of horizontal perturbations affected the amplitude of responses to rotational perturbations. Surface EMG recordings from lower leg muscles (gastrocnemius (GAS) and tibialis anterior (TA)) were recorded in twelve subjects, and the amplitudes of the responses were statistically analyzed. We did not find differences between the responses to rotational perturbations which preceded or followed horizontal perturbations. This finding did not support the hypothesis that differences in the order of presentation of the different types of perturbations accounted for the discrepancies in the literature. Furthermore, our design did not show the progressive elimination of the GAS response within three to five sequential trials. Instead, we found a slow but significant response amplitude reduction over ten trials without yielding a permanent disappearance of the response. When analyzing the GAS responses to the rotational perturbations only, we found two components that contributed to the response reduction: 1) an initial reduction between trials one and subsequent trials, which could be due to habituation of a startle-like response; and 2) a second reduction which was more gradual. Our results also showed an immediate change in the response amplitude on the first trial, when the type of perturbation was changed. This is inconsistent with the view that ankle musculature stretch and joint movement are the primary inputs driving the postural responses. Since small ankle dorsiflexing rotations produced by the platform translations caused large GAS responses while large ankle dorsiflexing rotations produced by direct platform rotations caused small GAS responses, this suggests that multiple sensory inputs contribute to the responses. We propose that an initial compensation to a new perturbation type occurs within the first trial by the integration of these divergent sensory inputs.