Letter to the Editor: On "Advantages and disadvantages of stiffness instructions when studying postural control" by C.T. Bonnet: You just can't win: Advantages and disadvantages of the postural stability requirement.

In the examination of postural control, instructions to stand as still as possible are common and promote a relatively unnatural sway pattern. The validity of the stability requirement is discussed in the present commentary in response to the discussion initiated by Cedrick T. Bonnet. The advantages of using the stability requirement include: evaluating unbiased postural control, reducing variability in postural sway, manipulating focus of attention, examining the ability to maintain an upright stance, and ecological validity of testing. The disadvantages include: constraining natural postural sway, increasing the complexity of the control condition, promoting an internal focus of attention, and reducing the ability to detect exploratory behaviour. After evaluating the aforementioned advantages and disadvantages, the present commentary suggests that researchers should strive to provide specific instructions to maintain feet, arm and eye position without specifically requiring participants to reduce their postural sway.

Functional balance and dual-task reaction times in older adults are improved by virtual reality and biofeedback training.

Virtual reality (VR) training has been used successfully to rehabilitate functional balance and mobility in both traumatic brain injury (TBI) survivors and elderly subjects. Similarly, computer-based biofeedback (BF) training has resulted in decreased sway during quiet stance and decreased reaction times during a dual-task reaction time paradigm in elderly subjects. The objective of this study was to determine the effect of VR and BF training on balance and reaction time in older adults. Two groups of twelve healthy older adults completed 10-week training programs consisting of two 30-min sessions per week. VR training required that participants lean sideways to juggle a virtual ball. Participants in the BF group viewed a red dot representing their center of gravity on a screen and were required to move the dot to the four corners of the monitor. Measures of functional balance and mobility (Community Balance and Mobility Scale [CB&M]), sway during quiet stance, and reaction time during a dual task paradigm were recorded before training, as well as 1 week and 1 month after the end of the program. Both groups showed significant improvements on the CB&M, as well as decreased reaction times with training. Postural sway during quiet stance did not change significantly.

Outcomes of intervention programs using flatscreen virtual reality.

Virtual reality (VR) has the potential to offer experiences which are engaging and rewarding. In VR, the focus is shifted from the person's efforts in producing a movement or completing a task to that of interaction with the virtual environment. We have found that participants place value and meaning on and enjoy the activities programmed. Virtual reality interventions have been shown to improve cognitive function and concentration through an individual's interaction with a pleasant activity. Importantly, the enjoyment experienced while working with VR may increase the level of participation. In addition to generating realistic situations for testing, intervention and collection of data, the provision of immediate and positive feedback through VR has been shown to increase self esteem and empowerment. We will report outcomes from several intervention and feasibility trials using a flat screen virtual reality system with survivors of traumatic brain injury, community living older adults and children with spastic cerebral palsy. Gross motor movements were elicited through various game-like VR applications without the need for head-mounted displays or other peripherals. The impact of VR exercise participation ranged from improvements in clinical measures of functional balance and mobility, time on task, as well as participant and care provider perceptions of enjoyment, independence and confidence. Although still preliminary, our data suggest that simple applications of virtual reality have significant impacts on physical and psychosocial variables. Possibilities for and benefits of home and community-based access to virtual reality based programs will be explored.

Predicting falls within the elderly community: comparison of postural sway, reaction time, the Berg balance scale and the Activities-specific Balance Confidence (ABC) scale for comparing fallers and non-fallers.

Simple reaction time, the Berg balance scale, the Activities-specific Balance Confidence (ABC) scale and postural sway were studied in order to determine cut-off scores as well as develop a model used in the prevention of fallers within the elderly community. One hundred and twenty-five subjects, 45 fallers and 80 non-fallers were evaluated throughout the study and results indicated that non-fallers have significantly faster reaction times, have higher scores on the Berg balance scale and the ABC scale as well as sway at slower frequencies when compared to fallers. Furthermore, all risk factors were subsequently entered into a logistic regression analysis and results showed that reaction time, the total Berg score and the total ABC score contributed significantly to the prediction of falls with 89% sensitivity and 96% specificity. A second logistic regression was carried out with the same previous variables as well as all questions of the Berg and ABC scales. Results from the logistic analysis revealed that three variables were associated with fall status with 91% sensitivity and 97% specificity. Results from the following study would seem rather valuable as an assessment tool for health care professionals in the identification and monitoring of potential fallers within nursing homes and throughout the community.

Comparison of the reaction time, the Berg Scale and the ABC in non-fallers and fallers.

Postural sway, reaction time, the Berg Scale and the Activities-specific Balance Confidence Scale (ABC) were administered to 80 elderly subjects (40 fallers and 40 healthy non-fallers) in order to determine reliable predictor for those at risk of falling. Non-fallers showed significantly faster reaction times when compared with fallers experimental group. Postural sway results show a higher oscillation displacement in the Y axis (lateral) than the X axis (anterior-posterior) and that fallers oscillate at a higher frequency than non-fallers. All variables were submitted to a multiple regression analysis. Only three of the four variables were found to be significant predictors of fall status. The variables in order of their inclusion are reaction time, Total Berg Balance Score and mean ABC. It was further determined that reaction time was indeed the best predictor of fall status in the present study.

Functional community ambulation requirements in incomplete spinal cord injured subjects.

STUDY DESIGN: A group of people with incomplete spinal cord injuries (SCI) were evaluated and compared with able-bodied individuals during several walking conditions. OBJECTIVES: To evaluate the functional community ambulation and estimated energy expenditure in persons with incomplete SCI and able-bodied individuals. METHODS: A list of criteria was used to evaluate functional community ambulation among participants. Physiological variables, such as the heart rate, oxygen uptake and the lactate concentration, were also measured. RESULTS: Three of nine incomplete SCI subjects and all able-bodied subjects were able to meet all the criteria measured. The required velocity to safely cross an intersection was the criterion that the incomplete SCI group had the most difficulty reaching. The able-bodied subjects had a comfortable walking velocity twice that of the incomplete SCI subjects' preferred velocity. When walking at the same velocity (incomplete SCI subjects' preferred velocity), the incomplete SCI subjects had a rate of oxygen uptake 26% greater than the healthy subjects and were 200% less efficient. The lactate concentration also proved to be a useful tool when evaluating the incomplete SCI subjects' walking efficiency. The incomplete SCI subjects lactate concentration increased after walking at their preferred velocity, meaning that the anaerobic pathways were used to meet energy demands. CONCLUSION: Rehabilitation centers should adapt their evaluation forms and increase their criteria requirements to more suitable criteria that are found in the SCI patient's community. The physiological cost should also be taken into consideration when evaluating the SCI patient's functional ambulation.

Attentional requirements of walking in spinal cord injured patients compared to normal subjects.

STUDY DESIGN: Recent developments in the field of rehabilitation and the increase in the number of incomplete spinal cord injured subjects (SCI) emphasize the importance of studying the walking behavior of that population. OBJECTIVES: Attentional requirements were measured in spinal cord injured subjects during sitting, standing and walking and compared to a healthy group for the same tasks and walking at a similar speed. METHODS: Auditory stimuli and verbal responses were used to measure the attentional demands in the three experimental tasks. RESULTS: For both static tasks, SCI subjects were faster than the normal group. During walking at preferred speed, the normal group is significantly faster than the SCI subjects are. No difference was found between the two groups when the normal group walked at a similar speed. However, SCI subjects need to allocate significantly more attentional resource to walking than the normal. SCI patients also showed slower reaction times when the stimuli were presented during the single-support phase of walking. Kinematics analysis revealed that SCI subjects produced longer cycle duration than the normal group even when they walked at a match speed. Although variability in the cycle duration and the cycle length were comparable between the two groups, when the normal group walked at a slower speed, they were significantly more variable than the SCI subjects. CONCLUSION: Walking for SCI patients is cognitively challenging. Walking speed seems to be an important factor associated with attentional demands.

The effects of different knowledge of results spacing and summarizing techniques on the acquisition of a ballistic movement.

The present study crossed three knowledge of results summarizing techniques (single-trial KR, summary KR, and average KR) with two spacing conditions (KR on every fifth trial-20%- and KR on every trial-100%). Participants (n = 10 per group) performed 80 acquisition trials of a ballistic movement task involving both a temporal and spatial goal, followed by 30 immediate (10 min) and 30 delayed (2 days) no-KR transfer trials. For the spatial goal, performance was less accurate (absolute constant error) for the 20% spacing condition than the 100% condition during acquisition, but more accurate during delayed transfer. No effects were significant for variable error. For the temporal goal, performance was more accurate for the summary and average conditions than the single-trial KR condition; however, this effect was only present within the 20% spacing condition and only during Block 1 of acquisition. A similar effect held for variable error as well, except that the effect persisted for acquisition and transfer. It was concluded that the spacing of KR is more influential in promoting spatial accuracy than the summarizing of KR.

Gait problems in diabetic neuropathic patients.

OBJECTIVE: To examine whether a reduced peripheral sensibility caused by diabetic neuropathy increases the attentional demands necessary for controlling and regulating gait. DESIGN: Nonrandomized control trial. SETTING: University motor performance laboratory. SUBJECTS: Twelve diabetic patients with peripheral neuropathy and 7 control subjects, all volunteers. INTERVENTIONS: All subjects first performed a control seated reaction time task. For the walking task, auditory stimuli were randomly presented in the third, fourth, or fifth walking cycle on left foot toe off on left foot heel contact. The subject's task was to respond verbally as fast as possible to the auditory stimulus, while maintaining progression. MAIN OUTCOME MEASURES: Simple reaction times and kinematics of the gait pattern (cycle amplitude, cycle duration, cycle speed, cadence and percentage of time spent in the single support phase) were evaluated. RESULTS: For the walking task, diabetic neuropathic patients had a smaller cycle amplitude, cycle speed, and percentage of time spent in the single support phase than control subjects. Also, reaction times while walking were higher for diabetic neuropathic patients than for control subjects. CONCLUSIONS: Diabetic neuropathic patients show a less destabilizing and more conservative gait than control subjects. The increased attentional demands in gait for the diabetic neuropathic patients, along with their more conservative gait pattern, suggest that a lack of proprioception from the legs affects the control of gait. Diminished sensory information makes gait control more cognitively dependent in diabetic neuropathic persons than in control subjects.

Gait of a deafferented subject without large myelinated sensory fibers below the neck.

We evaluated the gait pattern of a deafferented subject who suffered a permanent loss of large sensory myelinated fibers below the neck following an acute episode of purely sensory neuropathy 21 years ago. The subject has developed several strategies to achieve a secure gait, namely: (1) a reduction of the degrees of freedom by freezing the knee articulations during the stance phase, (2) a preservation of body balance by enlarging his base of support, and (3) visual monitoring of his step by stabilizing the head-trunk linkage together with a characteristic forward tilt. As a result, the gait of the deafferented subject lacks the fluidity of normal gait. Compared with normal subjects, the gait pattern of the deafferented subjects is characterized by a shorter cycle length, a longer cycle duration, a slower speed, and a lower cadence. Using a dual-task paradigm, the attentional demands for walking were particularly important (as indexed by longer probe reaction times) during the double-support phase, suggesting that the deafferented subject uses the double-support phase as a transitory stable phase to update cognitively the postural features necessary for generating his next step.

Upright standing and gait: are there changes in attentional requirements related to normal aging?

This study evaluates attentional requirements for maintaining an upright posture and for walking among young and elderly persons to determine if, with normal aging, there is a deficit and/or a modification in the allocation of the attentional resources necessary for balance control. Eight young adults and 8 elderly persons were asked to respond to an auditory reaction time (RT) task (secondary task) while in a seated position, while in a broad-support or narrow-support upright standing position, and while walking (primary tasks). Reducing the base of support yielded slower RTs for the elderly than for the young persons. When walking, the elderly persons adopted a slower speed than young persons. They also had a shorter stride length. These adaptations have been reported to produce a more secure gait. Even so, they responded to the probe RT task with greater delays than young adults. Together, the results suggest that normal aging requires that a greater proportion of attentional resources be allocated to the balance demands of postural tasks.

Weight judgment. The discrimination capacity of a deafferented subject.

A weight discrimination study was undertaken to test (i) the capacity of controls and a deafferented subject (deprived of large sensory myelinated fibres from nose down), to discriminate weights with and without vision; (ii) the capacities of observers to discriminate weights while watching the deafferented and control subjects' lifting movements; (iii) the contribution of supplementary sources of sensory information (e.g. vestibular afferents) to the deafferented subject's discrimination capacity. With vision, G.L.'s liminal discrimination of weights was similar to that of the controls. In contrast, precluding vision impaired massively, but not completely, G.L.'s discrimination capacity, so emphasizing the importance of visual kinaesthetic cues in G.L. and incidently the importance of large myelinated sensory function in weight discrimination in controls. Kinematics recordings of G.L.'s lifting movements with vision revealed a significant correlation between weight and peak velocity of the lifting movement. This reflects a specific strategy used by G.L. to generate movements, allowing her to judge the weight of a lifted object visually. Peak velocity rather than amplitude of movement appears to be the main cue for G.L. since there was a lack of correlation between amplitude and weight lifted. For controls, none of the correlations (weight versus amplitude or weight versus velocity) was significant, whether vision was available or not. When watching G.L.'s lifting performance, external observers were able to use similar cues to establish their judgments, but they were far less accurate in doing so when watching control subjects. This suggests that controls were using a strategy different from G.L.'s.(ABSTRACT TRUNCATED AT 250 WORDS)

Attentional demands for static and dynamic equilibrium.

Upright standing and walking tasks require the integration of several sources of sensory information. In a normal and highly predictable environment, locomotor synergies involving several muscles may take place at lower spinal levels with neural circuitry tuned by local loops of assistance or self-organizing processes generated in coordinative networks. When ongoing regulation of gait is necessary (obstacles, changes in direction) supraspinal involvement is necessary to perform movements adapted to the environment. Using a classical information processing framework and a dual-task methodology, it is possible to evaluate the attentional demands for performing static and dynamic equilibrium tasks. The present experiment evaluates whether the attentional requirements for a control sitting condition and for standing and walking conditions vary with the intrinsic balance demands of the tasks. The results show that standing and walking conditions required more attention than sitting in a chair. The attentional cost for walking was also significantly greater than for standing. For the walking task, reaction times when subjects were in single-support phase (small base of support) were significantly longer than those in double-support phase, suggesting that the attentional demands increased with an increase in the balance requirements of the task. Balance control requires a continuous regulation and integration of sensory inputs; increasing balance demands loads the higher level cognitive system.

Mirror drawing in a deafferented patient and normal subjects: visuoproprioceptive conflict.

Results on a mirrow drawing task showed that a deafferented patient had no problem completing the pattern, whereas normal subjects needed more than four trials to attain a similar performance. The results suggest the presence of integrated visual and proprioceptive maps. The inversion of visual coordinates requires the need for a recalibration. Without proprioception, the task is more like a simple visual tracking task.

Role of afferent information in the timing of motor commands: a comparative study with a deafferented patient.

The accuracy of the motor system in synchronizing simultaneous movements initiations was tested in two conditions: (1) when the motor commands were triggered by an external signal (reactive condition), and (2) when subjects self-paced their movement onsets (self-paced condition). The task consisted of initiating simultaneously ipsilateral finger extension and heel raising. Eight normal subjects and a deafferented patient were tested. In the reactive condition, both normal subjects and the deafferented patient exhibited a precession of finger initiation over heel raising. This delay corresponds to the difference observed in the reaction time of the two limbs when measured independently. It reflects the difference in conduction times of the efferent pathways, as if the two motor commands were released simultaneously through a common triggering signal in the motor cortex. In contrast, in the self-paced condition normal subjects showed precession of heel over finger onsets, suggesting that synchrony is based upon the evaluation of afferent information. Unlike normal subjects, the patient showed no heel precession in the self-paced condition. These findings suggest that reactive and self-paced responses are produced through two different control modes and that afferent information contributes to the timing of motor commands in the self-paced mode.