Comparative gait initiation kinematics between simulated unilateral and bilateral ankle hypomobility: Does bilateral constraint improve speed performance?

Improvement of motor performance in unilateral upper limb motor disability has been shown when utilizing inter-limb coupling strategies during physical rehabilitation. This suggests that 'default' bilateral central motor commands are facilitated. Here, we tested whether this bilateral motor control principle may be generalized to the lower limbs during gait initiation, which involves alternate bilateral actions. Disability was simulated by strapping to produce ankle hypomobility. Healthy adult subjects initiated gait at a self-paced speed with no ankle constraint (control), or with the stance, swing or bilateral ankles strapped. The duration of the anticipatory postural adjustments lengthened and the center of mass instantaneous progression velocity at foot-off decreased when the ankle was strapped. During the step execution phase, progression velocity at foot-contact was higher when both ankles were strapped compared to unilateral strapping of the stance ankle. These findings suggest that bilateral central motor commands are favored during walking tasks. Indeed, unilateral constraint of the stance ankle should compel the central nervous system to adapt specific commands to the constraint and normal sides whereas the 'default' bilateral motor commands would be utilized when both ankles are strapped leading to better kinematics performance. Bilateral in-phase upper limb coordination and bilateral alternating lower limb locomotor movements may share similar control mechanisms.

Duration of step initiation predicts freezing in Parkinson's disease.

OBJECTIVES: In some individuals with idiopathic Parkinson's disease (PD), freezing of gait episodes develops as the disease progresses. The neural mechanism underlying freezing in PD is poorly understood. Here, we report a 2-year follow-up on the novel discovery of prolonged step initiation duration as a potential marker of impending freezing. METHODS: Non-freezing PD participants in stages 2.5-4 of the Hoehn and Yahr disease severity scale were recruited from an earlier study which determined the effect of semi-virtual cues on walking. Responders were those who completed the first step faster in the presence of the virtual cues while non-responders either did not change or took longer to complete the first step. Both groups of participants were interviewed 2 years later to determine who had developed freezing of gait. RESULTS: Participants in the responder group had a 13-fold risk of developing freezing of gait within 2 years following the cueing study (OR=13.3, 95% CI=1.1-167). A cutoff score of -2.6% (i.e., a decrease in the duration of the first step with visual cues by 2.6% relative to no cues) gave a sensitivity and specificity of 100% and 89%, respectively. CONCLUSIONS: To the best of our knowledge, this is the first novel discovery of a physical predictor of freezing in PD. The time to complete the first step is a simple test to administer in the clinic or at home and may therefore be easily incorporated into a fall prevention training program for PD before the inception of freezing.

Diagnostic value of the rapid assessment of postural instability in Parkinson's disease (RAPID) questionnaire.

BACKGROUND: People with idiopathic Parkinson's disease (PD) develop postural instability in the later stages of the ailment. Postural instability has traditionally been quantified with the Pull test even though its face validity is limited. We previously established cut-off scores for a three-part rapid assessment of postural instability (RAPID) questionnaire as a non-physical complement to the physical test. In the current study, the questionnaire was administered to a new group of PD subjects to evaluate the diagnostic value of the instrument. METHODS: Sensitivity and specificity values were calculated for single and combined sections of the questionnaire by using the Pull test as the gold standard for assessing the presence of postural instability. RESULTS: The questionnaire when used in its entirety gave the highest sensitivity (.71), whereas specificity was highest in the activities of daily living (.74) and fear of falling sections (.74). Net specificity decreased to .44 when the scores from the three sections of the questionnaire were combined. CONCLUSIONS: The high sensitivity of the RAPID questionnaire suggests that it may be used as an adjunct to the Pull test or solely if it is not convenient or contraindicated. The questionnaire may also be adapted for use via the telephone or internet. The limitation of the Pull test in revealing postural instability may explain the low specificity of the questionnaire, i.e. the questionnaire correctly identifies patients as unstable when the Pull test indicates normal postural control. It is hoped that the rapid identification of postural instability in PD may lead to increased awareness of the disease progression and fewer falls.

Rapid assessment of postural instability in Parkinson's disease (RAPID): a pilot study.

BACKGROUND: The Fahn's pull (or retropulsion) test is an item in the motor section of the Unified Parkinson's Disease Rating Scale, which is used almost exclusively to classify postural instability in Parkinson's disease (PD). However, the test is hard to standardize and is often performed incorrectly, making it hard to interpret. Moreover, it may not be safe to administer in patients who experience pain in the shoulders, neck, trunk and/or lower extremities. Identifying and grading postural instability in PD without requiring a physical challenge would not only be useful for the clinician but would assist patients and caregivers in its recognition. We propose the use of the rapid assessment of postural instability in Parkinson's disease (RAPID) questionnaire as a non-physical assessment tool. METHODS: We determined the associations between the pull test and items on a risk-assessment questionnaire that consisted of three parts: activities of daily living, fear of falling, and frequency of falling. RESULTS: Significant correlations were found between the pull test and the predictor variables, which ranged between 0.51 and 0.56 whilst the correlations amongst the predictor variables ranged between 0.58 and 0.70. The three parts of the questionnaire, when used in combination, produced a 96% sensitivity in the classification of postural instability. CONCLUSIONS: The RAPID questionnaire can be used as an adjunct to the pull test or solely if the pull test is contraindicated. It may also be possible to administer the questionnaire via the telephone or Internet. It is hoped that the rapid identification of postural instability would lead to fewer falls.

Influence of sensory inputs and motor demands on the control of the centre of mass velocity during gait initiation in humans.

Human gait requires the simultaneous generation of goal-directed continuous movement (locomotion) and the maintenance of balance (postural control). In adults, the centre of mass (CoM) oscillates in the vertical plane while walking. During the single support phase of gait initiation, its vertical (vCoM) velocity increases as the CoM falls and is actively reversed prior to foot-contact. In this study we investigated whether this active control, which is thought to reflect balance control during gait initiation, is controlled by visual and somatosensory inputs (Experiment 1) and whether it is modified by a change in motor demands, two steps versus one step (Experiment 2). In all healthy adults, the vCoM velocity was braked, or controlled, by contraction of the soleus muscle of the stance leg. The elimination of visual input alone had no effect on braking, although its amplitude decreased when somatosensory inputs were disrupted (-47%), and further decreased when both visual and somatosensory inputs were disrupted (-83%). When subjects performed only one step, with no trailing of the stance foot, the vCoM velocity braking also decreased (-42%). These results suggest that active braking of the CoM fall during the transition to double support, an indicator of balance control, is influenced by both multisensory integration and the demands of the current motor program. The neural structures involved in this mechanism remain to be elucidated.

The strategies to regulate and to modulate the propulsive forces during gait initiation in lower limb amputees.

We used the framework of motor program adaptability to examine how unilateral above-knee (AK) or below-knee (BK) amputee subjects organize the global and local biomechanical processes of generation of the propulsive forces during gait initiation to overcome the segmental and neuro-muscular asymmetry. The organization of the global biomechanical process refers to the kinematics behavior of the couple center of foot pressure (CoP) and center of mass (CoM); the organization of the local biomechanical process refers to the propulsive forces generated by the prosthetic or intact limb during the anticipatory postural adjustment phase and the step execution phase. Specifically, we examined: i) the strategy to regulate the progression velocity, i.e., to maintain it comparably when the leading limb changed from the prosthetic limb to the intact limb; and ii) the strategy to modulate the progression velocity, i.e., to increase it when gait was initiated with the prosthetic limb vs. intact limb. The kinematics of the CoM and CoP in the amputees showed the same global biomechanical organization that is typically observed in able-bodied subjects, i.e., the production of the forward disequilibrium torque was obtained by a backward shift of the CoP, followed by a forward acceleration of the CoM. However, gait initiation was achieved by using a different local strategy depending on which limb was used to initiate the step. For the regulation of the CoM progression velocity, when the gait was initiated with the intact limb, the slope of the progression velocity during the anticipatory postural adjustment phase (APA) was steeper and lasted longer, the step execution duration was shorter, and the variation of the CoM speed was lower. In other words, to regulate the speed of progression, the amputee subjects controlled the spatial and temporal parameters of the propulsive forces. In the modulation of the CoM progression velocity, when the gait was initiated with the intact limb, the amputees controlled only the intensity of the propulsive forces during both the APA and step execution phases. In contrast, when the gait was initiated with the prosthetic limb, the modulation resulted mainly from the propulsive forces generated during the step execution phase. These different strategies are discussed in terms of the subject's capacity to adapt the motor program for gait initiation to new constraints.

Does balance control deficit account for walking difficulty in Parkinson's disease?

Patients with Parkinson's disease (PD) walk slowly, in part to compensate for their balance control deficit. We tested the effect of balance support to determine if walking performance in PD patients would improve. The sample consisted of unmedicated older adults with idiopathic Parkinson's disease who had poor balance control but no stooped posture, arthritis or muscle weakness. There was no difference in walking speed between unsupported and supported walking. The speeds were between those reported for disease-free older adults and older adults with muscle weakness and a history of falling. PD patients' walking difficulties, even while using a balance aid, may be partly explained by their set-changing problems. They frequently hold the cane off the ground when walking, suggesting their set-changing difficulty may be severe enough that using it aggravates their walking difficulty. Treatment of walking difficulty in PD patients should consider interventions other than those dealing only with balance control.

Source of improvement in balance control after a training program for ankle proprioception.

Previous research on healthy individuals reported improvements in balance control following a purported ankle proprioception-training program. The training may have resulted in a general rather than a specific enhancement of ankle proprioception. To test this hypothesis, subjects were constrained at the hips and trunk with a custom-made thoracolumbosacral orthosis and performed a one-leg standing test with eyes closed and head tilted back, so that they had to rely primarily on their ankle musculature to keep their balance. Subjects were retested after training on the BAPS three times a week for 4 weeks, following the training recommendations of the manual. Subjects' bodies were not constrained during the training. Analysis showed that subjects made improvements during training in performing more difficult tasks on the board. On the one-leg test, however, there were no improvements in sway velocity, number of touchdowns, or falls relative to pretest scores. Improvements observed during training likely resulted from diffuse enhancement of proprioception in other body segments such as the knees, hips, spine, and upper extremities. A training program in control of general balance does not specifically target ankle proprioception.

Initial evidence for the mixing and soft assembly of the ankle, suspensory, and hip muscle patterns.

The onset latencies of automatic muscle responses to backward translations of the support surface in standing subjects were assessed to test two longstanding hypotheses: that (1) the ankle, hip, and suspensory muscle patterns represent discrete synergies; and (2) the suspensory pattern cannot be mixed with the ankle and the hip patterns (based on theoretical modeling). Muscle responses were recorded at the leg (medial gastrocnemius and tibialis anterior), thigh (hamstring and quadriceps), lower trunk (paraspinals and abdominals), and the upper trapezius muscles. A principal factor analysis with orthogonal varimax rotation was conducted to describe the variation among the muscle onset latencies by means of the smallest possible set of newly defined variables, or factors. Muscles that were correlated with each other were grouped into a factor. The analysis produced three factors with eigenvalues > or =1, which accounted for 81% of the total variance. Based on the first muscle response in each factor, the muscle patterns resembled the hip, ankle, and suspensory synergies. Muscle responses within a synergy were activated before the completion of the muscle responses in the other synergies, indicating that the synergies, though mixed, are triggered separately. This is consistent with the hypothesis that the ankle, hip, and suspensory patterns represent discrete synergies. On the other hand, the results do not support the hypothesis that the three patterns cannot be combined.

Parkinson's disease impairs the ability to change set quickly.

We tested the hypothesis that basal ganglia dysfunction in Parkinson's disease impairs the ability to quickly change set. The ability to change set was inferred by measuring the change in the amplitude of automatic gastrocnemius or tibialis anterior muscle responses in standing subjects: (1) when the direction of a surface perturbation changed from a backward translation to a toes up rotation; and (2) when subjects were instructed to 'give' or 'resist' while responding to the translations and rotations. In experiment 1, a change in sensorimotor set was assessed by the suppression of gastrocnemius responses to toes up rotations following a series of backward translations. Unlike healthy young and older subjects, Parkinson subjects did not change sensorimotor set immediately to the first rotation, but needed several rotations to change their responses. When required to alternate their responses between backward translations and toes up rotations, Parkinson subjects showed a smaller amplitude change in gastrocnemius responses. In experiment 2, Parkinson subjects had more difficulty in using cognitive set to modify their responses, especially when instructed to 'resist' the perturbations. A small number of healthy older subjects also had difficulties changing set quickly, but to a lesser extent than the Parkinson subjects. Levodopa medication did not improve the Parkinson subjects' ability to change set quickly. These results suggest that the basal ganglia, which are affected in Parkinson's disease, are critical neural substrates in the ability to change set quickly.

Factors influencing the quick onset of stepping following postural perturbation.

It has been shown that the stepping to recover balance following a forward fall occurs at a constant time (on average 293 ms) (Do et al. Journal of Biomechanics 15, 1982, 933-939). In this study, we tested the hypothesis according to which programming to make fast movement could trigger the movement earlier than when programming self-pace movement. The same experimental paradigm of forward fall was used (see Do et al., 1982) to induce stepping. Different extents of stepping were manipulated by instructions: Subjects were instructed to step to recover their balance naturally (control condition); to make shorter steps than in the control condition; longer steps; faster steps. Lastly, a fast step was also induced by the biomechanical constraint on the initial posture, i.e. by inclining the subject forward at his maximum capacity. Data were collected from 12 subjects. The variables analyzed were the onset latency of step execution and other classical parameters (time of heel-contact, duration of the swing phase, step length, center of mass progression velocity, and step velocity). The results showed that the onset of stepping was unchanged in the longer- and faster-step conditions, relative to the control condition (mean control value = 280 ms). In contrast, the onset of stepping was significantly earlier in the short-step condition, and when the initial inclination was greater (250 and 252 ms, respectively). The swing phase duration in these two conditions averaged 140 and 185 ms, was significantly shorter than in the other conditions, whereas step length was obviously expected to be shorter in the shorter-step condition and longer in the longer-step condition than in the other conditions. Step length was similar between the other conditions. We conclude that neither step length or step velocity programming could induce an earlier onset latency of stepping. Step programming in relation to these specific instructions seemed to concern the extent of step execution and not the time of triggering of the stepping. We suggest that the control of short swing phase duration resulted in an earlier onset latency of stepping to recover the balance. This control depends on the combination of biomechanical constraints and cognitive processes, including subject's interpretation of the instructions and evaluation of the risk of fall.

Sensory organization for balance: specific deficits in Alzheimer's but not in Parkinson's disease.

BACKGROUND: The cause of frequent falling in patients with dementia of the Alzheimer type (AD) is not well understood. Distraction from incongruent visual stimuli may be an important factor as suggested by their poor performance in tests of shifting visual attention in other studies. The purpose of this study was to determine whether AD patients have difficulty maintaining upright balance under absent and/or incongruent visual and other sensory conditions compared to nondemented healthy elderly persons and individuals with Parkinson's disease (PD). METHODS: Seventeen healthy older adults, 15 medicated PD subjects, and 11 AD subjects underwent the Sensory Organization Test protocol. The incidence of loss of balance ("falls"), and the peak-to-peak amplitude of body center of mass sway during stance in the six sensory conditions were used to infer the ability to use visual, somatosensory, and vestibular signals when they provided useful information for balance, and to suppress them when they were incongruent as an orientation reference. Vestibular reflex tests were conducted to ensure normal vestibular function in the subjects. RESULTS: AD subjects had normal vestibular function but had trouble using it in condition 6, where they had to concurrently suppress both incongruent visual and somatosensory inputs. All 11 AD subjects fell in the first trial of this condition. With repeated trials, only three AD subjects were able to stay balanced. AD subjects were able to keep their balance when only somatosensory input was incongruent. In this condition, all AD subjects were able to maintain balance whereas some falls occurred in the other groups. In all conditions, when AD subjects did not fall, they were able to control as large a sway as the healthy controls, except when standing with eyes closed in condition 2: AD subjects did not increase their sway whereas the other groups did. In the PD group, the total fall incidence was similar to the AD group, but the distribution was generalized across more sensory conditions. PD subjects were also able to improve with repeated trials in condition 6. CONCLUSION: Patients with dementia of the Alzheimer type have decreased ability to suppress incongruent visual stimuli when trying to maintain balance. However, they did not seem to be dependent on vision for balance because they did not increase their sway when vision was absent. Parkinsonian patients have a more general balance control problem in the sensory organization test, possibly related to difficulty changing set.

Postural set for balance control is normal in Alzheimer's but not in Parkinson's disease.

BACKGROUND: It has been suggested that patients with dementia of the Alzheimer type have abnormalities in the basal ganglia, and thus, may have similar sensorimotor problems as patients with basal ganglia degeneration from Parkinson's disease. Whether the similarity extends to balance control is unknown. One distinguishing feature of balance disorder in Parkinson's disease is difficulty with changing postural set in terms of adapting the amplitude of leg muscle activity as a function of support condition. We, therefore, tested whether patients with Alzheimer's disease without extrapyramidal signs would show a similar problem in changing postural set as patients with Parkinson's disease. METHODS: The ability to quickly change postural set was measured by comparing leg muscle activity under two conditions of support (free stance, versus grasping a frame, or sitting) during backward surface translations, during toes up surface rotations, and during voluntary rise to toes. Results were compared among 12 healthy adults, 8 nondemented Parkinson's patients on their usual dose of medication, and 11 Alzheimer patients without extrapyramidal signs. RESULTS: Subjects with Alzheimer's, but not Parkinson's, disease performed similarly to the healthy control subjects. They changed postural set immediately, by suppressing leg muscle activity to low levels when supported. Parkinson subjects did not change postural set immediately. They did not suppress the tibialis anterior in voluntary rise to toes when holding, nor the soleus in perturbed sitting as much as the healthy control and Alzheimer subjects in the first trial. Instead, the Parkinson subjects changed set more slowly, over repeated and consecutive trials in both protocols. The onset latencies of soleus responses to backward surface translations and perturbed sitting, as well as tibialis anterior responses to toes up rotations, were the same for all three groups. CONCLUSION: Alzheimer patients without extrapyramidal signs, unlike nondemented Parkinson's disease patients, have no difficulty in quickly changing postural set in response to altered support conditions. Our results, therefore, do not support the hypothesis that Parkinson's and uncomplicated Alzheimer's diseases share common postural set problems that may contribute to disordered balance control.

Time-dependent influence of sensorimotor set on automatic responses in perturbed stance.

These experiments tested the hypothesis that the ability to change sensorimotor set quickly for automatic responses depends on the time interval between successive surface perturbations. Sensorimotor set refers to the influence of prior experience or context on the state of the sensorimotor system. Sensorimotor set for postural responses was influenced by first giving subjects a block of identical backward translations of the support surface, causing forward sway and automatic gastrocnemius responses. The ability to change set quickly was inferred by measuring the suppression of the stretched antagonist gastrocnemius responses to toes-up rotations causing backward sway, following the translations. Responses were examined under short (10-14 s) and long (19-24 s) inter-trial intervals in young healthy subjects. The results showed that subjects in the long-interval group changed set immediately by suppressing gastrocnemius to 51% of translation responses within the first rotation and continued to suppress them over succeeding rotations. In contrast, subjects in the short-interval group did not change set immediately, but required two or more rotations to suppress gastrocnemius responses. By the last rotation, the short-interval group suppressed gastrocnemius responses to 33%, similar to the long-interval group of 29%. Associated surface plantarflexor torque resulting from these responses showed similar results. When rotation and translation perturbations alternated, however, the short-interval group was not able to suppress gastrocnemius responses to rotations as much as the long-interval group, although they did suppress more than in the first rotation trial after a series of translations. Set for automatic responses appears to linger, from one trial to the next. Specifically, sensorimotor set is more difficult to change when surface perturbations are given in close succession, making it appear as if set has become progressively stronger. A strong set does not mean that responses become larger over consecutive trials. Rather, it is inferred by the extent of difficulty in changing a response when it is appropriate to do so. These results suggest that the ability to change sensorimotor set quickly is sensitive to whether the change is required after a long or a short series of a prior different response, which in turn depends on the time interval between successive trials. Different rate of gastrocnemius suppression to toes-up rotation of the support surface have been reported in previous studies. This may be partially explained by different inter-trial time intervals demonstrated in this study.

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

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.

Heart rate, blood pressure, and running speed responses to mesencephalic locomotor region stimulation in anesthetized rats.

The decerebrate rat locomotor preparation described in a previous study requires extensive brain surgery with the possibility of significant blood loss. The purpose of this study was to improve on the previous model by using lightly anesthetized instead of decerebrated rats. After initial surgery consisting of boring a small hole through the parietal bone, the animals were maintained on low levels of halothane anesthetic. The mesencephalic locomotor region was then located by physiological criteria using stereotaxic coordinates from the previous study. Locomotor speed, blood pressure and heart rate responses were then measured over a wide range of stimulation currents that elicited a maximal running speed. Stimulation currents ranged from 36 microA for walking to 82 microA for fast galloping. Locomotor speeds ranged from 20 m/min for walking to 64 m/min for fast galloping. Some animals easily achieved galloping speeds beyond 100 m/min. Blood pressure and heart rate increased with increasing stimulation currents. Blood pressure also increased during stimulation after muscular paralysis. This was not due to current spread, suggesting that the mesencephalic locomotor region might be involved in central command mechanisms. Heart rate did not increase after paralysis. This supports other multi-joint dynamic studies suggesting that movement per se may be necessary to induce heart rate changes, presumably via joint mechanoreceptors. The range of locomotor patterns and cardiovascular responses were obtained under self-supported conditions. By defining the mesencephalic locomotor region via physiological criteria, and by grouping blood pressure and heart rate measurements by gait rather than by stimulation currents, the potential use of the intact model for cardiovascular control studies was demonstrated. The animals were able to run and gallop at high speeds considering they were anesthetized. The simplified preparation will be useful for more complex cardiovascular experiments requiring intact and self-supported conditions.