Dynamic posturography using a new movable multidirectional platform driven by gravity

Negotiating ground level perturbations in walking: Visual perception and expectation of curb height modulate muscle activity

To negotiate visible and unpredictable changes in ground level, humans use different control strategies depending on the visibility. In case of fully visible perturbations, humans can anticipate the occurrence and the magnitude of the perturbation. In case of a camouflaged perturbation, they can anticipate the occurrence based on the camouflage cover but need to predict the magnitude from experience, as it is not visible. The purpose of this study was to investigate the anticipatory muscular control strategy humans employ when walking down curbs of different height and to investigate how this strategy differs if the step down is fully visible or camouflaged. The activity of five bilateral lower limb muscles (M. gastrocnemius medialis, M. soleus, M. tibialis anterior, M. biceps femoris and M. vastus medialis) of eight healthy subjects was recorded during walking down visible (0, -10 and -20 cm) and camouflaged curbs (0 and -10 cm). The results reveal that the M. gastrocnemius shows a clear anticipatory adaptation to visible curbs in the contralateral and partly also the ipsilateral leg, which further depends on the curb height. Furthermore, in case of a camouflaged perturbation, M. gastrocnemius activity of the contralateral leg shows an adaptation that indicates an average prediction of the curb height, presumably based on previous experience.

Walking with perturbations: a guide for biped humans and robots

This paper provides an update on the neural control of bipedal walking in relation to bioinspired models and robots. It is argued that most current models or robots are based on the construct of a symmetrical central pattern generator (CPG). However, new evidence suggests that CPG functioning is basically asymmetrical with its flexor half linked more tightly to the rhythm generator. The stability of bipedal gait, which is an important problem for robots and biological systems, is also addressed. While it is not possible to determine how biological biped systems guarantee stability, robot solutions can be useful to propose new hypotheses for biology. In the second part of this review, the focus is on gait perturbations, which is an important topic in robotics in view of the frequent falls of robots when faced with perturbations. From the human physiology it is known that the initial reaction often consists of a brief interruption followed by an adequate response. For instance, the successful recovery from a trip is achieved using some basic reactions (termed elevating and lowering strategies), that depend on the phase of the step cycle of the trip occurrence. Reactions to stepping unexpectedly in a hole depend on comparing expected and real feedback. Implementation of these ideas in models and robotics starts to emerge, with the most advanced robots being able to learn how to fall safely and how to deal with complicated disturbances such as provided by walking on a split-belt.

First trial and StartReact effects induced by balance perturbations to upright stance

Postural responses (PR) to a balance perturbation differ between the first and subsequent perturbations. One explanation for this first trial effect is that perturbations act as startling stimuli that initiate a generalized startle response (GSR) as well as the PR. Startling stimuli, such as startling acoustic stimuli (SAS), are known to elicit GSRs, as well as a StartReact effect, in which prepared movements are initiated earlier by a startling stimulus. In this study, a StartReact effect paradigm was used to determine if balance perturbations can also act as startle stimuli. Subjects completed two blocks of simple reaction time trials involving wrist extension to a visual imperative stimulus (IS). Each block included 15 CONTROL trials that involved a warning cue and subsequent IS, followed by 10 repeated TEST trials, where either a SAS (TESTSAS) or a toes-up support-surface rotation (TESTPERT) was presented coincident with the IS. StartReact effects were observed during the first trial in both TESTSAS and TESTPERT conditions as evidenced by significantly earlier wrist movement and muscle onsets compared with CONTROL. Likewise, StartReact effects were observed in all repeated TESTSAS and TESTPERT trials. In contrast, GSRs in sternocleidomastoid and PRs were large in the first trial, but significantly attenuated over repeated presentation of the TESTPERT trials. Results suggest that balance perturbations can act as startling stimuli. Thus first trial effects are likely PRs which are superimposed with a GSR that is initially large, but habituates over time with repeated exposure to the startling influence of the balance perturbation.

Fast responses to stepping on an unexpected surface height depend on intact large-diameter nerve fibers: a study on Charcot-Marie-Tooth type 1A disease

The contribution of reflexes from the large myelinated afferents in the control of normal and perturbed gait in humans is a highly debated issue. One way to investigate this topic is by studying normal and perturbed gait in patients lacking large myelinated fibers in the distal limb (Charcot-Marie-Tooth [CMT] type 1A disease). Such patients should have delayed and decreased reflexes if the latter depend on these large myelinated fibers. To elicit the reflexes, both patients and controls had to step on a platform that was either at the same level or lowered by 5 cm. In control subjects, landing on a level surface induced short-latency responses in the biceps femoris and tibialis anterior muscles, whereas such responses were largely absent in the patients. Similarly, stepping down unexpectedly induced a very fast muscle synergy, leading to a brake of the forward propulsion in the controls, which was significantly reduced and delayed (on average 32 ms) in the patients. The observed changes correlated with both sensory and motor deficits. Nevertheless, it is concluded that the results are primarily related to the sensory deficits, since the delayed or absent responses appeared in both upper and lower leg muscles, whereas only the latter showed motor deficits. The data are taken as evidence that large-diameter afferents from the distal leg are essential for fast reflex activations induced by stepping on a level or lowered surface unexpectedly.

Development of a three-degrees-of-freedom moveable platform for providing postural perturbations

The purpose of this study was to design and develop a three-degrees-of-freedom moveable platform to provide postural perturbations for balance assessment and training. The platform consists of three motion mechanisms, which can provide forward-backward translation, upward-downward tilt, and clockwise-counterclockwise rotation. This platform can move in any of its degrees of freedom separately or simultaneously. The precision and accuracy of the platform movement were examined by calculating the standard deviations in repeated trials and comparing the real amplitude and velocity of the movement with the preset values. All the standard deviations in repeated trials were small in that the variation coefficients were less than 2 per cent, except that in the highest-velocity test, and all the mean differences were less than 1 mm for translational and 1 degree for tilt or rotational perturbations. The results demonstrated that the platform is a reliable and valid instrument for providing postural perturbations. The preliminary investigation of the kinematic postural responses to translational and tilt perturbation showed that this platform is a useful apparatus for balance research. Potential applications of this platform include investigation of the postural responses to yaw rotation or any combination of its degrees of freedom and studying the effects of perturbation-based balance-training programmes provided by this platform.

Hitting a support surface at unexpected height during walking induces loading transients

The impact phase during walking is sometimes characterised by an early loading peak, termed 'transient', followed by a brief decline in the force profile, termed 'unloading phase'. It was hypothesized that transients occur more frequently when subjects are unaware of the landing condition, and that the unloading phase represents a yield of the leg. This was tested experimentally by introducing an unexpectedly lowered or level support surface height during walking. Furthermore, associations between the unloading phase and type of foot placement, load-rate, kinesiology and centre of pressure were investigated. The transient occurred more frequently when subjects were unaware of the surface height. The amplitude of unloading was higher in flatfooted (combined), as compared to heel and toe landings. The percentage of combined landings, as well as the amplitude and duration of unloading were highest in the first unexpected level trials (UL1) and gradually decreased in the subsequent level trials, when subjects adapted to the situation. Following the UL1 unloading phase, the foot roll-off was halted, the ipsilateral knee flexed, the onset of the contralateral swing phase was delayed, and the double support phase increased. The unloading amplitude correlated significantly with the load-rate and knee flexion. It is concluded that an unexpected surface height frequently induces an early stance transient that is followed by an unloading phase, flexion response and halt in foot roll-off. These characteristics deserve further study in the context of the frequent falls induced by uneven surfaces during walking.

Sex and age differences of relationships among stepping parameters for evaluating dynamic balance in the elderly

This study aimed to examine the relationships among various stepping parameters, sex, and age in the elderly. Healthy elderly Japanese individuals 60-85 years old (50 males and 61 females) performed 4 types of stepping motions for 20 s. Stepping motions included bilateral stepping (back/forth and right/left) and unilateral stepping (back/forth and right/left). The number of steps, the average connecting time of a foot during one step, and the average time of both feet touching the floor at the same time (bilateral connecting time) were measured with a foot switch sheet. The trial-to-trial reliability was very high (above 0.86) except for the bilateral connecting time in the bilateral stepping back/forth test for 70-85 year olds (males: 0.67, females: 0.68). With age, the number of steps was significantly smaller, and the average connecting time and the bilateral connecting time were shorter in all stepping tests. There were significant sex differences in bilateral connecting time for bilateral stepping right and left and the number of steps for the bilateral stepping back and forth and the unilateral stepping right and left tests. The number of steps and average connecting time showed high correlations between bilateral stepping right/left and back/forth (r=0.71-0.94) and between unilateral stepping back/forth and right/left (r=0.87-0.99). There were significant correlations of the average connecting time between bilateral and unilateral stepping motions (r=0.51-0.83), but both stepping motions are considered to have different motion properties from the viewpoint of center of gravity sway. The correlations between the bilateral connecting time and the number of steps in bilateral stepping were relatively low (males: /r/<0.70, females: /r/<0.57). The bilateral connecting time was near 0 s in many males; thus, it may depend greatly on individual or sex differences in stepping strategy. These results suggest that the stepping motions used in this study can evaluate dynamic balance ability, and that the unilateral test may be useful for the elderly who cannot walk independently with ease.

Kinematic response characteristics of the CAREN moving platform system for use in posture and balance research

The CAREN system is a new and unique device for use in postural and balance research in clinical settings due to its ability to independently perturb the support surface in each of six degrees of freedom. Users of this system need knowledge of its technical performance which is not available. The aim of this study was to determine the technical performance of the CAREN system by defining its kinematic response characteristics to two commonly used input functions (sine and ramp) for each of its six translational and rotational axes. The translational and rotational displacement, velocity and acceleration limits of the CAREN system suggest that it is a mid-range system with regard to single degree of freedom moving platform devices reported in the literature. The maximum average displacement cross-talk was 1.5% of the viable working range in any specified direction. The maximum average velocity cross-talk was 3.3% of its maximum velocity in any specified direction. The CAREN system was able to respond to ramp input functions within its displacement and velocity limits although, for short duration ramps, there was evidence that target velocity was not reached. It is concluded that the CAREN system is an appropriate device for postural and balance research with some unique features. This specification of its technical performance should help researchers to identify the tasks for which it is most suitable.

Muscle reflexes and synergies triggered by an unexpected support surface height during walking

An important phase in the step cycle is foot contact. When the moment of foot contact differs from the one expected, a fast response is needed. Such a mismatch can be caused by hitting a support surface earlier or later than expected. To study this, experiments were performed with healthy young adults who walked on a platform that was unexpectedly at a lowered (5 cm) or at a level height. Glasses blocked the lower visual field. In the unexpectedly lowered trials, the absence of expected heel contact triggered responses in the ipsilateral anti-gravity muscles [ipsilateral medial gastrocnemius (MGi), ipsilateral rectus femoris (RFi)] and contralateral flexor muscles [contralateral tibialis anterior (TAc), contralaterial biceps femoris (BFc)] with latencies of 47-69 ms. After the delayed heel contact, enhanced activity was found in the MGi, RFi, and TAc muscles. This specific muscle synergy was presumably activated to arrest the forward propulsion of the body. In contrast, when the surface was unexpectedly at level height, the subjects expected to step down, and the leg briefly yielded. A muscle synergy was activated at 46-81 ms that flexed the ipsilateral knee (TAi, BFi, RFi) and extended the contralateral one (MGc, BFc) to unload the perturbed leg and delay the contralateral swing phase. Both conditions triggered a fast functionally relevant muscle synergy because of a mismatch between the expected and actual sensory feedback at the moment of foot contact. The results are consistent with an internal model that compares the expected with the actual sensory feedback. The short latency of the response suggests a subcortical, possibly cerebellar pathway.

Study on the usefulness of precise and simple dynamic balance tests for the evaluation of recovery from intravenous sedation with midazolam and propofol

BACKGROUND AND OBJECTIVE

Dynamic balance involving movement of the centre of gravity is important for the evaluation of street fitness after sedation. The purpose of this study was to compare the recovery of dynamic balance after intravenous sedation with propofol or midazolam, and to investigate the usefulness of simple dynamic balance tests in evaluating the recovery.

METHODS

Fourteen young male volunteers underwent intravenous sedation with propofol and midazolam for 1 h each at an interval of more than 1 week. Computerized dynamic posturography using a multi-axial tilting platform, the 10-m maximum-speed walking test and the timed 'up & go' test (subjects stand up from a chair, walk 5 m and back with maximum speed and sit down again) were performed before and after sedation. The increase in each variable of the tests described above represents a reduction of function.

RESULTS

The score of the computerized dynamic posturography was significantly lower in propofol sedation than that in midazolam sedation until 40 min after the end of sedation (P = 0.006). The scores of maximum-speed walking test and timed 'up & go' test were significantly lower in propofol sedation than those in midazolam sedation till 60 min after the end of sedation, respectively (P = 0.035 and 0.042). The timed 'up & go' and maximum-speed walking tests were well and significantly correlated with computerized dynamic posturography in midazolam sedation (timed 'up & go' test vs. computerized dynamic posturography: r = 0.66, P < 0.01; and maximum-speed walking test vs. computerized dynamic posturography: r = 0.53, P < 0.01).

CONCLUSION

The timed 'up & go' and maximum-speed walking tests are useful simple dynamic balance tests well correlated with precise computerized dynamic posturography for the evaluation of the recovery of dynamic balance from midazolam sedation in younger adults.

A method for manipulating a movable platform's axes of rotation: a novel use of the CAREN system

The functionality of movable platforms used in human balance studies is limited as they allow rotations around pre-defined axes, which typically run close to the platform's surface and so cannot be used to directly investigate control mechanisms of proximal joints. A new six degrees of freedom platform (CAREN, Motek, Amsterdam) is now available which in principle could be programmed to rotate around any axis of rotation. The location of the default axes of rotation for this device are not documented and the algorithm to move the axes has not yet been defined. The purpose of this study was to (1) locate the platform's default axes of rotation, (2) implement an algorithm for relocating its axes of rotation and (3) evaluate the algorithm. A simplified method was developed to locate the bounding rectangles within which the default axes of rotations were located. The three axes of rotation were found to be at x=1.13+/-0.69 mm, z=-204.22+/-0.63 mm in the roll plane, y=-2.67+/-0.59 mm, z=-211.38+/-0.63 mm in the pitch plane and x=0.43+/-0.70 mm, y=-4.72+/-0.65 mm in the yaw plane (X: left, Y: rear, Z: up), relative to the centre of its surface, with the maximum bounding rectangle of dimensions 2.50mm by 2.42 mm. Relocation of the platform's axes of rotation was achieved by the use of compensatory corrections, which were determined using a translation algorithm. Evaluation of the algorithm involved pitching the platform around three newly defined axes in the sagittal plane, representing the ankle, knee and hip joints. The platform was able to rotate around the new axes while keeping the instantaneous axes of rotation within bounding rectangles of 1.87 mm x 0.81 mm (ankle), 3.04 mm x 1.23 mm (knee), 3.14 mm x 1.63 mm (hip). The ability to overcome the limitation of other moveable platforms makes the CAREN system a valuable tool in research on the role of individual joints in balance.