Design, control, and characterization of a Sliding Linear Investigative Platform for Analyzing Lower Limb Stability (SLIP-FALLS)

Development of a dynamic oriented rehabilitative integrated system

Moving platform are introduced in the field of the study of posturography since '70 years. Commercial platforms have some limits: a limited number of degrees of freedom; preconfigured protocols and usually they are expensive. In order to overcome these limits, we developed a robotized platform: DORIS. We aimed at realizing a versatile solution that can be applied both for research purpose but also for personalizing the training of equilibrium and gait. We reached these goals by means of a Stewart platform that was realized with linear actuators and a supporting plate. Each actuator is provided by a monoaxial ad hoc built load cell. Position and force control allow a multipurpose range of movement and a reactive interaction with the force applied by the subject. TCP/IP protocol guarantees the communication between the platform and other systems. Therefore, we integrated DORIS with motion analysis system, EMG system and virtual reality. The adopted solution offers the opportunity to manipulate available information by means of different coupling of visual, vestibular and plantar feet pressure inputs. The full control of its movement and of human dynamic interaction is a further benefit for the identification of innovative solutions for research and physical rehabilitation in a field that is strongly investigated, but still open.

Development of a Dynamic Oriented Rehabilitative Integrated System (DORIS) and Preliminary Tests

Moving platforms were introduced in the field of the study of posturography since the 1970s. Commercial platforms have some limits: a limited number of degrees of freedom, pre-configured protocols, and, usually, they are expensive. In order to overcome these limits, we developed a robotic platform: Dynamic Oriented Rehabilitative Integrated System (DORIS). We aimed at realizing a versatile solution that can be applied both for research purposes but also for personalizing the training of equilibrium and gait. We reached these goals by means of a Stewart platform that was realized with linear actuators and a supporting plate. Each actuator is provided by an ad hoc built monoaxial load cell. Position control allows a large range of movements and load cells measure the reactive force applied by the subject. Transmission Control Protocol/Internet Protocol (TCP/IP) guarantees the communication between the platform and other systems. We integrated DORIS with a motion analysis system, an electromyography (EMG) system, and a virtual reality environment (VR). This integration and the custom design of the platform offer the opportunity to manipulate the available information of the subject under analysis, which uses visual, vestibular, and plantar feet pressure inputs. The full access to the human movements and to the dynamic interaction is a further benefit for the identification of innovative solutions for research and physical rehabilitation purposes in a field that is widely investigated but still open.

Emergence of Lissajous Patterns as a Function of a Perturbation Frequency in Postural Responses to the Short Sinusoidal Translations of Varying Frequencies

The existence of in-phase and anti-phase postural responses to sinusoidal perturbations to the base of support is well known. In this study, we investigate if such coordinated postural responses exist at 'near-sway' perturbations where the perturbation amplitudes are kept within the range of normal sway lengths in healthy adults (n=10). The postural responses are analyzed via bursts of anterior-posterior (AP) 2.5 mm horizontal sinusoidal oscillations of the base of support at sequentially varying frequencies (0.25, 0.375, 0.5, 0.625, 0.75, 1 and 1.25 Hz). The parametric plots of the perturbation signal (platform position) and the response profiles (AP Center of Pressure [APCoP]) show the emergence of elliptical Lissajous patterns as the perturbation frequency is increased from 0.25 Hz to 1.25 Hz. The presence of such characteristic pattern shows the 'lock-in' behavior of APCoP with perturbation signal. These elliptical patterns become more apparent at the center frequencies (0.375 to 0.75 Hz). At the higher frequencies (1 and 1.25 Hz), the Lissajous patterns do exist but are dominated by low-frequency drift. The area and orientation of Lissajous patterns and the phase shifts between perturbation and APCoP show a strong nonlinear decreasing trend with increasing perturbation frequency for both, young (n=5) as well as mature (n=5) adults within the study group. This may suggest that such characteristic, frequency-locked, phased shifted response of healthy posture control could be a fundamental property of a healthy posture control's response to 'near-sway' sinusoidal translations in AP direction.

Using multiple placements of accelerometers to measure cardiovascular pulse transit times

To measure pulse transit times (PTT) and calculate pulse wave velocities (PWV), tri-axial and uniaxial accelerometers were placed in groups of 2 to 4 over the manubrium, xiphoid process, forehead, wrist and ankle, and/or over the carotid, femoral, and posterior auricular arteries in 11 consented supine subjects. Signals were sampled at 1 kHz and filtered. Radial vectors were calculated from the tri-axial measurements. A 3-lead ECG was simultaneously collected over the same 180 s window, as well as respiratory rate. Ensemble averages (with ±S.D.) and raster plots were generated for each filtered time series from 200 ms before to 800 ms after the peak of each ECG R-wave. Lag times between the R-wave peak (taken as t=0) and one or more prominent peaks (or valleys if appropriate) of the various accelerometer signals were calculated, by using the signal from the axis (or the radial vector) with the best signal to noise ratio. PWV was calculated from the regression of the distance measured versus the PTT between pairs, especially of the clinically-relevant carotid-femoral PTT. A spectral analysis of each ensemble was taken, with the hypothesis that in young adults the measures at the periphery would have less energy at higher frequencies than those of an older adult because of age-related changes in arterial distensibility.

Using fuzzy logic in psychophysical experiments to separate hits, false positives and guesses in posturally perturbed standing subjects

In a 2-Alternative Forced Choice Interval task (2AFCi), a standing subject is required to press a button once or twice to signal in which of two 4 s sequential intervals that (s)he thought that a short ≤ 16 mm postural perturbation had occurred. The perturbation might or might not result in transient changes of the subject's Anterior-Posterior Center of Pressure (APCOP) or in other measures. This paper used fuzzy inference to explore whether the correctness of a subject's stimulus detection can be gleaned from analyzing changes in one of more metrics related to changes in the APCOP. Also, distinguishing guesses from correct responses is a critical issue in any psychophysical detection paradigm. Biomechanical and psychophysical data are used to design a prediction model based on fuzzy inference that is able to discriminate correct responses from guesses.

Design of multiple axis robotic platform for postural stability analysis

This paper presents the design and implementation of IsiMove, a new dynamic posturography platform. It allows the evaluation of the static and dynamic balance of a human placed on a force plate. IsiMove is a robotic platform open kinematic with four degrees of freedom: anteroposterior tilt, mediolateral tilt, vertical rotation, and horizontal translation. It is capable of measuring the displacement of the center of pressure over time, with a resolution of 0.1 mm for each foot and support a human of about 120 kg. IsiMove can generate various types of balance perturbations based on parameters such as direction, amplitude, frequency and shape. In this paper, we will give a description of the mechanisms that constitute our platform. First, the technical specifications of the hardware and software architecture will be presented. Then, we will provide details related to extensive experimental evaluations of the platform in both static and dynamic condition as well as result of postural stability analysis with healthy subjects and stroke patients.

Dependence of elbow joint stiffness measurements on speed, angle, and muscle contraction level

Elbow joint stiffness is critical to positioning the hand. Abnormal elbow joint stiffness may affect a person's ability to participate in activities of daily living. In this work, elbow joint stiffness was measured in ten healthy young adults with a device adapted from one previously used to measure stiffness in other joints. Measurements of elbow stiffness involved applying a constant-velocity rotational movement to the elbow and measuring the resultant displacement, torque, and acceleration. Elbow stiffness was then computed using a previously-established model for joint stiffness. Measurements were made at two unique elbow joint angles, two speeds, and two forearm muscle contraction levels. The results indicate that the elbow joint stiffness is significantly affected by both rotational speed and forearm muscle contraction level.

Whole body motion-detection tasks can yield much lower thresholds than direction-recognition tasks: implications for the role of vibration

Earlier spatial orientation studies used both motion-detection (e.g., did I move?) and direction-recognition (e.g., did I move left/right?) paradigms. The purpose of our study was to compare thresholds measured with motion-detection and direction-recognition tasks on a standard Moog motion platform to see whether a substantial fraction of the reported threshold variation might be explained by the use of different discrimination tasks in the presence of vibrations that vary with motion. Thresholds for the perception of yaw rotation about an earth-vertical axis and for interaural translation in an earth-horizontal plane were determined for four healthy subjects with standard detection and recognition paradigms. For yaw rotation two-interval detection thresholds were, on average, 56 times smaller than two-interval recognition thresholds, and for interaural translation two-interval detection thresholds were, on average, 31 times smaller than two-interval recognition thresholds. This substantive difference between recognition thresholds and detection thresholds is one of our primary findings. For motions near our measured detection threshold, we measured vibrations that matched previously established vibration thresholds. This suggests that vibrations contribute to whole body motion detection. We also recorded yaw rotation thresholds on a second motion device with lower vibration and found direction-recognition and motion-detection thresholds that were not significantly different from one another or from the direction-recognition thresholds recorded on our Moog platform. Taken together, these various findings show that yaw rotation recognition thresholds are relatively unaffected by vibration when moderate (up to ≈ 0.08 m/s(2)) vibration cues are present.

Empirical mode decomposition/Hilbert transform analysis of postural responses to small amplitude anterior-posterior sinusoidal translations of varying frequencies

Bursts of 2.5mm horizontal sinusoidal anterior-posterior oscillations of sequentially varying frequencies (0.25 to 1.25 Hz) are applied to the base of support to study postural control. The Empirical Mode Decomposition (EMD) algorithm decomposes the Center of Pressure (CoP) data (5 young, 4 mature adults) into Intrinsic Mode Functions (IMFs). Hilbert transforms are applied to produce each IMF's time-frequency spectrum. The most dominant mode in total energy indicates a sway ramble with a frequency content below 0.1 Hz. Other modes illustrate that the stimulus frequencies produce a 'locked-in' behavior of CoP with platform position signal. The combined Hilbert Spectrum of these modes shows that this phase-lock behavior of APCoP is more apparent for 0.5, 0.625, 0.75 and 1 Hz perturbation intervals. The instantaneous energy profiles of the modes depict significant energy changes during the stimulus intervals in case of lock-in. The EMD technique provides the means to visualize the multiple oscillatory modes present in the APCoP signal with their time scale dependent on the signals's successive extrema. As a result, the extracted oscillatory modes clearly show the time instances when the subject's APCoP clearly synchronizes with the provided sinusoidal platform stimulus and when it does not.

Psychophysical detection thresholds in anterior horizontal translations of seated and standing blindfolded subjects

To help separate out the contributions of the somatosensory and vestibular systems to postural and sway control, short (1, 4 and 16 mm) anterior translations of lengths less than the normal sway path length were made of a platform upon which blindfolded young adult test subjects (n=12) stood or sat. Acceleration detection thresholds from these short moves were compared in standing vs seated conditions using a 2-Alternative [Interval] Forced-Choice psychophysical test protocol. A negative power law trading relationship was found between peak acceleration threshold and move length and duration for standing subjects. For these same subjects while seated, acceleration thresholds for all lengths were nearly constant, and showed a weak positive power law trade between threshold and move length or duration. This latter observation is consistent with that of Benson et al '86, who also observed a positive power law trade relationship between acceleration threshold and move duration for seated subjects. Thresholds were higher at 1mm for standing vs. seated tests; while at 16 mm, standing tests had lower thresholds compared to those obtained for the seated tests. These results suggest that the vestibular system provides the principal input for detecting these short translations while seated, but not while standing.

A 3-DOF parallel robot with spherical motion for the rehabilitation and evaluation of balance performance

In this paper a novel electrically actuated parallel robot with three degrees-of-freedom (3 DOF) for dynamic postural studies is presented. The design has been described, the solution to the inverse kinematics has been found, and a numerical solution for the direct kinematics has been proposed. The workspace of the implemented robot is characterized by an angular range of motion of about ±10° for roll and pitch when yaw is in the range ±15°. The robot was constructed and the orientation accuracy was tested by means of an optoelectronic system and by imposing a sinusoidal input, with a frequency of 1 Hz and amplitude of 10°, along the three axes, in sequence. The collected data indicated a phase delay of 1° and an amplitude error of 0.5%-1.5%; similar values were observed for cross-axis sensitivity errors. We also conducted a clinical application on a group of normal subjects, who were standing in equilibrium on the robot base with eyes open (EO) and eyes closed (EC), which was rotated with a tri-axial sinusoidal trajectory with a frequency of 0.5 Hz and amplitude 5° for roll and pitch and 10° for the yaw. The postural configuration of the subjects was recorded with an optoelectronic system. However, due to the mainly technical nature of this paper, only initial validation outcomes are reported here. The clinical application showed that only the tilt and displacement on the sagittal pane of head, trunk, and pelvis in the trials conducted with eyes closed were affected by drift and that the reduction of the yaw rotation and of the mediolateral translation was not a controlled parameter, as happened, instead, for the other anatomical directions.

Categorizing and comparing psychophysical detection strategies based on biomechanical responses to short postural perturbations

Background

A fundamental unsolved problem in psychophysical detection experiments is in discriminating guesses from the correct responses. This paper proposes a coherent solution to this problem by presenting a novel classification method that compares biomechanical and psychological responses.

Methods

Subjects (13) stood on a platform that was translated anteriorly 16 mm to find psychophysical detection thresholds through a Adaptive 2-Alternative-Forced-Choice (2AFC) task repeated over 30 separate sequential trials. Anterior-posterior center-of-pressure (APCoP) changes (i.e., the biomechanical response R_B) were analyzed to determine whether sufficient biomechanical information was available to support a subject's psychophysical selection (R_Ψ) of interval 1 or 2 as the stimulus interval. A time-series-bitmap approach was used to identify anomalies in interval 1 (a₁) and interval 2 (a₂) that were present in the resultant APCoP signal. If a₁ > a₂ then R_B = Interval 1. If a₁ < a₂, then R_B= Interval 2. If a₂ - a₁ < 0.1, R_B was set to 0 (no significant difference present in the anomaly scores of interval 1 and 2).

Results

By considering both biomechanical (R_B) and psychophysical (R_Ψ) responses, each trial run could be classified as a: 1) HIT (and True Negative), if R_B and R_Ψ both matched the stimulus interval (SI); 2) MISS, if R_B matched SI but the subject's reported response did not; 3) PSUEDO HIT, if the subject signalled the correct SI, but R_B was linked to the non-SI; 4) FALSE POSITIVE, if R_B = R_Ψ, and both associated to non-SI; and 5) GUESS, if R_B = 0, if insufficient APCoP differences existed to distinguish SI. Ensemble averaging the data for each of the above categories amplified the anomalous behavior of the APCoP response.

Conclusions

The major contributions of this novel classification scheme were to define and verify by logistic models a 'GUESS' category in these psychophysical threshold detection experiments, and to add an additional descriptor, "PSEUDO HIT". This improved classification methodology potentially could be applied to psychophysical detection experiments of other sensory modalities.

The effects of diabetes and/or peripheral neuropathy in detecting short postural perturbations in mature adults

Background

This study explored the effects of diabetes mellitus (DM) and peripheral neuropathy (PN) on the ability to detect near-threshold postural perturbations.

Methods

83 subjects participated; 32 with type II DM (25 with PN and 7 without PN), 19 with PN without DM, and 32 without DM or PN. Peak acceleration thresholds for detecting anterior platform translations of 1 mm, 4 mm, and 16 mm displacements were determined. A 2(DM) × 2(PN) factorial MANCOVA with weight as a covariate was calculated to compare acceleration detection thresholds among subjects who had DM or did not and who had PN or did not.

Results

There was a main effect for DM but not for PN. Post hoc analysis revealed that subjects with DM required higher accelerations to detect a 1 mm and 4 mm displacement.

Conclusion

Our findings suggest that PN may not be the only cause of impaired balance in people with DM. Clinicians should be aware that diabetes itself might negatively impact the postural control system.

A phase-locked loop model of the response of the postural control system to periodic platform motion

A phase-locked loop (PLL) model of the response of the postural control system to periodic platform motion is proposed. The PLL model is based on the hypothesis that quiet standing (QS) postural sway can be characterized as a weak sinusoidal oscillation corrupted with noise. Because the signal to noise ratio is quite low, the characteristics of the QS oscillator are not measured directly from the QS sway, instead they are inferred from the response of the oscillator to periodic motion of the platform. When a sinusoidal stimulus is applied, the QS oscillator changes speed as needed until its frequency matches that of the platform, thus achieving phase lock in a manner consistent with a PLL control mechanism. The PLL model is highly effective in representing the frequency, amplitude, and phase shift of the sinusoidal component of the phase-locked response over a range of platform frequencies and amplitudes. Qualitative analysis of the PLL control mechanism indicates that there is a finite range of frequencies over which phase lock is possible, and that the size of this capture range decreases with decreasing platform amplitude. The PLL model was tested experimentally using nine healthy subjects and the results reveal good agreement with a mean phase shift error of 13.7 degrees and a mean amplitude error of 0.8 mm.

Characterization of postural stability in a simulated environment of an earthquake using in-shoe plantar pressure measurement

An abled individual is believed to be capable of withstanding and overcoming the severe tremors of an earthquake as has been ascertained in a previous study. However, the event-related physiological mechanisms of human postural stability during an earthquake are subject to further investigation. Accordingly, the objective of this study is to further characterize postural stability in a simulated environment of an earthquake using a pedar-x (novel gmbh, Munich, Germany) in-shoe dynamic plantar pressure measurement system. A foot mask, dividing each of the insoles into seven plantar loading regions, was employed in this study. This paper reports preliminary results obtained from a normal adult female test subject with right side dominance and a normal foot arch. The test trial was comprised of 12 stages, ranging from quiet standing to simulated earthquake magnitude of 6.7 degrees on the Richter's scale, which is considered to be violent. The study metrics included: mean plantar pressure, foot-to-ground contact duration, insole loading area, and the position, displacement, and instantaneous velocity of the center of pressure. The study showed bilateral quantifiable changes in these metrics by foot-mask-region as a result of increasing magnitudes of simulated tremors. The subject was able to defy the overwhelming perturbations and maintain her balance and postural stability throughout the test period. The significance of this study lies in its ability to determine the threshold of falling within different subject populations in the event of an earthquake.

Moment measurement accuracy of a parallel spherical robot for dynamic posturography

This paper characterizes the moment measurement accuracy for a novel parallel spherical robot (SR) for dynamic posturography, controllable by position or impedance. The SR consists of three linear motors placed on a support base, a moving base, and three passive arms equipped with uniaxial load cells permitting impedance controlled perturbations. To evaluate the accuracy, a subject stood still on the SR, set in position control mode, while selected sinusoidal trajectories were applied. The moments computed by the load cells were compared to the value measured by a six-component force platform, placed on top of the rotating base. For the intended application of the SR, the errors were negligible with the worse case of only 4 Nm in a total of 15 trials (five conditions, three repetitions). The observed moment error was related mainly to the intrinsic accuracy of the sensors, equal to about 7 N. To demonstrate clinical applicability, the platform was set to impedance control mode and a protocol was tested with a 12-year-old girl with brain injury and a group of four healthy subjects. In total, 24 trials (eight conditions, three repetitions) were recorded for each subject. The results of this pilot study identified distinctive postural behaviors and therefore showed that the SR can be considered as an effective tool for dynamic posturography.

A quiet standing index for testing the postural sway of healthy and diabetic adults across a range of ages

A quiet standing index is developed for tracking the postural sway of healthy and diabetic adults over a range of ages. Several postural sway features are combined into a single composite feature C that increases with age a. Sway features are ranked based on the r(2)-values of their linear regression models, and the composite feature is a weighted sum of selected sway features with optimal weighting coefficients determined using principal component analysis. A performance index based on both reliability and sensitivity is used to determine the optimal number of features. The features used to form C include power and distance metrics. The quiet standing index is a scalar that compares the composite feature C to a linear regression model f(a) using C(')(a) = C/f(a). For a motionless subject, C(') = 0, and when the composite feature exactly matches the healthy control (HC) model, C(') = 1. Values of C(') >> 1 represent excessive postural sway and may indicate impaired postural control. Diabetic neurologically intact subjects, nondiabetic peripheral neuropathy subjects (PN), and diabetic PN subjects (DPN) were evaluated. The quiet standing indexes of the PN and DPN groups showed statistically significant increases over the HC group. Changes in the quiet standing index over time may be useful in identifying people with impaired balance who may be at an increased risk of falling.

Assessing the psychophysics of posture control and its physiological correlates

Graphical visualization methods are described that enable psychophysical detection data to be quantitatively correlated with underlying physiological data in postural control studies. Stitched, raster and ensemble averaged time-series plots are graphical tools that can guide later quantitative analysis. The examples presented point out the role that early Tibialis Anterior and later Gastrocnemius EMG activation might play in the 2-Alternative Forced Choice psychophysical detection of 16 mm horizontal anterior perturbations of a sliding platform on which a subject stands, and their linkage to AP and ML Center of Pressure changes brought about by a perturbation. These methods also give a preliminary indication that differing or no response patterns were seen at 4 and 1 mm.

A biomechanical model of human ankle angle changes arising from short peri-threshold anterior translations of platform on which a subject stands

This study modeled ankle angle changes during small forward perturbations of a standing platform. A two-dimensional biomechanical inverted pendulum model was developed that uses sway frequencies derived from quiet standing observations on a subject's Anterior Posterior Center of Pressure (APCoP) to track ankle angle changes during a 16 mm anterior displacement perturbation of a platform on which a subject stood. This model used the total torque generated at the ankle joint as one of the inputs, and calculated it assuming a PID controller. This feedback system generated a simulated ankle torque based on the angular position of the center of mass (CoM) with respect to vertical line passing through the ankle joint. This study also assumed that the internal components of the net torque were only a controller torque and a sway-pattern-generating torque. The final inputs to the model were the platform acceleration and anthropometric terms. This model of postural sway dynamics predicted sway angle and the trajectory of the center of mass. Knowing these relationships can advance an understanding of the ankle strategy employed in balance control.

Robotic platform for human gait analysis

A hydraulically actuated platform with 4-degrees of freedom (4-DOF) was designed to be able to apply velocity- or acceleration-controlled floor surface perturbations to freely walking human subjects. The apparatus was required to provide velocity-controlled translational perturbations over the floor surface, rotational perturbations about the ankle joint, and acceleration-controlled vertical translational perturbations. The apparatus was physically constructed, and tested by both measurements of dynamics and repeatability. Crossover of movement from one DOF to another was shown to be less than 1 mm or 0.5 degrees for all desired perturbations. Repeated perturbations were nearly identical with a standard deviation of less than 0.2 mm over translational axes. The application of the platform to human gait research was demonstrated with a protocol of midstance phase perturbations (n=8). For this, the platform controller was programmed to randomly select one out of three conditions: (1) no movement (control); (2) upward perturbation of 0.8 g, 50 mm, 300 ms after heel contact; (3) downward perturbation of 0.8 g, 50 mm, 300 ms after heel contact. In total, 90 trials (3 conditions x 30 repetitions) were recorded for each subject. By singling out the SOL EMG and normalizing and averaging over the subject population, it was shown that the upward and downward perturbations elicited at least two distinctive stereotypical reflex responses in the ankle extensors, opposite in sign. All subjects reported comfort with the apparatus and nobody fell.

Correction of the inertial effect resulting from a plate moving under low-friction conditions

The purpose of the present study was to develop a set of equations that can be employed to remove the inertial effect introduced by the movable platform upon which a person stands during a slip induced in gait; this allows the real ground reaction force (GRF) and its center of pressure (COP) to be determined. Analyses were also performed to determine how sensitive the COP offsets were to the changes of the parameters in the equation that affected the correction of the inertial effect. In addition, the results were verified empirically using a low friction movable platform together with a stationary object, a pendulum, and human subjects during a slip induced during gait. Our analyses revealed that the amount of correction required for the inertial effect due to the movable component is affected by its mass and its center of mass (COM) position, acceleration, the friction coefficient, and the landing position of the foot relative to the COM. The maximum error in the horizontal component of the GRF was close to 0.09 (body weight) during the recovery from a slip in walking. When uncorrected, the maximum error in the COP measurement could reach as much as 4 cm. Finally, these errors were magnified in the joint-moment computation and propagated proximally, ranging from 0.2 to 1.0 Nm/body mass from the ankle to the hip.

Variations in Anterior-Posterior CoP Patterns in Elderly Adults Between Psychophysically Detected and Non-Detected Short Horizontal Perturbations

Using an ultra-low-vibration Sliding Linear Investigative Platform for Assessing Lower Limb Stability (SLIP-FALLS), postural responses were evaluated while subjects stood on a platform that was given a short anterior perturbation presented in one of 2 sequential test intervals for a set of 30 trials. An adaptive 2-Alternative-Forced-Choice protocol required the subject to detect platform movement. Anterior-Posterior Centers-of-Pressure (AP CoP) were compared among the detected and non-detected trials for the Healthy Elderly Adults (HEA) and Diabetic Peripheral Neuropathy (DPN) elderly adults. Results indicate that there is a significant difference between the CoP patterns for a detected and non-detected trial. Also, the range of sway is found to be higher in the case of DPN elderly adults when compared to HEA. However, the AP CoP pattern for detected trials in both HEA and DPN were the same.

Amplitude demodulation of entrained sway to analyze human postural control

This paper presents an innovative technique to study postural control. Our translating platform, the Sliding Linear Investigative Platform For Analyzing Lower Limb Stability and Simultaneous Tracking, EMG and Pressure mapping (SLIP-FALLS-STEPm), makes precise, vibration movements under controlled conditions. We look at the psychophysical thresholds to the perception of a sinusoidally induced sway. In the Sine Lock experiments described, an induced sinusoidal perturbation locks the subject's natural sway pattern at the frequency of the perturbation. The input / output system is treated as an Amplitude Shift Key (ASK) modulated signal modulating a carrier frequency (at or about a subject's natural sway frequency). The Position signal (input) and the Anterior-Posterior Center of Pressure (APCOP) signal (output) or the ankle angle are demodulated by mixing them with the pure sine wave carrier at the frequency of underlying oscillation and then low-pass filtering it to detect the amplitude envelope. These detected envelopes elucidate that the square pulse increase in the position sine wave amplitude yields a triangular increase in APCOP demodulated signal.

Effect of lateral perturbations on psychophysical acceleration detection thresholds

Background

In understanding how the human body perceives and responds to small slip-like motions, information on how one senses the slip is essential. The effect of aging and plantar sensory loss on detection of a slip can also be studied. Using psychophysical procedures, acceleration detection thresholds of small lateral whole-body perturbations were measured for healthy young adults (HYA), healthy older adults (HOA) and older adults with diabetic neuropathy (DOA). It was hypothesized that young adults would require smaller accelerations than HOA's and DOA's to detect perturbations at a given displacement.

Methods

Acceleration detection thresholds to whole-body lateral perturbations of 1, 2, 4, 8, and 16 mm were measured for HYAs, HOAs, and DOAs using psychophysical procedures including a two-alternative forced choice protocol. Based on the subject's detection of the previous trial, the acceleration magnitude of the subsequent trial was increased or decreased according to the parameter estimation by sequential testing methodology. This stair-stepping procedure allowed acceleration thresholds to be measured for each displacement.

Results

Results indicate that for lateral displacements of 1 and 2 mm, HOAs and DOAs have significantly higher acceleration detection thresholds than young adults. At displacements of 8 and 16 mm, no differences in threshold were found among groups or between the two perturbation distances. The relationship between the acceleration threshold and perturbation displacement is of particular interest. Peak acceleration thresholds of approximately 10 mm/s² were found at displacements of 2, 4, 8, and 16 mm for HYAs; at displacements of 4, 8, and 16 mm for HOAs; and at displacements of 8 and 16 mm for DOAs. Thus, 2, 4, and 8 mm appear to be critical breakpoints for HYAs, HOAs, and DOAs respectively, where the psychometric curve deviated from a negative power law relationship. These critical breakpoints likely indicate a change in the physiology of the system as it responds to the stimuli.

Conclusion

As a function of age, the displacement at which the group deviates from a negative power law relationship increases from 2 mm to 4 mm. Additionally, the displacement at which subjects with peripheral sensory deficits deviate from the negative power law relations increases to 8 mm. These increases as a function of age and peripheral sensory loss may help explain the mechanism of falls in the elderly and diabetic populations.

Factors affecting reaction times to short anterior postural disturbances

One source of falls in the elderly may be an inability to sufficiently adjust to transient postural perturbations or slips. Identifying useful predictors of fall potential, as well as factors that affect the ability of an individual to detect a movement of the standing support surface may provide insight into postural stability and methods to increase stability in elders. The effects of acceleration, displacement, neurological status, and age on movement detection reaction times were studied in 25 individuals--1 young adults, seven neurologically intact elderly adults, and six elders with (diabetic) peripheral neuropathy. Acceleration detection thresholds for anterior perturbations of 1, 4, and 16 mm of the support surface was previously determined for each subject via a two-alternative forced choice (2AFC) protocol, with longer (16 mm) moves yielding lower 2AFC thresholds (12-39 mm/s(2)) that varied by group. Using the acceleration threshold value determined, and 125% of that threshold (suprathreshold), reaction times to the start of the platform movement were determined for all three displacements. Reaction times to an additional superthreshold movement (4 mm at 100 mm/s(2)) were also measured. Lower acceleration values over longer moves required longer reaction times for motion detection. Reaction times were also influenced by peak energy imparted to the subject through the move. The higher prevalence of falls in the elderly and elderly diabetic may be due to slowing reaction times compounded by larger amounts of imparted energy needed for detection of a slipping event.

A comparative study of reaction times between type II diabetics and non-diabetics

BACKGROUND

Aging has been shown to slow reflexes and increase reaction time to varied stimuli. However, the effect of Type II diabetes on these same reaction times has not been reported. Diabetes affects peripheral nerves in the somatosensory and auditory system, slows psychomotor responses, and has cognitive effects on those individuals without proper metabolic control, all of which may affect reaction times. The additional slowing of reaction times may affect every-day tasks such as balance, increasing the probability of a slip or fall.

METHODS

Reaction times to a plantar touch, a pure tone auditory stimulus, and rightward whole-body lateral movement of 4 mm at 100 mm/s2 on a platform upon which a subject stood, were measured in 37 adults over 50 yrs old. Thirteen (mean age = 60.6 +/- 6.5 years) had a clinical diagnosis of type II diabetes and 24 (mean age = 59.4 +/- 8.0 years) did not. Group averages were compared to averages obtained from nine healthy younger adult group (mean age = 22.7 +/- 1.2 years).

RESULTS

Average reaction times for plantar touch were significantly longer in diabetic adults than the other two groups, while auditory reaction times were not significantly different among groups. Whole body reaction times were significantly different among all three groups with diabetic adults having the longest reaction times, followed by age-matched adults, and then younger adults.

CONCLUSION

Whole body reaction time has been shown to be a sensitive indicator of differences between young adults, healthy mature adults, and mature diabetic adults. Additionally, the increased reaction time seen in this modality for subjects with diabetes may be one cause of increased slips and falls in this group.

Acceleration threshold detection during short anterior and posterior perturbations on a translating platform

Balance control systems have usually been studied under two conditions, during quiet standing or under large postural perturbations of a magnitude that requires a postural adjustment to prevent falling. Between these two extremes lie perturbations that can be repeated and measured while not forcing adaptive strategies from the postural control system. Unlike other studies of postural control, we employed very short translations with varying accelerations at the edge of psychophysical detectability. These perturbations were vibration-free anterior or posterior translations of the platform on which a subject stood. Using a full Latin-square design set of perturbations in the forward or backward direction, with a smooth or jerk acceleration profile, and of length 4 or 20 mm, were presented to five subjects. Perceptual peak acceleration thresholds were determined by an iterative psychophysical method that forced the subjects to choose in which of two sequential intervals that they perceived a stimulus to have been presented. The only factor found that significantly correlated with detection was perturbation length. The 4 mm peak thresholds averaged 14.51 mm/s2 while 20 mm thresholds averaged 8.55 mm/s2. For the short perturbations employed in this study, detection of motion thus was dependent upon the magnitude of the acceleration, but it was independent of the acceleration profile (jerk versus smooth) or movement direction. By understanding the influences on the ability to perceptually detect motion underfoot, we can begin to understand what elements of the postural control system might be involved in the second-to-second control of balance.