This article was made possible by NPRP grant #6-463-2-189 from the Qatar National Research and MOP grant #81280 from the Canadian Institutes of Health Research.

The authors have nothing to disclose.

Several steps are critical in performing these experiments to study human postural control. These steps are associated with the correct measurement of the signals and include: 1) Correct alignment of the shank ankle axis of rotation to that of the pedals, for the correct measurement of ankle torques. 2) Correct set-up of the range finders to ensure they work in their range and are not saturated during the experiments. 3) Measurement of EMG with good quality and minimal cross talk. 4) Application of appropriate perturbations, which evoke sufficient responses, but not disrupt the normal postural control. 5) Selection of an appropriate trial length, based on the intended analysis, while avoiding body shift and fatigue. In addition to the experiments, the analysis also must be done carefully. For the estimation of the intrinsic stiffness from data acquired in mechanically perturbed standing, it is critical to select the length of the intrinsic response in a way that ensures NO reflex torque (which starts soon after a burst of activity in TS muscles) is included. In addition, although many studies have assumed that the intrinsic stiffness does not change in standing11,14,15, a recent study showed that it is important to account for the modulation of the stiffness with changes in ankle torque associated with postural sway23,32. For determining the FR of the dynamic relation from any input to the output, the most important step is to correctly estimate the cross-spectrum and power spectrum by selecting the window length and overlap, appropriate to the record length.
Design of the perturbations is an important step in human standing experiments. Different types of mechanical and visual perturbations have been used for the study of postural control, given as the angle of the support surface or the angle of the visual field. These include multi-sine, low-pass filtered noise, pseudo-random ternary sequence (PRTS) and others3,9,10,12,18,24,31,33,34. However, the use of a pseudo random binary sequence (PRBS) is advantageous for mechanical perturbations, because: 1) For a given peak-to-peak amplitude, it provides the highest power over a wide range of frequencies, which can be controlled by selecting the switching rate3; 2) It is unpredictable, yet repeatable, making it possible to reduce noise by averaging; 3) A PRBS input with low absolute mean velocity generates reflex responses, allowing quantification of stretch reflexes in standing. For the visual system, step pulses evoke no significant postural responses, because the visual system cannot follow fast changes of the visual field. In addition, predictable inputs such as sinusoids with one frequency can generate anticipatory behavior. Multi-sine signals are not effective for the study of visual responses, because their fast and continuous changes are hard to follow and can cause subjects to become motion sick. PRTS signals have been used extensively to study visual system in standing, as it is an informative input; the movements of the visual field are discrete rather than continuous and their velocity can be controlled to generate coherent visual responses. Although, the PRTS performs well, it is a non-zero mean signal, which may cause non-stationarities in the postural control and makes identification difficult. Therefore, the TrapZ was designed to address this problem, which is unpredictable, discrete, and has a zero-mean (Figure 2B). Another important consideration in designing the experiments is the perturbation amplitude. Generally, perturbations with low amplitudes should be used when the objective is to perform linear analysis and not to deviate from an ankle strategy. The validity of ankle strategy can be checked analytically35, and if there are large deviations, which may be generated by larger perturbation amplitudes, nonlinear analysis methods, accompanied by multi-segment models of body in standing, may be required36.
Another consideration for perturbation design is trial length, which must be long enough to allow reliable estimates of the model parameters. However, very long trials are undesirable, because they may result in the subject shifting the body orientation, resulting in a non-stationarity that makes system modeling and identification difficult. A trial length between 2 and 3 minutes is optimal. This trial length does not generally result in fatigue, provided a sufficient resting period is enforced between trials. The analysis method also influences the required trial length. If a linear analysis using FR or impulse response function is used, then the lowest frequency of interest will determine the record length. The inverse of the window length is equal to the minimum frequency, so, if lower frequencies are to be examined, longer windows must be used. Moreover, the trial must be long enough to provide enough averaging to yield robust spectral estimates. Nonlinear analysis will, in general require even longer data records, because nonlinear models usually have more parameters than linear models.
The study of human postural control requires the selection of an appropriate identification method. Parametric and non-parametric linear identification methods can be used to study postural control10,12,18,19,20,28,31,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54. Non-parametric identification, using FR estimation, has been used extensively to study postural control, because it is well suited for the identification of data acquired in the closed-loop condition of standing24 and requires few a-priori assumptions (for the details of this method see24). The most commonly used method is to estimate the FR of the closed-loop system between an external (mechanical/sensory) perturbation and an output (e.g., body angle, ankle torque, or muscle EMG), which is a combination of controller, plant, and feedback. To provide physical significance and examine each component separately, many studies have used a parametric model of the closed-loop system and estimated the parameters that match the parametric model&#8217;s FR to that of the estimated output sensitivity10,18,31,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51. Parametric identification, on the other hand, assumes that the system input and output are related by some model structure with a limited number of parameters, known a-priori. The prediction error method is used to find the model parameters that minimize the error between the measured output and model prediction55. In contrast to FR models, where the external perturbation must be measured and used for the analysis, these methods can be applied directly to any two signals, as long as a separate noise model, which is adequately parametrized, is estimated as well56. This means there is no need to measure the external perturbation. Although, the model orders must be determined a-priori, parametric models usually have fewer parameters than the FR models and hence provide more robust parameter estimates. The main drawback of a parametric model is that a correct noise model must be used to obtain unbiased estimates of the parameters.
An important consideration in human postural control is its remarkable adaptability to new experimental and environmental conditions. This is achieved through multisensory integration, meaning that the CNS combines the information from somatosensory, visual, and vestibular systems, whereas it gives a larger weight to more accurate (and less variable) sensory inputs in any experimental conditions for postural control. For example, when proprioception is perturbed through foot rotation, the CNS relies more on visual and vestibular inputs. A method has been developed by Peterka31 to quantify multisensory integration. For a standing experiment with a specific external perturbation, he identified the FR of the closed loop system and then fitted a parametric model to it (as explained in the previous paragraph). The parametric model comprised a central control, whose input was the weighted sum of the inputs from the three sensory systems; the weights were used to provide a means to quantify the importance of each sensory source to postural control, i.e., the higher the weight, the more important the sensory input. Application of this method to the experimental data showed that the perturbed sensory system has a lower weight and lower importance due to inaccuracy of its input and therefore, contributes less to postural control31. This method has been used to show how the postural control also changes due to ageing and diseases38,39. A similar approach can be used with our experimental apparatus, where mechanical or/and visual perturbation are applied to investigate the role and interaction of the important sensory systems in postural control.
The presented methods have some limitations as the experimental and analytical methods are intended for the study of postural control when an ankle strategy is used. Therefore, the perturbations must be designed to avoid excessive body movement. However, when the perturbations are large or the support surface is compliant, a hip strategy is used, meaning both ankle and hip movements are significant. The hip strategy is characterized by anti-phase movement of the lower and upper body, which is specifically pronounced in frequencies larger than 1 Hz57. Study of hip strategy requires modeling the body with at least two links, i.e., a double-inverted pendulum model.

Human postural control is realized through complex interactions between the central nervous and musculoskeletal systems1. The human body in standing is inherently unstable, subject to a variety of internal (e.g., respiration, heartbeat) and external (e.g., gravity) perturbations. Stability is achieved by a distributed controller with central, reflex, and intrinsic components (Figure 1).
Postural control is achieved by: an active controller, mediated by the central nervous system (CNS) and spinal cord, which changes muscle activation; and an intrinsic stiffness controller that resists joint movement with no change in muscle activation (Figure 1). The central controller uses sensory information to generate descending commands that produce corrective muscle forces to stabilize the body. Sensory information is transduced by the visual, vestibular, and somatosensory systems. Specifically, the somatosensory system generates information regarding the support surface and joint angles; vision provides information regarding the environment; and the vestibular system generates information regarding the head angular velocity, linear acceleration, and orientation with respect to gravity. The central, closed-loop controller operates with long delays that may be destabilizing2. The second element of the active controller is reflex stiffness, which generates muscle activity with short latency and produces torques resisting joint movement.
There is a latency associated with both components of active controller; consequently, joint intrinsic stiffness, which acts with no delay, plays an important role in postural control3. Intrinsic stiffness is generated by passive visco-elastic properties of contracting muscles, soft tissues and inertial properties of the limbs, which generates resistive torques instantaneously in response to any joint movement4. The role of the joint stiffness (intrinsic and reflex stiffness) in postural control is not clearly understood, since it changes with operating conditions, defined by muscle activation4,5,6 and joint position4,7,8, both of which change with the body sway, inherent to standing.
Identifying the roles of the central controller and joint stiffness in postural control is important, as it provides the basis for: diagnosing the etiology of balance impairments; the design of targeted interventions for patients; assessment of the risk of falls; the development of strategies for fall prevention in the elderly; and the design of assistive devices such as orthotics and prosthetics. However, it is difficult, because the different sub-systems act together and only the overall resulting body kinematics, joint torques, and muscle electromyography can be measured.
Therefore, it is essential to develop experimental and analytical methods that use the measurable postural variables to evaluate each subsystem&#8217;s contribution. A technical difficulty is that the measurement of postural variables is done in closed-loop. As a result, the inputs and outputs (cause and effect) are interrelated. Consequently, it is necessary to: a) apply external perturbations (as inputs) to evoke postural reactions in responses (as outputs), and b) employ specialized mathematical methods to identify system models and disentangle cause and effect9.
The present article focuses on postural control when an ankle strategy is used, that is, when the movements occur primarily about the ankle joint. In this condition, upper body and lower limbs move together, consequently, the body can be modeled as a single-link inverted pendulum in sagittal plane10. The ankle strategy is used when the support surface is firm and the perturbations are small1,11.
A standing apparatus capable of applying appropriate mechanical (proprioceptive) and visual sensory perturbations and recording the body kinematics, kinetics, and muscle activities has been developed in our laboratory12. The device provides the experimental environment needed to study the role of ankle stiffness, central control mechanisms, and their interactions by generating postural responses using visual or/and somatosensory stimuli. It is also possible to extend the device to study the role of vestibular system by the application of direct electrical stimulation to the mastoid processes, that can generate a sensation of head velocity and evoke postural responses12,13.
Others have also developed similar devices to study human postural control, where linear piezo electric actuators11, rotary electrical motors14,15, and linear electrical motors16,17,18 were used to apply mechanical perturbations to ankle in standing. More complex devices also have been developed to study multi-segment postural control, where it is possible to apply multiple perturbations to ankle and hip joints simultaneously19,20.
Standing apparatus
Two servo-controlled electrohydraulic rotary actuators move two pedals to apply controlled perturbations of ankle position. The actuators can generate large torques (&#62;500 Nm) needed for postural control; this is especially important in cases such as forward lean, where the body&#8217;s center of mass is far (anterior) from ankle axis of rotation, resulting in large values of ankle torque for postural control.
Each rotary actuator is controlled by a separate proportional servo valve, using pedal position feedback, measured by a high-performance potentiometer on the actuator shaft (Table of Materials). The controller is implemented using a MATLAB-based xPC real-time, digital signal processing system. The actuator/servo-valve together have a bandwidth of more than 40 Hz, much larger than bandwidth of the overall postural control system, ankle joint stiffness, and the central controller21.
Virtual reality device and environment
A virtual reality (VR) headset (Table of Materials) is used to perturb the vision. The headset contains an LCD screen (dual AMOLED 3.6&#8217;&#8217; screen with a resolution of 1080 x 1200 pixels per eye) that provides the user with a stereoscopic view of the media sent to the device, offering three-dimensional depth perception. The refresh rate is 90 Hz, sufficient to provide a solid virtual sense to the users22. The field of view of the screen is 110&#176;, enough to generate visual perturbations similar to real world situations.
The headset tracks the rotation of the user&#8217;s head and alters the virtual view accordingly so that the user is fully immersed in the virtual environment; therefore, it can provide the normal visual feedback; and it can also perturb vision by rotating the visual field in sagittal plane.
Kinetic measurements
Vertical reaction force is measured by four load cells, sandwiched between two plates beneath the foot (Table of Materials). Ankle torque is measured directly by torque transducers with a capacity of 565 Nm and a torsional stiffness of 104 kNm/rad; it also can be measured indirectly from the vertical forces transduced by the load cells, using their distances to ankle axis of rotation23, assuming that horizontal forces applied to the feet in standing are small2,24. Center of pressure (COP) is measured in sagittal plane by dividing the ankle torque by the total vertical force, measured by the load cells23.
Kinematic measurements
Foot angle is the same as pedal angle, because when an ankle strategy is used, the subject&#8217;s foot moves with the pedal. Shank angle with respect to the vertical is obtained indirectly from the linear displacement of the shank, measured by a laser range finder (Table of Materials) with a resolution of 50 &#956;m and bandwidth of 750 Hz25. Ankle angle is the sum of the foot and shank angles. Body angle with respect to the vertical is obtained indirectly from the linear displacement of the mid-point between the left and right posterior superior iliac spines (PSIS), measured using a laser range finder (Table of Materials) with a resolution of 100 &#956;m and bandwidth of 750 Hz23. Head position and rotation are measured with respect to the global coordinate system of the VR environment by the VR system base stations that emit timed infrared (IR) pulses at 60 pulses per second that are picked up by the headset IR sensors with sub-millimeter precision.
Data acquisition
All signals are filtered with an anti-aliasing filter with a corner frequency of 486.3 and then sampled at 1000 Hz with high performance 24-bit/8-channel, simultaneous-sampling, dynamic signal acquisition cards (Table of Materials) with a dynamic range of 20 V.
Safety mechanisms
Six safety mechanisms have been incorporated into the standing apparatus to prevent injuries to subjects; the pedals are controlled separately and never interfere with each other. (1) The actuator shaft has a cam, which mechanically activates a valve that disconnects hydraulic pressure if the shaft rotation exceeds &#177; 20&#176; from its horizontal position. (2) Two adjustable mechanical stops limit the range of motion of the actuator; these are set to each subject&#8217;s range of motion prior to each experiment. (3) Both the subject and the experimenter hold a panic button; pressing the button disconnects hydraulic power from the actuators and causes them to become loose, so they can be moved manually. (4) Handrails located at either side of the subject are available to provide support in case of instability. (5) The subject wears a full body harness (Table of Materials), attached to rigid crossbars in the ceiling to support them in case of a fall. The harness is slack and does not interfere with normal standing, unless the subject becomes unstable, where the harness prevents the subject from falling. In the case of fall, the pedal movements will be stopped manually either by the subject, using the panic button or by the experimenter. (6) The servo-valves stop the rotation of the actuators using fail-safe mechanisms in case of electrical supply interruption.

<table><tbody><tr><td>5K potentiometer</td><td>Maurey</td><td>112P19502</td><td>Measures actuator shaft angle</td></tr><tr><td>8 channel Bagnoli surface EMG amplifiers and electrodes</td><td>Delsys</td><td></td><td>Measures the EMG of ankle muscles</td></tr><tr><td>AlienWare Laptop</td><td>Dell Inc.</td><td>P69F001-Rev. A02</td><td>VR-ready PC laptop</td></tr><tr><td>Data acquisition card</td><td>National instruments</td><td>4472</td><td>Samples the analogue signals from the sensors</td></tr><tr><td>Directional valve</td><td>REXROTH</td><td>4WMR10C3X</td><td>Bypasses the flow if the angle of actuator shaft goes beyond &amp;plusmn;20&amp;deg;</td></tr><tr><td>Full body harness</td><td>Jelco</td><td>740</td><td>Protect the subjects from falling</td></tr><tr><td>Laser range finder</td><td>Micro-epsilon 1302-100</td><td>1507307</td><td>Measures shank linear displacement</td></tr><tr><td>Laser range finder</td><td>Micro-epsilon 1302-200</td><td>1509074</td><td>Measures body linear displacement</td></tr><tr><td>Load cell</td><td>Omega</td><td>LC302-100</td><td>Measures vertical reaction forces</td></tr><tr><td>Proportional servo-valve</td><td>MOOG</td><td>D681-4718</td><td>Controls the hydraulic flow to the rotary actuators</td></tr><tr><td>Rotary actuator</td><td>Rotac</td><td>26R21VDEISFTFLGMTG</td><td>Applies mechanical perturbations</td></tr><tr><td>Torque transducer</td><td>Lebow</td><td>2110-5k</td><td>Measures ankle torque</td></tr><tr><td>Virtual Environment Motion Trackers</td><td>HTC inc.</td><td>1551984681</td><td>Tracks the head motion</td></tr><tr><td>Virtual Reality Headset</td><td>HTC inc.</td><td>1551984681</td><td>Provides visual perturbations</td></tr></tbody></table>

experimental methods to study human postural control

Many components of the nervous and musculoskeletal systems act in concert to achieve the stable, upright human posture. Controlled experiments accompanied by appropriate mathematical methods are needed to understand the role of the different sub-systems involved in human postural control. This article describes a protocol for performing perturbed standing experiments, acquiring experimental data, and carrying out the subsequent mathematical analysis, with the aim of understanding the role of musculoskeletal system and central control in human upright posture. The results generated by these methods are important, because they provide insight into the healthy balance control, form the basis for understanding the etiology of impaired balance in patients and the elderly, and aid in the design of interventions to improve postural control and stability. These methods can be used to study the role of somatosensory system, intrinsic stiffness of ankle joint, and visual system in postural control, and may also be extended to investigate the role of vestibular system. The methods are to be used in the case of an ankle strategy, where the body moves primarily about the ankle joint and is considered a single-link inverted pendulum.

Pseudo random ternary sequence (PRTS) and TrapZ signals
Figure 2A shows a PRTS signal, which is generated by integrating a pseudo random velocity profile. For each sample time <img alt="Equation 76" src="/files/ftp_upload/60078/60078eq76.jpg" />, the signal velocity may be equal to zero, or acquire a pre-defined positive or negative value, <img alt="Equation 77" src="/files/ftp_upload/60078/60078eq77.jpg" />. By controlling <img alt="Equation 77" src="/files/ftp_upload/60078/60078eq77.jpg" /> and <img alt="Equation 78" src="/files/ftp_upload/60078/60078eq78.jpg" />, PRTS inputs with a wide spectral bandwidth can be generated and scaled to different peak-to-peak amplitudes. Furthermore, the PRTS is periodic, but unpredictable, which is desirable for the study of postural control. The reader is referred to the following article for detailed explanation of the PRTS signal31.
Figure 2B shows a TrapZ signal. It starts at a zero value and after a random period <img alt="Equation 79" src="/files/ftp_upload/60078/60078eq79.jpg" /> (whose minimum is <img alt="Equation 80" src="/files/ftp_upload/60078/60078eq80.jpg" />), the signal ramps up randomly to its maximum amplitude (<img alt="Equation 81" src="/files/ftp_upload/60078/60078eq81.jpg" />) with a velocity <img alt="Equation 82" src="/files/ftp_upload/60078/60078eq82.jpg" /> or ramps down to its minimum amplitude (<img alt="Equation 83" src="/files/ftp_upload/60078/60078eq83.jpg" />) with a velocity <img alt="Equation 84" src="/files/ftp_upload/60078/60078eq84.jpg" />. The signal stays at its maximum or minimum for a random period, <img alt="Equation 85" src="/files/ftp_upload/60078/60078eq85.jpg" /> (minimum of <img alt="Equation 80" src="/files/ftp_upload/60078/60078eq80.jpg" />) and then returns to zero with velocity <img alt="Equation 82" src="/files/ftp_upload/60078/60078eq82.jpg" /> or <img alt="Equation 84" src="/files/ftp_upload/60078/60078eq84.jpg" />. The loop starts again from zero. It is evident that unlike the PRTS, the TrapZ is a zero-mean signal, and therefore, does not cause non-stationarity in the postural response. In addition, it is unpredictable, as the timing of change of the signal value and the direction of the change (i.e., positive or negative velocity) are random.
Identification of the body angle to visual perturbations system
Figure 3 shows the signals from a typical standing trial with TrapZ visual perturbations. Figure 3A shows the VR perturbation, where the field of view rotates from 0 to &#177; 0.087 rad (5&#176;) in the sagittal plane. Figure 3C,E shows the ankle and body angles, which are very similar, since the foot angle is zero, and shank and upper body move together. Figure 3G shows the ankle torque, which is correlated with the shank and body angles. Figure 3B,D,F,H shows the EMGs from the ankle muscles. It is evident that SOL and LG are continuously active, MG periodically generates large bursts of activities with body sway, and TA is silent.
Figure 4 shows the FR of the transfer function relating the visual input to the body angle for the data in Figure 3. The first step is to examine the coherence, because gain and phase are meaningful only when the coherence is high (when the coherence is 1, there is a linear noise- free relationship between the input and the output; a coherence less than 1 happens when the input output relationship is nonlinear or the data is noisy). The coherence is the highest at low frequency, between 0.1&#8722;1 Hz and drops significantly at higher frequencies. The gain increases initially from 0.1 Hz to 0.2 Hz and then decreases till 1 Hz, showing the expected low-pass behavior due to body&#8217;s high inertia. The phase also starts at zero and decreases almost linearly with frequency, indicating that the output is delayed with respect to the input.
Identification of ankle intrinsic stiffness parameters
Figure 5 shows the signals measured for a typical perturbed standing trial. Figure 5A shows the pedal perturbation&#8213;a PRBS with a peak-to-peak amplitude of 0.02 rad and a switching interval of 200 ms. The pedal position switches between two values (-0.01 and 0.01) at integer multiples of the switching interval. Figure 5C shows the ankle angle, where the fast changes are due to the foot movement while the other changes are the result of shank movement with sway. Figure 5E shows the body angle in response to the perturbation with a peak-to-peak movement of around 0.04 rad. Figure 5G shows the measured ankle torque; two components are evident: the modulation of the torque with body sway, and large downward peaks, showing the stretch reflex torque response (generally happening after a dorsiflexing pulse). Figure 5B,D,F,H shows the SOL, MG, LG and TA EMGs. It is clear that the TS muscles are continuously active and display large bursts of activity due to stretch reflex responses. TA is mostly silent, except for a few peaks, which seem to be crosstalk from TS muscles, because they occur simultaneously with stretch reflex activity of TS muscles.
Figure 6 shows a typical pulse position perturbation, its velocity and the corresponding SOL EMG and torque response. The intrinsic response starts 25 ms before and last until 40 ms after the peak foot velocity; the peak in the SOL EMG shows the presence of a reflex response. The pre-response segment, starting 50 ms before the peak velocity is used to find the background torque.
Figure 7 shows the intrinsic stiffness as a function the COP position for the left and right sides of the subject shown in Figure 5; the stiffness was estimated using the analysis method presented. It is evident that the intrinsic stiffness is not constant but changes significantly with postural sway. These changes appear functionally appropriate, because the stiffness increases as the COP moves farther from ankle axis of rotation, where there is higher possibility of fall23.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/60078/60078fig1v3.jpg" /> 
Figure 1: Postural control model: the body is inherently unstable and subject to destabilizing gravity torque (<img alt="Equation 87" src="/files/ftp_upload/60078/60078eq87.jpg" />) and disturbances. Stable upright posture is maintained by corrective muscle forces, generated by a central controller, spinal stretch reflexes, and intrinsic mechanical joint stiffness. Muscle activation due to stretch reflex and central contributions is evident in the EMG activity. Only the signals in red can be measured, whereas black signals cannot be measured. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60078/60078fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/60078/60078fig2v3.jpg" /> 
Figure 2: Generation of PRTS and TrapZ signals. (A) PRTS signal. A stimulus is created from a 242-length PRTS sequence, which includes values of 0, 1, and 2, corresponding to fixed velocities of 0, +v, and -v for a fixed duration of <img alt="Equation 88" src="/files/ftp_upload/60078/60078eq88.jpg" />. The velocity is integrated to generate the position, which is used as the perturbation signal. The period of the perturbation signal is equal to <img alt="Equation 89" src="/files/ftp_upload/60078/60078eq89.jpg" />, where m is the stage number of the shift registrar, determining the sequence of the velocity. (B) TrapZ signal. The signal starts at zero; after a random time interval (<img alt="Equation 79" src="/files/ftp_upload/60078/60078eq79.jpg" />), it ramps up or down to its maximum (<img alt="Equation 81" src="/files/ftp_upload/60078/60078eq81.jpg" />) or minimum value (<img alt="Equation 90" src="/files/ftp_upload/60078/60078eq90.jpg" /> with a constant velocity; the signal goes back to zero after a random time interval (<img alt="Equation 85" src="/files/ftp_upload/60078/60078eq85.jpg" />) and the whole loop starts again. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60078/60078fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/60078/60078fig3v3.jpg" /> 
Figure 3: Typical experimental trial with TrapZ visual perturbation; the peak-to-peak perturbation amplitude is 0.174 rad, and the velocity is 0.105 rad/s. (A) VR perturbation angle, showing the rotation of the field of view in sagittal plane. (C) Ankle angle, which is the same as shank angle, as the foot does not move. (E) Body angle. (G) Ankle torque. (B, D, F, H) Raw rectified EMG of SOL, MG, LG, and TA; SOL and LG are continuously active, while MG shows burst of activity associated with body sway, and TA is silent. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60078/60078fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/60078/60078fig4v3.jpg" /> 
Figure 4: Frequency response of the dynamic relation of body angle to visual perturbation estimated from the data presented in Figure 3. Gain (top panel) shows ratio of the amplitude of the output to the input as a function of frequency; it shows a low pass behavior. Phase (middle panel) shows the difference between the input and output phase as a function of frequency. Coherence (bottom panel) provides an index measuring how much of the output power is linearly related to the input power at each frequency. A coherence of 1 shows perfect linear input-output relationship; however, the presence of noise or nonlinearity reduces it. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60078/60078fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/60078/60078fig5v3.jpg" /> 
Figure 5: Typical PRBS position perturbation trial; the peak-to-peak perturbation amplitude is 0.02 rad, and the switching interval is 200 ms. (A) Foot angle, which is the same as the position perturbations since the foot moves with the pedal. (C) Ankle angle; the random changes are due to shank movement with sway. (E) Body angle, obtained assuming the body acts as an inverted pendulum. (G) Ankle torque measured form the load cells data. (B, D, F, H) Raw EMG of SOL, MG, LG, and TA; the TS muscles are all continuously active, while the large peaks reflect stretch reflex activity; TA is mostly silent. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60078/60078fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/60078/60078fig6v4.jpg" /> 
Figure 6: An individual pulse from the trial shown in Figure 5, on an expanded time scale. (A) Foot angle, (B) foot velocity, (C) SOL EMG, and (D) ankle torque. The vertical dotted lines separate the response into the pre-response (25 ms), intrinsic response (65 ms), and reflex response (300 ms); positive torque and angles correspond to dorsiflexion. The data for this figure are taken from Amiri and Kearney23. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60078/60078fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/60078/60078fig7v4.jpg" /> 
Figure 7: Estimated normalized intrinsic stiffness as a function of COP position for the left and right side of a typical subject, obtained from the data shown in Figure 5. Bars indicate the 95% confidence intervals of the stiffness values. The data for this figure are taken from Amiri and Kearney23. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60078/60078fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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Experimental Methods to Study Human Postural Control

This article presents an experimental/analytic framework to study human postural control. The protocol provides step-by-step procedures for performing standing experiments, measuring body kinematics and kinetics signals, and analyzing the results to provide insight into the mechanisms underlying human postural control.

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9.4K Views.  McGill University. This article presents an experimental/analytic framework to study human postural control. The protocol provides step-by-step procedures for performing standing experiments, measuring body kinematics and kinetics signals, and analyzing the results to provide insight into the mechanisms underlying human postural control.

Video: Experimental Methods to Study Human Postural Control

Methods Development for Blood Borne Macrophage Carriage of Nanoformulated Antiretroviral Drugs

Nanoformulated drugs can improve pharmacodynamics and bioavailability while serving also to reduce drug toxicities for antiretroviral (ART) medicines. To this end, our laboratory has applied the principles of nanomedicine to simplify ART regimens and as such reduce toxicities while improving compliance and drug pharmacokinetics. Simple and reliable methods for manufacturing nanoformulated ART (nanoART) are shown. Particles of pure drug are encapsulated by a thin layer of surfactant lipid coating and produced by fractionating larger drug crystals into smaller ones by either wet milling or high-pressure homogenization. In an alternative method free drug is suspended in a droplet of a polymer. Herein, drug is dissolved within a polymer then agitated by ultrasonication until individual nanosized droplets are formed. Dynamic light scattering and microscopic examination characterize the physical properties of the particles (particle size, charge and shape). Their biologic properties (cell uptake and retention, cytotoxicity and antiretroviral efficacy) are determined with human monocyte-derived macrophages (MDM). MDM are derived from human peripheral blood monocytes isolated from leukopacks using centrifugal elutriation for purification. Such blood-borne macrophages may be used as cellular transporters for nanoART distribution to human immunodeficiency virus (HIV) infected organs. We posit that the repackaging of clinically available antiretroviral medications into nanoparticles for HIV-1 treatments may improve compliance and positively affect disease outcomes.

Nanoparticles of indinavir, ritonavir, efavirenz and atazanavir were manufactured using wet milling, homogenization and ultrasonication. These nanoformulations, collectively termed nanoformulated antiretroviral therapy (nanoART), assessed macrophage-based drug delivery. Monocyte-derived macrophage nanoART uptake, retention and sustained release were determined. These preliminary studies suggest the potential of nanoART for clinical use.

Nanoformulated drugs can improve pharmacodynamics and bioavailability while serving also to reduce drug toxicities for antiretroviral (ART) medicines. To ...

The Three-Dimensional Human Skin Reconstruct Model: a Tool to Study Normal  Skin and Melanoma Progression

Most in vitro studies in experimental skin biology have been done in 2-dimensional (2D) monocultures, while accumulating evidence suggests that cells behave differently when they are grown within a 3D extra-cellular matrix and also interact with other cells (1-5). Mouse models have been broadly utilized to study tissue morphogenesis in vivo. However mouse and human skin have significant differences in cellular architecture and physiology, which makes it difficult to extrapolate mouse studies to humans. Since melanocytes in mouse skin are mostly localized in hair follicles, they have distinct biological properties from those of humans, which locate primarily at the basal layer of the epidermis. The recent development of 3D human skin reconstruct models has enabled the field to investigate cell-matrix and cell-cell interactions between different cell types. The reconstructs consist of a "dermis" with fibroblasts embedded in a collagen I matrix, an "epidermis", which is comprised of stratified, differentiated keratinocytes and a functional basement membrane, which separates epidermis from dermis. Collagen provides scaffolding, nutrient delivery, and potential for cell-to-cell interaction. The 3D skin models incorporating melanocytic cells recapitulate natural features of melanocyte homeostasis and melanoma progression in human skin. As in vivo, melanocytes in reconstructed skin are localized at the basement membrane interspersed with basal layer keratinocytes. Melanoma cells exhibit the same characteristics reflecting the original tumor stage (RGP, VGP and metastatic melanoma cells) in vivo. Recently, dermal stem cells have been identified in the human dermis (6). These multi-potent stem cells can migrate to the epidermis and differentiate to melanocytes.

In this report, we describe the three-dimensional skin reconstruct model which mimics human skin in architecture and composition. Melanocyte physiology, melanoma progression and the fate of dermal stem cells have been investigated using the skin reconstruct model. The model is also useful as a preclinical tool for drug assessment.

Most in vitro studies in experimental skin biology have been done in 2-dimensional (2D) monocultures, while accumulating evidence suggests that cells ...

Sit-to-stand-and-walk from 120% Knee Height: A Novel Approach to Assess Dynamic Postural Control Independent of Lead-limb

Individuals with sensorimotor pathology e.g., stroke have difficulty executing the common task of rising from sitting and initiating gait (sit-to-walk: STW). Thus, in clinical rehabilitation separation of sit-to-stand and gait initiation - termed sit-to-stand-and-walk (STSW) - is usual. However, a standardized STSW protocol with a clearly defined analytical approach suitable for pathological assessment has yet to be defined.
Hence, a goal-orientated protocol is defined that is suitable for healthy and compromised individuals by requiring the rising phase to be initiated from 120% knee height with a wide base of support independent of lead limb. Optical capture of three-dimensional (3D) segmental movement trajectories, and force platforms to yield two-dimensional (2D) center-of-pressure (COP) trajectories permit tracking of the horizontal distance between COP and whole-body-center-of-mass (BCOM), the decrease of which increases positional stability but is proposed to represent poor dynamic postural control.
BCOM-COP distance is expressed with and without normalization to subjects&#39; leg length. Whilst COP-BCOM distances vary through STSW, normalized data at the key movement events of seat-off and initial toe-off (TO1) during steps 1 and 2 have low intra and inter subject variability in 5 repeated trials performed by 10 young healthy individuals. Thus, comparing COP-BCOM distance at key events during performance of an STSW paradigm between patients with upper motor neuron injury, or other compromised patient groups, and normative data in young healthy individuals is a novel methodology for evaluation of dynamic postural stability.

Here, we present a novel protocol to measure positional stability at key events during the sit-to-stand-to-walk using the center-of-pressure to the whole-body-center-of-mass distance. This was derived from the force platform and three-dimensional motion-capture technology. The paradigm is reliable and can be utilized for the assessment of neurologically compromised individuals.

Individuals with sensorimotor pathology e.g., stroke have difficulty executing the common task of rising from sitting and initiating gait (sit-to-walk: ...

Behavior

A Vibrotactile Feedback Device for Seated Balance Assessment and Training

Postural perturbations, motion tracking, and sensory feedback are modern techniques used to challenge, assess, and train upright sitting, respectively. The goal of the developed protocol is to construct and operate a sitting platform that can be passively destabilized while an inertial measurement unit quantifies its motion and vibrating elements deliver tactile feedback to the user. Interchangeable seat attachments alter the stability level of the device to safely challenge sitting balance. A built-in microcontroller allows fine-tuning of the feedback parameters to augment sensory function. Posturographic measures, typical of balance assessment protocols, summarize the motion signals acquired during timed balance trials. No dynamic sitting protocol to date provides variable challenge, quantification, and sensory feedback free of laboratory constraints. Our results demonstrate that non-disabled users of the device exhibit significant changes in posturographic measures when balance difficulty is altered or vibrational feedback provided. The portable, versatile device has potential applications in rehabilitation (following skeletal, muscular, or neurological injury), training (for sports or spatial awareness), entertainment (via virtual or augmented reality), and research (of sitting-related disorders).

A sitting platform has been developed and assembled that passively destabilizes sitting posture in humans. During the user&#39;s stabilizing task, an inertial measurement unit records the device&#39;s motion, and vibrating elements deliver performance-based feedback to the seat. The portable, versatile device may be used in rehabilitation, assessment, and training paradigms.

Postural perturbations, motion tracking, and sensory feedback are modern techniques used to challenge, assess, and train upright sitting, respectively. ...

An Instrumented Pull Test to Characterize Postural Responses

Impairment of postural reflexes, termed postural instability, is a common and disabling deficit in Parkinson's disease. To assess postural reflexes, clinicians typically employ the pull test to grade corrective responses to a backward perturbation at the shoulders. However, the pull test is prone to issues with reliability and scaling (score/4). Here, we present an instrumented version of the pull test to more precisely quantify postural responses. Akin to the clinical test, pulls are manually administered except pull force is also recorded. Displacements of the trunk and feet are captured by a semi-portable motion tracking system. Raw data represent distance traveled (in millimeter units), making subsequent interpretation and analysis intuitive. The instrumented pull test also detects variabilities influencing pull test administration, such as pull force, thereby identifying and quantifying potential confounds that can be accounted for by statistical techniques. The instrumented pull test could have application in studies seeking to capture early abnormalities in postural responses, track postural instability over time, and detect responses to therapy.

Impairment of postural reflexes, termed postural instability, is difficult to quantify. Clinical assessments such as the pull test suffer issues with reliability and scaling. Here, we present an instrumented version of the pull test to objectively characterize postural responses.

Impairment of postural reflexes, termed postural instability, is a common and disabling deficit in Parkinson's disease. To assess postural reflexes, ...

Computerized Dynamic Posturography for Postural Control Assessment in Patients with Intermittent Claudication

Computerized dynamic posturography with the EquiTest is an objective technique for measuring postural strategies under challenging static and dynamic conditions. As part of a diagnostic assessment, the early detection of postural deficits is important so that appropriate and targeted interventions can be prescribed. The Sensory Organization Test (SOT) on the EquiTest determines an individual&#39;s use of the sensory systems (somatosensory, visual, and vestibular) that are responsible for postural control. Somatosensory and visual input are altered by the calibrated sway-referenced support surface and visual surround, which move in the anterior-posterior direction in response to the individual&#39;s postural sway. This creates a conflicting sensory experience. The Motor Control Test (MCT) challenges postural control by creating unexpected postural disturbances in the form of backwards and forwards translations. The translations are graded in magnitude and the time to recover from the perturbation is computed.
Intermittent claudication, the most common symptom of peripheral arterial disease, is characterized by a cramping pain in the lower limbs and caused by muscle ischemia secondary to reduced blood flow to working muscles during physical exertion. Claudicants often display poor balance, making them susceptible to falls and activity avoidance. The Ankle Brachial Pressure Index (ABPI) is a noninvasive method for indicating the presence of peripheral arterial disease and intermittent claudication, a common symptom in the lower extremities. ABPI is measured as the highest systolic pressure from either the dorsalis pedis or posterior tibial artery divided by the highest brachial artery systolic pressure from either arm. This paper will focus on the use of computerized dynamic posturography in the assessment of balance in claudicants.

Individuals with intermittent claudication exhibit poor balance compared to healthy controls. Computerized dynamic posturography is an objective method for measuring an individual's postural responses to balance disturbances. This provides an objective reflection of the person&#x2019;s ability to respond to situations under which sensory stimuli are altered and unexpected perturbations occur.

Computerized dynamic posturography with the EquiTest is an objective technique for measuring postural strategies under challenging static and dynamic ...

Medicine

A Modified Lean and Release Technique to Emphasize Response Inhibition and Action Selection in Reactive Balance

Assessment of reactive balance traditionally imposes some type of perturbation to upright stance or gait followed by measurement of the resultant corrective behavior. These measures include muscle responses, limb movements, ground reaction forces, and even direct neurophysiological measures such as electroencephalography. Using this approach, researchers and clinicians can infer some basic principles regarding how the nervous system controls balance to avoid a fall. One limitation with the way in which these assessments are currently used is that they heavily emphasize reflexive actions without any need to revise automatic postural reactions. Such an exclusive focus on these highly stereotypical reactions would fail to adequately address how we can modify these reactions should the need arise (e.g., avoiding an obstacle with a recovery step). This would appear to be a glaring omission when one considers the enormous complexity of the environments we face daily. Overall, the status quo when evaluating the neural control of balance fails to truly expose how higher brain resources contribute to preventing falls in complex settings. The present protocol offers a way to require suppression of automatic, but inappropriate corrective balance reactions, and force a selection among alternative action choices to successfully recover balance following postural perturbation.

Here we offer a protocol that allows the user to selectively change affordances and/or constraints on movements that are relevant for recovering balance after postural perturbation.

Assessment of reactive balance traditionally imposes some type of perturbation to upright stance or gait followed by measurement of the resultant ...

Quantitative Static and Dynamic Assessment of Balance Control in Stroke Patients

In patients with stroke, damage to the central nervous system (CNS) can affect the postural stability and increase the risk of falling. Therefore, accurately assessing the balance is important to understand the type, extent, and causes of balance deficit, and to identify individualized interventions. Clinical assessment methods for balance function can be broadly divided into observation, scale assessment, and balance instrument testing. Here, a clinical protocol is presented for static and dynamic balance assessment in stroke patients, which includes three semiquantitative balance function scale assessments (i.e., Berg Balance Scale, Timed Up and Go Test, and Fugl-Meyer Assessment) and three quantitative instrumental balance evaluation (i.e., Stability Assessment Module, Proprioceptive Assessment Module, and Limit of Stability Module). It is recommended that clinicians consider the use of both classic clinical balance scales and instrumental balance measurements when assessing stroke patients to improve the accuracy of assessments, leading to a better individualized treatment plan.

Presented here is a quantitative, clinical balance assessment method suitable for stroke patients with balance disorders.

In patients with stroke, damage to the central nervous system (CNS) can affect the postural stability and increase the risk of falling. Therefore, ...

Evaluating Postural Control and Lower-extremity Muscle Activation in Individuals with Chronic Ankle Instability

Computerized dynamic posturography (CDP) is an objective technique for the evaluation of postural stability under static and dynamic conditions and perturbation. CDP is based on the inverted pendulum model that traces the interrelationship between the center of pressure and the center of gravity. CDP can be used to analyze the proportions of vision, proprioception, and vestibular sensation to maintain postural stability. The following characters define chronic ankle instability (CAI): persistent ankle pain, swelling, the feeling of &#8220;giving way,&#8221; and self-reported disability. Postural stability and fibular muscle activation level in individuals with CAI decreased due to lateral ankle ligament complex injuries. Few studies have used CDP to explore the postural stability of individuals with CAI. Studies that investigate postural stability and related muscle activation by using synchronized CDP with surface electromyography are lacking. This CDP protocol includes a sensory organization test (SOT), a motor control test (MCT), and an adaption test (ADT), as well as tests that measure unilateral stance (US) and limit of stability (LOS). The surface electromyography system is synchronized with CDP to collect data on lower limb muscle activation during measurement. This protocol presents a novel approach for evaluating the coordination of the visual, somatosensory, and vestibular systems and related muscle activation to maintain postural stability. Moreover, it provides new insights into the neuromuscular control of individuals with CAI when coping with real complex environments.

Individuals with chronic ankle instability (CAI) exhibit postural control deficiency and delayed muscle activation of lower extremities. Computerized dynamic posturography combined with surface electromyography provides insights into the coordination of the visual, somatosensory, and vestibular systems with muscle activation regulation to maintain postural stability in individuals with CAI.

Computerized dynamic posturography (CDP) is an objective technique for the evaluation of postural stability under static and dynamic conditions and ...

Neuroscience

Postural Organization of Gait Initiation for Biomechanical Analysis Using Force Platform Recordings

Gait initiation (GI), the transient phase between orthograde posture and steady-state locomotion, is a functional task and an experimental paradigm that is classically used in the literature to obtain insight into the basic postural mechanisms underlying body motion and balance control. Investigating GI has also contributed to a better understanding of the physiopathology of postural disorders in elderly and neurological participants (e.g., patients with Parkinson&#39;s disease). As such, it is recognized to have important clinical implications, especially in terms of fall prevention.
This paper aims to provide scholars, clinicians, and higher education students information on the material and method developed to investigate GI postural organization via a biomechanical approach. The method is based on force platform recordings and the direct principle of mechanics to compute the kinematics of the center of gravity and center of pressure. The interaction between these two virtual points is a key element in this method since it determines the conditions of stability and whole-body progression. The protocol involves the participant initially standing immobile in an upright posture and starting to walk until the end of an at least 5 m track.
It is recommended to vary the GI velocity (slow, spontaneous, fast) and the level of temporal pressure - gait may be initiated as soon as possible after the deliverance of a departure signal (high level of temporal pressure) or when the participant feels ready (low level of temporal pressure). Biomechanical parameters obtained with this method (e.g., duration and amplitude of anticipatory postural adjustments, step length/width, performance, and stability) are defined, and their computation method is detailed. In addition, typical values obtained in healthy young adults are provided. Finally, critical steps, limitations, and significance of the method with respect to the alternative method (motion capture system) are discussed.

This paper describes the material and method developed to investigate the postural organization of gait initiation. The method is based on force platform recordings and on the direct principle of mechanics to compute center of gravity and center of pressure kinematics.

Gait initiation (GI), the transient phase between orthograde posture and steady-state locomotion, is a functional task and an experimental paradigm that ...