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.

All experimental methods have been approved by the McGill University Research Ethics Board and subjects sign informed consents before participating.
1. Experiments
NOTE: Each experiment involves the following steps.
<ol>
	<li>Pre-test
	<ol>
		<li>Prepare a definite outline of all trials to be performed and make a checklist for data collection.</li>
		<li>Provide the subject with a consent form with all the necessary information, ask them to read it thoroughly, ans...

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 perturbati...

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 &#177;20&#176;</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_uplo...

Watch this Scientific Journal Video about Experimental Methods to Study Human Postural Control at JoVE.com

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.

Many components of the nervous and musculoskeletal systems act in concert to achieve the stable, upright human posture. Controlled experiments accompanied ...

These experimental and analytical methods provide guidelines that aim to understand to role of the many components of the central nervous and musculoskeletal systems in human postural control. Postural control models with meaningful physical parameters can be used to investigate the role and interaction of the sensory systems and their changes due to disease and aging. These methods can be used to assess patient balance problems, to reveal the ideology of the impairment, and to aid in the design of interventions for improving postural control.These methods can also be used to study the interactions between sensory motor pathologies and balance control. For example, for fall prevention in the elderly. This protocol provides a means to investigate the relative contributions of sensory modalities including proprioceptive and visual systems and their interactions as well as muscle passive contributions to postural control.To prepare a subject for the measurement of electromyography from ankle muscles, use single differential electrodes with an interelectrode distance of one centimeter. Mark the medial gastrocnemius at the most prominent bulge of the muscle, the lateral gastrocnemius at one third of the line between the head of the fibula and the heel, the soleus at two thirds of the line between the medial condyles of the femur and the medial malleolus, and the tibialis anterior at one third of the line between the tip of the fibula and the tip of the medial malleolus. When all of the points have been marked, use a razor to shave each area and clean the skin with alcohol.When the skin has dried, use double sided tape to attach one electrode to each area taking care that each electrode is fixed to the skin securely. To prepare the subject for a kinematic measurement, first use a strap to attach a reflective marker as high as possible on the subject's shank and have the subject put on the body harness. Use a strap to attach a reflect marker to the subject's waist and have the subject climb on to the standing apparatus.Adjust the subject's foot position to align the lateral and medial malleoli of each leg to the pedal axis of rotation and use a marker to outline the foot positions. Instruct the subject to keep their feet in the same locations during the experiments and adjust the vertical position of the laser range finders to point to the center of the reflective markers. Then, adjust the horizontal distance between the laser range finder and the reflective markers so that the range finders work in their mid range and do not saturate during standing experiments.Before beginning the experiment, inform the subject of what to expect for each trial condition. Instruct the subject to stand quietly with hands at the side while looking forward maintaining their balance as they do so when faced with the real world perturbations. For a quiet standing trial, have the subject stand still for two minutes with no perturbations.For perturbed experiments, if the objective is to investigate the role of somatosensory system or ankle stiffness in standing, apply pedal perturbations for two to three minutes while recording the data. If the objective is to examine the role of vision in postural control, apply visual perturbations by rotating the visual field using the virtual reality headset for two to three minutes while recording the data. If the objective is to examine the interaction of the two systems in postural control, apply the visual and pedal perturbations simultaneously.For non-parametric identification of the dynamic relation of the body angle to visual perturbations after loading the visually perturbed trial data into a suitable analysis software program, use the commands as indicated to decimate the raw body angle and the visual perturbation signals and remove the means from the decimated signals. Determine the decimated sampling frequency, then select the lowest frequency of interest to determine the window length and choose the degree of overlap for the estimation of power spectra. Define the vector of frequencies at which the frequency response is to be estimated.Use the TF Estimate function to find the frequency response of the system as indicated and find the gain and phase of the estimated frequency response as demonstrated. Then, use the command as indicated to calculate the coherence function and plot the gain, phase, and coherence as a function of frequency. In this example of a typical standing trial with visual perturbations, a trapezoidal signal applied by the virtual reality headset can be observed where the field of view rotates from zero to plus or minus 0.087 rad in the sagittal plane.The ankle and body angles were very similar in this analysis since the foot angle is zero and the shank and upper body move together. The ankle torque was also correlated with the shank and body angles. Electromyographs from the ankle muscles demonstrate that the soleus and the lateral gastrocnemius are continuously active, but the medial gastrocnemius periodically generates large bursts of activity with body sway and that the tibialis anterior is silent.Here, a frequency response of the transfer function relating the visual input to the body angle for the standing trial data is shown. In this experiment, the coherence was high at low frequencies up to around one hertz and dropped significantly at higher frequencies, meaning the frequency response is meaningful up to one hertz. The gain initially increased from 0.1 to 0.2 hertz before decreasing to one hertz, demonstrating the expected low pass behavior due to the body's high inertia.The phase also started at zero and decreased almost linearly with the frequency indicating that the output was delayed with respect to the input. Take care to align the ankle axis of rotation with that of the actuator. Make sure that the subject does not generate extra movements and ensure that the appropriate mechanical and visual perturbations are used.First time users may have difficulty in establishing a consistent repeatable experimental paradigm and in using the appropriate identification methods that account for close loop, non-linear, and time varying effects in postural control. These methods have been used to investigate healthy postural control and its adaptation as well as to quantify changes in balance control under a variety of experimental and clinical conditions.

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