An Instrumented Pull Test to Characterize Postural Responses

The overall goal of this procedure is to assess postural responses more precisely using an instrumented version of the clinical pull test. This is achieved by using a semi-portable, motion-tracking system to capture postural responses to a manual backward pull. First, apply the harness and attach sensors to the trunk and feet.

The next step is to ensure the participant is standing in comfortable stance along line markings, looking forwards. Next, the instrumented pull test procedure is performed. Pull force is recorded during each trial.

Magnitude and reaction time of the trunk and step responses are computed. The final step is to assist the participant out of the harness and detach the sensors from the trunk and feet. Ultimately, the instrumented pull test is used to characterize postural responses that are produced by the clinical pull test.

It can also be used to determine variabilities that influence pull test performance. The main advantage of this method is its capability to provide precise quantification of postural responses to the clinical pull test. Using semi-portable motion capture, the instrumented pull test offers a method of measurement that is more accessible compared to conventional laboratory techniques.

This allows researchers to easily adopt this technique in the measurement of postural responses to the pull test. The instrumentation of clinical pull test has potential utility in research studies that seek to quantify postural instability in for example, over time in a cohort of patients or in response to therapies for balance problems, particularly in conditions such as Parkinson's disease. Visual demonstration of this method is critical because it is important for investigators to understand the sequence of events.

These include equipment setup, patient preparation, and administration of instrumented pull test by an examiner. In short, all equipment is set up correctly prior to data collection. Prepare the electromagnetic motion tracker with three miniature motion sensors according to manufacturer guidelines.

Each sensor is sampled at minimum 250 hertz with displacement measured in millimeters and rotation in degrees. All internal filtering must be disabled and the position of the sensor set to reference of static origin. Affix load cell to patient harness at shoulder level using rope with minimum diameter of 10 millimeters.

Connect the load cell to data acquisition unit. Depending on load cell specifications, a pre-amplifier and separate power supply may be required. Connect the trigger output from the data acquisition unit into the trigger input of the motion tracker to ensure synchronized recording.

Match the sampling rate of the motion tracker and disable any filtering. The trigger may also be used to time the delivery of auditory or visual stimuli for further characterization of balance mechanisms. The experiment should be conducted in a quiet room to minimize distractions during the assessment.

Allow sufficient space for the participant to take several corrective steps to regain balance. Foam mats may be placed on the floor as a precautionary measure. Begin by obtaining informed consent from the participant.

Participants appropriate for the instrumented pull test procedure include those who are able to stand independently and generate a corrected balance response not requiring assistance to recover. Request the participant wears comfortable, loose clothing on the day of the experiment and remove shoes for the test procedure. Assist the participant to wear the harness.

Clip the buckles around the chest and waist. Ensure straps are adequately tightened, not allowing more than 50 millimeters of slack in the harness when pulling on the rope. Attach motion sensors using medical tape to the sternal notch and on the feet of the right and left ankle malleolus.

All cables must be routed carefully to avoid trip hazards. Position the participant on the mat with feet aligned along the markings in comfortable stance. The participant should note their feet position on the lines so that they're able to return to the same position after each pull.

The assessor should prompt the participant to return to the original feet position if any deviations are observed. Instruct the participant to focus on artwork 1.5 meters ahead at eye level with hands by their side to minimize distractions between pulls. The instrumented pull test is performed according to the clinical pull test guidelines described in the Unified Parkinson's Disease Ratings scale.

First, explain the test procedure, and let the participant know stepping is allowed following the backward pull. Discourage anticipatory responses such as forward leaning of the trunk prior to the pull. Ensure the participant is attentive, standing upright, with eyes open, and feet at the designated markers in a comfortable stance.

Stand behind the participant and apply a quick, forceful pull on the rope. Ensure the rope is held perpendicular to the shoulder level of the participant. After each pull, ensure the participant returns to the original feet positioning on the floor, and repeat the test 35 times.

Allow the participant a short rest up to a minute after every 10 pulls. Request the participant refrain from speaking in between trials until a rest is allowed, unless expressing discomfort or requiring an earlier break. The assessor should be prepared to catch the participant at all times.

An assistant is required for safety when participants with known postural instability are assessed. Detach sensors and assist the participant out of the harness following completion of the instrumented pull test procedure. Data recorded during the study must be analyzed with suitable data science software.

Use trigger signals to align the motion tracker and load cell data. High pass filter or motion tracking and load cell data with a 0.05 hertz cut-off frequency to remove baseline grift. Double differentiate the trunk motion tracking displacement data to obtain trunk velocity and acceleration.

Separate the continuous recording into individual trial epochs. Reject trials where anticipatory truncal movement is observed prior to pull administration. Determine posture reaction time as the difference between the onset of trunk displacement following the pull and the turning point of the trunk velocity curve, which indicates the beginning of trunk deceleration.

Determine the magnitude of the postural response as the peak deceleration of the trunk. Stepping is defined as the foot moving past the stance foot in the backwards direction. Calculate step reaction time as the difference between the onset of truncal displacement to the initial movement of the stepping limb.

Determine step response magnitude by calculating the total displacement of the feet in millimeters from initial foot lift-off to contact of the stepping limb arresting backward retropulsion. Exclude steps less than 50 millimeters as the changing base of support is considered negligible. Calculate peak pull force and rate of force development from the load cell.

The peak pull force indicates the instantaneous maximum force delivered, whereas the force rate is the slope of the force versus time curve indicating how rapidly the force was generated. Postural responses from a cohort of young healthy participants were assessed using the instrumented pull test. 35 trials were performed with an auditory stimulus delivered concurrently with each pull at either 90 decibels or 116 decibels.

Referred to as the start react effect, the startling 116 decibels stimulus is expected to speed reaction time beyond the lesser intensity 90 decibels stimulus. Of the 35 trials, the first was kept to analyze unhabituated responses, and four subsequent trials discarded to allow for practice effects. Subsequent trials analyzed comprised of 20 normal intensity trials at 90 decibels and 10 loud trials at 116 decibels that were randomly intermixed.

Analysis was conducted using linear mixed models due to multiple contributing factors that could influence trunk and step postural responses. These factors include pull force and participant height and weight. The instrumented pull test was able to distinguish first trial effects, where responses from the very first trial were different compared to subsequent habituated trials.

During the first trial, step reaction time was slower, and step response magnitude was larger. StartReact effects were only present in the trunk to subsequent habituated pulls. A loud auditory stimulus accelerated truncal reaction time and increased truncal response magnitude.

Examiner peak pulled force was found to influence types of stepping responses and trunk reaction times. Participant weight influenced step reaction times. Otherwise, participant weight and height did not influence results.

Depending on the participant's physical function and balance abilities, the instrumented pull test procedure should take approximately 20 minutes. This also allows for up to a minute's rest after every 10 pull test trials. In patients with known postural instability, it is important to prevent an actual fall during the experiment.

An assistant should be present and the participant needs to be constantly monitored for fatigue and their ability to participate. Instrumentation of the clinical pull test has yielded a method to objectively quantify postural instability. This is potentially useful in tracking postural instability over time and also in the detection of emerging treatments for postural instability.