TITLE: unexpected perturbations while walking in virtual reality environment
AUTHORS: Uri Rosenblum, Lotem Kribus-Shmiel, Gabi Zeilig, Yotam Bahat, Shani Kimel-Naor, Itshak Melzer, Meir Plotnik

[DESCRIPTION]
<div class = "text-blocks"><div class = "text-block">Walking perturbations are a well-established tool to study human performance of balance tasks. We aimed to characterize walking and balance reactions in adults with and without neurological disease. Participants walked on self-paced treadmill while being exposed to unexpected surface displacement Perturbations. these were introduced at different phases of the gait cycle, on both legs and in anterior-posterior or medio-lateral directions, in random order. </div><div class = "text-block"><div class = "justify" style = "text-align:left"><span>Two types of perturbations were implemented: (1) medio-lateral platform perturbations were achieved by displacing the moving platform 15 cm, to the left or to the right, over 0.92 seconds. The platform held its new position for 30 seconds and then returned gradually to its original position over 3 seconds; (2) anterior-posterior treadmill belt perturbations were achieved by reducing the speed of one of the treadmill belts by 1.2 m/s with a deceleration of 5 m/s</span><span style = "vertical-align:super;">2</span><span style = "vertical-align:super;">.</span></div></div><div class = "text-block"><div class = "justify" style = "text-align:left">Relevant gait phase, gait parameters of step length and width as well as walking speed were detected and calculated from force plate and markers data in the sagittal plane. </div></div><div class = "text-block"><div class = "justify" style = "text-align:left">Force plate data was filtered using a low-pass filter with cutoff frequency of 5Hz and a force threshold of 25N is used; no filters were applied to the marker data.</div></div><div class = "text-block">The protocol has two steps: (1) self-pace learning and (2) walking with unexpected surface displacement perturbations.</div></div>

[STEPS]
?. [Recruit Participants]
Iclusion CriteriaParticipants can be included in the study if they are:* Usually healthy* age 18-90Exclusion criteria Participants cannot be included in the study if they present the following conditions:* obesity (body mass index (kg/m2)> 30 [1]) * orthopedic condition affecting gait and balance (e.g., total knee replacement, total hip replacement, ankle sprain, limb fracture, etc.).* cognitive or psychiatric conditions that could jeopardize the participant’s ability to participate in the study (Mini-Mental State Exam score * heart condition, such as non-stable ischemic heart disease or moderate to severe congestive heart failure. * severe chronic obstructive pulmonary disease. * neurological disease associated with balance disorders (e.g., multiple sclerosis, myelopathy, etc.)
?. [Collect demographic information from participants]
Participant Codeacronym of first and last name + 3 last digits of ID (e.g. UR193)
?. [Collect demographic information from participants]
Date
?. [Collect demographic information from participants]
SexSex of participant.If male, code as 1.If female, code as 0.Missing data should be marked with 555
?. [Collect demographic information from participants]
Date of birthMissing data should be marked with 555
?. [Collect demographic information from participants]
Highthight in meters, two decimal places precisionMissing data should be marked with 555
?. [Collect demographic information from participants]
Leg lengthLength of the right limb from the greater trochanter to the floor in cm.Missing data should be marked with 555
?. [Collect demographic information from participants]
WeightWeight in Kg.Missing data should be marked with 555
?. [Collect demographic information from participants]
BMIBody mass index calculated using the formula kg/m2Missing data should be marked with 555
?. [Collect demographic information from participants]
Years of Educationrecord the number of years of Education for the participantMissing data should be marked with 555
?. [Collect demographic information from participants]
Falls in the last yearDid the participant fall in the last year?If yes, code as 1if no, code as 0Missing data should be marked with 555
?. [Collect demographic information from participants]
If fell, how many times in the last year?Record number of falls in the last yearIf not relevant leave empty.
?. [Cognitive tests]
For older adults perform the Mini Mental State Examination
?. [Cognitive dual task ]
Teach and train participant on cognitive taskPaced auditory sirial addition task of 15 numbers that are read every .the participants are asked to add the numbers quitly in their head. at the end of the reading they have to give the sum of the numbers that were read to them. In the table below are the number series for practice and baseline measure.  ABCDEF1List 1List D'List C'List B'List A'     2  8    9    1    4    7    1  3  5    4    4    6    9    2  4  4    4    7    4    6    3  5  5    4    5    9    6    4  6  3    6    3    7    8    5  7  5    5    2    3    4    6  8  5    2    4    1    3    7  9  9    6    5    1    4    8  10  7    5    2    5    8    9  11  2    5    6    6    5    10  12  9    7    4    6    8    11  13  2    5    6    2    4    12  14  2    6    3    7    3    13  15  7    8    1    7    9    14  16  8    9    8    5    9    15  17  81    85    61    73    93  Sum18                         SumRep
ABCDEF1List 1List D'List C'List B'List A'     2  8    9    1    4    7    1  3  5    4    4    6    9    2  4  4    4    7    4    6    3  5  5    4    5    9    6    4  6  3    6    3    7    8    5  7  5    5    2    3    4    6  8  5    2    4    1    3    7  9  9    6    5    1    4    8  10  7    5    2    5    8    9  11  2    5    6    6    5    10  12  9    7    4    6    8    11  13  2    5    6    2    4    12  14  2    6    3    7    3    13  15  7    8    1    7    9    14  16  8    9    8    5    9    15  17  81    85    61    73    93  Sum18                         SumRep
?. [Main Experiment]
Prepare participant for the experiment* Fit participant with a harness* Explain to the participant that they are tied and cannot fall.* Callibrate vicon system to the participant's range of motion.
?. [Cognitive dual task ]
Record score for List 1Cognitive score List 1 = 100-((|81-X|)/5.4)*6.66
?. [Main Experiment]
Stage I: Self-paced walking – learning and acclimationThe trials starts with acclimation sessions during which participants are introduced to the self-paced walking paradigm. Participants are asked to maintain their comfortable walking speed for one minute, and then to dynamically modify their gait speed, including accelerating, decelerating, coming to a full stop, and resuming their comfortable walking speed. This is repeated until they reef confident and perform the transition from standing to comfortable walking speed smoothly.
Self-paced walking modeThe treadmill was operated in self-paced mode, in which the motor is operated as a function of the instantaneous position and speed of the participant, which are sampled using a motion capture system (see more details in [3]).
?. [Main Experiment]
Stage II: Introducing physical perturbations during walkingThe protocol consisted of walking trials in two different scenes (park and a no hands hanging bridge over a deep canyon). Every scene was used in 4 walking trials of single task and dual task conditions (see Fig. 3 for more details).Once the participant reached steady state walking velocity, one of several types of unexpected perturbations was randomly presented (see Fig.4) in two trials conditions: (a) the single task condition, in which no additional cognitive load was introduced; (b) the dual task condition, in which participants were asked to perform concurrent arithmetic calculations they learned in step 16, higher cognitive load. Participants were instructed to react naturally to the perturbations, to prevent themselves from falling.A total of 18 possible surface perturbations were used in random order to reduce learning effects anticipatory reactions from trial to trial. At least 30 seconds of baseline walking were implemented before and after every perturbation to allow full recovery of the kinematic gait parameters.
Physical  perturbations Perturbation types were classified according to the following criteria: (1) perturbation direction –medio-lateral (i.e. right or left platform displacement) or anterior-posterior (i.e. deceleration of one of the treadmill belts); (2) gait cycle phase – immediately after initial contact, mid-stance, or towards toe off, as detected in real time from foot markers and force plates data; and (3) foot of reference (right or left foot that were detected in the relevant gait phase). An example for perturbation type by criteria would be: (1) platform left in (2) initial contact (3) right =a displacement of the platform to the left was executed when initial contact phase was detected for the right foot. Medio-lateral platform perturbations were achieved by displacing the moving platform 15 cm, to the left or to the right, over 0.92 seconds (perturbation level 12, see Fig. 4). The platform held its new position for 30 seconds, to allow full recovery of the gait parameters (i.e., step length and step width), and then returned gradually to its original position over 3 seconds; (2) anterior-posterior treadmill belt perturbations were achieved by reducing the speed of one of the treadmill belts by 1.2 m/s with a deceleration of 5 m/s2(perturbation level 12, see Fig. 4). To enable anterior-posterior perturbations, the treadmill speed was fixed to the measured average self-paced walking speed 5 seconds prior to perturbation onset. The perturbation was presented when the relevant gait phase was detected, and 3 seconds later the treadmill was once again set to self-paced mode (see perturbation profiles, Fig 4). Relevant gait phase was detected using force plate data and toe marker data in the sagittal plane. Force plate data was filtered using a low-pass filter with cutoff frequency of 5Hz and a force threshold of 25N is used; no filters were applied to the marker data. Initial contact was detected when the vertical force from the foot in front (leading limb) exceeded a minimum for of 35N (minimum force was chosen to avoid force plate baseline noise); mid-stance was detected 200ms after initial contact; and toe off was detected when the vertical force from the force plate on the side of the trailing limb went under the minimum force. Once the relevant gait phase was detected the main system computer gave a command to the platform or the treadmill belt to execute the perturbation. Since there was a lag time between the detection of the relevant gait phase and perturbation execution, we re-evaluated the actual perturbation type post-hoc. Gait phases and leading limb were defined using the heel markers’ position in the sagittal plane [4]. Initial contact and toe off were detected when the heel marker was at its maximal forward or backward position, respectively, with 0.025 seconds margins. Perturbation that was introduced beyond this margins was considered a mid-stance perturbation. Perturbation magnitude and timing (see experimental procedure below) were controlled and modified by the VR system computer.
Capturing gait parametersGait speed was obtained from an encoder on the drive shaft of the treadmill motor. Spatial-temporal gait parameters (i.e., step times, step length, and step width) were obtained from a Vicon (Vicon Motion Systems, Oxford, UK) motion capture system. Gait cycle phases were detected using foot marker and force plate data.
?. [Cognitive tests]
Colors Trails Test: the test consists of two parts. In part 1 the participant is asked to connect the numbers 1-25 with lines as fast as they can. In part 2 the objective is the same but every number appears twice, in yellow and pink colors. The participant has to alternate between the colors as they connect the numbers from 1-25. Participants performed part 1 and part 2.
?. [Cognitive tests]
Digit Span Test: the participant is required to repeat sequences of numbers read to them by the examiner. First they repeat them forward (from start to end) and then they have to repeat them backwards (from end to start).
?. [Cognitive tests]
NeuroTrax (NeuroTrax Corporation, NY, USA) computerized battery of cognitive tests which assesses different cognitive domains related to motor and postural control.
?. [Clinical walking tests ]
Time Up and Go Test: The test starts with the participant seated in a chairs with arm rests. Than the participant is asked to get up and walk 3 meters to a mark on the floor, turn around and go back to sit in the chair, in their comfortable speed. The test is ended when the participant is seated in the chair. The test is times from the moment the participants starts to get up to the time they seat back in the chair at the end. The test was Performed twice in comfortable walking speed.
?. [Clinical walking tests ]
10 Meter Walking Test:  The participant is asked to walk in their comfortable walking speed and the result of the test is the time to complete 10 meters in the middle of a 30-meter-long corridor. The test was repeated 4 times.
?. [Clinical walking tests ]
10 Meter Walking Test with a concurrent cognitive task: Participants performed the 10 meter walk test with a concurrent cognitive task of subtracting 7 from a random three digits number. The test was repeated 4 times.
?. [Clinical walking tests ]
2 Minute Walking Test: the participant has to cover the greatest distance they can in two minutes of walking back and forth in a 30-meter-long corridor. The result of the test in the distance covered during the trial.
?. [Clinical walking tests ]
High Level Mobility Assessment Tool (HiMAT): a high level functional test that includes tasks such as fast walking, running, climbing stairs and jumping.
?. [Clinical walking tests ]
Dynamic gait index (DGI) test: A high level walking balance assessment tool that includes tasks such as walking with looking in different directions, changing walking directions, stepping over an obstacle, turning and climbing stairs.
?. [Main Experiment]
Prepare participant for the experiment* Fit participant with a harness* Explain to the participant that they are tied and cannot fall.* Callibrate vicon system to the participant's range of motion.
?. [Main Experiment]
Stage I: Self-paced walking – learning and acclimationThe trials starts with acclimation sessions during which participants are introduced to the self-paced walking paradigm. Participants are asked to maintain their comfortable walking speed for one minute, and then to dynamically modify their gait speed, including accelerating, decelerating, coming to a full stop, and resuming their comfortable walking speed. This is repeated until they reef confident and perform the transition from standing to comfortable walking speed smoothly.
Self-paced walking modeThe treadmill was operated in self-paced mode, in which the motor is operated as a function of the instantaneous position and speed of the participant, which are sampled using a motion capture system (see more details in [3]).
?. [Main Experiment]
Stage II: Introducing physical perturbations during walkingThe protocol consisted of walking trials in two different scenes (park and a no hands hanging bridge over a deep canyon). Every scene was used in 4 walking trials of single task and dual task conditions (see Fig. 3 for more details).Once the participant reached steady state walking velocity, one of several types of unexpected perturbations was randomly presented (see Fig.4) in two trials conditions: (a) the single task condition, in which no additional cognitive load was introduced; (b) the dual task condition, in which participants were asked to perform concurrent arithmetic calculations they learned in step 16, higher cognitive load. Participants were instructed to react naturally to the perturbations, to prevent themselves from falling.A total of 18 possible surface perturbations were used in random order to reduce learning effects anticipatory reactions from trial to trial. At least 30 seconds of baseline walking were implemented before and after every perturbation to allow full recovery of the kinematic gait parameters.
Physical  perturbations Perturbation types were classified according to the following criteria: (1) perturbation direction –medio-lateral (i.e. right or left platform displacement) or anterior-posterior (i.e. deceleration of one of the treadmill belts); (2) gait cycle phase – immediately after initial contact, mid-stance, or towards toe off, as detected in real time from foot markers and force plates data; and (3) foot of reference (right or left foot that were detected in the relevant gait phase). An example for perturbation type by criteria would be: (1) platform left in (2) initial contact (3) right =a displacement of the platform to the left was executed when initial contact phase was detected for the right foot. Medio-lateral platform perturbations were achieved by displacing the moving platform 15 cm, to the left or to the right, over 0.92 seconds (perturbation level 12, see Fig. 4). The platform held its new position for 30 seconds, to allow full recovery of the gait parameters (i.e., step length and step width), and then returned gradually to its original position over 3 seconds; (2) anterior-posterior treadmill belt perturbations were achieved by reducing the speed of one of the treadmill belts by 1.2 m/s with a deceleration of 5 m/s2(perturbation level 12, see Fig. 4). To enable anterior-posterior perturbations, the treadmill speed was fixed to the measured average self-paced walking speed 5 seconds prior to perturbation onset. The perturbation was presented when the relevant gait phase was detected, and 3 seconds later the treadmill was once again set to self-paced mode (see perturbation profiles, Fig 4). Relevant gait phase was detected using force plate data and toe marker data in the sagittal plane. Force plate data was filtered using a low-pass filter with cutoff frequency of 5Hz and a force threshold of 25N is used; no filters were applied to the marker data. Initial contact was detected when the vertical force from the foot in front (leading limb) exceeded a minimum for of 35N (minimum force was chosen to avoid force plate baseline noise); mid-stance was detected 200ms after initial contact; and toe off was detected when the vertical force from the force plate on the side of the trailing limb went under the minimum force. Once the relevant gait phase was detected the main system computer gave a command to the platform or the treadmill belt to execute the perturbation. Since there was a lag time between the detection of the relevant gait phase and perturbation execution, we re-evaluated the actual perturbation type post-hoc. Gait phases and leading limb were defined using the heel markers’ position in the sagittal plane [4]. Initial contact and toe off were detected when the heel marker was at its maximal forward or backward position, respectively, with 0.025 seconds margins. Perturbation that was introduced beyond this margins was considered a mid-stance perturbation. Perturbation magnitude and timing (see experimental procedure below) were controlled and modified by the VR system computer.
Capturing gait parametersGait speed was obtained from an encoder on the drive shaft of the treadmill motor. Spatial-temporal gait parameters (i.e., step times, step length, and step width) were obtained from a Vicon (Vicon Motion Systems, Oxford, UK) motion capture system. Gait cycle phases were detected using foot marker and force plate data.