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

The protocol described below follows the guideline of the human research ethics committee of the Université Paris-Saclay. Participants approved and signed a consent form.

1. Participants

1. Include at least 15 healthy young adult participants in the experiment (aged 20 to 40 years old).
   NOTE: This recommended number of subjects corresponds to what is classically considered in the literature on GI.
2. Exclude participants with walking aids, visual, hearing, or orthopedic problems, identified neurological disorders, dementia, cognitive impairments (i.e., a score < 25 on the Mini Mental State Exam), and a medical history of falling.
3. Ask participants to provide written consent after informing them of the nature and purpose of the experiment.
4. Ensure that the experiment conforms to the standards set by the Declaration of Helsinki.

2. Laboratory preparation

1. Ensure that the force platform is long enough to have the entire swing foot land on it at the end of the first step. If it is not, use two small distance force platforms, with participants standing in the initial posture on the first one and striking their swing foot on the second one placed in front of the first²¹. In both cases, ensure that the force platform(s) is/are embedded in a track at least 5 m long to ensure that steady-state walking is reached.
   NOTE: A force platform that registers the 3D moments and forces is necessary to compute the whole set of experimental variables (see section 5).
   1. As a safety measure, affix a harness to the ceiling and center it to the grand axe of the force platform in case the experiment includes frail patients (e.g., neurological patients).
2. Calibrate the force platform(s). Click on the auto-zero button.
3. Importing the journals
   1. Open Qualisys Track manager.
   2. Choose and open the "Project" folder.
4. Create a patient folder.
   1. Click on Add, then select patients.
   2. Enter labels: Patient ID, First name, Last name, Date of birth, Sex, and Comment if needed.
   3. Click on Add, then select Gait session.
   4. Enter labels: Case ID, Test operator, Comments if needed, Diagnosis, Secondary Diagnosis, Affected Side, Gross Motor Function Classification, Functional Mobility Scale, Height, Weight, Leg length left, Leg length right, Knee width left, Knee width right, Ankle width left, Ankle width right, Sole delta left, Sole delta right, Shoulder offset left , Shoulder offset right, Elbow width left, Elbow width right, Wrist width left, Wrist width right, Hand thickness left, Hand thickness right, and Marker diameter.
   5. Click on Add, then select Markerless session.
   6. Enter labels: Test Condition, Prothesis_Orthosis, External aid, External aid side, Personal aid, Personal aid side, Comments if needed, Test operator, and Event mode (choose multiple force plate).
5. Check Force plate auto-zero.
   1. Select Tools.
   2. Click on Force Plates.
   3. Click on On preview start in the label box "Force plate auto-zero".
6. Ensure that the baseline signals from the force platform (forces and moments) are at zero when it is uncharged.
   1. Click on New or use shortcut Ctrl+N.
   2. Click on Data info Window 1 or use shortcut Ctrl+D.
   3. Click on Display Force Data or use shortcut Ctrl+D.
   4. Click on Force and select Plot.

3. Experimental procedure

1. Ask participants to stand barefoot and immobile on a force platform in their natural upright posture, with the arms hanging loosely against their sides, and their gaze directed to a target at eye level at least 5 m away (Figure 1).
   NOTE: Delineate the position of the feet on the force platform in the initial posture (e.g., with chalk). Carefully check that the participants reposition their feet on these marks after each trial. This point is important since the initial foot position influences the APA features of GI.
2. Determine the participants' preferential starting leg by pushing lightly against the participants' back while in the initial posture with their eyes closed to provoke a step forward.
3. Explain to the participants that the task they are to perform is to initiate gait from the standing posture with the preferred leg, to continue walking to the end of the track, and then to return quietly to the initial standing posture.
   NOTE: If during the experiment gait is not initiated with the identified preferred leg in a given trial, repeat the trial.
4. Explain that gait is to be initiated following two successive signals (acoustic, visual, or tactile): a preparatory signal and a departure signal (see steps 3.6 and 3.7).
5. Explain the instructions on velocity and temporal pressure (see steps 3.8-3.10).
6. Deliver the first (preparatory) signal to the participants. Instruct them to stand immobile and avoid anticipating GI at this first signal.
7. Deliver the second (departure) signal following a random 2-5 s delay after the preparatory signal.
   1. Ensure that the participants are visually immobile before delivering this second signal. Check immobility online with the time plots of the anteroposterior or mediolateral center of pressure displacement
      NOTE: If they are not immobile, detection of APA onset (step 5.1.1) may be difficult.
8. Instruct the participants to either initiate gait i) as soon as possible (i.e., in a reaction time condition), or ii) only once they feel ready (i.e., in a self-initiated condition) following the departure signal.
9. Vary the conditions of "temporal pressure" imposed on GI (i.e., low temporal pressure (self-initiated condition) and high temporal pressure (reaction time condition)).
10. Vary the conditions of GI velocity (slow, spontaneous, fast conditions).
    1. To limit the number of experimental conditions and thus avoid fatigue, instruct the participants to perform only two conditions of GI velocity (e.g., slow and fast) under a low or high temporal pressure condition, or the reverse (i.e., GI at a slow or fast velocity under a high and a low temporal pressure condition).
       NOTE: Repeat the instructions on temporal pressure and GI velocity frequently.
11. Instruct the participants to perform series of 10 successive trials in each experimental condition.
    NOTE: Series of five trials are sufficient for elderly subjects or patients with Parkinson's disease²².
    1. Randomize the conditions of GI velocity and temporal pressure across the participants to avoid order effects.
12. Impose a rest of at least 2 min between successive conditions to avoid the effects of fatigue.
13. In each condition, allow participants to perform two familiarization trials before the recordings.
14. Trigger data acquisition from the force platform a few seconds before the onset of the preparatory signal and stop once the participant has left the force platform.

Figure 1: Experimental setup. The participants initially stand on a force platform (1) embedded in a track at least 5 m long (2), with the gaze directed toward a target at eye level (3). Please click here to view a larger version of this figure.

4. Processing of force platform kinetics recordings

1. Filter data from the force platform using a no-lag low-pass Butterworth order with a 15 Hz cut-off frequency.
   1. Import the file.
   2. Open Visual3D.
   3. Choose and open the file "Project".
   4. Processing
      1. Click on Pipeline or use shortcut F11.
      2. Select Signal Filter.
      3. Select Lowpass_Filter.
      4. Click on Execute.
2. Collect data from the force platform at a rate of 100 Hz.
   1. Click on Pipeline or use shortcut F11.
   2. Select File Save/Export.
   3. Select Export_Data_To_Acsii_File.
   4. Click on Edit.
   5. Enter 100 in the label Number of Points for Normalization.
   6. Click on Execute.
3. Compute the time plots of the instantaneous center of gravity accelerations along the anteroposterior (x''G), mediolateral (y''G), and vertical (z''G) directions from the 3D ground reaction forces obtained with the force platform (see Supplemental Figure S1) using Newton's second law^10,23.
   NOTE: According to Newton's second law, the sum of the external forces applied to a system is equal to the mass of this system (m) multiplied by the acceleration of its center of gravity. Thus, with the GI protocol described in this study, the sole external forces applied to the participants are body weight (BW) and ground reaction forces (R). Equations (1), (2), and (3) can be written:
   x''G = Rx / m     (1)
   y''G = Ry / m     (2)
   z"G = (Rz – BW) / m     (3)
   Where Rx, Ry, Rz are the instantaneous anteroposterior, mediolateral, and vertical components of the vector ground reaction force, respectively. Typical plots of x''G, y''G, and z''G are shown in Figure 2.
4. Compute the 3D time plots of the center of gravity velocity by means of a simple numerical integration of the 3D center of gravity acceleration time plots, using integration constants equal to zero (i.e., 3D initial center of gravity velocity considered as null¹⁰). See Figure 2 for typical time plots of anteroposterior, mediolateral, and vertical velocity of the center of gravity (x'G, y'G, and z'G, respectively).
5. Perform an additional integration of the y'G time plot to obtain the displacement of the center of gravity along the mediolateral direction. Use this quantity to compute the "margin of stability" (see step 5.3.5.2).
6. Compute the mediolateral (yP) and anteroposterior (xP) displacement of the center of pressure from force platform data using equations (4) and (5):
        (4)
        (5)
   Where Mx and My are the instantaneous moments around the anteroposterior and mediolateral directions, respectively; Rx, Ry, and Rz are the instantaneous anteroposterior, mediolateral, and vertical ground reaction forces, respectively; and dz is the distance between the surface of the force platform and its origin (provided by the manufacturer). Typical time plots of xP and yP are shown in Figure 2 (see also Supplemental Figure S2).

5. Experimental variables

NOTE: Each experimental variable described below must be extracted from the experimental time plots obtained for each trial.

1. Detection of the timing events of gait initiation
   1. Onset of APA
      1. Display the time plots of the center of pressure displacement along the mediolateral and anteroposterior directions.
      2. Compute the mean value of the mediolateral and anteroposterior center of pressure time plot during the 250 ms time window preceding the second signal delivered to the participants.
         NOTE: These values correspond to the "baseline values" of these time plots.
      3. Detect the instants following the second signal when the mediolateral and the anteroposterior center of pressure displacement trace deviates 2.5 standard deviations from the baseline value for at least 50 ms.
         NOTE: These two instants correspond to the onset of APA along the mediolateral and anteroposterior directions (t0ML and t0AP, respectively; Figure 2). These two instants may also be identified as the instants when the time plots of the mediolateral and anteroposterior center of gravity acceleration reach 10% of their respective peak value.
      4. Ensure that, in the reaction time condition, the onset of APA ranges between 150 ms and 300 ms after the second (Go) signal. If not, repeat the trial and the instructions on temporal pressure.
         NOTE: If it is less than 150 ms, the participants have anticipated. If it is greater than 300 ms, the participants were not focused on the task.
      5. Ensure that in the self-initiated condition, the onset of APA is greater than 300 ms. If it is not, repeat the trial and the instructions on temporal pressure as the participants may have initiated gait in a reaction time condition.
   2. Swing heel-off time
      1. Display the time plots of the vertical center of gravity velocity and anteroposterior center of pressure displacement.
      2. Identify the instant when the trace of the vertical center of gravity velocity first peaks downward after APA onset as the swing heel-off time²⁴ (Figure 2). Alternatively, identify the instant when the time plot of the anteroposterior center of pressure displacement shows a rapid drop toward the baseline (i.e., toward the toes; Figure 2) or place a foot switch (an inexpensive tool) at the swing heel.
   3. Swing toe-off time
      1. Display the time plots of the mediolateral and anteroposterior center of pressure displacement and of the anteroposterior velocity of the center of gravity.
      2. Identify the instant when the time plot of the mediolateral center of pressure displacement reaches a first (quasi) plateau directed toward the stance foot side as the swing toe-off time (Figure 2). Alternatively, identify the instant following swing heel-off when the time plot of the anteroposterior center of pressure displacement reaches 90% of the maximum backward value, or place a foot switch at the swing toe.
   4. Swing foot contact time
      1. Display the time plots of the anteroposterior center of pressure displacement.
      2. Identify the instant when the anteroposterior center of pressure is abruptly shifted forward (Figure 2) as the swing foot contact time. If this time plot is derived, identify the swing foot contact time as the instant when this derived time plot increases sharply from its baseline level value. Alternatively, place a foot switch at the swing heel to detect this instant.
         NOTE: A method similar to that previously described above for APA detection (based on the computation of a mean baseline level value; step 5.1.1.2) can be used here.
   5. Rear foot-off time
      1. Display the time plot of the mediolateral center of pressure displacement.
      2. Identify the instant when the time plot of the mediolateral center of pressure displacement reaches a second (quasi) plateau, directed in the opposite direction as the first one (step 5.1.3.2; Figure 2), the rear foot-off time²⁵. Alternatively, place a foot switch at the rear to detect this instant.
2. Computation of temporal variables
   1. Compute the delay between the onset of APA (t0ML and t0AP) and the swing heel-off time (tHO) for both the mediolateral and anteroposterior directions, which correspond to the duration of APA along the mediolateral (dAPAML) and the anteroposterior directions (dAPAAP). See equations (6) and (7).
      dAPAML = tHO – t0ML     (6)
      dAPAAP = tHO – t0AP     (7)
   2. Compute the delay between swing toe-off time (tTO) and swing heel-off time (tHO), which corresponds to the "unloading phase" duration (UNLd; Figure 2) using equation (8).
      UNLd = tTO – tHO     (8)
   3. Compute the delay between swing toe-off time (tTO) and swing foot contact (tFC), which corresponds to the "swing phase" duration (SWINGd; Figure 2) using equation (9).
      SWINGd = tFC – tTO    (9)
3. Computation of spatial variables
   1. Initial position of the center of pressure
      1. Display the time plots of the center of pressure displacement along the mediolateral and anteroposterior directions.
      2. Compute the mean values of the mediolateral (yP0) and anteroposterior (xP0) center of pressure positions during the 250 ms time window preceding the second (departure) signal delivered to the participants, which are representative of the center of pressure position in the initial posture (or "baseline" value).
         NOTE: The spatio-temporal features of APA described above are sensitive to the position of the center of pressure in the initial posture²⁶. Hence, it is important to check that any change in the APA features between experimental conditions (e.g., a condition with an obstacle to clear vs. a condition without an obstacle to clear) or between experimental populations (e.g., healthy participants vs. neurological participants) cannot be ascribed to a "simple" change in the center of pressure position in the initial posture, but rather to the factor being investigated.
   2. Amplitude of APA
      1. Display the time plots of the center of pressure displacement and center of gravity velocity along the mediolateral and anteroposterior directions.
      2. Detect the instant when each of these four time plots reaches a maximal value during the APA time window (Figure 2).
      3. Subtract the mean center of pressure baseline value computed in step 5.3.1.2 (i.e., the xP0 and yP0 values) from the maximal center of pressure value detected during the APA time window (for each direction; i.e., compute using equations (10) and (11)).
         xPAPA = xPMAX – xP0     (10)
         yPAPA = yPMAX – yP0     (11)
         Where xPAPA and yPAPA are the amplitude of APA (center of pressure) along the anteroposterior and mediolateral directions, respectively; xPMAX and yPMAX are the maximal anticipatory center of pressure displacement along the anteroposterior and mediolateral directions, respectively.
         ​NOTE: No such baseline subtraction is necessary for center of gravity velocity since it is considered that the participants are initially immobile (the initial center of gravity velocity is therefore null; see step 4.4). The four values obtained are representative of the amplitude of APA (two values per direction).
   3. Step length and step width
      1. Display the time plot of the center of pressure displacement along the anteroposterior direction.
      2. Detect the most backward position of the center of pressure position, xPBACK.
      3. Detect the center of pressure position at the time of rear foot-off, xPRFO (Figure 2 and step 5.1.5).
      4. Compute the spatial difference between these two quantities, which corresponds to step length, L⁴¹, using equation (12).
         L = xPBACK – Xprfo     (12)
      5. Display the time plot of the center of pressure displacement along the mediolateral direction.
      6. Detect the most lateral position of the mediolateral center of pressure position obtained during the first plateau of the time plot, yPSTANCE ("stance", because the center of pressure is located under the stance foot at that time; see Figure 2).
      7. Detect the lateral center of pressure position at the rear foot-off time, yPRFO (Figure 2 and step 5.1.5).
      8. Compute the spatial difference between these two quantities, which corresponds to step width, W²⁵, using equation (13).
         ​W = yPSTANCE – yPRFO     (13)
   4. Performance of gait initiation
      1. Display the time plot of the center of gravity velocity along the anteroposterior direction (Figure 2).
      2. Detect the instant when the participants strike the force platform with the swing foot (step 5.1.4, Figure 2) and note the velocity of the center of gravity at this instant as a criterion of GI performance.
         NOTE: The peak value of this time plot, which is reached a few milliseconds after swing foot contact, can also be considered as a criterion of GI performance. Step length and swing phase duration can also be considered as criteria of GI performance. The longer and the shorter these quantities are, respectively, the better the performance is.
   5. Stability control parameters
      1. For braking index, display the time plot of the center of gravity velocity along the vertical direction. Detect the peak downward center of gravity velocity of the time plot (z'GMIN) and the center of gravity velocity at the swing foot contact time (z'GFC, Figure 2). Compute the difference between these two quantities, termed the "braking index" (BI), as an indicator of stability control, using equation (14).
         BI =      (14)
         NOTE: The BI was introduced by Do and colleagues and provides evidence that the central nervous system anticipates the swing foot strike with the support surface by decreasing the vertical center of gravity velocity during the swing phase of gait initiation^4,5,27. This active braking facilitates stability maintenance after foot strike. The greater the BI, the better the stability control.
      2. For the margin of stability, display the time plots of the center of gravity velocity and displacement along the mediolateral direction. Detect the velocity (y'GFC) and the displacement of the center of gravity (yGFC) at swing foot contact time (Figure 2). Compute the mediolateral component of the margin of stability (MOS) at foot contact using equation (15).
              (15)
         Where BOSmax is the mediolateral boundary of the base of support (BOS) and ω0 is the eigenfrequency of the body, modeled as an inverted pendulum. During GI, participants systematically land on the force platform first with the swing heel, then with the toe. Under such a foot landing strategy, the BOSmax can be estimated with the mediolateral center of pressure position at the time of rear foot-off (step 5.1.5). The eigenfrequency of the body can be computed using equation (16).
              (16)
         Where g = 9.81 m/s² is the gravitational acceleration and l is the length of the inverted pendulum, which corresponds to 57.5% of body height.
         NOTE: The quantity in brackets in equation (15) is termed the "extrapolated center of mass"¹⁸. The condition of the stability at foot contact implies that the extrapolated center of mass is located within the base of support. This condition corresponds to a positive MOS value. If the MOS is negative, corrective postural adjustments are required to recover balance.