Measurement of Spatial Stability in Precision Grip

To obtain a comprehensive understanding of the Grip Force control of grasping, it is important to evaluate a finger force direction. We suggest a new simpler technique for measuring spatial stability in precision grasping, to evaluate finger force direction in a clinical setting. Demonstrating the procedure, will be Ryoto Teshima, a PhD student from our laboratory.

Before beginning an analysis, connect two sensor connector cables to the USB ports of a computer and pull up the lever attached to one sensor connector, to allow insertion of the sensor tab into the insertion slot. Return the attached lever to its original position, and open the sensor software in the computer. If the sensor sheets are correctly connected, real time pressure distribution maps will appear on the monitor.

To adjust the pressure, insert the sensing areas of the sensor sheet one by one into a compressor rig and turn on the air valve of the compressor controller. Begin applying the pressure and adjust the regulator to the appropriate load value. While pressure is being applied, select Tools and Equilibration and click Equilibrate1 and start, in the equilibration dialog box.

The color of equilibration of the window, should change to gray. At the end of the equilibration, select Save Eq.File, enter the file name and click Save, to save the equilibration. Click Okay.

Select Tools and Calibration and click Add, to enter the load value in Newtons in the Applied Force box. Click Start and check the calibration result, to confirm that the calibration was performed correctly. Here, the Newton value is 134.33 and the value of the loaded cells matches that of the sensor sheets being used.

Then click SaveCal. File, enter the file name and click Save, to save the calibration. Place three to five millimeter long pieces, of double-sided tape, onto each of the four corners of the iron cube.

Use the tape to attach the pressure sensitive parts of the two sensor sheets, to both sides of the iron cube and place the iron cube on top of a setting stand, on a table. Set up the devices for a measurement. Then open the equilibration and calibration files in the software.

After arranging the measurement environment, select Options and Acquisition Parameters in the software. In the Data Acquisition Parameter dialog box, enter 36, 000 in the Movie Frames box, 0.01 in the Period box and 100 in the Frequency box, and click Okay. To perform a measurement, have the participant sit in front of a table and adjust the table height to the participant's shoulder joint flection of zero degrees, and a 90 degree elbow joint flection position.

Set the iron cube and setting stand 30 centimeters from the participant, in the mid sagittal plane on the table, and wipe the participant's finger pulps with an alcohol swab or towelette. Instruct the participant to use minimal force with their thumb and index finger, to grasp both sides of the iron cube to which the sensor sheets are attached, and to lift the cube approximately five centimeters above the setting stand. Holding it for five to seven seconds, before placing it back on the setting stand.

When the participant is ready, click Record and Center of Force trajectory, to monitor the Center of Pressure in the software and give them a cue to start the task. When the task has been completed, click Stop, and save the recorded movie data. To assess task performance in the presence of visual interference, first allow the participant to touch the sensors without exceeding 0.5 Newtons of pressure, before instructing the participant to close their eyes and use minimal force with their thumb and index finger, to lift the iron cube approximately five centimeters above the setting stand.

Then hold the cube for five to seven seconds, before placing it back on the setting stand. To assess task performance in the presence of cognitive interference, instruct the participant as a calculation task, continuously subtract seven from 100 as accurately as possible, while using minimal force with their thumb and index finger, to lift the iron cube approximately five centimeters above the setting stand, for five to seven seconds, before placing it back on the setting stand. To assess task performance in the presence of contralateral hand movement interference, place a peg board 30 centimeters away from the participant, next to the iron cube in the mid sagittal plane, and place the appropriate size and number of pegs, according to the task difficulty.

Then instruct the participant to manipulate the iron cube with minimal force using their thumb and index finger, and to lift and hold the iron cube approximately five centimeters above the setting stand with one hand while, inverting the pegs with the other hand. To analyze the Grip Force, open the movie file for analysis in the dialog box. When the recorded pressure distribution map appears, click Multiple Windows View, to open the graph window.

Locate the point in time at which the load starts to be applied in each lift and note the time with reference to the graph. Save the Grip Force data in ASCII format and open the movie file. Locate the frame number at which the load started to be applied in each lift in the spreadsheet, and note the frame number in reference to the time point at which the load was applied.

Then calculate the total Grip Force used in a range, which is the sum of the values from the cell that was applied. To analyze the Center of Pressure, open the movie file for analysis, to display the Center of Pressure trajectory, on the pressure distribution map. With the recorded pressure distribution map active, click Play Forward, to play the movie.

Click Stop and advance frame by frame, until the frame at which the Center of Pressure begins to appear in each lift, on the pressure distribution map. Note the frame number and save the Center of Pressure data in ASCII format, making sure that Center of Force and Whole Movie are selected. Open the movie file and locate the frame number at which the Center of Pressure begins to appear in each lift on the spreadsheet, with reference to the noted frame number.

Then calculate the Center of Pressure trajectory length between frames. Here the Center of Pressure trajectories and Grip Force traces of the dominant index finger, in single and dual tasks for representative young and elderly adults are shown. In this representative analysis, the Grip Force increased in the contralateral hand movement interference, while the Center of Pressure trajectories tended to decrease.

Be sure to inform that participants to use a minimal force when performing the tasks. Otherwise, they will tend to use a relatively strong Grip Force, to avoid a spatial instability.