September 27th, 2018 Copyright Motus Global Inc. Page 1 of 4

VALIDATION OF AN IMU SYSTEM FOR MEASURING ANKLE ROM AND GAIT MECHANICS FOR RUNNING ANALYSIS

Motus Global, Rockville Centre, NY

INTRODUCTION

This report outlines the validity of measures collected using a Motus IMU system, by comparing data outputs to equivalent measures captured with a 16-camera motion capture system. The purpose of this tool is to evaluate ankle kinematics (flexion & pronation) during static range of motion (ROM) trials as well as during running (GAIT).

METHODS

Two healthy recreational runners (one male: 30 yrs, 91 kg, 180 cm, one female: 28 yrs, 45 kg, 142 cm) reported to the Motus Biomechanics Laboratory for testing. Upon arrival, participants were outfitted with 30 retro-reflective markers bilaterally on anatomical landmarks of the shank and foot as well as marker clusters surrounding the IMU sensors (Figure 1). Kinematic data were collected at 480 Hz for the MAC system and 1000 Hz for the Motus IMU system.

Figure 1. Experimental setup of marker and sensor placement.

Once participants were outfitted with markers they were led through a series of ankle ROM tests that lasted 5 seconds each. Figure 2 shows an example of these tests being completed. Following the ROM testing subjects were asked to run on a treadmill at a pace of 7 mph. After running for at least 30 seconds a 10 second window of data was collected to evaluate running kinematics.

Figure 2. Example of a neutral FLX and maximum PRN test.

Data Processing & Analysis

Raw positional data from both the MAC and IMU systems were up sampled to 1600 Hz. An across trial average was take for each ROM trial for the entire 5 second collection period. Gait trial data were synced using a custom algorithm for comparison.

All subjects’ ROM data was compiled and validity was examined using a Pearson Correlation (R), Mean Error (ME), and Mean Difference (Bias). The gait mechanics data was compared using a Pearson Correlation (R), Root-Mean-Squared-Error (RMSE), and Mean Difference (Bias).

RESULTS AND DISCUSSION

The descriptive statistics for ankle flexion ROM are reported in Table 1 and a scatter plot of ankle flexion ROM can be seen in Figure 3. The IMU measurement system was able to measure these kinematics with a Bias of 0.01°, a mean error of 2.91°, and a validity of 0.981. These values indicate the IMU as a valid tool for measuring ankle flexion during these ROM trials.

Table 1. Descriptive Statistics for Ankle Flexion using motion capture (MAC) and the motus IMU sensor (IMU).

Metric Value UnitsBias (Mean Difference) 0.01 degStandard Error 2.91 degPearson Correlation (R Value) 0.981

September 27th, 2018 Copyright Motus Global Inc. Page 2 of 4

Figure 3. Scatter plot of ROM Ankle Flexion as measured by motion capture (MAC) and motus IMU sensor (IMU)

The descriptive statistics for ankle pronation ROM are reported in Table 2 and a scatter plot of ankle pronation ROM can be seen in Figure 3. The IMU measurement system was able to measure these kinematics with a Bias of -0.07°, a mean error 2.62°, and a validity of 0.980. These values indicate the IMU as a valid tool for estimating ankle pronation during these ROM trials.

Table 2. Descriptive Statistics for Ankle Pronation using motion capture (MAC) and the motus IMU sensor (IMU).

Figure 4. Scatter plot of ROM Ankle Pronation as measured by motion capture (MAC) and motus IMU sensor (IMU)

The descriptive statistics for the GAIT trial segment Right side sagittal plane rotations (pitch) and frontal plane rotations (roll) are reported in Table 3. These statistics show Bias below 2 degrees for all values, RMSE below 10 degrees, and validity of pitch above 0.97 and Roll above 0.636.

Table 3. Descriptive statistics of Right Leg rotations during running (GAIT) using motion capture (MAC) and the motus IMU sensor (IMU).

The descriptive statistics for the GAIT trial segment Left side sagittal plane rotations (pitch) and frontal plane rotations (roll) are reported in Table 3. These statistics show Bias below 3 degrees for all values, RMSE below 14 degrees, and validity of pitch above 0.932 and Roll above 0.719.

Table 4. Descriptive statistics of Left Leg rotations during running (GAIT) using motion capture (MAC) and the motus IMU sensor (IMU).

The descriptive statistics for the GAIT trial kinematics (Ankle Flexion & Pronation) right and left side can be found in Table 4. These statistics show a Bias of less than 3.5°, a RMSE of 14° or less, and validity above 0.662 for flexion and 0.601 for pronation.

Table 5. Descriptive statistics of Right and Left leg ankle flexion (FLX) and ankle pronation (PRN) using motion capture (MAC) and motus IMU sensor (IMU).

For each segment during the GAIT trial, the motus IMU sensor was plotted against the motion capture (MAC) data and these plots can be found in Appendix A for the Left Leg, and Appendix B for the right leg.

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Metric Value UnitsBias (Mean Difference) -0.07 degStandard Error 2.62 degPearson Correlation (R Value) 0.980

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Metric Pitch Roll Pitch Roll UnitsBias (Mean Difference) 0.71 -0.18 2.00 -0.48 degRoot Mean Squared Error (RMSE) 7.56 3.70 10.62 3.78 degPearson Correlation (R Value) 0.973 0.786 0.960 0.636

Shin Foot

Metric Pitch Roll Pitch Roll UnitsBias (Mean Difference) 0.63 -0.17 2.76 -3.21 degRoot Mean Squared Error (RMSE) 2.98 2.08 13.99 5.55 degPearson Correlation (R Value) 0.996 0.935 0.932 0.719

Shin Foot

Metric FLX PRN FLX PRN UnitsBias (Mean Difference) 0.63 -0.17 2.76 -3.21 degRoot Mean Squared Error (RMSE) 2.98 2.08 13.99 5.55 degPearson Correlation (R Value) 0.872 0.601 0.662 0.607

Right Left

September 27th, 2018 Copyright Motus Global Inc. Page 3 of 4

APPENDIX A. Left Leg

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Figure 5. Left Leg frontal plane kinematic plots of Left Shin Roll (Top), Left Foot Roll (Middle), and Left Ankle Pronation (Bottom). The blue line represents the IMU measure and the red line represents the MAC measure.

Figure 6. Left Leg sagittal plane kinematic plots of Left Shin Pitch (Top), Left Foot Pitch (Middle), and Left Ankle Flexion (Bottom). The blue line represents the IMU measure and the red line represents the MAC measure.

September 27th, 2018 Copyright Motus Global Inc. Page 4 of 4

APPENDIX B. Right Leg

Figure 7. Right Leg frontal plane kinematic plots of Right Shin Roll (Top), Right Foot Roll (Middle), and Right Ankle Pronation (Bottom). The blue line represents the IMU measure and the red line represents the MAC measure.

Figure 8. Right Leg sagittal plane kinematic plots of Right Shin Pitch (Top), Right Foot Pitch (Middle), and Right Ankle Flexion (Bottom). The blue line represents the IMU measure and the red line represents the MAC measure.