. 2020 Aug 10;20(16):4474. doi: 10.3390/s20164474

Table A1.

The full feature set (84 features) that was used in the machine-learning training.

No. of Features	Feature Description	Back Sensor	Leg Sensors
Time domain
6	Mean( $x$ )	Acceleration, $x \in {V, AP}$	$\frac{\min (\| {\bar{x}}_{RL} \|, \| {\bar{x}}_{LL} \|)}{\max (\| {\bar{x}}_{RL} \|, \| {\bar{x}}_{LL} \|)}$ $\| {\bar{x}}_{RL} - {\bar{x}}_{LL} \|$ Acceleration, $x \in V$ Gyroscope, $x \in ML$
6	SD( $x$ )	Acceleration, $x \in {V, AP}$	$\max (\| SD (x_{RL}) \|, \| SD (x_{LL}) \|)$ $\| {\bar{x}}_{RL} - {\bar{x}}_{LL} \|$ Acceleration, $x \in V$ Gyroscope, $x \in ML$
6	Correlations( $x, y$ )	Acceleration, $(x, y) \in {V-ML, V-AP, AP-ML}$	$\max (corr (x_{RL} {, y}_{RL}), corr (x_{LL} {, y}_{LL}))$ Gyroscope, $(x, y) \in {V-ML, V-AP, AP-ML$ }
1	Correlations between the right and left leg	.	$corr (x_{RL} {, x}_{LL})$ Gyroscope, $x \in ML$
7	Range(cumulative sum ( $x$ ))	Acceleration, $x \in {V, AP}$ Gyroscope: $x \in ML$	$\min (\begin{matrix} range (cumulative sum (x_{RL})), \\ range (cumulative sum (x_{LL})) \end{matrix})$ $\max (\begin{matrix} range (cumulative sum (x_{RL})), \\ range (cumulative sum (x_{LL})) \end{matrix})$ Acceleration, $x \in V$ Gyroscope, $x \in ML$
2	RMS ( $x$ )	Acceleration, $x \in {V, AP}$	.
2	RMS ( $x, y, z)$ ; total RMS across all axes	Acceleration, $(x, y, z) \in {V-AP-ML}$	$\max (RMS (x_{RL} {, y}_{RL} {, z}_{RL}), RMS (x_{LL} {, y}_{LL} {, z}_{LL}))$ Gyroscope, $(x, y, z) \in {V-AP-ML}$
1	$\frac{\max (RMS (x_{RL} {, y}_{RL} {, z}_{RL}), RMS (x_{LL} {, y}_{LL} {, z}_{LL}))}{RMS (x_{back} {, y}_{back} {, z}_{back})}$	Legs: Gyroscope, $(x, y, z) \in {V-AP-ML}$ Back: Acceleration, $(x, y, z) \in {V-AP-ML}$
1	$\frac{\max (RMS (x_{RL} {, y}_{RL} {, z}_{RL}), RMS (x_{LL} {, y}_{LL} {, z}_{LL}))}{RMS (x_{back} {, y}_{back} {, z}_{back})}$	Legs: Gyroscope, $(x, y, z) \in {V-AP-ML}$ Back: Gyroscope, $(x, y, z) \in {V-AP-ML}$
Frequency domain
16	SD( $x$ ); SD of the frequency amplitude in specific bands	Acceleration, $x \in {\begin{matrix} V_{0.5 - 3 Hz, 3 - 8 Hz}, \\ {AP}_{0.5 - 3 Hz, 3 - 8 Hz} \end{matrix}}$	$\min (SD (x_{RL}), SD (x_{LL}))$ $\max (SD (x_{RL}), SD (x_{LL}))$ Acceleration, $x \in V_{0.5 - 3 Hz, 3 - 8 Hz}$ Gyroscope, $x \in {ML}_{0.5 - 3 Hz, 3 - 8 Hz}$
24	Peak amplitude of $x$ and its frequency across all frequencies and in specific bands	Acceleration, $x \in {\begin{matrix} V_{all, 0.5 - 3 Hz, 3 - 8 Hz}, \\ {AP}_{all, 0.5 - 3 Hz, 3 - 8 Hz} \end{matrix}}$	${\max (x}_{RL} {, x}_{LL})$ Acceleration, $x \in {\begin{matrix} V_{all, 0.5 - 3 Hz, 3 - 8 Hz}, \\ {AP}_{all, 0.5 - 3 Hz, 3 - 8 Hz} \end{matrix}}$ Gyroscope, $x \in {\begin{matrix} V_{all, 0.5 - 3 Hz, 3 - 8 Hz}, \\ {ML}_{all, 0.5 - 3 Hz, 3 - 8 Hz} \end{matrix}}$
3	Entropy of $x$ in specific bands	Acceleration, $x \in {AP}_{0.5 - 3 Hz, 3 - 8 Hz}$ Gyroscope, $x \in V_{0.5 - 3 Hz, 3 - 8 Hz}$	.
1	Freezing index ( $x$ ); $\frac{integrated power spectral density in 3 - 8 Hz}{integrated power spectral density in 0.5 - 3 Hz}$	.	Acceleration, $x \in {AP}_{0.5 - 3 Hz, 3 - 8 Hz}$ $freezing index = \max (\begin{matrix} {freezing index}_{RL}, \\ {freezing index}_{LL} \end{matrix})$ $freezing index = {\begin{matrix} 1 & if freezing index > 2.5 \\ 0 & else \end{matrix}$
1	Total power: the average power of $x$	While turning: Gyroscope, $x \in V$ Otherwise: Acceleration, $x \in AP$	.
5	Skewness of $x$ in specific bands	Acceleration, $x \in {\begin{matrix} V_{0.5 - 3 Hz, 3 - 8 Hz}, \\ {AP}_{0.5 - 3 Hz, 3 - 8 Hz} \end{matrix}}$ Gyroscope, $x \in {ML}_{0.5 - 3 Hz}$	.
4	Kurtosis of $x$ in specific bands	Acceleration, $x \in {\begin{matrix} V_{0.5 - 3 Hz, 3 - 8 Hz}, \\ {AP}_{0.5 - 3 Hz, 3 - 8 Hz} \end{matrix}}$	.

RL, right leg; LL, left leg; V, vertical axis; AP, anterior-posterior axis; ML, medio-lateral axis; SD, standard deviation; corr, correlation; RMS, root-mean-square.