TABLE II.

RMSE (ml/kg/min) and R² for VO₂ estimation from different feature sets of SCG (Amplitude, Ampl, Frequency, Freq and Time, Time) and ECG using XGB_oost, and HR using linear regression model

		Treadmill Protocol		Outside Walking Protocol
Feature Set		RMSE	R2	RMSE	R2
Ampl		4.06±1.06	0.76	4.8±1.53	0.52
Freq		3.68±0.98	0.77	4.85±1.31	0.57
Time		5.42±1.39	0.45	5.13±1.18	0.45
ECG		7.48±1.83	0.17	5.81±1.07	0.4
Ampl+ECG		4.24±1.18	0.72	4.46±1.56	0.58
Freq+ECG		3.99±1.28	0.71	4.30±1.47	0.64
Time+ECG		5.07±1.79	0.5	4.89±1.66	0.47
Ampl+Freq		3.78±0.98	0.78	4.79±1.53	0.54
Ampl+Time		4.04±1.27	0.68	4.93±1.62	0.47
Freq+Time		3.9±1.33	0.67	4.92±1.41	0.48
Ampl+Freq+ECG		3.98±1.27	0.75	4.52±1.52	0.59
Ampl+Time+ECG		4.27±1.54	0.64	4.81±1.67	0.48
Freq+Time+ECG		4.27±1.61	0.61	4.69±1.65	0.51
Ampl+Freq+Time		3.87±1.18	0.73	4.95±1.57	0.47
Ampl+Freq+Time+ECG		4.12±1.4	0.68	4.75±1.66	0.5
HRa		6.31±1.72	0.44	5.94±1.76	0.35
Selected 25 Featuresb		3.76±1.15	0.77	4.28±1.44	0.63

^aA simple linear regression model was used for HR only. For other feature sets, XGBoost regression model was used to generate the reported results.

^bSelected using SFS method