Table 8.

Features commonly extracted and applied in the wearable-based AR.

	Features
ACC	Time-domain: Statistical features (e.g., mean, median, standard deviation, absolute integral, correlation between axes), first and second derivative of acceleration energy Frequency-domain: Power ratio (0–2.75 Hz and 0–5 Hz band), peak frequency, entropy of the normalised power spectral density References: [45,137,153,154]
ECG/PPG	Time-domain: Statistical features (e.g., mean, median, 20th and 80th percentile), HR, HRV, statistical features on HRV (e.g., Root Mean Square of Successive Differences (RMSSD), Standard Deviation of the RR Intervals (SDNN)), number and percentage of successive RR intervals differing by more than 20 ms (NN20, pNN20) or 50 ms (NN50, pNN50), pNN50/pNN20 ratio, Frequency-domain: Ultra low (ULF, 0–0.003 Hz), very low (VLF, 0.003–0.03 Hz), low (LF, 0.03–0.15 Hz), and high (HF, 0.15–0.4 Hz) frequency bands of HRV, normalised LF and HF, LF/HF ratio Non-linear: Lyapunov exponent, standard deviations ($S{D}_1$ and $S{D}_2$) from Poincaré plot, $S{D}_1/S{D}_2$ ratio, sample entropy Geometrical: triangular interpolation index Multimodal: respiratory sinus arrhythmia, motion compensated HR , respiration-based HRV decomposition References: [56,63,89,95,96,155]
EDA	Time-domain: Statistical features (mean, standard deviation, min, max, slope, average rising time, mean of derivative, etc.) Frequency-domain: 10 spectral power in the 0–2.4 Hz bands SCL features: Statistical features, degree of linearity SCR features: Number of identified SCR segments, sum of SCR startle magnitude and response durations, area under the identified SCRs References: [56,63,147,148,149,156]
EMG	Time-domain: Statistical features, number of myoresponses Frequency-domain: Mean and median frequency, energy References: [34,35,69]
RESP	Time-domain: Statistical features (e.g., mean, median, 80th percentile) applied to: inhalation (I) and exhalation (E) duration, ratio between I/E, stretch, volume of air inhaled/exhaled Frequency-domain: Breathing rate, mean power values of four subbands (0–0.1 Hz, 0.1–0.2 Hz, 0.2–0.3 Hz and 0.3–0.4 Hz) Multimodal: RSA References: [34,46,95,157,158]
TEMP	Time-domain: Statistical features (e.g., mean, slope), intersection of the y-axis with a linear regression applied to the signal References: [13,113]