-50 -100 -60 0 5 10 15 Time (seconds) (a) 20 100 0 -100 -200 -70 200 100 100 Frequency (Hz) 200 0 -100 0 5 10 15 Time (seconds) (c) 200 100 100 -100 -200 0 5 10 15 Time (seconds) (e) 200 100 100 -100 -200 0 5 10 15 Time (seconds) (g) 0 5 10 15 Time (seconds) (d) 20 0 5 10 15 Time (seconds) (f) 20 0 5 10 15 Time (seconds) (h) 20 0 -100 -200 20 20 -100 200 0 10 15 Time (seconds) (b) 0 -200 20 5 -100 200 0 0 0 -200 20 Frequency (Hz) Frequency (Hz) Frequency (Hz) Frequency (Hz) -40 0 -200 Frequency (Hz) -30 100 -200 200 -20 Frequency (Hz) Frequency (Hz) 200 figure 4. Spectrograms of typical motion profiles [9]. (a) Fall backward. (b) Fall backward with arm motion. (c) Fall forward. (d) Fall forward with arm motion. (e) Sit and stand. (f) Fast sit and stand. (g) Bend and stand up. (h) Fast bend and stand up. (Figure reproduced with permission of the Institution of Engineering and Technology.) IEEE SIgnal ProcESSIng MagazInE | March 2016 | 77