  The applicability of Doppler radar for gait analysis is investigated by
quantitatively comparing the measured biomechanical parameters to those
obtained using motion capturing and ground reaction forces. Nineteen
individuals walked on a treadmill at two different speeds, where a radar system
was positioned in front of or behind the subject. The right knee angle was
confined by an adjustable orthosis in five different degrees. Eleven gait
parameters are extracted from radar micro-Doppler signatures. Here, new methods
for obtaining the velocities of individual lower limb joints are proposed.
Further, a new method to extract individual leg flight times from radar data is
introduced. Based on radar data, five spatiotemporal parameters related to
rhythm and pace could reliably be extracted. Further, for most of the
considered conditions, three kinematic parameters could accurately be measured.
The radar-based stance and flight time measurements rely on the correct
detection of the time instant of maximal knee velocity during the gait cycle.
This time instant is reliably detected when the radar has a back view, but is
underestimated when the radar is positioned in front of the subject. The
results validate the applicability of Doppler radar to accurately measure a
variety of medically relevant gait parameters. Radar has the potential to
unobtrusively diagnose changes in gait, e.g., to design training in prevention
and rehabilitation. As contact-less and privacy-preserving sensor, radar
presents a viable technology to supplement existing gait analysis tools for
long-term in-home examinations.
