Ahmad Najmuddin Yasuhiro Fukuoka Shota Ochiai Experimental development of stiffness adjustable foot sole for use by bipedal robots walking on uneven terrain. 248-253 2012 SII https://doi.org/10.1109/SII.2012.6426940 conf/sii/2012 db/conf/sii/sii2012.html#NajmuddinFO12