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A perturbation technique for determining linear time-invariant systems with an unknown input using ensemble data is presented. The procedure divides the unknown input and the perturbation through high-pass filtering and transforms the multi-input single-output system identification into single-input single-output identification. Experiments indicated that mechanical characteristics of the human elbow joint change with voluntary movement speed and that the mean stiffness with voluntary movement is in the range of the posture and is higher than previously reported.

This paper investigates whether torso force feedback on treadmill-style locomotion interfaces can substitute for treadmill tilt to simulate walking on smooth inclines. The experimental platform is the Sarcos Treadport, whose active mechanical tether can apply horizontal forces to the user to simulate the gravity forces experienced in slope walking. The authors show that users are extremely sensitive to slope while walking, being able to discriminate a 0.5 degree slope change. Comparisons are then made between walking on a tilted treadmill platform versus walking on a level platform but with tether force application. Psychophysical experiments show that users select tether forces that are predicted by the gravity forces, although at a 65% fractional force level. These results demonstrate definitively that torso force feedback can realistically simulate gravity forces during smooth slope walking. [ABSTRACT FROM AUTHOR]