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Lower-Limb Prosthetics 253

1500

Viscous damping B (Nm/s) 900
1200

600

300

0
0 0.5 1 1.5 2 2.5
Average walking speed v (m/s)
Fig. 8 Estimated viscous damping B vs average walking speed v. (From Bertos, G.A.,
Childress, D.S., Gard, S.A., 2005. The vertical mechanical impedance of the locomotor system
during human walking with applications in rehabilitation. In: IEEE 9th International Con-
ference on Rehabilitation Robotics. IEEE, New York, pp. 380–383.)

The values of the proposed walking model along with estimates of
damping ratio ζ, stiffness k, and damping B of other activities of human like
bouncing and running are illustrated in Table 1 (Bertos, 2006).
Following this work, Smyrli et al. (2018) performed simulations with a
passive biped model with leg compliance and damping, and semicircular
feet, and studied its stability as a function of a set of nondimensional
parameters.
A unified walking theory model of able-bodied walking is needed for the
prostheses of the future to take advantage of nature’s optimized evolution.
Nevertheless, for unilateral amputees, the prosthetic side must match the
unaffected side. In addition, it will generate more symmetry between the
two sides, if the prosthesis matches a unified walking model.

4.1 Design Intelligence of Human Legs

A philosophical argumentative quest for where the “design intelligence” of
human legs as designed by nature comes from is given by Blickhan et al.
(2007). Is it due to structural mechanics? Does the foot serve a purpose
for walking? Blickhan et al. (2007) noted: “By placing the foot with its heels
and shifting the point of pressure toward the toes, the foot acts like the rim of
a wheel” which is in support of the roll-over shape that was introduced by
Gard and Childress (1997a,b) in Section 4.