Summary of Article: An Engineering Approach To Ambulation, Without The Use Of External Power Sources, Of Severely Handicapped Individuals

R. X. Spielrein, A.M.I.E. Aust.

R. E. Spielrein, A.M.I.E. Aust., is Senior Engineer of the Central Development Unit, Repatriation Department, South Melbourne, Australia. His article, "An Engineering Approach to Ambulation, Without the Use of External Power Sources, of Severely Handicapped Individuals", appeared originally in The Journal of the Institution of Engineers, Australia, in December, 1963. The following summary of the article, by Maurice Bluestein, Project Engineer, Body Parameters Study, Research Division, School of Engineering and Science, New York University, is printed with the permission of Mr. Spielrein and the publishers.

Mr. Spielrein's article describes the theoretical and practical considerations involved in the development of the CDU (Central Development Unit)* Ambulator, a prosthetic walking aid for legless people.** The prototype is shown schematically in Fig. 1 . The Ambulator consists of a bucket-type socket on a base resembling that of a side profile of a hocking chair. Forward movement is attained by rocking to one side, thereby unloading one pylon, and pivoting forward about the contact point under the other pylon. Hence taking a step requires two stages of energy input: (1) the energy to provide the lateral impulse to cause rocking, and (2) the twisting energy to effect torsion about the contact point.

With respect to the first stage, the minimum energy consumption of a system such as described here would be achieved if the side-to-side sway of the body could be made to coincide with the natural frequency of its oscillations. On the basis of prior experience, a period of about two seconds was considered acceptable as a starting point for an average adult patient on whom the experimental work was to be carried out.

The other consideration involved the method to be employed in rotating the body with respect to the feet, and more specifically, the manner in which each foot was to be returned to its neutral (starting) position upon the transfer of the body weight to the other foot. Both these factors were given prime consideration in the design of the Ambulator.

Two Requirements

The lower surfaces of the "feet" are curved in an arc resembling a section of the rim of a wheel. The circumference of such a wheel would constitute a so-called "rolling circle". When the system (subject and Ambulator) rocks sideways to an angle off the vertical neutral axis, this corresponds to the wheel rolling through the same angle. Two requirements must be met to ensure stability during this rocking procedure :

1. The effective center of the rolling circle must be located on the vertical axis above the center of gravity of the system,

2. The functional width of the rolling circle at the feet must be such that at the maximum rock angle, a plumb line dropped from the center of gravity will not fall outside the foot-base.

As an illustration of the sequence of events involved in forward movement, consider the subject in Fig. 1 rocking to his right. The weight of the system is now applied to the floor at the contact point on the foot surface under the right pylon. The subject then swings his left side forward, or clockwise around the point of contact. He then shifts his weight to the left, causing contact to occur under the left pylon. He now swings counterclockwise, brings the right side forward, and continues in this manner to duplicate walking.

Bearings are inserted at the "ankle" to facilitate rotation. Thus when the subject pivots around the right foot, for example, the entire system (except the right foot) swings on the bearing about the stationary right foot. However, the foot is now misaligned with the rest of the system; to bring the foot back to the neutral position with respect to the pylon, torsion bars of rubber are provided. These are depicted in Fig. 2 . Thus, as the load is taken off the right foot, the bar is free to twist back to its neutral (starting) position, bringing the foot into the same aspect as the pylon.

Two Sets of Oscillations

Note that two sets of oscillations are involved: (1) side-to-side rocking and (2) the twisting of the pylon about the foot, transmitted through the torsion bar.

Two parameters of the system must be determined in order to satisfy the required stability and oscillatory conditions. These are the center of gravity of the system and its mass moment of inertia. The procedures involved are based on the theory of the lever, and the theory of the compound pendulum.

In order to determine the center of gravity of the system, the subject and Ambulator are placed on a wooden plank supported at the ends on two knife edges. The left support is a weighing scale and the right support a rigid surface (Fig. 3 and Fig. 4 ). By the principle of moments, the left support force (found by the scale reading) can be related to the position of the center of mass of the system. Thus, taking cognizance of the bending moment about the right support due to the action of (1) the plank with the subject and Ambulator, and (2) the plank alone, the equation to determine the Center of Mass of the subject and Ambulator (h) is defined by Equation 1 , where c is the distance between supports; W1 is the scale reading for plank, Ambulator and subject; w1 is the scale reading for the plank alone; and w is the weight of subject and Ambulator.

Mass Moment of Inertia

To find the mass moment of inertia, a pendulum as depicted in Fig. 5 and Fig. 6 is set up. The subject is fixed to a rigid, swinging platform and oscillated about a horizontal axis for small displacement angles. By measuring the period of oscillation, the mass moment of inertia of the total system about an axis through point A can be found. Swinging the pendulum alone provides its moment of inertia, which can be then subtracted from the above figure to get the moment of inertia for the subject and Ambulator alone. Knowing the mass and the center of mass of the subject and Ambulator, the mass moment of inertia about the axis through the mass center can be found.

The mass center location and the mass moment of inertia, together with the value of the desired period of vibration for the rocking motion, can be used to find the required radius (R) of the rolling circle.

The required parameters were determined for a 16-year-old girl, born almost without legs, who acted as an experimental subject. The subject, fitted into the Ambulator, is depicted in Fig. 7 and Fig. 8 .

In a walking trial, it was observed that correct posture of the patient had been achieved, along with sufficient stability of the system. The resultant overall height was found acceptable by both the medical officers and the patient. After an extensive walking period, the patient reported that she did not feel fatigued, indicating that a low expenditure of energy was involved in maintaining such a motion.


The design and construction of the new type of prosthesis described in this report have now reached a stage where its suitability and advantages for use with severely handicapped legless people are quite apparent. Considerably more work will have to be carried out both in the field of training of the wearers and on the mechanical improvements to the prosthesis itself.

* Prosthetic Unit, Repatriation Department, Commonwealth of Australia.

** Mr. Spielrein acknowledges the contributions of Dr. Eugene F. Murphy and Mr. Anthony Staros, United States Veterans Administration, to the original concept of the item.