Abstract

The stability of all animals is an extensive feedback network which operates on all the muscles and joints in the body. The challenges that bipedal animals face is much more complex than those associated with four legged animals. Because of the nature of such a feedback system, it is natural that oscillations will occur as a human attempts to balance and perform even the simplest of maneuvers. A close observation of any person will show that they are swaying about a point as they stand.

Background

Because of the nature of the oscillations and the similarity of structure among humans, it is possible to use a measurement of stability to identify neuromuscular disorders, injuries and other problems that would manifest themselves in how well a person is able to hold a position.

The body is in constant motion. There is quite a bit of noise that is generated by the body itself just by attempting to remain still. Only a certain amount of the muscles are contracting at any given time, making the force being generated a probabilistic event as opposed to a constant value. Other contributing factors will be breathing, heart rate, and any type of environmental considerations.

The major oscillations are what I considered in the experiment. I began with the proposition that each major oscillation is directly related to a muscle or muscle group (i.e. hamstrings and quadriceps) that oppose each other on a joint. On a simple level, as the quadriceps extends the knee, eventually, it extends too far at which point the hamstring begins to contract and the quadriceps relax until the process must reverse itself in the opposite direction. The real system is much more complex than that, as what the calf muscle, hip flexors and virtually every other muscle in the body has an effect on what the hamstring must do in order to stabilize the body.

Figure 1

Simplified Feedback and Input For Stability Control

If a basic stabilization system could be discovered, it would be of critical important to many people around the world. If a device could be developed which helps a person increase their neuromuscular response to a sudden imbalance, it could help elderly people that are prone to falling be more active and lead to an enhancement of the quality of their lives.

Concepts of Bipedal Stance

As mentioned earlier, the ability of a human to stand on two feet is much more challenging than four feet. This is due to the position of the center of mass. Our feet have evolved to be longer than those of a four legged animal. For example, the bones in a horse’s hoof is a pair of fused toes. The center of mass for a horse does not need to be between two of these small areas, but over the four points at which the feet touch the ground. A human must keep his center of mass over the area between or underneath his two feet. The length of his feet multiplied by the width of his stance is the size of this area, whereas the length of the horse’s hooves has little effect on the size of this area.

Because of this, each step requires a significant shift in the center of mass with a bipedal gait. The horse can lift a leg with little correction for the center of mass which makes movement much easier. Researchers who are working on bipedal robots are familiar many of the challenges that are inherent in this type of locomotion. Every step needs to be carefully considered by the processors and are in controlled environments. This is a far cry from the real world situations which our bodies deal with on a daily basis.

Experimental Procedure

For this case, I am only concerned about the movements related to specific movements. I had my test subjects go through a series of maneuvers with short rest periods in between each. The subject would stand still, perform the test until their body reached an equilibrium and then return back to standing normally on two feet. The tests were performed in this order:

Stand on right foot

Stand on left foot

Eyes closed

Eyes closed on right foot

Eyes closed on left foot

Standing on toes

Standing on toes with eyes closed

Arms extended out perpendicular to the body

Arms extended out perpendicular to the body with eyes closed

Bent forward at a 90° angle at the waist

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This work is licensed by Shane Hudak under a Creative Commons Attribution License (CC-BY 2.0), and is an Open Educational Resource.

Last edited by Shane Hudak on May 15, 2008 7:23 pm -0500.