The task for physical therapists and biomechanic researchers was deceptively simple: Measure the force, position, and orientation of a subject's feet to determine walking or running gait characteristics.
Multiple footstrikes can be used to assess rehabilitation effectiveness, perform motion studies, and adjust athletic performance. Placing a couple of force-measuring strike plates on a walkway seemingly could do the job. Wrong.
Several idiosyncrasies in dealing with real patients dictate a more ingenious approach. For starters, a patient may not be able to accurately hit the plates or may alter his gait to strike a plate, thus invalidating the results. Likewise, a subject may step on a plate abnormally hard to insure recording data, again altering results. Finally, because a weakened patient or one in pain may only be capable of recording few strikes before having to be off her feet to rest, valid data must be quickly gathered.
While using more strike plates can alleviate some of these difficulties, it is a costly solution at up to $20,000 each. A walkway has its own limitations: Depending on its length, only one or two consecutive strides may be recorded before the patient has to turn around. A new approach taken by treadmill-maker Kistler Instrument Corp. (Amherst, NY) embeds a pair of piezoelectric force plates under the front and rear of a moving treadmill belt to produce repeated gait information. Thus multiple footstrike data is gathered continuously in a short period. This treadmill, called the Gaitway(reg), is smaller than a walkway. The latter takes a minimum of 40 ft, because of "start-up" and "slow-down" areas, according to researcher Scott White of the State University of New York at Buffalo.
Each of the two force plates extends the full width of the treadmill. The supports at each of the plate corners contain a piezo transducer to measure force. System software processes the force data from each plate as a foot contacts the front plate under the belt, transitions from the front to rear plate immediately behind it, and lifts off the rear sensor. Data from the four corner transducers in each plate are also integrated with belt speed to derive footfall center-of-pressure location and travel.
An infrared sensor detects the position of a retroreflective tape on the patient's ankle, allowing the system software to distinguish individual left/right limb data. A practitioner can thus quantify any limb-to-limb discrepancies.
Ken Wagener, biomechanics product manager at Kistler Instruments, says while the force plates are basic to the Gaitway's utility, the patented foot-resolution software algorithm, separating right and left foot forces, is the key to the design. By positioning the subject over the interface between the front and rear plates, only one foot is in contact with a plate at any one time. By the time one foot is moving back across the joint between plates, pressing on both simultaneously, the other foot is in the air, in the "swing phase." It moves up to contact the front plate only after the rear foot is entirely on the back plate.
There are a few limitations, Wagener notes. "This method only measures the vertical component of force, neglecting shear forces. An accurate measurement of shear is not yet available due to the effects of belt movement and slipping."
But because the vertical forces are more than 10 times the shears, the vertical component is the far more dominant and important. He adds, "Still, many would like all three components of force to see the effects of braking, acceleration, and side-to-side movement." Also, a shuffling gait cannot be accommodated. In order for the sensors to delineate separate footstrikes, the subject must place the heel of one foot completely in front of the other, starting the gait on the front plate without the rear foot touching the same plate.
Scott White notes further advantages of using the treadmill instead of an overground walkway. "Steady-state locomotion speeds are selectable, and successive, repeated strides can be documented expeditiously." Reduced is the trial-to-trial variability, increasing the test-retest reliability, that can be a problem in motion analysis of overground gait, he adds.
The Gaitway is being used to classify prospective knee and hip replacement patients; for physical therapy to monitor recovery from various surgeries or prosthetics accommodation; and even in quantifying injuries for workman's compensation assessments. Sports medicine uses include studies by sneaker manufacturers, and baseline gait determination of athletes in preseason so that recovery after any injury can be more accurately pinpointed.
Biomechanics piezoforce-sensor applications
- Physical-therapy evaluation
- Force measurement within magnetic resonance imaging fields
- Denture force-loading research
- Sport-shoe and playing-surface evaluation
- Athlete stress-indicating loading pattern determination
- Athlete physical-limit training
- Weight-lifting recording
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