The human body is a very unstable system of rigid components connected by supple moving joints. When a man raises his arm, the observer notes that particular motion. Not so apparent is the stabilization of all the other joints in that man's body necessary to maintain balance. The ability to measure and quantify these motions in real time may contribute to the early diagnosis of degenerative neuromuscular conditions like Parkinson's disease or multiple sclerosis.

That's because the stand-still movement of a healthy person falls within a predictable range, or normalcy sphere. Tremors associated with Parkinson's and similar conditions result in movements outside this sphere. Not only are the deviations distinct and identifiable, they increase in amplitude as the disease progresses.

Such is the rationale behind, as well as the observations resulting from, a Computer Assisted Rehabilitation ENvironment known by its acronym CAREN. Developed by Motek Motion Technology (Amsterdam, The Netherlands; Manchester, NH) under a European Union funded program, CAREN combines disparate technologies to help medical experts view and analyze balance and coordination disorders as they happen in an interactive, controlled environment.

"There is no standard network or communicative protocol for treating balance disorders," states Motek CEO Edward Costello. "Current evaluations are done visually. Because there are no generic tools for diagnosis, there is little, if any, knowledge transfer." CAREN, he claims, is the only system available to register and evaluate human balance behavior in a repeatable and controllable manner.

Combined technologies. Five parts make up CAREN: An SGI (Silicon Graphics Inc.) Dual CPU Octane MXE computer, custom configured for Motek; a flight-simulator-like motion platform; optical or magnetic motion capture equipment; a human body model for simulation; and enabling software that ties the different components together.

Called D-Flow, the patented software processes data from the motion capture system, maps that information into the human body model, and drives the motion platform. In addition, it performs all of these functions faster than real time, defined by Motek as the visual observation rate of 30 frames per second.

"To evaluate the subtleties of human balance behavior," explains Oshri Even-Zohar, Motek's vice president and chief technology officer, "you need a system that is an order of magnitude faster than the dynamics you are trying to measure."

Rexroth Hydraudyne B.V. (Boxtel, The Netherlands) supplies CAREN's six-degree-of-freedom motion platform. Built to Motek specifications, the hybrid system incorporates both hydraulic and electric actuators to meet CAREN's required response times. During operation, the person under evaluation stands on the platform which follows motion instructions from the central computer while simultaneously generating new data sets based on real-time biofeedback. It is this controllable and interactive environment that gives doctors the means for analyzing human motion.

A corresponding motion capture system, developed in The Netherlands at the Technical University of Delft, employs infrared cameras, passive optical markers, and a host computer to collect the subject's exact movements. The retro-reflective markers, sewn into a bodysuit or strategically attached to the subject's clothing, send IR signals back to the cameras, positioned about the platform.

This information passes first to the host computer for processing into position/orientation data, and then to CAREN's central computer via high-speed Ethernet link. The proprietary D-Flow algorithm turns the collected data into the degrees of freedom applied to Motek's 3D human body model-a stored library of common skeletal and muscle motions that resides in the computer. "CAREN's ability to reconstruct the moments of force around the joints of the subject, and compare them against a standard database in real time," says Even-Zohar, "enables a doctor to view and measure a subject's balance compensation strategies."

Faster rehabilitation. CEO Costello adds that because CAREN identifies and quantifies how a subject stresses certain muscle groups in response to platform motion, physical therapists can use the system to devise appropriate treatment programs. Platform movements can also be performed in close synchrony with projected images to simulate every-day-life environments for aiding physical therapy.

"The real-time feedback loop allows the operator to change the environment as the subject is interacting with it," Costello points out. Platform motion, he says, can be independent of the subject's movements (as when standing on a very large boat), or co-dependent with the subject's movements (as when standing in a very small boat).

For example, a person with a broken leg naturally favors that leg during rehabilitation, exerting less pressure than normal. By tilting the platform slightly higher, a therapist can make the patient step down sooner than anticipated. More pressure is applied to the leg, shortening the rehabilitation process, and lowering treatment costs.

Even-Zohar envisions even more far-reaching medical benefits: retraining people that have lost mobility. "We have a data motion capture sequence of a healthy person walking," he explains. "Using D-Flow, we convert data from the feet, knees, and hips into force feedback instructions. Now, we situate the person who is immobile over the motion platform with a harness, and attach their shoes to the platform with Velcro. The platform's inverse kinematic motion, according to the force feedback input, establishes a normal walking pattern in the subject's limbs that may aid in retraining patients, for example, after spinal injury.

Indeed, such experiments are presently underway at The University of Groningen, The Nether- lands, where the CAREN system has been integrated with other motion control and testing equipment into a movement research laboratory. Run in cooperation with Beatrixnoord, a Dutch rehabilitation center, the laboratory may provide new insight into physiotherapy, orthopedics, neurology, and the early diagnosis of a wide range of balance and coordination disorders.

CAREN is also being tested at the University of Amsterdam, and the city's Central Orthopedic Technique.

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