A new microcomputer system that may someday bring paralyzed limbs to life again -- allowing some paraplegics to walk and stroke victims to use their arms once more -- is now being tried with preliminary success on animals.

The system, being developed by Dr. Jerrold Petrofsky of Wright State University, is designed for those whose limbs would be normal except that their nerve link to the brain has been broken by a spinal injury or a stroke.

The computer package, installed above the paralyzed limb, acts like an outpost of the brain. It is strapped on and linked to a set of implanted electrodes that actually trigger the muscle movement on command of the computer. It not only provides nerve signals to the formerly inactive muscle, but listens to feedback from the muscles, keeping constant track of motion so that complex movements can be made smoothly.

Researchers have for a century been facinated by, and attempted to use in a practical way, the fact that the signals which make muscles move are ordinary electrical currents.

In recent decades, medical researchers have developed devices such as one that can automatically contract the bladder in patients with urinary trouble, or a device that can rhythmically trigger contractions of the diaphragm for patients whose breathing circuitry has failed.

But the successful devices made so far can produce only simple, on-off muscle contraction. Making subtler, coordinated movements with artificial stimulation has turned out to be a far more difficult problem.

Walking, for example, involves many muscles contracting and relaxing in rapid sequence, using feedback from the muscles and the brain to keep a steady motion while swinging the body's weight forward from foot to foot.

Using a microcomputer to govern nerve and muscle action, Petrofsky has now been able to move smoothly and to coordinate the eight muscles necessary for walking forward in the leg of a cat.

The experiment with the cat took place on a lab table with the cat anesthetized and held in place upright while the computer moved its leg. Within the next few months, Petrofsky hopes to demonstrate his system by getting a paralyzed cat to walk.

Dozens of researchers have been working toward the goal of artificially stimulating natural muscle movement, and over the years different researchers at places such as UCLA and Case Western Reserve University have contributed different elements to the final solutions that are emerging now.

The first difficulty in artificial stimulation of muscles is to get them to contract evenly, at a controlled speed, rather than in a sudden or jerky manner. Many experimenters accomplished this by using strong, even pulses of electricity to the muscles, because weaker pulses produce jerky movement.

Petrofsky, following the lead of some British researchers, wrapped a set of three electrodes directly around the motor nerve and was able to use about one-fiftieth of the voltage to get smooth muscle movement.

These electrodes are implanted permanently, and the skin closed over them, and then are run by a tiny radio frequency transmitter which in turn is linked to the microcomputer pack.

A larger obstacle to building the device has been the sophiscated, rapid control necessary to coordinate the movement of many muscles together. Petrofsky, after studying the motiom of cat's legs, has made a computer program that will give movement signals to the paralyzed leg based on information from two sources -- first, the motion of the opposite leg will be translated into signals triggering motion in the paralyzed leg; and second, the paralyzed leg as it moves will provide feedback about its own position and muscle tension to continue motion smoothly.

The first test of Petrofsky's computer system will come this spring when he implants a set of electrodes and straps the computerized harness on a paralyzed cat to see if the cat is able to walk in a near normal manner with the help of the device.

There are some 3,000 spinal injuries in auto accidents alone every year and there are tens of thousands of other victims around the country who have the kind of paralysis that might be relieved by a device like Petrofsky's.