ABERDEEN PROVING GROUND, Md. (Army News Service) -- One of a Soldier's most fundamental and valued skills is the ability to shoot a weapon accurately.

In order to do that, several sub-skills are required, including trigger control -- the ability to smoothly squeeze the trigger, causing the weapon to discharge a round.

Trigger control can be learned and improved upon with practice, but there's a component of it that Soldiers don't have complete control of, and that has to do with involuntary muscle twitch that is often imperceptible to the shooter, said Dr. Matt Tenan.

That twitch, which is caused by imperfect signals transmitted from the brain via neurons down to the finger muscle, can instruct the muscle to contract and thus cause the Soldier to pull the trigger, explained Tenan, a kinesthesiologist with the Army Research Laboratory, who spoke Thursday, July 28, during a media day here.

After a full day's rucksack march in brutal heat, a Soldier's ability to smoothly pull the trigger and accurately hit a target has been shown to decline as the signal from the brain weakens due to fatigue, he said.

Tenan has been working to attenuate muscle twitch to allow a smoother trigger pull, sometimes referred to as trigger squeeze, even as fatigue sets in. The research involves using an electrical device to optimize how the brain controls muscles.

ABOUT THE DEVICE

The device Tenan is working on doesn't have a name yet. But it uses an artificial trigger containing a force transducer, which measures the amount of a person's trigger pull.

That's just one of four components that make up the device. The second is a piezoelectric vibrator hooked up to the person's wrist. The third is a wire -- twice the thickness of an acupuncture wire -- that is stuck into the person's muscle, without causing pain or blood.

The wire is in turn connected to the fourth component, an ultrasound device that detects muscle activity and outputs that activity in the form of sine waves on a computer screen.

Smoothly curved sine waves indicate smooth trigger squeeze and chopped or jerky sine waves indicate erratic trigger squeeze.

KEY STIMULI

The key component of the device is the vibrator, Tenan said. When the vibrator is turned on, it recruits more muscle cells to contract. That signal is strengthened because the neurons connecting the muscle to the brain are outputting a greater chemical discharge to the muscle fibers.

The vibration, he said, is set at a sub-sensory level, meaning the person the device is hooked up to cannot detect it.

"We've known the basics of how human movement has been controlled for 20 to 30 years," he said. "But here, we're trying to optimize it in a specific muscle," that being the trigger finger.

That vibration smooths trigger pull is known because of research at the Army Research Laboratory, but according to Tenan no one understands why it occurs.
"We just know it happens," he said.

Vibration is just one of several kinds of stimuli that can smooth a trigger squeeze. Researchers have found that electrical stimulation and noise stimulation have similar effects when tested separately.

Researchers at Army Research Laboratory are honing in on the vibration stimuli, Tenan said, because it's simpler to use a battery and an oscillator to produce the vibration in battlefield conditions.

FUTURE POSSIBILITIES

Tenan said it's not the job of his lab to produce a working device for Soldiers. Instead, researchers conduct the basic research that could someday be used to meet Army requirements, if such a requirement is ever expressed.

"Something like this might be built into the handgrip of a weapon that may apply stimulation to the human that they can't feel, but is increasing the steadiness and probability of hitting a target," he said.

Some have compared Tenan's device to the Da Vinci Surgical System used by surgeons to operate in a more smooth and controlled fashion.

While the goals of the Da Vinci and the trigger-pulling device are the same, the techniques are different, he explained. With Da Vinci, a robot controls the movement. The trigger pull device, on the other hand, actually modulates the human nervous system to control movement.