By Larine Barr
Air Force Research Laboratory Public Affairs

WRIGHT-PATTERSON AIR FORCE BASE, Ohio  -- For many Air Force aircrews, discomfort caused by sitting on extended missions is just part of the job.

Col. Kerry Keithcart, a KC-135 Stratotanker pilot for the 434th Air Refueling Wing at Grissom Air Reserve Base, Ind., said he and his crew do what they can to make those longer missions more bearable.

"Some of us try to stretch and that's of some value. We also drink a lot of water to stay hydrated," Keithcart said. "But when you're sitting in one position for as many as 13 hours at a time, you're going to experience different aches and pains."

Keithcart and many others in the aircrew population have experienced specific physiological problems associated with sitting on seat cushions in current Air Force aircraft ranging from muscle soreness to overall fatigue. To address those problems, scientists at the Air Force Research Laboratory’s human effectiveness directorate here are studying how different seat cushion materials and designs affect those problems.

Test subjects were photographed while sitting on the seat cushions for various periods of time, said project manager Joseph Pellettiere.

"Photographs reveal dangerous pressure points that can develop when people sit for long periods of time," Pellettiere said.

But these are not your ordinary photographs. They are collected from a device called an “Xsensor” which is made up of 900 sensors that respond to pressure.

"The device works on capacitance technology, and as the space inside the sensors changes, an output voltage is recorded by a computer and displayed on a computer screen," Pellettiere said. "This pressure mat is only fractions of an inch thick and very pliable. As such, it conforms quite well to a variety of seat cushions and the human anatomy to give a reading on how much pressure is being exerted, and where the 'hot spots' are located for different people on different cushions."

The obvious question seems to be: Why does it matter what the pressure distribution is for a crewmember on a cushion? The answer lies in preventing deep vein thrombosis and trying to prevent sore spots that cause people to squirm, Pellettiere explained.

"The risk of DVT has been shown to increase with the amount of time the tissues in the body are compressed and leg muscle activity is reduced," he said. "This occurs because the deepest part of the calf muscles partially relies on muscle activity to promote good blood flow.

"Furthermore, with increased periods of pressure applied to the body's tissues, the capillaries begin to close off, reducing the amount of blood flow and causing a numbing sensation. Because of this reduced blood flow, it's possible to develop a clot that can be dislodged and travel to other areas of the body where it can be a big problem."

While the risk of developing DVT is low, the other adverse effects of sitting for long periods are quite common -- tingling and numbness in the extremities, leg and back pain and overall fatigue, Pellettiere said.

For commercial long-haul flights, passengers can generally get up out of their seats and move around if need be; however, most crewmembers are not afforded this same luxury.

"It's not uncommon for some missions to last from eight to 44 hours of seat time," Pellettiere said. "While there are some techniques such as stretching exercises that can alleviate some of the tension, the search has been on to develop new passive technologies that can improve comfort and reduce fatigue with the goal of improving the overall performance and well-being of the crewmember."

Lab scientists have worked closely with volunteer test subjects to measure pressure distribution and how different people interface with different types of cushion materials.

"During tests, a subject puts on a flight suit and settles into a mock-up ejection seat that's fitted with (an) Xsensor," Pellettiere said. "After about 10 minutes, which is the time it takes for the cushion to warm up and conform to the body surface, a snapshot of how that subject is sitting on the cushion is collected. The test subject then sits on that cushion for an extended period of time while performing some cognitive tasks to measure performance and filling out some surveys to rate his overall comfort or discomfort."

As part of the study, scientists have tested candidate seats for the fixed aircrew seat standardization program, which is developing crew and mission seats for C-135 Stratolifter, C-130 Hercules and E-3 Sentry aircraft. A contract is also in place to develop a generic seat cushion specification for both fixed and ejection seats.

"Our data is feeding directly into those programs," Pellettiere said. "All the data will be gathered to develop cushion guidelines that seat developers can apply to new seat cushion designs to ensure the continued success of our crewmembers."

That is good news to Keithcart and his KC-135 crew.

"I think the concept is great," he said. "We have enough challenges in the aircrew world like sleep cycles and long days, and anything the Air Force Research Lab can do to make our mission more reasonable is time and money well spent." 

(Second Lt. Christy Stravolo of AFMC Public Affairs contributed to this story.)