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HYPERSOLE: A Focus on Feet

Researchers will "tickle" the feet of eight spacebound astronauts in hypersensitivity tests to further our knowledge of aging on Earth

Few body parts suffer as much daily abuse, with such little appreciation, as the lowly foot. Whether burdened for hours supporting the full load of our weight, crammed into ill-fitting shoes, or forced to walk too far – and often not far enough – our feet bear the brunt of our excesses as well as our neglect.

One of the human body's primary depots of sensory mechanoreceptors, the foot has a large impact on balance and postural control. Skin receptors on the foot sole act as intuitive messengers that, in tandem with the vestibular (inner ear) system, send the information to the brain that gives us a sense of where we are in our gravitational environment. In effect, they tell us which end is "up."

In the weightlessness of space, feet may seem somewhat superfluous. Yet a research team from the University of Guelph's College of Biological Sciences (including a scientist from Wilfrid Laurier University) is counting on space to teach us how valuable our feet really are – above and beyond their critical task of keeping us upright and mobile on Earth.

The Hypersole experiment will study eight astronauts from the crews of NASA's three remaining shuttle missions (Atlantis in May; Discovery in September and Endeavour in November) who will submit their foot soles to a series of tests prior to and immediately after their flights.

Using vibration devices and filaments to "tickle" reactions from the astronauts, researchers will then compare the two sets of "before and after" data in order to identify which receptor has been affected by the astronauts' time in microgravity. Combined with a standard balance test, the information will help to establish how hypersensitivity contributes to balance control.

The impact of sensory systems and skin receptors on balance control in Earth's gravitational environment are well documented, most notably in studies that focus on the natural deterioration of vestibular reflexes during the aging process or as a result of prolonged periods of bed rest.

In sharp contrast, research into how input from the skin on the foot sole changes in relation to space flight was non-existent until Dr. Leah Bent, a University of Guelph associate professor of Human Health and Nutritional Sciences, heard in 2007 about a specific experience that happened to Canadian astronaut Dave Williams on his first flight into space in 1998.

Williams, an emergency doctor and neuroscience researcher, told Bent that he had felt a tingling sensation in his feet during, and for a brief period after, his mission. Nor was he the only one: other astronauts he had spoken to had felt the same physical phenomenon.

The unusual tingling raised an intriguing scientific dichotomy. Normally, the changes experienced by otherwise healthy astronauts while in space tend to mimic changes that occur over time as we age, often at a dramatically accelerated pace. Among those changes: the loss of bone mass, atrophying muscles and hardened blood vessels.

With skin sensitivity, the opposite seemed to be true. On Earth, skin sensitivity decreases along with many other sensory system functions, including vestibular reflexes. In space, however, anecdotal evidence from Williams and other astronauts indicates that skin sensitivity actually increases in correlation with balance deficiencies related to vestibular information.

For Bent, the principal investigator of Hypersole, the differing reactions opens the door to a fascinating possibility: to compensate for the lack of vestibular input while in space, the central nervous systems selectively "turns up" one or more skin receptors in the astronauts' foot soles, in effect naturally re-weighting the sensory systems.

"If there is a selective increase in sensitivity, we're hoping that this may identify one or two of the most important receptors," says Bent. "That way, if we start to look at an aging population that has a decrease in all the skin receptors, maybe we can target to activate one or two receptors, rather than all of them."

If the research team's hypotheses are proven valid, Hypersole – and the tingling foot that inspired its creation – will add an entirely new dimension to the study of hypersensitivity, the aging process and, importantly, to the search for ways to arrest or perhaps reverse the body's loss of critical sensory input.