Touching the "Soles" of Astronauts:
An Interview with Dr Leah Bent
HYPERSOLE – a deft combination of hypersensitivity and the sole of the foot – is a unique research project to document changes in the skin sensitivity of eight NASA astronauts before and after their space flights. Sponsored by the Canadian Space Agency, the project is led by Dr. Leah Bent, associate professor of Human Health and Nutritional Sciences at the University of Guelph's College of Biological Sciences. On the eve of Space Shuttle mission STS-132, she explained what the Hypersole team was up to in the hours before the launch of Atlantis and what they would be doing in the critical moments after the shuttle returned to Earth.
Q. How did Hypersole come about?
A. Research into skin and vestibular input in balance has been an area of interest to me for quite some time. But it hadn't occurred to me to test healthy astronauts until Canadian astronaut Dave Williams mentioned in 2007 that he had experienced some tingling in his feet during a shuttle flight in 1998 and for a short period of time when he returned home. He told us that he knew of other astronauts who had also experienced the same sensation when they stood. Since then, we've talked to others who have been to space. One astronaut recounted how she'd bungee-cord herself in to run on a treadmill and every time her foot hit the treadmill it was suddenly tingly. She noted that it mostly happened when the foot was fully loaded [in full contact with the treadmill surface].
Q. And that anecdotal evidence led to a formal test of eight astronauts on the three remaining NASA shuttles?
A.Yes, thanks to the sponsorship of the Canadian Space Agency and the support and coordination of NASA. We start pre-flight and post-flight trials on three astronauts on NASA's STS-132, the Atlantis shuttle scheduled to launch on May 14. Then we will conduct identical tests on two more astronauts set to fly on STS-133 Discovery in November (rescheduled for December) and on three of the crew members of the last shuttle, STS-134 Endeavour, in February, 2011.
Q. How long does the sensation last after the astronauts return to Earth?
A. We really don't have a sense of that yet. It varies for each person. After a flight of long duration, it's a couple of days at most. Short-term crews may experience the sensation for about a day before they return to baseline levels.
Q. What will these tests give you in terms of hard scientific data?
A. We start with several hypotheses based on past research and what we've already learned from informal conversations with crew members. One is that skin sensitivity will be increased post-space-flight. Another is that those increases will correlate with balance deficiencies that are related to the vestibular (inner ear) information that maintains our equilibrium.
Q. You can't tell for certain that these are more than hypotheses?
A. So far, this kind of increased sensitivity in space is based on perceptions. Our tests are designed to provide the first formal documentation of how input from the skin on the foot sole changes in relation to spaceflight as well as the potential consequences of those changes to balance control. In short, they will help us to determine whether or not this hypersensitivity is actually happening. We achieve this by first establishing a baseline – what each astronaut is experiencing before the launch – and then compare the data to how sensitive they are when they come back.
Q. What does anecdotal evidence suggest is happening?
A. We think that some of the different sensory systems are compensating for each other. Because the astronauts are in space, in a microgravity environment, their vestibular system functions differently. They're not getting information from the inner ear, so they have to rely more on other sensory systems, such as visual information or tactile cues.
Here on Earth, both the vestibular system and the cutaneous, or skin, information from the feet give us a sense of where we are in a gravitational environment – they tell us which end is “up.” In microgravity, we think the skin might be compensating for that lack of vestibular input. The central nervous system says, “All right, we have no vestibular input, we need to turn up all our other system to compensate.” If the body then selectively tunes, or turns up, one or more of these individual skin receptors that will give us an idea which ones may play a more important role for balance and locomotion and stability here on Earth.
Q. What will this research tell us about the aging process?
A. A lot of changes that occur in space travel are analogous to changes we see in aging. The elderly rely more heavily on vision, for instance, to compensate for a deterioration of vestibular reflexes – the sensory system that contributes to our balance and sense of spatial orientation. Similarly, since spaceflight reduces the gravitational stimulus to the vestibular system, astronauts' systems must also re-weigh other sensory input.
What we lack is formal documentation about skin sensitivity in relation to space flight. Here on Earth, our skin sensitivity decreases as we age. One of the interesting things about the skin during space travel is that we see changes in the opposite direction. Skin sensitivity actually increases.
It's also not known how information is weighted in an elderly population. Or if an increase in skin input actually benefits postural stability.
Thus spaceflight provides a unique opportunity to examine an increase in skin sensitivity as a means of identifying the mechanism by which we use skin in postural control.
In fact, we expect that research targeting healthy astronauts who experience physical changes as a result of weightlessness will provide a model for future balance investigations with aging.
Q. Why does skin sensitivity change with age?
A. As we get older, the reduction of information relayed by sensors in our skin can lead to a loss of balance control and an increased incidence of falls. We're still trying to figure out if this is caused by changes in the skin itself or changes in the receptors. Some information suggests that there are fewer receptors as we get older. There is also evidence to suggest that the information coming from these receptors is slowing down. So they may be firing as they were before, but the information isn't travelling as quickly to the brain or to the spinal cord.
Q. What exactly is a receptor?
A. There are four different types of receptor in the glabrous (or hairless) skin of the foot that provide information regarding skin contact, indentation pressure, slips and skin stretch. Two receptors are in a group called corpuscles, which can be described as fluid filled balloons. The pacinion corpuscle, in particular, responds to force in a way that is similar to a water balloon. Force on the outside is easily transmitted to the centre, where the nerve ending is. The nerve ending will fire and then the water will become still. If you take your finger off the balloon, the water will again be displaced and the receptor will fire once more. These are called fast-adapting receptors and they respond to contact with a surface.
A third receptor, called a merkel cell disc, has a multiple number of endings and acts more like a strain gauge. If you compress the skin it will respond to the amount of compression causing it to fire more. The last type of receptor, the Ruffini ending, responds to skin stretch along the skin's surface.
Q. Under what conditions are the astronauts tested?
A. The pre-flight test will take place on site prior to each launch. Astronauts will be comfortably seated, one sock off, while we run through a set of three different tests. There are, of course, no in-flight tests – some of the equipment is too cumbersome – but if all goes well that might change in the future. Within an hour or so of the shuttle's return, we'll conduct the same tests again. Time is critical here: we need them to remain off their feet as much as possible. The objective is to document changes in skin sensitivity that have occurred during the flight, before the astronauts re-adjust to the earth's gravitational environment.
Q. What is the first test?
A. It's called a vibration sensitivity threshold test. It measures the vibrotactile sensitivity across the foot. The astronauts place their foot on a footplate. Under that plate is a shaker, shaped like a cylinder. A probe, rounded at the tip, moves up through a hole in the footplate until it nestles against the foot. During testing the shaker will vibrate the probe and the crew will tell us whether they feel the vibration by pushing a trigger button. It starts at high enough amplitude – 1 millimetre of displacement – so they'll feel it. We then decrease the amplitude until they can't feel the vibration at all. Testing will be performed at four different frequencies to attempt to capture the sensitivity of the four different receptors.
The origins of this test go back to microneurography studies first conducted on the hand in the early 1980s. Researchers were able to determine how individual skin receptors respond to different frequencies of vibrations. They found that each type of receptor had a specific frequency that it was most responsive to. When we test the astronauts we can use this information (which frequency they were most sensitive to) to establish which of the four types of receptors are most heavily influenced by microgravity.
Q. Do the vibrations tickle?
A. The astronauts all asked that! It does tickle a little bit, especially at the higher frequency. But we have a way to ease any possible discomfort. At the start, we pre-load the foot sole with about two Newtons [1 kilogram corresponds approximately to 10 Newtons]. By that I mean we push the probe into the foot just a little bit, so it indents the skin slightly. When it reaches a load of 2 Newtons, which isn't very much, we start the vibrations. If we vibrated it when the probe was just barely touching the foot it would be considerably more ticklish.
Q. Just the thought of it tickles.
A. Actually, there is another good reason to pre-load. We want each one of the receptors to start from the same playing field. Some fast-adapting receptors respond on contact. If we contact and indent all of the receptors at the same intensity, they all start at a level baseline.
Q. What is the static sensitivity test?
A. This is a secondary test to provide information on static thresholds. We use a set of 17 monofilaments – they look like nylon fishing lines – to poke the skin of the foot. The device looks much like a toothbrush. At the tip is a nylon monofilament that protrudes perpendicularly. Each monofilament is applied to the skin and is calibrated to buckle at a known force. Astronauts will indicate verbally when they feel each poke and this lets us know how much force the astronauts can sense at each of the sites we test.
Q. And finally, the balance test?
A. The balance test is a standardized NASA test conducted as part of medical procedures. It's called a sensory organization test. For our purposes, astronauts stand on a footplate that is “sway-referenced”. When they push down on the plate, it will tilt with them. They'll close their eyes to remove their ability to use vision. When the plate tilts about their ankles, the sway referencing will cause them to lose their ability to use muscle stretch to help them maintain balance. This increases their reliance on their vestibular information, the inner ear, to tell them where vertical is. We can see how well they balance in that situation.
It will be quite challenging for some astronauts post-flight because the vestibular system is still adjusting back to a gravitational environment. We want to see if there is a correlation between greater or changed skin sensitivity and changes in balance. This will tell us that skin receptors may be compensating for that lack of vestibular information.
Q. What are some of the benefits of this research?
A. For one, it will build on our knowledge base. Sensitivity threshold tests have been done on elderly individuals and on young people. Insight into balance challenges as a result of heightened skin input will enhance current theories on the skin's contribution to postural control. It will also help address current interventions that involve increasing the input to skin receptors.
It could be that the body naturally selects to turn, or increase the gain of information that comes from one or two of these specific receptors, as opposed to turning all of them up. If there is a selective increase in sensitivity we're hoping this may identify one or two of the most important receptors. 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.
There are similarities between spaceflight and bed rest. One of them is obvious: in both there is no weight on the feet. A research colleague has conducted several bed rest studies, with a primary interest in blood pressure regulation. We were able to perform some sensitivity testing on these individuals at the same time, and these tests helped us to establish structured testing ranges for the Hypersole project. In turn, information gained from Hypersole will be appended to existing bed-rest protocols.
There is also potential for athletes. I'm not sure how much more you can improve upon the system for someone operating at their peak. But we do know that receptors don't work as well when they're cold. A skier, for example, often gets cold feet. Olympic skiers would certainly be interested in ways to bring these receptors back up to a normal or healthy functioning level.
Q. How will this help future space travel?
A. We're hoping to get a better sense of whether or not there is a strong relationship between change in sensitivity and balance. If we can see that there is – is it an improvement or a deficit?
Certainly, skin sensitivity is not the entire picture. There are many of other sensory systems involved. Still, this will give us a little bit more information as to whether or not there are changes so we can begin to think about how we're going to address them. Right now, there are only questions: Is it something we need to be concerned with, or something that might affect an astronaut's ability to perform an EVA (extra-vehicular activity)? If that's the case, what is the best way to help them compensate?
Q. Do you have any suggestions to help keep your feet and skin receptors functioning and healthy?
A. Well, my Nana is 97 and she rubs cream into the bottom of her feet every day and every night. And I tell her, that's why you're still able to walk around! I'm sure it helps, since there are changes in the integrity of our skin as we age. So get yourself a good bottle of cream.
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