The foot in microgravity and space flight: an exploratory review

Podiatry Now, Feb, 2008 by Evelyn C. Weir, Cameron Kippen, Gary A. Dalrymple

The research area of lower-limb musculoskeletal function in spaceflight is predicted to grow over the coming decades as the amount of man hours spent in space flight increases and the distance travelled is poised to lengthen. This article examines the impact of microgravity and space flight on the human lower limb, with particular reference to three key areas of potential degenerative risk: muscular change, alterations in gait observations and interference with circulatory performance.

As man ventures further and for longer into the space environment, the notion of habitability must be considered. As the health of the flight crew is always of paramount importance, flight crew are observed, monitored and tested in many aspects of physiological performance both before, during and after episodes of flight. Establishing health and performance of the flight crew, particularly in terms of physiological change, is a major priority (1).

The effects of space flight on the human body have been much researched, and studies of the impact of such space flight can be conducted in a number of ways. Simulated reduced gravity or microgravity, in the form of parabolic flight, ground-based microgravity simulators or even simple bed rest can attempt to emulate or replicate the situations flight and space station crew may find in their daily activities.

In addition, in-flight and post-flight research on astronauts themselves will undoubtedly yield pertinent data regarding the physiological changes produced by conditions of low or zero gravity. The number of these in-flight studies is increasing, but may have been hindered in part until the last decades by the relatively small numbers of potential subjects (1).

As the human leg and foot play an important role in Earth gravity kinematics, it is inevitable that future research consideration will examine lower-limb performance and foot health in astronauts. As space flight will inevitably expand and develop in the coming decades, an understanding of the effects of changes in gravity is of interest to lower-limb specialists. The effects of gravity on Earth and the reduction in gravity in space flight need to be examined carefully in order to construct effective exercise countermeasures to the reduction in gravitational influence. In addition, the effects of the return to Earth following the influence of microgravity is a further area of research consideration as flight crew readapt to the gravity on Earth, and return to gravity can present a special range of difficulties (2). In general, it is observed that space flight will influence multifactorial elements of the human body, including degeneration of the cardiovascular system, reduction in fluid volume, loss of bone minerals and associated muscle atrophy (3).

Using literature primarily from the last five years, this review article examines the impact of microgravity and space flight on the human lower limb, with particular reference to three key areas of potential degenerative risk, namely; lower-limb muscular change, alterations in gait observations and interference with circulatory performance.

FOOTWEAR IN SPACE FLIGHT

During space flight, for example during a space shuttle mission, flight crew wear a variety of footwear on their feet and legs. Common problems with these types of footwear is often their bulkiness and lack of flexibility along with size and weight. In 2008, shoes weighing less than 140g with a slanted sole and a hallux divider will be tested with some of the shuttle crew. These shoes aim to 'work' the gastrocnemius during the space flight.

During space 'walks', flight crew wear additional foot protection. Extravehicular space walks themselves present a set of lower-limb risks that have been identified. The space suits themselves can be prohibitively complicated for normal gait, and contact with the footwear can result in injury. In a space walk training exercise in a neutral buoyancy laboratory, in over 770 submersions it was found that 11% of spacesuit symptoms occurred in the feet, with the severest symptoms occurring in the shoulders, hands and feet (4). While the majority of the symptoms reported in this study were mild, reversible and transient, hard boot contact was implicated as a causal factor for foot problems.

MUSCULAR CHANGE

In general, it is observed that space flight may result in atrophy of skeletal muscle, particularly in the thigh, leg and foot (3). Countermeasures to reduce the impact of this potential degeneration are therefore of paramount importance (5), and muscle wastage and bone density loss are key issues for pre-flight, during-flight and post-flight assessment.

In a parabolic flight experiment, four subjects utilised an interim resistance exercise device to examine performance in 1G gravity versus 0G gravity. It was noted that crew members performing simple weightbearing exercises such as squatting and deadlift in 0G will require a much greater external resistive force than in 1G to provide enough stimulus to retain mass and subsequent muscle tissue (6).