Rationale

The cardiovascular system seems to adapt well to microgravity but is significantly compromised on re-establishment of gravitational forces, resulting in orthostatic intolerance and a reduction in work capacity. Many of the physiological consequences of weightlessness and the cardiovascular abnormalities on return from space could be due, at least in part, to alterations in the regulation of the autonomic nervous system. Changes in, relative contributions to and adaptations of the autonomic nervous system can be monitored by recording heart rate variability, which has been shown to be a reliable and non-invasive probe for the identification of autonomic control mechanisms. Previous studies investigating the effects of microgravity on the autonomic system have revealed conflicting results and have been unable to conclude whether autonomic control differs between a 1-G and a 0-G environment.

To our knowledge, the literature provides no reports of a closely monitored and regulated exercise regimen during space flight, and tends to focus more on pre- and post-flight measurements. There is a substantial amount of evidence to suggest that post-flight loading of the lower limbs results in eccentric-like ultra structural muscle damage. It has thus been suggested that by performing unaccustomed eccentric exercise during the pre-flight phase, the muscle weakness, stiffness and delayed onset muscle soreness that accompanies this post-flight muscle damage may be attenuated. Because humans tend to predominantly use their upper limbs for stability, it would be logical to focus this exercise training on the lower limbs. It is also possible that by participating in regular in-flight strength and endurance training, lower limb vasoconstrictor properties will be maintained and will therefore aid in preventing post-flight orthostatic intolerance to a certain degree.

To date, studies investigating the impact of micro-gravity on total daily energy expenditure have used the doubly labeled water method, a costly, invasive technique, which requires astronauts to diligently take daily urine and saliva samples for analysis. A far more cost-effective and less invasive technique is that of heart rate monitoring. This is applicable for the South African population as it provides an accurate means of measuring total energy expenditure in large sample groups whilst still being a cost effective modality. Should this technique prove to be accurate in a microgravity environment where there are known and reported cardiovascular changes, the validity of this technique will be strengthened in South Africa.

A large proportion of the laboratory measurements that have been conducted in space have not been reported in peer-reviewed journals and have thus been questioned in terms of scientific rigor (West, 2000). Similarly, a large amount of the microgravity literature has been predominantly descriptive in nature and limited in terms of physiological application to humans during and after space flight.