Lumbar disc volume measured by MRI: effects of bed rest, horizontal exercise, and vertical loading

Nutrition to the cells in the disc is partly dependent on fluid flowing out during the day and flowing in during bed rest. In spaceflight there are little or no such diurnal changes, since the gravitational load is essentially zero.

The questions we asked were: 1) How much fluid does the disc gain during the night and how quickly does the disc lose fluid during the following day? 2) Is it possible to carry out, in a reasonable amount of time, an exercise regimen on a spacecraft that would be rigorous enough to expel from the disc the equivalent amount of fluid lost during a normal day's activity?

in five normal subjects, magnetic resonance imaging was used to measure the volume changes (and the corresponding fluid changes) in the lumbar intervertebral discs (L1/L2, L2/L3, L3/L4, L4/L5) before and after a night's bed rest and again at specific intervals during the course of the day while carrying out three different protocols: walking, carrying a backpack, and exercising in a horizontal position.

1) On average the disc gained 10.6% of its volume (or 0.90 cm3 of fluid) during an overnight bed rest; 2) on rising in the morning and after 8 h (using our walking protocol) the disc volume did not decrease to the volume measured at the end of the previous day; and 3) wearing a backpack that weighed 40% of body weight produced a volume decrease equivalent to the decrease in volume obtained at the end of the day, but it took 4 h to do it.

The ability of the disc to retain water is substantial. It would be difficult to carry out an exercise regimen on a spacecraft that would be rigorous enough to expel from the disc the equivalent amount of fluid lost during a "normal" day's activity on Earth.