Effects of spaceflight on the musculoskeletal system: NIH and NASA future directions.

Prolonged bed rest, undertaken by volunteers or resulting from injury and disease, can impair bone and muscle function and structure; extended travel in space also induces these effects. Fluid shifts and disrupted fluid balance may also contribute to observed musculoskeletal aberrations in the weightless environment. Some molecular and cellular events involved in the loading and unloading of the musculoskeletal system are under neural and endocrine influence or control, whereas other events are influenced by local growth factors.

Myosin subfragment-1 attachment to actin. Expected effect on equatorial reflections.

The characteristic equatorial X-ray pattern from a relaxed vertebrate skeletal muscle changes when the muscle is activated. In particular, there is a simultaneous decrease in the intensity of the first reflection (I10) and increase in the intensity of the second (I11). This observed change is almost reciprocal.

X-ray diffraction of actively shortening muscle.

Low angle x-ray diffraction patterns were obtained from resting and activated frog sartorius muscles by means of a position-sensitive detector. Although the intensity ratio I10/I11 decreased many-fold upon activation, it was nearly the same during isometric and isotonic contraction. Thus, motion has a much smaller effect on the low order equatorial pattern than the transition from rest to activity.