When a muscle is immobilized in a shortened position (as can happen when someone is in a cast, or bedridden), sarcomeres, which are the active, contractile components of the muscle fibers, disappear, making the muscle shorter. In addition, the collagen fibrils inside the muscle increase as a ratio to sacromeres, which increases the stiffness of the muscle. This does not happen to a muscle that is immobilized in a lengthened position, but unfortunately, when one muscles is lengthened, its antagonist is shortened and it is this muscle that will undergo stiffening and weakness.
However, if a yin-like stress is applied to the shortened muscle, loss of length is avoided and the muscle does not become stiffer. This study looked at three groups: one control group; one group that had a joint immobilized for 10 days; one group that was also immobilized for 10 days, but every 2 days the cast was removed the muscles was statically stretched for 15 minutes (a long dorsiflexion was held at the ankle). At the end of the test, the subjects that had yin yoga done to the muscle had considerably greater range of motion than the second group, however it was not the same as controls who were not immobilized. There was no significant difference in the number of sarcomeres between group 2 and 3, however and both groups had far less sacromeres than the control group.
Conclusion: in this study, applying a yin yoga stress for 15 minutes every second day prevent a dramatic loss of range of motion to an immobilized muscle, but being a yin-stress, it did nothing to maintain the muscle’s strength.
See P.E. Williams “Effect of intermittent stretch on immobilized muscle” in Ann Rheum Dis. 1988 Dec; 47(12): 1014–1016. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1003657/