Harold,
A lot depends on what type of tissue you are freezing. I have never seen
this problem with skeletal muscle, at least within the muscle fibers,
although sometimes the tissue can crack within the intermyofibrillar
connective tissue, if this is pathologically excessive. 
Tim mentions that leaving tissue in frozen isopentane for long periods
of time can lead to cracks. I have never experienced that. When working
in Glasgow, Scotland, we used to have to travel for several hours in a
car to do the muscle biopsies, and the tissue was kept in frozen
isopentane for >two hours on the return journey with no tissue damage.
However, cracking is common with CNS tissue snap-frozen in isopentane.
We solved this problem by freezing in hexane, pre-cooled in liquid
nitrogen. Hexane freezes at a higher temperature, -90C if my memory
serves me right.
 
Ronnie Houston
Cytochemistry & Molecular Pathology
Texas Scottish Rite Hospital for Children
2222 Welborn Street
Dallas, TX 75219