Rigor development is accompanied by a decrease in fiber diameter as myofilaments are drawn closely together by the active heads of myosin molecules. Fibers also may lose water as their pH declines towards the isoelectric point of myofibrillar proteins. If fibers are removed from a muscle prior to the onset of rigor mortis, they may contract in a manner that is difficult to control. Even if a muscle sample is clamped at a predetermined length, there is no guarantee that some parts of the sample will not contract strongly enough to stretch other parts. The measurement of radial fiber dimensions must be coupled with some method to reduce all data to a common muscle length related to sarcomere length.
Histological fixation introduces an extra set of experimental variables.
Fixation is necessary to preserve tissue structure, but some fixatives cause tissue shrinkage while others cause expansion. The radial growth of muscle fibers is associated with an increase in the apparent number of myofibrils seen in cross section. The apparent number of myofibrils is a useful measure of radial growth, even though it is rather an artificial parameter. Ribosomes are present within the myofilament lattice where presumably they are able to synthesize new myofibrillar proteins. As new myofilaments are added and the myofibril increases in diameter, myofibrils may split longitudinally, but seldom are the splits continuous along the complete length of a muscle fiber.