In the past several years there has been increasing interest in different "fiber types" of skeletal muscle in various animals. Considerable progress has been made in gaining a better understanding of some of their metabolic and mechanical properties. At the same time, great confusion due to an absence of a common approach to the problem has limited progress on research dealing with skeletal muscles of mammals. Techniques are now available which can be utilized to resolve many of the fundamental questions that currently limit our basic understanding of fiber types. Resolving these questions is now a matter of combining existing histochemical, biochemical, mechanical and electromyographic skills in a coordinated approach with a proper interpretation of information currently available on fiber types. There are two major objectives to be pursued in this project: 1. Relate current classification schemes for motor units of hindlimb skeletal muscle to those of the forelimb and specialized muscles such as the masseter and temporalis in the mammals: guinea pig, cat and lesser bushbaby. 2. To gather information regarding how these mechanically and metabolically distinct types of muscle fibers are used in simplified isometric and isotonic contractions and in more complex movements such as walking, running, jumping and weight lifting.