Fiber functional properties, peak force, contraction velocity, resistance to fatigue, oxidative and glycolytic capacities, and actino-myosin ATPase activities, fall across a broad spectrum. Nonetheless, it is possible to divide this continuum into a few clusters.

Based on observations of the contractile properties of motor units (force, velocity and fatiguability), Burke and coworkers created four motor unit types. Histochemical assays of the motor unit fibers striking similarites within a unit.

Slow

The most distinct type had long twitch times, low peak forces and high resistance to fatigue. Biochemically, these fibers were found to be high in oxidative enzymes, but low in glycolytic markers and ATPase activity. These have been termed "slow" fibers.

Fast, Fatigue Resistant

Of the fibers with faster contraction times, some were found to maintain their force production even after a large number of contractions. They tend to be high in oxidative and glycolytic enzymes and ATPase activity. These have been termed Fast Resistant (FR) or (histochemically) Fast Oxidative-Glycolytic (FOG).

Fast Fatiguable

The last clearly definable group displayed high contraction rates and extremely large forces, but was unable to maintain these tensions for more than a few contractions without rest. These properties correlated with high ATPase and glycolytic activities and low oxidative capacity. These have been termed Fast Fatiguable (FF) or Fast Glycolytic (FG) fibers.

Fast Intermediate

Basically a catch-all group for a small number of fibers that didn't clearly belong to the other fast groups. These fibers have fast contraction times and maintain some, though not a great amount of their force production with repeated activity.

With the development of techniques capable of identifying specific proteins (or even isoforms of the same family), specifically, antibody techniques and gel electrophoresis, it has been found that these functional properties are closely related to the myosin heavy chain (MHC) isoform. In fact, most of the proteins of the contractile machinery exist in several isoforms, with one or two being associated with each MHC. It should be noted that there is not a one-for-one correspondence between the functional classification and the MHC based classification schemes: you can find fibers that contract quickly but express the slow myosin isoform (and vice versa). These are relatively uncommon, though.

There are at least nine different mammalian MHC isoforms. Two are developmental, termed embryonic and neonatal, based on the time of their expression. Two are "slow" forms, expressed in the heart and termed cardiac alpha and beta. The cardiac beta is also found in slow skeletal muscle fibers (in which case it is called type 1). The remaining forms are found in fast skeletal muscle. Type 2a is found in most FOG fibers, and type 2b and 2x in FG fibers. The last two are relatively rare and appears to be expressed primarily in the extraocular, laryngial and jaw muscles.