Skeletal muscle fibers are classified as ""fast"" or ""slow"" based upon what factor?

Skeletal muscles fibre are classified base on how the produce energy.

Explanation:

Skeletal muscles fibres consist of bundles of cells that form muscles which contain myobrills.

Skeletal muscles are classified based on how the produce energy;

Type 1 or slow pitch muscle fibres are more efficient and last for a long period of time. They are use for postural maintenance or endurance. It use aerobic respiration to produce energy or ATP.

Type 11 or fast twitch muscle fibres use anaerobic respiration and are for short speed and fatigue more easily than type 1.

The speed of the myosin ATPase.

Explanation:

Myosin ATPase is actually considered as molecular motor of muscle cells because it converts chemical energy into directional movement.

ATP comes and bind with the myosin. Now, it is in high-energy state. ATPase hydrolyzed the ATP into ADP by releasing of inorganic phosphate (Pi).

When myosin binds with the actin, then ATPase convert ATP into ADP and this chemical energy converts into directional movement (contraction) of muscle cells.

Myosin ATPase speed depends on the shortening of respective muscle. ATPase activity are higher when intrinsic speed increases.