Physiological cross-sectional area

In a non-pennate muscle the fibers are parallel to the longitudinal axis, and therefore PCSA and ACSA coincide.

A better estimate is provided by the total area of the cross-sections perpendicular to the muscle fibers (green lines in figure 1).

This measure is known as the physiological cross-sectional area (PCSA), and is commonly calculated and defined by the following formula, developed in 1975 by Alexander and Vernon:[4][5][6] where ρ is the density of the muscle: PCSA increases with pennation angle, and with muscle length.

Namely, the maximum (tetanic) force of a muscle fiber simply depends on its thickness (cross-section area) and type.

Hence, it cannot be described as the total area of the cross-sections perpendicular to the muscle fibers (green lines in figure 1).

Figure 1 Pennate muscle fiber arrangements. The green lines represent PCSA; the blue lines represent ACSA