Question

What is the most important characteristic of muscle architecture that dictates its maximal force production?

a.) Tendon length b.) Number of sarcomeres in series c). Anatomical cross-sectional area d). Physiological cross-sectional area

Answer 1

D) Physiological cross-sectional area

The muscle cross-sectional area (also known as anatomical cross-section area, or ACSA) does not accurately represent the number of muscle fibers in the muscle. A better estimate is provided by the total area of the cross-sections perpendicular to the muscle fibers,This measure is known as the physiological cross-sectional area (PCSA).

The total force exerted by the fibers in their oblique direction is proportional to PCSA. If the specific tension of the muscle fibers is known (force exerted by the fibers per unit of PCSA), it can be computed as follows

However, only a component of that force can be used to pull the tendon in the desired direction. This component, which is the true muscle force (also called tendon force), is exerted along the direction of action of the muscle

The other component, orthogonal to the direction of action of the muscle (Orthogonal force = Total force × sinΦ) is not exerted on the tendon, but simply squeezes the muscle, by pulling its aponeuroses toward each other.

Notice that, although it is practically convenient to compute PCSA based on volume or mass and fiber length, PCSA (and therefore the total fiber force, which is proportional to PCSA) is not proportional to muscle mass or fiber length alone.

The maximum (tetanic) force of a muscle fiber simply depends on its thickness (cross-section area) and type. By no means it depends on its mass or length alone.