Effects of Preload on Cardiac Fiber Shortening

When allowed to shorten against a fixed afterload (isotonic contraction), an increase in muscle length prior to contraction (i.e., increased preload) increases the degree of shortening (ΔL) and the rate of length change (dL/dt; velocity of shortening) as shown in the figure at right. With increased preload, a normal cardiac muscle fiber will shorten to the same minimal length. The effect of preload on velocity of shortening alters the slope of the force-velocity relationship for cardiac muscle.

As described above, increasing the preload increases the magnitude of the muscle fiber shortening. This can also be depicted using a length-tension diagram as shown at right, below.

With the muscle preload length at L₁, the muscle will develop tension until a fixed afterload tension is reached, then the muscle will shorten (black, dashed line) with no further increase in tension to a minimal length (L_min, blue line). If the preload is increased to L₂ and the muscle contracted with the same fixed afterload, the muscle will shorten (green, solid line) to the same L_min. Therefore, a normal cardiac muscle fiber will contract to the same minimal length regardless of the preload, as long as the preload is within a normal physiologic range.

Revised 04/02/2007