Figure 2. Schematic illustration of force enhancement following stretch according to the sarcomere length non-uniformity theory.

A muscle is stretched on the descending limb of the force–length relationship from an initial mean sarcomere length (○) to a final mean sarcomere length (▪). During stretching, it is assumed that a few sarcomeres are stretched much more than average (• right) while most sarcomeres are stretched less than average (• left). The sarcomeres that are stretched less than average are stronger than the average sarcomere would be because of the slope of the force–length relationship. The sarcomeres that are stretched more than average become weaker initially, lose overlap between actin and myosin, and are ‘caught’ by passive elements. They elongate until the passive force is at equilibrium with the force of the ‘short’, actively force-producing sarcomeres. This force equilibrium (dashed line connecting the filled circles) is greater than the expected force at the final average sarcomere length. If we assume now that in a purely isometric contraction, sarcomere lengths are ‘uniform’, then this mechanism could potentially account for the experimentally observed force enhancement (FE).