What is the effect of preload and after load on muscle twitch?

Preload is the load placed on a muscle before the muscle contracts. It serves to stretch the muscle sarcomeres, thus producing a passive tension across the muscle.

This passive tension increases muscle contraction in two ways:

(i) It adds an elastic recoil force to the muscle during its contraction;

(ii) It stretches the muscle to its resting length, producing the optimum length-tension relationship for active force generation. In real life situations, it is a common practice to pre-stretch a muscle using antagonist muscles. Through experience, we learn how much force our muscles generate at different lengths and unconsciously, we adjust muscle length before initiating a movement to develop the power we want. After load is an opposing force which the muscle encounters immediately after it starts contracting. It is a force that a muscle must overcome before an observable shortening of the muscle can occur.

The contraction of an after-loaded muscle is comparatively lesser. In real life situations, after-loaded contractions tend to occur in novel situations when the magnitude of the load is not known or during rapid adjustments to unexpected perturbations in load.