Learning Objective

By the end of this section, you should be able to define preload and afterload, distinguish between passive and active tension, and explain how these factors influence isotonic and isometric muscle contractions.

Preload

Preload refers to the load placed on a muscle while it is in a relaxed state—before contraction begins. Applying preload to a muscle produces two important effects:

• Stretching of the muscle:
  This stretch is transmitted to the sarcomeres. As preload increases, the sarcomeres are stretched to a greater extent.
• Generation of passive tension:
  Because muscle contains elastic components (e.g., titin), it resists being stretched. This resistance creates passive tension within the muscle—similar to the recoil seen when a rubber band is stretched and released. Increasing preload leads to greater passive tension.

Afterload

Afterload is the load that a muscle must overcome during contraction. For example, if a muscle attempts to lift a 10 kg weight, that weight represents the afterload. Two outcomes are possible:

• If the muscle generates a force greater than 10 kg, the muscle shortens, and the weight is lifted.
  → This is an isotonic contraction.
• If the muscle generates a force less than 10 kg, the muscle does not shorten, and the weight remains stationary.
  → This is an isometric contraction.

Types of Muscle Tension

• Passive tension: Produced by the elastic properties of muscle in response to preload
• Active tension: Generated by actin–myosin cross-bridge cycling during contraction
• Total tension: The sum of passive and active tension

Activity