Final answer:
Passive muscle force increases when muscle tissue stretches, due to its elastic properties. This differs from active muscle tension, such as a twitch force, which requires neural activation and ATP for cross-bridge cycling during contraction.
Step-by-step explanation:
The increase in passive muscle force when a muscle is stretched can be explained by the elasticity and extensibility properties of muscle tissue. As the muscle stretches, the passive tension increases due to the elastic fibers within muscles which resist the stretch and recoil to their original length. This is different from active muscle tension or twitch force, which is generated by the contractility of muscle fibers when activated by the nervous system through a series of action potentials that lead to cross-bridge formation between actin and myosin within the sarcomeres.
During an active contraction, such as a twitch, the muscle actively generates force. This involves neurological stimulation, calcium release, and the sliding filament mechanism resulting in muscle shortening and tension. By contrast, passive force doesn't require neural input or energy consumption in the form of ATP but is a product of the muscle's inherent elastic properties. When the muscle stretches to a certain point, it reaches an optimal overlap of actin and myosin, providing maximal power during active contraction, but beyond that, the excessive stretch diminishes force production.
The concepts of graded muscle response, summation, and tetanus further explain how active force can be modulated in response to increased frequency of action potentials and recruitment of additional motor units.