Question

Explain the following two statements: Muscles can only actively contract; muscle fibers lengthen passively

Short answer

Muscles function by actively contracting and generating force, which requires energy in the form of ATP. This occurs when nerve impulses release calcium ions, causing muscle fibers to slide past each other and shorten. Conversely, muscle fibers lengthen passively, without using energy, as they return to their resting length. This passive lengthening relies on the force generated by the contracting muscle and the elastic properties of the muscle tissue. In a pair of muscles, one muscle actively contracts while the other relaxes and passively lengthens, allowing for controlled movement and force generation.

Step-by-step solution

Introduction to Muscles

Muscles are specialized tissues in our body composed of muscle fibers that allow for movement and force generation. In order to understand the two statements given, it is important to know how muscles function at the cellular level. Muscles typically work in pairs, where one muscle contracts while the other relaxes, allowing for movement.

Understanding Active Contraction

Muscles can only actively contract, which means they generate force and tension when they shorten. This process is initiated by nerve impulses that trigger the release of calcium ions within muscle cells. The calcium ions bind to specific molecules in the muscle fibers, which in turn causes the muscle fibers to slide past each other and shorten, ultimately leading to muscle contraction. This is an active process because it requires energy in the form of ATP (adenosine triphosphate) for the muscle to contract.

Passive Lengthening of Muscle Fibers

Muscle fibers lengthen passively, which means that they return to their resting length without using energy. When a muscle contracts, the opposing muscle relaxes and is passively lengthened by the force generated by the contracting muscle. Essentially, the contracting muscle is doing the work to pull the lengthening muscle back to its original position. This process is passive because the muscle fibers do not require energy to lengthen; instead, they rely on the force generated by the contracting muscle and the elastic properties of the muscle tissue.

In Conclusion

To summarize, muscles can only actively contract and generate force, which requires energy. Muscle fibers lengthen passively, meaning they return to their resting length without using energy. When one muscle in a pair contracts, the force generated by this contraction causes the opposing muscle to relax and lengthen passively. This combination of active contraction and passive lengthening allows for controlled movement and force generation in the body.