Final answer:
Sarcomeres, the basic functional units of muscle fibers, are responsible for muscle shortening, as they contain actin and myosin filaments that slide over each other during contraction, shortening the muscle fiber and generating movement.
Step-by-step explanation:
The functional units that make a muscle shorten, or contract, are known as sarcomeres. Sarcomeres are the basic functional units of skeletal and cardiac muscles, comprising overlapping actin and myosin protein filaments. The process of muscle contraction involves the sliding of these filaments over each other, which leads to the shortening of the sarcomere and thus the muscle fiber. As a multitude of sarcomeres within a muscle fiber simultaneously shorten, the whole muscle fiber contracts. This shortening is critical for muscle movement and the generation of force.
During muscle contraction, the interaction between actin and myosin filaments involves the myosin heads attaching to actin to form cross-bridges, which pull the actin filaments inwards. This sliding motion reduces the overall length of the sarcomere, creating muscle tension. The shortening of the H zone and maintenance of the A band length are characteristic changes that occur during this process. A muscle fiber contracts fully, and the cumulative contraction of numerous fibers determines the overall force of the muscle. The strength of muscle contraction is directly proportional to the number of fibers activated.