Final answer:
When a skeletal muscle fiber contracts, myosin heads attach to actin to form cross-bridges and pull the actin, resulting in sarcomere shortening. The length of the sarcomere influences the force generated during shortening.
Step-by-step explanation:
When a skeletal muscle fiber contracts, myosin heads attach to actin to form cross-bridges followed by the thin filaments sliding over the thick filaments as the heads pull the actin, and this results in sarcomere shortening, creating the tension of the muscle contraction. The cross-bridges can only form where thin and thick filaments already overlap, so that the length of the sarcomere has a direct influence on the force generated when the sarcomere shortens. This is called the length-tension relationship.