Final answer:
The sliding filament theory explains how muscles contract through the interaction between thick myosin and thin actin filaments within muscle fibers, leading to the shortening of sarcomeres and ultimately muscle contraction, as seen when lifting a barbell.
Step-by-step explanation:
The Sliding Filament Theory
The lifting of a barbell from the waist to the shoulders is made possible through the process of muscle contraction described by the sliding filament theory. This theory states that muscle fibers contract when the thick myosin filaments repeatedly bind to and pull on thin actin filaments, causing the sarcomeres to shorten and thus the muscle to contract. A sarcomere is the functional unit of a muscle fiber, demarcated by Z-lines. The cross-bridges formed by the myosin heads walking along the actin filaments are powered by ATP, which allows for this shortening to happen.
How Muscle Contraction Occurs
When a nerve impulse stimulates muscle fibers, calcium ions are released, which enables the myosin-binding sites on the actin filaments to become exposed. The myosin heads then attach to these sites and pivot, pulling the actin filaments towards the center of the sarcomere. This sliding action of myosin on actin effectively shortens the sarcomere, which results in muscle contraction. Sequential contractions of many sarcomeres in a muscle fiber can produce significant movement, such as lifting a barbell.
Diagram 1: At rest, sarcomeres are in an extended position with myosin and actin filaments partially overlapping.
Diagram 2: During contraction, the sarcomeres shorten as the myosin pulls the actin toward the center.