Final answer:
The basic contractile unit of a muscle according to the Hill model is the sarcomere, which contracts through the sliding of actin and myosin filaments as explained in the sliding filament model.
Step-by-step explanation:
The basic contractile unit of a muscle in the Hill model is the sarcomere. The sarcomere is the region from one Z-line to the next and is composed of thin and thick protein filaments, namely actin and myosin. During muscle contraction, as described by the sliding filament model, these filaments slide across each other to shorten the sarcomere, leading to muscle shortening and movement.
The interaction and formation of cross-bridges between actin and myosin, powered by ATP hydrolysis, result in the muscle's ability to generate tension and produce movement. This phenomenon was extensively described in Huxley's seminal work, explaining the mechanisms of ATP-dependent acto-myosin binding and the resulting power stroke that occurs in the myosin heads during the contraction cycle.