What is the basic unit of muscle contraction?

Sarcomere is the basic unit of muscle contraction in striated muscle, which includes skeletal and cardiac muscles. It is a highly organized structure made up of a series of thick and thin protein filaments. The thick protein filaments are mainly composed of the protein myosin. The thin protein filaments are mainly composed of the protein actin along with two other regulatory proteins –tropomyosin and troponin. The myosin and actin filaments are organized in an alternating arrangement, giving skeletal and cardiac muscles their distinctive striated appearance. 

The tropomyosin and troponin proteins control the interaction between the thick and thin filaments, thereby regulating the process of muscle contraction.  

According to the Sliding Filament Theory of Muscle Contraction, muscle contraction occurs when the thin actin filaments slide past the thicker myosin filaments causing the individual sarcomere to contract.