Final answer:
The basic unit of contraction in a skeletal muscle is the motor unit, and the force of contraction is increased through recruitment. Small motor units are activated first for low strength tasks, and larger motor units are recruited for tasks requiring more strength.
Step-by-step explanation:
The basic unit of contraction in an intact skeletal muscle is the motor unit. The force of contraction within a skeletal muscle is increased by additional motor units through a process known as recruitment. In skeletal muscles, there is a wide range of motor units which allows the nervous system to exert a wide range of control over the muscle. The smallest and most excitable motor units activate first, resulting in a small degree of contractile strength. As the need for more strength arises, larger motor units with higher-threshold motor neurons are recruited to activate larger muscle fibers, increasing the muscle contraction. The largest motor units can significantly increase the contractile force, and the recruitment of these units allows muscles to pick up a light object as easily as lifting a heavy one.