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In vivo, skeletal muscles produce graded and sustained contractions through the concept of motor units, which consist of a motor neuron and the muscle fibers it innervates. Graded contractions are achieved by motor unit recruitment, where smaller motor units are activated first for minimal force and larger units are recruited as more force is needed, and by frequency of stimulation, where the muscle can produce stronger contractions through wave summation or tetanic contractions if stimuli are provided before the muscle fully relaxes. Sustained contractions, or muscle tone, are maintained by the brain continuously activating a rotating pool of motor units to prevent fatigue. Overall, motor unit recruitment and varied frequency of stimuli allow muscles to finely control the force of contraction and maintain contraction for extended periods without fatigue.