Final answer:
The discharge frequency of motor units affects muscle output by varying the number of action potentials sent to muscle fibers, thereby controlling muscle tension and force production.
Step-by-step explanation:
Motor Unit Discharge Frequency and Muscle Output
The discharge frequency of a motor unit, which is the number of action potentials sent to muscle fibers per second, plays a crucial role in determining the muscle's output.
During low-intensity tasks such as picking up a pencil, fewer motor units are activated, leading to minimal muscle fiber response.
However, when a high-intensity task like lifting a heavy object is performed, more motor units are activated, resulting in higher muscle fiber participation and thus maximum force production.
This force can be further enhanced by increasing the action potential frequency, causing more calcium to flood the tropomyosin and increasing the potential for force production.
Moreover, different motor unit sizes in the muscle allow for fine-tuned control over muscle contraction strength. Small motor units, with more excitable, lower-threshold motor neurons, are activated first for light contractions, while larger motor units are recruited for more powerful contractions.
This process is known as graded muscle response or recruitment, allowing for a broad range of muscle contraction strength based on the needs of the task at hand.
At high frequencies of stimulation, muscle tension increases up to the point of incomplete tetanus, with muscle fibers undergoing quick contraction cycles.
If stimulation frequency is exceptionally high, relaxation phases diminish, leading to complete tetanus where contractions become sustained and continuous, maximizing muscle tension.