Final answer:
A Time to Peak vs Proportion of MU graph explains the relationship between the time take to reach peak muscle tension during a twitch, and the percentage of motor units recruited during voluntary contraction, which correlates with the force produced by the muscle.
Step-by-step explanation:
When discussing voluntary contraction and muscle response, a key concept is the Time to Peak vs Proportion of Motor Units (MU) graph. In this context, the 'Time to Peak' refers to the time taken for a muscle to reach peak tension during a twitch (which is a single, quick contraction of a muscle fiber following a nerve impulse).
The 'Proportion of MU' indicates the percentage of motor units being recruited during a voluntary contraction. As voluntary muscle contraction begins, motor units are progressively recruited to increase muscle tension. The increase in the proportion of motor units recruited correlates with the increased force produced by the muscle.
Initially, fewer motor units are needed for tasks requiring less force, such as picking up a pencil. However, as the demand for force increases, for example, when trying to lift something heavy like a piano, more motor units are activated until potentially all available motor units are recruited, signifying maximal force production.
Furthermore, the graph illustrates that as the frequency of motor neuron stimulation increases, the muscle tension can reach up to three to four times the tension of a single twitch in a phenomenon known as incomplete tetanus.
Should the frequency of stimulation be so high that there is no relaxation phase, the muscle enters a state known as complete tetanus, resulting in continuous contraction.