Final answer:
Larger motor units are recruited and fire at a higher frequency, leading to a stronger muscle contraction when there is a need for increased grip strength.
Step-by-step explanation:
Observing an EMG signal at different grip strengths shows that larger motor units are firing at higher frequency when you clench harder. This phenomenon is part of the recruitment and rate coding, which are two mechanisms that our nervous system uses to increase the force production of muscles. Small motor units, which contain small, excitable motor neurons that control a few muscle fibers, are activated first at a low contractile force. As the required strength increases, larger motor units with higher-threshold motor neurons that control more muscle fibers are recruited, resulting in a stronger muscle contraction. Finally, as the frequency of action potentials increases, this can lead to a state called complete tetanus, where muscle contractions become continuous, further increasing the force production.