The contraction strength of muscles is regulated by motor neurons, which modulate the frequency of action potentials dispatched to the motor units based on the body's requirements. This process of varying the muscle stimulation frequency allows muscles to contract with a force that is precisely tailored to the needs of the moment, whether lifting a feather or a heavy box.
Wave summation
At low firing rates, motor neurons induce individual twitch contractions in muscle fibers. These twitches occur when the muscle fibers have time to fully relax between stimuli, resulting in sporadic, gentle contractions. If the neuron fires before the muscle fibers fully relax after the first twitch, the second contraction is added to the previous one, resulting in wave summation and a more robust overall contraction. The resultant phenomenon is called temporal summation or wave summation, wherein a more powerful second contraction occurs if a second stimulus is received while the relaxation phase continues.
Incomplete tetanus
The duration of a single twitch determines the maximum time available for wave summation. For example, if a twitch lasts for 20 milliseconds, subsequent stimuli must be separated by less than 20 milliseconds, which means a stimulation rate of more than 50 stimuli per second. This rate is usually expressed in terms of stimulus frequency, the number of stimuli per unit of time. As the frequency of stimulation increases, the muscle enters a state known as incomplete tetanus. During incomplete tetanus, the muscle fibers only partially relax between twitches, leading to a fluttering yet more forceful contraction. This state is often sufficient for many everyday tasks that require moderate strength.
Complete tetanus
At even higher frequencies, the muscle reaches complete tetanus, where individual twitches become indistinguishable, and the muscle fiber is in a state of continuous, maximal contraction. This occurs at frequencies of around 80 to 100 times per second and is typically utilized in situations requiring maximum force. In complete tetanus, the muscle does not relax at all between stimuli, resulting in a smooth and sustained contraction plateau that represents the peak force a muscle can generate.
Treppe
Stronger, more sustained contractions can also occur through the Treppe or the staircase effect. This occurs when consecutive stimuli are provided at a frequency that allows complete relaxation between twitches, yet each subsequent twitch produces a slightly greater force. This effect is attributed to a gradual increase in calcium ions in the sarcoplasm and increased muscle enzyme efficiency.
