A single brief action potential in a skeletal muscle fiber can produce a transient mechanical response called a twitch.

The electrical action potential is much shorter than the resulting tension transient because Ca$^{2+}$ release, cross-bridge cycling and Ca$^{2+}$ reuptake continue after membrane repolarization.

A twitch can be divided conceptually into:

1. a latent period, when excitation–contraction coupling has begun but measurable tension has not yet risen much;
2. a contraction phase, when cytosolic Ca$^{2+}$ is elevated and active tension rises;
3. a relaxation phase, when Ca$^{2+}$ is returned toward the SR and active tension falls.

Repeated action potentials can summate mechanically

If a second action potential arrives before the fiber has fully relaxed, it releases additional Ca$^{2+}$ while Ca$^{2+}$ and cross-bridge activity from the first response are still elevated.

The second mechanical response therefore begins from a partially activated state and can produce more tension. This is temporal or wave summation.

widely spaced spikes → separate twitches
closer spikes         → overlapping Ca2+ transients → summed tension

The action potentials themselves do not fuse into one larger action potential. The mechanical responses summate because Ca$^{2+}$ handling and contraction are slower than the electrical spikes.

High stimulation frequencies produce tetanus

As action-potential frequency rises, cytosolic Ca$^{2+}$ may remain elevated between stimuli.

• In unfused (incomplete) tetanus, tension remains high but still oscillates because partial relaxation occurs between stimuli.
• In fused (complete) tetanus, stimuli are frequent enough that little detectable relaxation occurs and tension forms a sustained plateau.

Here tetanus means sustained skeletal-muscle contraction from high-frequency activation; it is not the infectious disease of the same name.

Force depends on activation history

A twitch therefore does not define one fixed maximum force for a muscle fiber. The same fiber can produce greater average tension when action potentials arrive more frequently because the intracellular Ca$^{2+}$ signal is temporally integrated.

This provides one mechanism by which the nervous system grades muscle force without changing the amplitude of individual action potentials.