Incomplete tetanus is produced when a muscle receives repeated electrical stimuli fast enough that individual twitches overlap but not so fast that they fuse into one smooth contraction. The key factor is stimulus frequency: signals arrive before the muscle has fully relaxed from the previous contraction, causing the twitches to stack on top of each other in a jerky, rising pattern of force. This happens at moderate frequencies, roughly 10 to 35 Hz depending on the muscle type, and it reflects what your muscles actually do during most everyday movements.
How Stimulus Frequency Creates the Effect
A single electrical signal to a muscle fiber produces a single twitch: a quick contraction followed by full relaxation. If a second signal arrives after the fiber has completely relaxed, you simply get another identical twitch. But if the second signal arrives while the fiber is still partially contracted, the new twitch builds on top of the residual tension from the first. This stacking process is called wave summation, and it’s the direct mechanism behind incomplete tetanus.
As the frequency of stimulation increases, the overlap between successive twitches grows larger. The force output climbs in a wavy, sawtooth pattern where peaks and valleys are still visible but the overall tension is much higher than a single twitch would produce. This distinguishes incomplete (or “unfused”) tetanus from complete tetanus, where the frequency is high enough that individual twitches merge entirely and the muscle produces a smooth, maximal contraction. In lab preparations, incomplete tetanus in fast muscle fibers has been studied at frequencies around 35 Hz, while complete tetanus in some muscle preparations requires frequencies approaching 200 Hz to generate peak force output.
The Role of Calcium Inside the Muscle Fiber
The underlying reason twitches can stack is calcium. Each time a nerve signal reaches a muscle fiber, the internal storage system (called the sarcoplasmic reticulum) releases a burst of calcium ions into the cell. Calcium is what allows the protein machinery inside the fiber to grab on and generate force. After the signal passes, the sarcoplasmic reticulum pumps that calcium back in, and the fiber relaxes.
During incomplete tetanus, the next signal arrives before all the calcium from the previous release has been pumped back into storage. This means the baseline calcium concentration inside the fiber stays elevated. Each new release adds more calcium on top of what’s already there, so the contractile proteins can generate progressively more force. Research on frog skeletal muscle has shown that an internal calcium buffer also plays a role: during sustained stimulation, this buffer becomes saturated with calcium and can no longer absorb the excess. The result is a longer-lasting elevation of calcium inside the cell, which keeps the fiber partially contracted between stimuli and produces the characteristic rippling tension of incomplete tetanus.
Why Different Muscles Have Different Thresholds
Not all muscles transition to incomplete tetanus at the same frequency. The threshold depends largely on how fast a given fiber type contracts and relaxes. Fast-twitch fibers complete a single twitch quickly, so they need a higher stimulus frequency before twitches start to overlap. Slow-twitch fibers take longer to relax, meaning even relatively low-frequency stimulation can produce summation.
This distinction matters in research. In fast motor units, the initial force production during unfused tetanus shows a characteristic “boost” where the first few responses (the second through fourth contractions in the fastest fibers) generate disproportionately high force before settling into a lower, more stable level. In slightly slower fast-fatigable-resistant units, this peak force occurs across the second through eighth responses. After the boost, fast motor units often show a phenomenon called “sag,” where force temporarily declines before stabilizing. Slow motor units generally don’t display sag, which researchers use as a practical way to classify fiber types.
Energy Cost and Fatigue
Maintaining incomplete tetanus costs energy, but significantly less than complete tetanus. The muscle is cycling between partial contraction and partial relaxation, so it gets brief windows to restore some of its fuel supply between peaks. Fast-twitch fibers consume their energy currency (ATP) faster than they can regenerate it during intense stimulation, which is a core driver of fatigue. Glycogen, the primary energy reserve in muscle, depletes rapidly during high-intensity anaerobic work and more gradually during aerobic activity.
Fatigue during repeated tetanic contractions in fast-twitch fibers follows a predictable three-phase pattern. First, force drops by 10 to 20 percent even as calcium levels inside the fiber actually increase. Then there’s a plateau where force holds relatively steady. Finally, both force and calcium levels decline sharply. How long that stable middle phase lasts varies dramatically between individual fibers and depends heavily on their capacity for aerobic metabolism. Fibers with better oxygen utilization maintain the plateau far longer.
This matters practically because the intermittent stimulation pattern that produces incomplete tetanus closely mimics what happens during natural activities like walking, running, and breathing. The built-in relaxation intervals between contractions allow for some metabolic recovery, producing a much slower rate of fatigue than continuous high-frequency stimulation would.
Incomplete Tetanus in Everyday Movement
Your nervous system rarely commands a muscle to produce single isolated twitches or maximal complete tetanus. Most voluntary movements rely on motor neurons firing at frequencies that produce incomplete tetanus. This gives the nervous system fine control over force output: by adjusting the firing rate up or down, even by a few pulses per second, the brain can smoothly increase or decrease the force a muscle generates. The wavy, partially fused contractions of incomplete tetanus are what allow you to hold a coffee cup with just enough grip, maintain posture while standing, or control the speed of your stride while walking.
The staircase effect (also called treppe) adds another layer. When a muscle that’s been resting is first activated, the initial contractions produce roughly half the force of later ones. With repeated stimulation, each successive contraction grows slightly stronger as calcium accumulates in the cell and the contractile machinery warms up. This graded increase in efficiency means your muscles perform better a few contractions into an activity than they do on the very first one, which is one physiological basis for the familiar experience of a warm-up improving performance.