What is Rate coding?

What is Rate coding?

Rate coding is the modulation of motor-unit firing frequency, and it is the second of the two levers your nervous system pulls to control how hard a muscle contracts. The first lever is recruitment: adding more motor units to the job. The second is rate coding: making the units you have already switched on fire their action potentials faster.

A motor unit is a single motor neuron plus every muscle fibre it commands, and it works all-or-none. When the neuron sends one impulse, all its fibres twitch once and relax. Send impulses closer together and the twitches begin to overlap and add up, so the same motor unit produces far more force at a high firing rate than at a low one.

That is the whole idea: the same number of active fibres, but more force per fibre, purely because the signals arrive faster. Both mechanisms usually operate at the same time, and the balance between them shifts with the size of the muscle and the level of force. In small muscles of the hand, such as the first dorsal interosseous and adductor pollicis, recruitment finishes at a low fraction of maximum and rate coding then grades almost all further force.

In large muscles such as the biceps brachii and deltoid, new units keep being recruited up to around 85 percent of maximum, so rate coding shares the work across a wider range. Understanding rate coding matters because it explains why you can produce a fast, forceful contraction without recruiting a single extra fibre, and why explosive and heavy training change how your muscles behave even before they change in size.

How it works

Rate coding works through the summation of twitches into tetanus. A single action potential produces one brief twitch that rises and falls over roughly 100 milliseconds. If a second impulse arrives before the first twitch has fully relaxed, the two forces add together, and as firing frequency climbs the twitches fuse into a larger, steadier plateau.

At low frequencies you get an unfused, rippling tetanus; at high frequencies the ripples disappear into a smooth, near-maximal fused tetanus. The force-frequency relationship that describes this is sigmoidal: force rises steeply across a middle band of frequencies and then flattens once the muscle is close to fully fused. When a motor unit is first recruited it usually fires slowly, near 5 to 8 impulses per second (Hz), and the nervous system then raises that rate to grade force.

Maximum discharge rates are muscle-specific: slow muscles such as the soleus top out near 10 to 11 Hz, while the biceps brachii and adductor pollicis reach around 30 Hz, and averaged across young adults maximal rates sit near 50 Hz. During ballistic, maximal-speed efforts the story changes: the first few impulses can arrive at instantaneous rates well above 100 Hz, and units often begin with a doublet, two spikes only a few milliseconds apart, which sharply boosts the rate of force development.

Lower-threshold units also tend to sustain higher firing rates than higher-threshold units at a given force, a pattern De Luca called the onion-skin scheme, and persistent inward currents driven by neuromodulators such as serotonin and noradrenaline amplify how briskly a motor neuron will fire for a given synaptic input. Force at any instant is therefore the product of how many units are active and how fast each one is firing.

The formula

Muscle force ≈ (number of motor units recruited) × (their firing rates)

There is no single equation for rate coding; force follows a sigmoidal force-frequency curve. The reference scale below shows the typical firing frequencies and what the muscle does at each, for a limb muscle during a slow to hard contraction.

How to apply it

• Minimum firing rate at recruitment: Every motor unit switches on at a low, fairly consistent rate, usually 5 to 8 Hz. This gives the nervous system a soft entry point so that adding a unit does not jolt force upward, keeping low-level contractions smooth and controllable.
• Progressive rate increase: Once a unit is active, its firing rate rises as you push harder. Because the force-frequency curve is steep in its middle range, small increases in discharge rate produce large increases in force, which is how you finely grade effort mid-contraction.
• Twitch summation to tetanus: Impulses arriving before the previous twitch relaxes stack their forces. Low rates give a rippling unfused tetanus; higher rates fuse the ripples into a smooth plateau. This summation is the physical event that turns faster firing into more force.
• Doublets at onset: In fast efforts a motor unit often opens with two spikes only 2 to 5 milliseconds apart, a doublet. That tiny interval produces a disproportionate jump in force and speed, which is why doublets are central to a high rate of force development.
• Onion-skin firing pattern: Lower-threshold units recruited early tend to reach and hold higher firing rates than higher-threshold units at the same force. This layered scheme, described by De Luca, keeps the earliest, most fatigue-resistant units doing steady work.
• Neuromodulation of firing: Serotonin and noradrenaline generate persistent inward currents in motor neurons that amplify their response to synaptic input. The same descending drive then produces higher firing rates, so brainstem neuromodulation sets the ceiling on how fast units can code force.

Types

Worked example

How the two mechanisms split the work of grading force in a large limb muscle such as the biceps brachii, from a light hold to an all-out effort. The percentages are approximate and vary between muscles, but the pattern, recruitment first and rate coding taking over at higher forces, is consistent.

In small hand muscles the crossover happens much earlier, near 50 percent of maximum, because recruitment finishes sooner. The takeaway is the same: once you run out of new units to recruit, faster firing is the only remaining way to produce more force.

Rate coding vs recruitment

Recruitment and rate coding are the two halves of neural force control, not competing options. Recruitment brings units online in a fixed small-to-large order; rate coding then squeezes more force from whatever units are already firing. Both operate together across almost every contraction.

By goal

• Strength and powerlifting: Lift heavy, near 85 to 100 percent of maximum, where recruitment is essentially complete and further force depends on driving firing rates up. Intending to move the bar fast even when it is heavy trains the high-rate, high-doublet firing that separates a maximal grind from a missed lift.
• Power and explosiveness: Train ballistic, maximal-intent efforts such as jumps, throws, and speed pulls. These raise the initial discharge rate above 100 Hz and add doublets, the exact firing changes shown to increase the rate of force development after explosive training, even when the load itself is light.
• Beginners and general fitness: Early strength gains come largely from neural adaptations, including better recruitment and higher firing rates, before muscles visibly grow. Progressive, challenging sets with the intent to produce force are enough to drive these changes; you do not need to train to failure to improve rate coding.

Common misconceptions

• "More force always means recruiting more muscle fibres." Recruitment is only one lever. Once all available units are active, which happens near 50 percent of maximum in small muscles and about 85 percent in large ones, extra force comes entirely from rate coding, the units firing faster, not from any new fibres joining in.
• "Rate coding and recruitment are two competing strategies." They work together in almost every contraction. Recruitment sets how many units are on and rate coding sets how fast they fire. At low forces recruitment leads; at high forces and in fast efforts rate coding dominates, but both are usually changing at once.
• "Motor units fire faster and faster with no ceiling." Firing rates saturate. Sustained maximal rates are muscle-specific, roughly 10 Hz in the soleus and near 30 to 50 Hz in limb muscles, because a fully fused tetanus produces no extra force. Only the brief onset of ballistic efforts pushes instantaneous rates above 100 Hz.
• "Only heavy lifting can improve firing rates." Explosive training against light loads raises firing rates too. Dynamic ballistic training increased the discharge rate of the first few impulses by about 38 percent and added doublets, improving contraction speed through rate coding rather than through heavier absolute loads.

Related terms

Rate coding FAQ

What is rate coding in simple terms?

Rate coding is your nervous system producing more force by making already-active motor units fire faster. Faster impulses make each unit's twitches overlap and add up into a stronger, smoother contraction. It works alongside recruitment, which adds more units, to control how hard a muscle pulls.

What is the difference between rate coding and recruitment?

Recruitment changes how many motor units are switched on, adding them in a small-to-large order. Rate coding changes how fast the active units fire. Recruitment dominates at lower forces, while rate coding takes over at high forces and in fast, explosive efforts. Both usually operate together.

How does rate coding increase muscle force?

When a motor unit fires faster, each new impulse arrives before the previous twitch relaxes, so the twitch forces stack up, a process called summation. As firing climbs the twitches fuse into a steady, near-maximal plateau called tetanus, producing far more force from the same active fibres.

What is a normal motor unit firing rate?

Motor units are recruited firing slowly, around 5 to 8 times per second, then speed up as force rises. Sustained maximal rates are muscle-specific: near 10 Hz in the soleus and around 30 to 50 Hz in limb muscles. Brief ballistic efforts can spike instantaneous rates above 100 Hz.

When does rate coding matter more than recruitment?

Rate coding dominates once recruitment is nearly complete. In small hand muscles that happens near 50 percent of maximum, and in large muscles like the biceps around 85 percent. Above those points, and during fast explosive efforts, faster firing is the main way to make more force.

Can you train or improve rate coding?

Yes. Heavy lifting and explosive, maximal-intent training both raise firing rates. Dynamic ballistic training increased the discharge rate of the first impulses by about 38 percent and added doublets. These neural changes help explain early strength and power gains before muscles visibly grow.

What is a doublet in rate coding?

A doublet is two action potentials from the same motor unit fired only about 2 to 5 milliseconds apart, usually at the very start of a fast contraction. That short interval produces a large jump in force and speed, making doublets important for a high rate of force development.

How is rate coding related to rate of force development?

Rate of force development, how quickly force rises, depends heavily on how fast motor units fire at contraction onset. High initial discharge rates and doublets let force climb rapidly. This is why explosive training that raises early firing rates improves rate of force development and athletic power.

Do all muscles use rate coding the same way?

No. The balance depends on muscle size and function. Small hand muscles finish recruiting early and rely on rate coding for most force. Large muscles keep recruiting to high intensities. Slow muscles like the soleus cap out near 10 Hz, while limb muscles reach much higher firing rates.

What is the onion-skin scheme in rate coding?

The onion-skin scheme, described by researcher Carlo De Luca, is the observation that earlier-recruited, lower-threshold motor units sustain higher firing rates than later-recruited units at a given force. The firing-rate curves stack like onion layers, keeping the most fatigue-resistant units doing steady work.

References

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2. Del Vecchio A, et al. The increase in muscle force after 4 weeks of strength training is mediated by adaptations in motor unit recruitment and rate coding. J Physiol, 2019. PMC6441907
3. Del Vecchio A, et al. The identification of extensive samples of motor units in human muscles reveals diverse effects of neuromodulatory inputs on rate coding. eLife, 2024. Article 97085
4. Van Cutsem M, Duchateau J, Hainaut K. Changes in single motor unit behaviour contribute to the increase in contraction speed after dynamic training in humans. J Physiol, 1998. PubMed 9782179
5. Del Vecchio A, et al. Maximal discharge rate of motor units determines the maximal rate of force development during ballistic contractions in human. Front Hum Neurosci, 2014. PMC4001023
6. Del Vecchio A, et al. How motor unit recruitment speed and discharge rates determine the rate of force development. J Physiol, 2019. PMC6487922
7. Bellemare F, Woods JJ, Johansson R, Bigland-Ritchie B. Motor-unit discharge rates in maximal voluntary contractions of three human muscles. J Neurophysiol, 1983. PubMed 6663333