• Introduction
  • The basics of neural adaptations to strength training
  • Motor unit recruitment
  • Rate coding
  • Final thoughts 
  • Sources
  • Action potential: a brief reversal of electric polarization of the membrane of a nerve cell or muscle cell.
  • Agonist muscle: the main muscle responsible for a movement.Also known as the “prime mover”.
  • Antagonist muscle: the muscle that opposes the action of another.
  • Central nervous system: the nervous system consisting of the brain and spinal cord.
  • Motor unit: a motor neuron and all muscle fibers innervated by it.
  • Motor unit synchronization: the ability to activate motor units with minimal (<5ms) delay.
  • Motor pool: a collection of motor units.
  • Rate coding: the rates at which motor units discharge action potentials.


Systematic training produces a myriad of structural and functional changes in the body. These so-called adaptations can be roughly divided into two categories; acute (short-term) and chronic (long-term) adaptations. Acute responses to resistance training occur mainly in the neurological, muscular, and endocrine (hormonal) systems, whereas chronic adaptations can be seen in the muscular, skeletal, endocrine, cardiovascular, and neurological systems. 

Training adaptations are often directly proportional to the volume (quantity), intensity (load), and frequency of physical demands placed on the body. If the body is presented with a greater stimuli that it is accustomed to, and is given sufficient time to recover, it adapts to these demands by becoming stronger. In short, consistent training allows the body to adapt to increasing amounts of physical work, resulting in beneficial changes in body composition and physical performance. 

It was previously thought that the cross-sectional area (CSA) of a muscle had the biggest impact on overall force production. This was due to the assumption that larger muscles can produce stronger contractions. However, more recent studies have highlighted the nervous system’s essential role as the primary component in maximal strength.

This post focuses on the neural adaptations to strength training, and why it plays such an integral part in strength training.

The basics of neural adaptations to strength training

During a muscle contraction, the brain sends electrochemical signals through the somatic nervous system (part of the peripheral nervous system associated with voluntary movements and sensory processing) to motor neurons that innervate muscle fibers. This combination of muscle fibers and the motor neuron that innervates them is known as a motor unit. When a person performs strenuous exercise, such as resistance training, the number and intensity of signals transmitted to that muscle are increased until the muscle becomes fatigued. 

The main neurological factors that contribute to the muscle’s capacity to produce force are motor unit recruitment and rate coding (i.e. firing frequency). The former refers to the amount of motor units activated during contraction, whereas the latter refers to the rate at which neural impulses are conducted to the individual motor units that make up the muscle. As the muscles grow more tired with each repetition, the rate coding becomes impaired and the firing sequence becomes less precise. Studies also suggest that motor unit synchronization (the ability to activate motor units with minimal (<5ms) delay) plays a small role in strength expression. However, it seems that this does not have as big of an impact as the other two factors.

In addition to the adaptations that occur in muscle fibers, resistance training also affects the influence of the nervous system on the muscle. The most concrete evidence of this is the increased electromyographic (EMG) signal across the muscle as a result of only a few weeks of consistent strength training. This illustrates that the muscles are activated more effectively via the nervous system. However, it is important to remember that this cannot determine whether the increased signal is due to the recruitment of more motor units, increased firing frequency, or even muscle hypertrophy. After all, the electrical potential is related to fibre cross-sectional area (CSA). Nevertheless, the resulting chronic neurological adaptations aim to:

  1. Generate a more efficient sequence of recruitment of motor units, resulting in a reduced tendency to fatigue due to neuromuscular factors.
  2. Increase motor unit firing and decrease co-contraction of the antagonist muscles (muscles with the opposite action of the prime mover), which occurs when both agonist (muscle responsible for a specific action. e.g. prime mover muscle) and antagonist muscles fire simultaneously.

Additionally, the nervous system must also learn to process the additional sensory feedback from the muscles and joints (proprioception). Together, these neural factors allow muscles to contract with greater force and increased efficiency.

Adaptations in intramuscular coordination (the interaction in between the nervous system and muscle) transfer well from one exercise to another, granted that the motor pattern is familiar to the athlete (e.g. maximal strength exercise and sport-specific skill). In a periodized seasonal training program, maximal strength macrocycles are used to increase motor unit recruitment of the prime mover muscles. Thus, increasing the amount of force produced during a specific movement. Power macrocycles are incorporated closer to the competitive season, and mostly used to improve rate coding. This increases how quickly the muscles are able to produce force.

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Neural Adaptations to Strength Training

Increased firing rate of motor unitsIncreased motor unit synchronizationLower activation threshold of motor unitsDecreased co-contraction of antagonist muscles when both agonist muscles are activatedIncreased central drive from the nervous system

Motor unit recruitment

Skeletal muscles are able to turn chemical energy into mechanical output in the form of muscle movement. Instead of firing all muscle fibers simultaneously, muscles must be activated gradually to conserve energy. By changing the firing rate and recruiting motor units with different metabolic properties, the muscles are able to produce varying levels of force. 

When stimulated by a motor neuron, all muscle fibers in a motor unit will fire at the same time. This  phenomenon is known as the all-or-none law. In simple terms, the motor unit will always give a maximal response or none at all when the stimulus exceeds a certain activation threshold.

Motor units are also recruited in an order from smallest to largest. This is known as the size principle. Since slow motor units are naturally smaller in size, they also have the lowest activation threshold. Therefore, they are the first motor units that are activated during any given movement. Larger motor units are only activated when slow motor units are unable to produce enough force. As a result of this activation pattern, it takes a relatively long time (0,5s-2,5s) for muscles to reach their maximum strength output.

Chronic adaptations of strength training in motor unit recruitment includes increased motor unit synchronization (several motor units fire simultaneously), increased motor unit firing rate, as well as decreased motor unit activation threshold.

Rate coding

In addition to the number of motor units recruited for a specific action, the force exerted by a muscle during a voluntary contraction also depends on the rates at which these motor units discharge action potentials (a rapid sequence of changes in the voltage across a membrane). This is phenomenon is known as rate coding. Over most the operating range of the muscle, the nervous system controls force production by adjusting both motor unit recruitment and rate coding. However, the relative contribution of rate coding and motor unit recruitment to overall force production varies across the operating range of the muscle. 

The recruitment of motor units remains relatively similar during gradually increasing contractions (ramp contractions), as well as during rapid contractions. The discharge rate of motor units increase progressively during slow ramp contractions, whereas fast contractions are associated with high initial discharge rates that decrease afterwards. These maximal discharge rates tend to reach values of 20–50 Hz during slow isometric contractions, and up to >100 Hz during fast contractions.

Compared to the relatively modest changes in discharge rate during ramp contractions, ballistic isometric contractions involve instantaneous discharge rates of 60–120 Hz. These type of contractions are characterized by a high initial discharge rate during muscle activation, followed by lower successive discharge rates. This evidence suggests that recruitment is the more significant factor at low forces, whereas rate coding becomes increasingly more important at intermediate and high forces, as well as during fast contractions.

The neural adaptations to strength training are usually attributed to changes in the neural drive to muscle, which in turn are a result of adaptations at the cortical or spinal level. Most studies suggest that the recruitment order of motor units remains relatively similar as a result of strength training. However, significant increases in the discharge rates of the initial action potentials have been observed as a result of consistent training. These findings are supported by the fact that rate coding often decreases after a period of reduced activity.

The magnitude of force produced by a muscle depends on the number of motor units activated and the rates at which motor neurons discharge action potentials.

Final thoughts 

As mentioned previously, exercise induces various adaptations to the musculoskeletal system, as well as the nervous and endocrine systems. The degree of these adaptations are often dependent on the volume, intensity, and frequency of physical activity. For example, endurance training has entirely different long-term effects when compared to strength training. Endurance training relies more on slow-twitch motor units and improves their ability to utilize fat for fuel, and contract efficiently. This, in turn, improves movement economy especially during longer activities. 

Strength training offers very different training adaptations, mainly in contraction speed and contraction force, and muscle mass. Interestingly, these adaptations mainly take place in fast-twitch motor units, which have the capacity to grow as well as produce significant amounts of force. Consistent strength training has been shown to improve the nervous system’s ability to recruit motor units and rate coding. Both of which translate to improved strength expression and athletic performance.

Did you learn anything new about the neural adaptations to strength training? Let us know in the comments.


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