Neural Adaptations to Strength Training

Introduction

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.