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The Secret of Motor Units: How to Adjust the Strength and Speed of Muscle Contraction to Make Your Exercise More Effective?
In every movement, the performance of muscles depends on how effectively our nervous system mobilizes muscle units. Motor unit recruitment refers to the activation of more motor units in order to increase the force of muscle contraction. Each motor unit consists of one motor neuron and all the muscle fibers it innervates. The adjustment of muscle force and speed is achieved through the effective recruitment of these motor units.
The recruitment of motor units is fundamental to understanding muscle movement. It affects not only the strength of a single contraction, but also the effectiveness of the entire movement.
What is Motor Unit Recruitment?
Simply put, a motor unit consists of a motor neuron and the multiple muscle fibers it controls, which are intertwined with each other. When a neuron is activated, all the muscle fibers it innervates contract simultaneously. This means that the first firing of a neuron will result in a relatively weak contraction, and as more neurons are fired, the strength of the muscle contraction increases. This is a simple physiological mechanism, but an important cornerstone of motor learning and training.
Henneman's Size Principle
Many studies have shown that motor unit recruitment occurs in order of size, starting with the smallest neurons and gradually reaching the largest neurons. This is called Henneman's size principle. This means that when we perform regular strength training, the small, slow-twitch muscle fibers are recruited first, followed by the larger, fast-twitch muscle fibers. This affects not only the intensity of exercise, but also its duration.
According to Henneman's theory, small neurons are more easily activated, which allows muscle movement to show a balance between efficiency and economy.
Mechanisms of neuronal recruitment
Henneman proposed that small motor neurons have higher membrane resistance due to their smaller surface area, which allows them to generate larger voltage changes when receiving excitatory postsynaptic potentials (EPSPs). This mechanism has led researchers to further explore the neuronal recruitment process, although the field remains controversial.
Motor Unit Type Recruitment
According to the classification of researcher Burke, motor units can be divided into three categories: S (slow-twitch fibers), FR (fast, fatigue-resistant) and FF (fast, fatigue-prone). These brands play a key role in the recruitment of motor units, but the latest research suggests that the motor units of human muscles may be more complex than previously thought, so this division remains criticized.
As Burke notes, classifying motor units may be too rigid, but such classifications are essential for scientific communication.
Frequency coding of muscle force
The force generated by a single motor unit depends in part on the number of muscle fibers within that unit, but more importantly on the frequency of nerve stimulation. The firing rate of motor units increases with increased muscle effort, a process that results in stronger muscle contractions called fusion contractions. This means that as strength increases, the firing frequency of neurons reaches a peak, making muscle strength more stable and sustained.
Proportional control of muscle strength
Regarding the distribution of motor units, it is generally believed that it is inversely proportional to the size of the motor units, that is, there are more small motor units and fewer large motor units. When strength is low, increasing the recruitment of motor units produces relatively small increments in force. However, when performing a powerful contraction, the incremental gain from each additional motor unit is significantly greater. It is a delicate balance between strength and recruitment.
Clinical Applications and Electrodiagnostic Testing
During electrodiagnostic testing of patients with muscle weakness, careful analysis of the size, shape, and recruitment pattern of the “motor unit action potential” (MUAP) can help differentiate between myopathy and neuropathy. These analyses have important clinical significance in determining the patient's specific condition.
In the pursuit of more efficient movement, it is not just about muscle strength and speed, but also about how to fine-tune the recruitment of motor units and the control of neurons. How does all this affect your athletic performance?
