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The Secret Weapon of Muscle Strength: Do You Know How to Increase Strength Through Motor Unit Recruitment?
The strength of a muscle depends not only on its size, but also on the recruitment of motor units. Motor unit recruitment refers to the activation of more motor units in order to increase the force of muscle contraction. A motor unit consists of one motor neuron and all the muscle fibers it innervates. These muscle fibers are intertwined and spread throughout the muscle, and when a motor neuron is activated, all the involved muscle fibers are stimulated and contract.
The activation of one motor neuron will result in a weak but discrete muscle contraction, but as more motor neurons fire, more muscle fibers will be activated, leading to a stronger muscle contraction.
The higher the recruitment of motor units, the stronger the force of the muscle contraction will be. According to Henneman's size principle, motor units are generally recruited in order from small to large: this means that small motor neurons are activated first, followed by larger motor neurons, and finally the largest motor neurons. Yuan.
Neural recruitment mechanisms
Henneman proposed that smaller motor neurons have less surface area and therefore higher membrane resistance. This results in smaller motor neurons being able to produce larger voltage changes (depolarizations) when excitatory postsynaptic potentials arrive. However, as the research progressed, scientists found that this view was not irrefutable. Burke et al. also found that the amplitude of synaptic activity decreases exponentially from small to large motor neurons.
There is some controversy regarding the classification of motor units. Burke and colleagues divided movement units into three major categories: S (slow), FR (fast endurance), and FF (fast fatigue). This classification was once widely used, but modern research suggests that human muscle motor units may be more complex.
Types of Motor Units Recruited
During muscle contraction, motor unit recruitment patterns may change, thought to be due to an interplay between excitatory and inhibitory neural inputs. According to Henneman's size rule, the smallest motor units (S-type) are recruited first, followed by FR-type, and finally FF-type for tasks requiring high energy.
Frequency coding of muscle force
The force generated by a single motor unit depends in part on the number of muscle fibers in that unit. What is more important is how often the muscle fibers are stimulated by the neurons. The firing frequency of motor units can range from low (producing a single contraction) to high (producing a strong sustained contraction). Such changes can increase the force of the muscles during hard work by 2 to 4 times.
Proportional control of muscle strength
The motor unit size distribution shows an inverse relationship between the number of motor units and the force produced by each motor unit. This means that there are more small motor units and fewer large motor units. As recruitment increases, the increments in strength will get larger and larger.
Applications of electrodiagnostic testing
In medical electrodiagnostic testing, analyzing the size, shape, and recruitment pattern of motor unit action potentials can help determine whether a patient has muscle or neuropathy.
Muscle strength is indeed the result of precise control by the nervous system, which raises an important question: Have you ever wondered why some athletes are able to demonstrate extraordinary strength?
