When you train with weights all of your body’s systems are affected in some way, and these systems overlap and connect. To build a better body you must understand how weight training influences the body so that you can understand the benefits that training with free weights provides. Nervous system adaptations to strength training are crucial.
Neural Adaptations and Strength Training
Your body is filled with intricate spiderwebs of nerves that connect every part of your body to the primary nerve center, the spinal cord, and the brain. Every nerve has a different job to do. The nerves that control muscles are called motor nerves.
The smallest part of the motor nerve is a single cell called a motor neuron. The neuron has many nerve fibers that branch out to provide nerve impulses to individual muscle fibers. The area of innervation is called the neuromuscular junction (the place where the nerve and muscle join).
Although each muscle fiber only has one neuromuscular junction, the motor neuron can innervate hundreds of fibers. The motor neuron and the fibers it innervates are collectively called a motor unit. When the motor neuron is stimulated, all of that motor unit’s muscle fibers will contract.
The basis of muscle stimulation starts with the motor unit – a motor neuron and the muscle fibers it stimulates. A motor unit can be small or large, depending on the number of muscle fibers that it stimulates. Small motor units are comprised of slow-twitch fibers (Type I); larger motor units are comprised of fast-twitch fibers (Type II).
The muscle fibers in a motor unit are not all located adjacent to each other but are spread out in the muscle in microbundles of about 3 to 15 fibers. Thus, adjacent muscle fibers do not necessarily belong to the same motor unit. Because of how the fibers of a motor unit are spread out within a muscle, when a motor unit is activated, fibers are activated throughout the whole muscle.
If fibers of a motor unit were all adjacent to each other, activating that motor unit would appear to stimulate only one segment of the muscle. When a muscle moves, those motor units that are not activated (and their associated muscle fibers) do not generate force; they only move passively through the range of motion as the muscle moves to follow the activated motor units.
The size of the motor units within a muscle varies, as does the number of fibers in a motor unit within different muscles. In the muscles of the eye, as few as 10 fibers may make up a motor unit, whereas 450 or more fibers may be included in a motor unit of the quadriceps.
The Role of Strength Training
The trick to strength training effectively is to get the most motor units working at one time. The more motor units that are working, the more muscle fibers are contracting, the more force you can produce. The more force you generate, the heavier the weights you can lift. And the heavier the weights, the more strength and muscle mass you can build.
During most daily activities, every motor unit in the muscle doesn’t need to be recruited. It takes less muscular force to sit at a computer or drive in a car than it does to lift heavy weight or move a piece of furniture. If these lazy motor units aren’t called on for duty very often, then it’s a case of use it or lose it. When you introduce weight training, you are calling into action motor units that have never been taxed before. Now the muscle activates fully and becomes more efficient at doing everyday chores as well.
Therefore, your body doesn’t instinctively throw every motor unit into a task just because you want it to. We typically can’t recruit all our motor units unless we’re faced with a life-or-death situation. We can, however, train our nervous system to recruit a greater percentage of our available motor unit pool.
It is possible for the nervous system to vary the force produced by a muscle over a wide range of intensities. For example, individuals may be able to curl a 10-pound dumbbell but can also work their way up the dumbbell rack to curl a 60-pound dumbbell with maximal effort. In the simplest sense, there are two mechanisms that the nervous system may use to vary, or grade, force production to accomplish these tasks. One method is to vary the number of motor units, and thus muscle fibers, that are activated. This is known as motor unit recruitment. The second method is to increase the firing rate of motor units already activated, a process known as rate coding.
Most of the significant strength gains that you see when you first embark on a weight-training program are due to neural factors. Your nervous system is very good at adaptation. It listens to what you want your body to do and responds appropriately.
When you begin training with weights or try any new sport or movement, your nervous system steps up to the plate and does everything in its power to allow you to accomplish the task at hand. It recruits more motor neurons, which in turn recruit more muscle fibers.
The strength of a muscle contraction depends upon the intensity of the nervous system stimulus, the number and size of motor units activated, and the types of muscle fibers that are stimulated. For example, if you are getting ready to lift a heavy weight, your central nervous system will send a stronger signal. This will activate a greater number of large fast motor units, resulting in a more forceful muscle contraction than if you were merely picking up an apple.
Changes in the way nerve pathways serve the motor units are called neuromuscular adaptation. It’s a type of motor learning – your body learns to assign more and more motor units to the movements and so you get stronger.
The increase in fiber recruitment and neural coordination leads to strength gains, without the muscles actually getting bigger (hypertrophy). Even advanced weightlifters have been shown to increase their strength and power, without increasing muscle size, when they change their exercise programs. This phenomenon can only be the result of neural adaptations and increased recruitment.
Nerve and muscle cells are partners. This alliance between nerve and muscle cells is called a motor unit.
- The command for muscle contraction comes from the cerebrum, the central area for contractions of skeletal muscle, reaching the anterior horn cells of the spinal cord.
- The transmission of nerve impulses to the skeletal muscles happens through dendrites. Dendrites are ramifications of nerve cells.
- At the motor end plate, the nerve impulse is transmitted to the muscle cell.
- As a result of this transmission, muscle cell contraction begins to set in.
Why lifting lighter weights with higher number of reps can never substitute lifting heavier weights with low reps?
This means that training with heavier weights always activates a higher number of motor units in comparison to weight training with lighter weights. It is also a fact that muscle cells only get activated when they are trying to overcome a given resistance. Consequently, training with heavier weights and performing eight to twelve reps per set, activates more muscle cells in comparison to using lighter weights for 15 to 20 reps per set. It is logical that the higher the number of contracted muscle fibers, the thicker and denser the muscles will be.
However, one has to keep in mind that bodybuilding is not primarily strength training. In contrast to weight lifting or power lifting, where the main emphasis is put on lifting the heaviest weight for one single rep, the goal in bodybuilding is muscle growth. This means that heavier is not always better. Using weights that can be moved between one and four reps per set will in fact improve strength ability. However, due to the short duration of training stimulus, a less than optimal effect is achieved in regard to muscle hypertrophy.
Weights that are too heavy don’t build optimum muscle mass, and the same rule applies for weights that are too light. Training with weights that enable you to do between 20 and 30 additional reps per set, do not provide a stimulus for building thick and dense muscles. For building massive and dense muscles, it is not possible to avoid heavy training with eight to twelve reps per set. Remember that training with light weights will always activate a lower number of motor units in comparison to heavy weight workouts.
Summing Up: Nervous System Adaptations to Strength Training
Without the numerous nervous system adaptations to strength training you can never build muscles or achieve anything in any sport. Your muscles are connected to your nervous system. They are fired, or activated, by motor nerves, and a single motor nerve may stimulate anywhere between one and several hundred muscle fibers. A nerve cell and a muscle fiber are called a motor unit. When a motor nerve is stimulated it causes all of the muscles fibers to contract. This is the all or nothing principle.
The number of motor units involved in a contraction depends on the load imposed on the muscle. With a light load (weight), only a few motor units – those activating the ST fibers – are pulled into action. As the load increases, progressively more motor units will be recruited – activating the FT fibers – until, with a maximal weight, all (or almost all) of the motor units will be recruited.
Therefore, to stimulate the whole muscle, you have to work with weights that require an all-out effort. Otherwise, the highest threshold motor units never get recruited. The arrangement of your motor units – the number of muscle fibers activated by each motor nerve – determines your rate of progress too. People who tend to gain strength and size very rapidly probably have an above-average number of muscle fibers in each motor unit. For the same effort, they generate a higher force output than the average person. This creates a bigger stimulus for muscle growth.