The chronic physiological adaptations that occur are specific to the anaerobic training program and occur to the neural, muscular, and endocrine systems.
Neuromuscular adaptations are seen early in training, typically after 6 to 10 weeks. They occur "along the neuromuscular chain", meaning they begin at the central nervous system and progress outward, toward the muscle spindles.
Central Nervous System Adaptations
There's an increase in activity of the primary motor cortex, which leads to an increase in the level of force developed and a greater ability to learn new movements.
Changes within the spinal cord increase the ability to recruit motor units. This includes an increase in muscle fiber size, which means that less stimulus is needed to activate motor units. Also, high-velocity power training helps the body adapt to recruit fast-twitch, Type II muscle fibers more quickly -- which is referred to as 'selective recruitment'.
Motor Unit Adaptations
An increase in muscle fiber size allows for a lower required neural activation. 10 larger muscle fibers require less stimuli to produce the same amount of force as the same number of muscle fibers of a smaller size.
There is also an increase in the rate of motor unit firing, which is essential to maximum force and power development; both of which are components of strength training.
An increase in synchronization is also expected, which may play a role in the timing and magnitude of force production.
Neuromuscular Junction Adaptations
Increase in endplate length, ACh receptors, and high intensity synapses.
Neuromuscular Reflex Potentiation
There is an increase in muscle spindle response, which is also referred to as reflex potentiation. There is also an increase in the rate and magnitude of force development.
Cross-education: training one limb can help to increase the strength in the other by up to 22%
Bilateral deficit in untrained individuals: the sum of the force produced by each limb, when trained unilaterally, is greater than both contracting simultaneously. Therefore, untrained individuals might want to train unilaterally in the beginning.
Muscular changes, especially hypertrophy, are typically the most emphasized and desired changes when it comes to anaerobic training and benefits.
The most popular adaptation is muscle hypertrophy, or muscle growth via increased cross sectional area or size of the muscle fibers. This is achieved by optimizing the levels of myosin and actin, which are the proteins that facilitate muscle movement on a microscopic level. Anaerobic training increases the production and decreases the degradation processes of these proteins. It also leads to an increase in the number of myofibrils, which are another component of the muscle cell.
However, it is unknown if anaerobic training leads to hyperplasia, which is the increase in the number, not size, of muscle fibers only because that would be too difficult to count.
The magnitude of protein synthesis and muscle growth depends on the training program, nutrition and hydration, and hormone receptor response. A strategy that includes a combination of mechanical and metabolic factors will optimize hypertrophy. Mechanical factors include heavy loads, eccentric actions, and low-to-moderate volumes. Metabolic factors focus on stressing the glycolytic energy system, which is said to kick in after 42 seconds of high-intensity, high volume activity with short rest periods.
Fiber type transition
Typically, the proportion of fiber types a person has is determined by genetics and relatively unchangeable. But hypertrophy can cause Type-II fibers to transition into Type-I fibers. Also, hypertrophy can cause Type II-x fibers to transform and behave more like Type II-a, which is seen as the ability to respond to a lower level of stimulus.
Structural and Architectural Changes
Structural changes enhance the muscle’s function and expression of strength. Anaerobic training results in increased volume of myofibrils, density of cytoplasm, and activity of Na-K ATPase. It also causes an increase in sarcoplasmic reticulum and t-tubule density.
There are two architectural changes that affect the way force is transmitted to tendons and bones. Increased muscle fascicle length, which is a bundle of muscle fibers. There is also an increase in the cross-sectional area of the muscle fiber, which results in an increased angle of pennation.
Additional Muscular Adaptations
Decreased mitochondrial and capillary density. The gross number of mitochondria and capillaries remains the same, but the density is decreased because of hypertrophy. Meaning there are less mitochondria and capillaries per muscle fiber, but the same total number as before hypertrophy. However, this doesn't impact aerobic performance because the efficiency of mitochondria and capillaries has improved.
Increased buffering capacity for more efficient acid-base balance. You can buffer out lactic acid more quickly, which is a by-product of the metabolic processes most heavily relied on during anaerobic exercise. This results in delayed fatigue during exercise and therefore the opportunity for longer, more productive training sessions.
Increased enzyme activity and substrate content. Overall muscle efficiency is improved because of the increase in storage of creatine phosphate, ATP, and glycogen. Also the enzymes that work to utilize these substrates during metabolic processes are more active and therefore more efficient.
Both acute and chronic changes caused by anaerobic training are seen in the endocrine system. Keep in mind that the magnitude of these changes are dependent on the training program.
During 15 to 30 minutes immediately following exercise, there is an increase in testosterone, growth hormone, and cortisol. There is also an increase in the release of catecholamines.
Long-term anaerobic training may actually elicit counterproductive endocrine “adaptations”, or maladaptations. There is no evidence suggesting that anaerobic training has a consistent impact on resting hormone levels. Long-term training has only shown to produce a lower amount of both testosterone and growth hormone by the same load, immediately following exercise.
However, long-term resistance training produces an effect referred to as upregulation, which is an increase in androgen receptor sensitivity that lasts for 48-72 hours after a training session.