Neuromuscular Adaptations and GAS

Neuromuscular Adaptations

• Resistance training, lasting 3-6 months, improves force production and maximal movement.
• Strength gains range from 25% to 100%.
• Neural control and muscle hypertrophy are involved in the process.
• Young males tend to have a greater strength gain potential.
• Muscle plasticity is high during this period.

Selye's General Adaptation Syndrome (GAS)

• GAS explains how organisms respond to training stress.
• The response to training stress can lead to adaptation or exhaustion.
• Three phases include alarm, resistance, and adaptation/exhaustion.
• The alarm phase is the initial response to training and performance decreases due to fatigue.
• The resistance phase involves adaptation, and the system either returns to baseline or improves.
• Overtraining can lead to further performance decrease if stressors are too high.

Muscle Damage and Adaptations

• Unaccustomed eccentric exercise (downhill running, slowly lowering weights) leads to high forces that damage the sarcolemma and release cytosolic enzymes and myoglobin
• Damage to muscle contractile myofibrils and noncontractile structures also occurs.
• Metabolites accumulate and cause more damage, decreasing force capacity.
• Inflammation process begins to heal and muscle becomes more resistant to damage in the future.

Glycogen Supercompensation

• Glycogen levels in muscles increase during the recovery period after exercise.
• The increased rate is termed glycogen supercompensation.

Adaptations to Resistance Training

• Various physiological adaptations occur in response to resistance training.
• These adaptations involve different systems/variables such as muscle fiber number, size, type, strength, and others.
• Some variables indicate an increase, some a decrease, and others have no change.

Adaptations in Force Gradation

• The force produced by a single muscle fiber depends on the number of crossbridges.
• Five factors contribute acutely to increased force generation: (1) number of motor units recruited, (2) frequency of motor unit discharge, (3) type of motor unit recruited, (4) activation of stretch reflex, and (5) speed of contraction.
• These factors are also affected by neuromuscular adaptations.

Neural Adaptations

• Anaerobic training may result in adaptations to the neuromuscular chain, beginning in higher brain centers like motor cortex, then continuing down to individual muscle fibers.
• High-intensity training leads to enhanced neural adaptations.
• Motor cortex activity increases when more force is developed and during learning of new exercises or movements.
• Increased maximal strength and power are due to enhanced recruitment, rate of firing, synchronization of firing, or a combination of factors in motor units.
• Untrained individuals can only voluntarily activate about 70% of their muscle tissue.
• Anaerobic training can cause an increased neuromuscular junction (NMJ) surface area, with dispersed, irregularly shaped synapses and greater nerve terminal branching length and area. Acetylcholine receptors are also more dispersed.

Proprioceptor Adaptations

• Anaerobic training enhances stretch reflex response, increasing the rate and magnitude of force development and improving muscle spindles and elasticity.
• It also causes a shorter amortization phase and an increase in Golgi tendon organ (GTO) threshold.
• Inhibitory impulses decrease.

Size Principle Adaptations

• With heavy resistance training, all muscle fibers, type I and II, recruit consecutively based on size, creating a higher force output.
• Advanced lifters may recruit motor units out of order.
• This may assist in increased power or speed during movements like plyometrics.

Muscular Adaptations

• Anaerobic training can cause muscle hypertrophy, increased strength and power, and increased connective tissue strength (tendons and fascia).
• Changes in muscle substrate content and glycolytic enzyme activity are also noted.
• Possible improvements in buffering capacities, mitochondrial density, and capillary density are also possibilities.
• Skeletal muscle adaptations are principally due to increased size (cross-sectional diameter), fiber type transitions, and enhanced biochemical and ultrastructural components.
• These changes result in enhanced muscular strength, power, and muscular endurance.

Muscular Adaptations Terms

• Hypertrophy: increase in the cross-sectional area (diameter) of existing muscle fibers.
• Hyperplasia: increase in the number of muscle fibers via splitting. Often questioned whether this happens in humans.
• Atrophy: decrease in muscle girth from disuse or other causes.
• Sarcopenia: age-related muscle atrophy.

How Muscles Hypertrophy

• Sarcoplasmic hypertrophy: increase in the amount of sarcoplasm and storage of substrates.
• Myofibrillar hypertrophy: increase in the size and number of myofibrils due to an increase in the number of contractile proteins and structural ones, strength of muscle units increasing because of more sarcomeres in parallel.

Key Point: Hypertrophy

• The process of muscle hypertrophy involves both an increase in the synthesis of contractile proteins (actin and myosin) and a rise in structural proteins in myofibrils.
• Additional myofilaments are added to the external layers of the myofibrils, resulting in increased diameter.

Satellite Cells and Hypertrophy

• Satellite cells serve as myogenic stem cells, vital for muscle regeneration.
• Acute damage or stretching of the muscle activates and proliferates satellite cells.
• Satellite cells migrate to the injured area and become new myonuclei, crucial for muscle growth.
• Maintaining an adequate myonuclear domain is essential for ongoing muscle hypertrophy.

Structural and Architectural Changes

• Resistance training increases myofibrillar volume, sarcoplasmic density, sarcoplasmic reticulum and T-tubule density, sodium-potassium ATPase activity.
• Sprint training enhances calcium release.
• Resistance training increases pennation angle.
• Other adaptations include decreased mitochondrial and capillary density, an increased buffering capacity, and changes in muscle substrates (e.g. glycogen) and enzyme activity.

Connective Tissue Adaptations

• Adaptations of tendons, ligaments, and fascia are triggered by mechanical forces during exercise.
• Collagen fibers have a similar striated appearance as muscle fibers.
• The adaptation's degree is in line with the exercise intensity.
• Fibroblasts create collagen fibers that strengthen connective tissue.
• Increased collagen fiber diameter, cross-links, and number contribute to strength increases.
• Load-bearing capacity increases at tendon and ligament junctions to skeletal muscle.

General Bone Physiology

• Trabecular bone responds more rapidly to stimuli compared to cortical bone.
• Minimal essential strain (MES) is the threshold stimulus for bone formation (approximately 1/10th of the force required to break bone).
• Progressive overload is necessary for strength increases.
• Increased muscle strength results in an increase in bone mineral density.

Bone Remodeling

• Longitudinal weight-bearing forces cause bending, stimulating new bone formation in areas of highest stress.
• Osteoblasts lay down additional collagen fibers along the periosteum.
• Previously dormant osteoblasts migrate to areas of strain.
• Collagen fibers become mineralized, effectively increasing bone diameter.

Why Do We See Specific Adaptations?

• The type of stress (overload) determines the specific adaptations.
• High-load resistance training prioritizes strength and power adaptations.
• High-volume/short-rest training prioritizes changes associated with metabolic stress and muscle damage.

Training Type and Adaptations

• The type of training stress (overload) dictates the prioritization of adaptations.
• High-load, lower volume training emphasizes strength adaptations, whereas high-volume, short rest training emphasizes adaptations associated with metabolic stress.

Review Questions

• Define the General Adaptation Syndrome (GAS) and the significance of progressive overload in a resistance training plan.
• Detail the significance of neural adaptations in strength improvements during a resistance training period.
• Explain how repetitions and load in a "strength" program augment adaptations.
• List seven discussed neural adaptations.
• Explain various muscular adaptations that result from anaerobic training.
• Detail the factors contributing to muscle fiber hypertrophy.

Figure & Notes References

• Books and journals relating to the presented topics are referenced.

Explore the details of neuromuscular adaptations related to resistance training and the General Adaptation Syndrome (GAS). This quiz will cover strength gains, neural control, muscle hypertrophy, and the phases of response to training stress. Test your knowledge on how these concepts impact physical performance and adaptations.