Adaptations to Resistance Training

CHAPTER 11: Adaptations to Resistance Training

Overview

  • Resistance Training and Gains in Muscular Fitness

    • Resistance training significantly improves muscular fitness and strength.

  • Mechanisms of Gains in Muscle Strength

    • Understanding the biological and physiological changes that occur with resistance training.

  • Interaction Between Resistance Training and Diet

    • The role nutrition plays in optimizing strength gains.

  • Resistance Training for Special Populations

    • Considerations for children, older adults, and other specific groups.

Resistance Training: Introduction

  • Strength Gains via Neuromuscular Changes

    • Significant improvements in strength are achieved through adaptations in the neuromuscular system.

  • Importance for Overall Fitness and Health

    • Resistance training is essential for maintaining overall physical fitness and promoting good health.

  • Critical for Athletic Training Programs

    • Integral component of training regimens for athletes to enhance performance capability.

Resistance Training: Gains in Muscular Fitness

  • Timeframe for Strength Gains

    • After 3 to 6 months of resistance training, individuals can expect a strength gain ranging from 25% to 100%.

  • Improvements in Force Production

    • Enhanced ability to achieve true maximal exertion.

  • Strength Gains According to Initial Levels

    • Strength gains occur similarly as a percentage of initial strength across different populations; however, absolute gains tend to be greater for young men compared to women, older men, and children.

  • Muscle Plasticity

    • The muscle’s ability to adapt and grow (or shrink) in response to training or lack of activity.

Mechanisms of Muscle Strength Gain

  • Hypertrophy Versus Atrophy

    • Hypertrophy: Increase in muscle size, leading to increased strength.

    • Atrophy: Decrease in muscle size, leading to reduced strength.

    • The relationship between size and strength is complex.

  • Sources of Strength Gains

    • Increases in muscle size and alterations in neural control contribute to strength gains.

Mechanisms of Muscle Strength Gain: Neural Control

  • Role of Neural Adaptations

    • Strength gains are primarily due to adaptations in the nervous system rather than just increases in muscle size.

    • Strength improvements can occur without muscle hypertrophy.

  • Essential Elements

    • Motor unit recruitment: the process of activating more muscle fibers.

    • Stimulation frequency: the rate at which motor units are fired.

    • Other neural factors: additional influences that affect muscle contraction strength.

Motor Unit Recruitment

  • Asynchronous vs. Synchronous Recruitment

    • Motor units are typically recruited asynchronously; however, synchronous recruitment leads to greater strength gains.

    • Benefits include:

      • Enhanced contraction facilitation.

      • More forceful contractions.

      • Improved rate of force development.

      • Enhanced ability to exert steady forces.

    • Resistance training promotes synchronous recruitment.

Motor Unit Recruitment: Greater Neural Drive

  • Enhanced Neural Drive and Rate Coding

    • Increased neural drive during maximal contraction enhances muscle recruitment.

    • Higher frequency of neural discharge leads to effective strength improvements.

    • Potential reduction in inhibitory impulses may contribute to increased strength.

    • Conclusion: A combination of improved synchronization and motor unit recruitment is critical for achieving strength gains.

Motor Unit Rate Coding

  • Evidence of Rate Coding Changes

    • Limited findings suggest that rate coding improves with resistance training, particularly in ballistic-type movements.

Autogenic Inhibition

  • Intrinsic Inhibitory Mechanisms

    • E.g., Golgi tendon organs inhibit muscle contractions when tendon tension is excessive, providing a protective mechanism.

    • Training reduces these inhibitory impulses, enabling muscles to generate greater force, which may account for extraordinary strength performances.

Other Neural Factors

  • Coactivation of Agonists and Antagonists

    • Normally, antagonist muscles oppose the force of agonists.

    • Reduced coactivation of antagonists can enhance strength gains.

    • Structural changes in the neuromuscular junction may also play a role.

Mechanisms of Muscle Strength Gain: Muscle Hypertrophy

  • Hypertrophy

    • Defined as the increase in muscle fiber size resulting in greater strength.

  • Transient vs. Chronic Hypertrophy

    • Transient Hypertrophy: Occurs as a result of temporary fluid accumulation after an exercise session, typically resolving within hours.

    • Chronic Hypertrophy: Long-term structural changes in muscle tissue, involving fiber hypertrophy (enlargement of existing fibers) and fiber hyperplasia (increase in the number of fibers).

Chronic Muscle Hypertrophy

  • Eccentric Training

    • High-velocity eccentric training can maximize hypertrophy by disrupting muscle structures (Z-line remodeling).

  • Intensity Impact

    • Muscle hypertrophy can be stimulated at intensities ranging from 30% to 90% of one-repetition maximum (1RM) and can occur through both high-rep (low-load) and low-rep (high-load) training regimens.

Fiber Hypertrophy

  • Components of Hypertrophy

    • Increased myofibrils, actin and myosin filaments, sarcoplasm, and connective tissue.

  • Protein Synthesis Dynamics

    • Resistance training increases protein synthesis, with muscle protein content being dynamically regulated.

    • During exercise, protein synthesis decreases while degradation increases. Post-exercise, synthesis increases while degradation decreases, facilitating muscle growth.

Hormones and Hypertrophy

  • Role of Anabolic Hormones

    • Testosterone: A natural anabolic steroid that facilitates fiber hypertrophy.

    • Synthetic Anabolic Steroids: Can induce substantial increases in muscle mass.

    • Other hormones involved include Growth Hormone (GH) and Insulin-Like Growth Factor 1 (IGF-1). Elevated post-exercise hormone levels are not strictly required for muscle growth.

Mechanisms of Muscle Strength Gain: Fiber Hyperplasia

  • Animal Studies on Fiber Splitting

    • In cats, intense strength training can lead to fiber splitting resulting in new fibers that grow akin to the original.

    • Other animals like chickens, mice, and rats primarily exhibit hypertrophy without hyperplasia, which may link to specific training approaches.

Fiber Hyperplasia in Humans

  • Human Adaptation

    • Most muscle hypertrophy in humans is attributed to hypertrophy rather than hyperplasia; however, some evidence of hyperplasia exists.

    • The degree of hypertrophy versus hyperplasia can be influenced by the intensity and load of resistance training. Higher intensity is more likely to induce (type II) fiber hypertrophy.

  • Mechanisms of Fiber Hyperplasia

    • Occurs through fiber splitting and activation of satellite cells, which are myogenic stem cells involved in muscle regeneration.

    • Satellite cells become activated in response to physical stress, proliferating, migrating, and fusing to form new muscle fibers.

Mechanisms of Muscle Strength Gain: Neural Activation and Hypertrophy

  • Short-term Muscle Strength Gains

    • Initial increases in muscle strength are often due to enhanced voluntary neural activation; most noticeable in the first 8 to 10 weeks of training.

  • Long-term Muscle Strength Gains

    • Associated with significant fiber hypertrophy; requires time and sustained increases in protein synthesis to manifest strength improvements. Hypertrophy becomes increasingly responsible for strength gains past the first 10 weeks.

Mechanisms of Muscle Strength Gain: Atrophy and Inactivity

  • Consequences of Reduced Activity

    • Cessation of activity leads to significant changes in muscle structure and function as evidenced by immobilization studies and detraining research.

Immobilization Effects

  • Short-term Effects on Muscle Use

    • Notable changes occur as soon as 6 hours post-immobilization, leading to decreased protein synthesis and initiating muscle atrophy processes.

    • Loss of strength can average 3%-4% per day during the first week.

    • Muscle atrophy is reversible with resumption of activity; type I fibers are generally more affected than type II.

Detraining Effects

  • Impact on 1RM

    • Detraining results in decreases in one-repetition maximum (1RM), though lost strength can typically be regained within approximately 6 weeks.

    • Maintenance resistance programs can effectively prevent detraining by ensuring continued strength and 1RM maintenance, allowing for reduced training frequency.

Mechanisms of Muscle Strength Gain: Fiber Type Alterations

  • Effects of Training Regimen

    • While training may not cause outright fiber type changing, different training approaches can affect the oxidative and anaerobic capacities of fibers.

    • Conversion of fiber types can occur under unique conditions, such as chronic stimulation or high-intensity training.

Fiber Type Transition Studies

  • Common Transition Patterns

    • The transition from Type IIx to Type IIa fibers is frequently observed with consistent heavy resistance training over a 20-week period, as well as in concurrent high-intensity anaerobic tasks.

Interaction Between Resistance Training and Diet

  • Protein Synthesis Enhancement

    • Resistance training increases the synthesis of muscle proteins.

  • Recommended Protein Intake

    • For muscle growth, an intake of 20 to 25 grams of protein immediately after resistance exercise is advised.

    • For overall muscle mass increase, 1.6 to 1.7 grams of protein per kilogram of body weight per day is recommended.

    • Small, consistent doses of approx. 20 grams every 2 to 3 hours are optimal for sustained protein synthesis stimulation.

Molecular Mechanisms of Increased Protein Synthesis

  • Role of IGF-1

    • Repeated muscle stretch leads to increased levels of IGF-1, which signals the activation of mTOR, a critical pathway for muscle protein synthesis that integrates signals from insulin and amino acids.

  • Translation Process of Protein Synthesis

    • Translation is the process wherein amino acids are utilized to build proteins with the assistance of messenger RNA (mRNA).

Resistance Training for Special Populations: Age

  • Children and Adolescents

    • There is a prevalent myth concerning resistance training and safety related to growth plate and hormonal changes; however, properly supervised resistance training is safe and effective for strength and mass gain in children.

  • Older Adults

    • Resistance training can counteract age-associated muscle loss, improving overall health quality and reducing the risks of falls.

Strength Training in Older Adults

  • Mechanisms of Strength Gain

    • Strength increases primarily result from neural adaptations, regardless of sex or racial background.

    • The response is similar to that of younger adults but may be reduced; the mTOR signaling response and gains in myofibrillar proteins are notably diminished.

    • A protein intake of 25-50 grams is essential to stimulate muscle protein synthesis in older adults.

Health Benefits of Resistance Training

  • Comprehensive Benefits

    • Improves mobility, cognitive processing speed, attention, executive functioning, and global cognitive function.

    • Positively affects body fat, glucose regulation, and cholesterol levels.

    • Significantly lowers all-cause and specific disease-related mortality rates (CVD, cancer).

Resistance Training for Sport

  • Considerations for Athletes

    • Advanced resistance training beyond basic strength, power, and endurance needs of the sport may not yield substantive performance benefits and could take away from valuable training time.

    • Results of such training should always be measured with sport-specific performance metrics.