Adaptations to Resistance Training

Chapter 11: Adaptations to Resistance Training

Overview

• Resistance training leads to gains in muscular fitness.
• Understanding the mechanisms of muscle strength gains is crucial.
• The interaction between resistance training and diet is important.
• Resistance training is beneficial for special populations.

Introduction to Resistance Training

• Resistance training leads to substantial strength gains via neuromuscular changes.
• It is important for overall fitness and health.
• It plays a critical role in athletic training programs.

Gains in Muscular Fitness

• After 3 to 6 months of resistance training:
  • Strength gain: $25\%$ to $100\%$. This leads to: Better force production. The ability to produce true maximal movement.
• Strength gains relative to initial strength are similar across different demographics.
• Young men experience greater absolute gains compared to young women, older men, or children, which is attributed to muscle plasticity.

Mechanisms of Muscle Strength Gain

• Hypertrophy versus atrophy:
  • Increase in muscle size leads to increased muscle strength ($\uparrow$ muscle size $\rightarrow$ $\uparrow$ muscle strength).
  • Decrease in muscle size leads to decreased muscle strength ($\downarrow$ muscle size $\rightarrow$ $\downarrow$ muscle strength).
  • The association is more complex than a simple relationship.
• Sources of strength gains:
  • Increase in muscle size.
  • Altered neural control.

Neural Control and Muscle Strength Gain

• Strength gain cannot occur without neural adaptations via plasticity.
• Strength gain can occur without hypertrophy.
• Strength is a property of the motor system, not just of muscle.
• Essential elements:
  • Motor unit recruitment.
  • Stimulation frequency.
  • Other neural factors.

Motor Unit Recruitment

• Motor units are normally recruited asynchronously.
• Synchronous recruitment leads to strength gains:
  • Facilitates contraction.
  • May produce more forceful contraction.
  • Improves rate of force development.
  • Improves capability to exert steady forces.
• Resistance training leads to synchronous recruitment.
• Strength gains may result from greater motor unit recruitment:
  • Increased neural drive during maximal contraction.
  • Increased frequency of neural discharge (rate coding).
  • Decreased inhibitory impulses.
• A combination of improved motor unit synchronization and motor unit recruitment likely leads to strength gains.

Motor Unit Rate Coding

• Limited evidence suggests that rate coding increases with resistance training.
• This is especially true for rapid-movement, ballistic-type training.

Autogenic Inhibition

• Normal intrinsic inhibitory mechanisms:
  • Example: Golgi tendon organs.
  • Inhibit muscle contraction if tendon tension is too high.
  • Prevent damage to bones and tendons.
• Inhibitory impulses are decreased by training:
  • Muscle can generate more force.
  • May also explain superhuman feats of strength.

Other Neural Factors

• Coactivation of agonists and antagonists:
  • Normally, antagonists oppose agonist force.
  • Reduced coactivation may lead to strength gain.
• Morphology of the neuromuscular junction.

Muscle Hypertrophy

• Hypertrophy: increase in muscle size.
• Transient hypertrophy (after exercise bout):
  • Due to edema formation from plasma fluid.
  • Gone within hours.
• Chronic hypertrophy (long term):
  • Structural change in muscle.
  • Fiber hypertrophy, fiber hyperplasia, or both.

Chronic Muscle Hypertrophy

• Maximized by high-velocity eccentric training, which disrupts sarcomere Z-lines (protein remodeling).
• Concentric training may limit muscle hypertrophy, strength gains.
• Stimulated by intensities as low as $30\%$ 1RM and as high as $90\%$. (RM = repetition maximum, the maximal weight that can be lifted for a specified number of repetitions.)
• Caused by both high-rep (low-load) and low-rep (high-load) training.

Fiber Hypertrophy

• More myofibrils.
• More actin and myosin filaments.
• More sarcoplasm.
• More connective tissue.
• Resistance training leads to increased protein synthesis:
  • Muscle protein content is always changing.
  • During exercise: synthesis decreases, degradation increases.
  • After exercise: synthesis increases, degradation decreases.

Hormones and Hypertrophy

• Fiber hypertrophy is facilitated by testosterone:
  • Natural anabolic steroid hormone.
  • Synthetic anabolic steroids lead to large increases in muscle mass.
• Growth hormone (GH).
• Insulin-like growth factor 1 (IGF-1).
• Elevated postexercise levels are not required for anabolism and strength.

Fiber Hyperplasia

• Cats:
  • Intense strength training produces fiber splitting.
  • Each half grows to the size of the parent fiber.
• Chickens, mice, rats:
  • Intense strength training produces only fiber hypertrophy.
  • But the difference may be due to the training regimen.
• Humans:
  • Most hypertrophy is due to fiber hypertrophy.
  • Fiber hyperplasia also contributes.
  • Fiber hypertrophy versus fiber hyperplasia may depend on resistance training intensity or load.
  • Higher intensity causes (type II) fiber hypertrophy.
  • Fiber hyperplasia may occur only in certain individuals under certain conditions.
• Can occur through fiber splitting.
• Also occurs through satellite cells:
  • Myogenic stem cells involved in skeletal muscle regeneration.
  • Activated by stretch, injury.
  • After activation: proliferate, migrate, fuse.

Neural Activation and Hypertrophy

• Short-term increase in muscle strength:
  • Substantial increase in 1RM.
  • Due to increase in voluntary neural activation.
  • Neural factors are critical in the first 8 to 10 weeks.
• Long-term increase in muscle strength:
  • Associated with significant fiber hypertrophy.
  • Net increase in protein synthesis requiring time to occur.
  • Hypertrophy is a major factor after the first 10 weeks.

Atrophy and Inactivity

• Reduction or cessation of activity leads to major change in muscle structure and function.
• Limb immobilization studies.
• Detraining studies.

Immobilization

• Major changes after 6 hours:
  • Lack of muscle use leads to reduced protein synthesis.
  • Initiates the process of muscle atrophy.
• First week: strength loss of $3\%$-$4\%$ per day.
  • Decrease in size (atrophy).
  • Decrease in neuromuscular activity.
• (Reversible) effects on type I and II fibers:
  • Cross-sectional area decreases, cell contents degenerate.
  • Type I is affected more than type II.

Detraining

• Leads to a decrease in 1RM.
• Lost strength can be regained (approximately 6 weeks).
• New 1RM matches or exceeds the old 1RM.
• Once the training goal is met, a maintenance resistance program prevents detraining.
  • Maintain strength and 1RM.
  • Reduce training frequency.

Fiber Type Alterations

• A training regimen may not outright change fiber type, but…
  • Type II fibers are more oxidative with aerobic training.
  • Type I fibers are more anaerobic with anaerobic training.
• Fiber type conversion is possible under certain conditions:
  • Cross-innervation.
  • Chronic low-frequency stimulation.
  • High-intensity treadmill or resistance training.
• Type IIx to type IIa transition is common.
• 20-week heavy resistance training program:
  • Static strength, cross-sectional area increases.
  • Percentage of type IIx decreases, percentage of type IIa increases.
• Other studies: type I to type IIa with high-intensity resistance work + short-interval speed work.

Interaction Between Resistance Training and Diet

• Resistance training increases protein synthesis.
• Consume 20 to 25 g of protein after resistance exercise for muscle growth.
• Consume 1.6 to 1.7 g of protein per kg of body weight per day to increase muscle mass.
• Small doses (20 g) every 2 to 3 hours are recommended for protein synthesis.
• Larger doses (20-25 g) are recommended immediately after resistance training.

Molecular Mechanisms of Increased Protein Synthesis

• Repeated muscle stretch leads to increased IGF-1 ($\uparrow$ IGF-1).
• Increased IGF-1 leads to increased mTOR ($\uparrow$ mTOR):
  • Integrates input from insulin, growth factors, amino acids.
  • Dictates transcription of mRNA.
  • Synthesizes ribosomes.
• Stimulated by insulin.
• Translation:
  • Amino acids are converted into protein via mRNA.

Resistance Training for Special Populations: Age

• Children and adolescents:
  • Myth: Resistance training is unsafe due to growth plate, hormonal changes.
  • Truth: It is safe with proper safeguards.
  • Children can gain both strength and muscle mass.
• Elderly persons:
  • Helps restore age-related loss of muscle mass.
  • Improves quality of life and health.
  • Helps prevent falls.

Strength Training in Older Adults

• Increases in strength are dependent primarily on neural adaptations.
• No difference across sex or race.
• Same response as in younger but blunted:
  • Decreased mTOR signaling response.
  • Smaller increases in myofibrillar protein and muscle size.
  • 25-50 g protein necessary to stimulate muscle protein synthesis

Resistance Training for Sport

• Training is not worth it beyond the basic strength, power, and endurance needs of the chosen sport.
• Training costs valuable time.
• Training results should be tested with sport-specific performance metrics.