Physiology of Resistance Training Notes

Strength Training Definitions

  • Muscular Strength:

    • Definition: The maximal force a muscle group can generate.
    • Testing: 1 Rep Max (1 RM).
    • Training: High-resistance (6-10 reps) aimed at increasing strength.

  • Muscular Endurance:

    • Definition: The ability to perform repeated contractions against a submaximal load.
    • Testing: Push-ups, Pull-ups, and similar tests.
    • Training: Low-resistance (35-40 reps) to enhance endurance.

Aging and Muscle Changes

  • Aging leads to significant loss of muscle mass, with major decline observed after the age of 50.
    • Sarcopenia: Condition of loss of muscle mass due to aging.
    • Atrophy primarily affects Type 2 muscle fibers; both Type 1 and Type 2 fiber counts decrease due to motor neuron loss.
    • Resistance training can promote muscle hypertrophy and strength in older adults, but gains may be lower compared to younger populations.

Resistance Training and Nervous System Adaptations

  • Neural Adaptations: Occur during the initial 2-8 weeks of resistance training, leading to strength gains without size changes.
    • Enhanced specific tension in muscle fibers and increased overall muscle mass.
    • Cross Education: Improvement in untrained limb strength due to training in the opposite limb.

Neural Pathways and Muscular Contraction

  • Muscle contraction involves a pathway: Higher Brain Center → Motor Cortex → Brain Stem → Spinal Cord → Motor Neurons → Muscle Fibers.
  • Force production is regulated by inhibitory signaling from the Central Nervous System (CNS).

Resistance Training and Neural Drive

  • Neural Drive: Represents the magnitude of efferent neural output from the CNS to muscle fibers.
    • Factors influencing adaptation include:
    • Increased motor unit recruitment and firing rates.
    • Decreased inhibition from Golgi Tendon Organ.
    • Enhanced motor unit synchronization and neural transmission across the Neuromuscular Junction (NMJ).

Muscle Fiber Properties and Force Production

  • Muscle force is directly proportional to the amount of Actin and Myosin present.
    • Larger muscles contain increased quantities of these proteins.
    • Hypertrophy: Increase in the size of existing muscle fibers.
    • Hyperplasia: Potential increase in the total number of muscle fibers, though debated in humans.

Summary of Resistance Training-Induced Adaptations

  • Physiological Variables affected by Resistance Training:
    • Nervous System: Increased neural drive and possible changes in agonist/antagonist activation ratios.
    • Muscle Mass: Overall increase, specifically in type 1 fibers.
    • Muscle Fiber Composition: Minimal shift from fast-to-slow fibers, unclear impact on oxidative capacity.
    • Tendons and Ligaments: Strength increases, leading to enhanced bone mineral content.

Muscle Protein Synthesis and Growth

  • Hypertrophy: Defined as the rate of protein synthesis exceeding protein breakdown.
    • Protein synthesis rates peak at 1-4 hours post-exercise.

Signaling Events Leading to Muscle Growth

  • Muscle protein synthesis is regulated by gene activation, mRNA production, and ribosomal function.
    • mTOR: Major regulator promoting protein synthesis via translation; requires leucine activation and presence of essential amino acids.

Hormonal Contributions to Muscle Growth

  • Testosterone, IGF-1, Growth Hormone: Potentially enhance muscle protein synthesis, although not mandatory for hypertrophy.

Role of Satellite Cells

  • Satellite cells contribute to increasing myonuclei and promote muscle hypertrophy in young adults.
  • In older adults, satellite cells are diminished, affecting the ability to hypertrophy significantly.

Genetic Influence on Muscle Mass

  • Genetic factors account for approximately 80% of the differences in muscle mass among individuals.
    • 47 genes relate closely to muscle mass, with variations in responders' capacity to activate protein synthesis.

Effects of Detraining

  • When strength training is halted, the rate of strength loss is slower than with endurance training.

Tools for Rehabilitation: Blood Flow Restriction

  • Blood Flow Restriction Training (BFRT): Utilizes pneumatic cuffs to restrict blood flow during low-load resistance training (20-30% of 1RM) to prevent atrophy and encourage strength gains during rehabilitation.
    • Effective for increasing size and strength comparable to traditional high-load training.

Muscle Atrophy Due to Prolonged Inactivity

  • Muscle Atrophy: Reduction in muscle fiber cross-sectional area due to inactivity.
    • Causes may include immobilization, prolonged bed rest, or space flight.

Concurrent Strength and Endurance Training

  • Training effects depend on intensity, volume, and frequency.
    • Can lead to neural factors and overtraining impacts affecting protein synthesis.