resistance training

Overload (Principle of Training)

A training effect occurs when a physiological system is exercised at a level beyond which it is normally accustomed.

Specificity (Principle of Training)

The training effect is specific to muscle fibers recruited, energy system involved (aerobic vs. anaerobic), velocity of contraction, and type of contraction (eccentric, concentric, isometric).

Reversibility (Principle of Training)

Gains are lost when the overload stimulus is removed.

Muscular Strength

The maximal force a muscle or muscle group can generate, measured as 1 repetition maximum (1-RM).

Muscular Endurance

The ability to make repeated contractions against a submaximal load.

High-Resistance Training

Strength training involving 2–10 repetitions maximum (RM), leading to gains in strength.

Low-Resistance Training

Strength training involving 20+ repetitions maximum (RM), leading to gains in endurance.

Anaerobic Exercise

A short-duration (10-30 seconds) all-out effort involving both type I and II muscle fibers. Energy is mainly supplied by the ATP-PC system for efforts under 10 seconds, and 80% anaerobically/20% aerobically for 20-30 second efforts.

Anaerobic Performance Increases

4-10 weeks of anaerobic training can increase peak anaerobic power by 3-28% across individuals.

Muscle Buffering Capacity Improvements

Improvements in muscle buffering capacity are achieved by increasing both intracellular buffers and hydrogen ion transporters.

Hypertrophy of Type II Muscle Fibers (Anaerobic Training)

Anaerobic training leads to hypertrophy of type II muscle fibers, increases enzymes involved in both the ATP-PC system and glycolysis. High-intensity interval training (>30 seconds) promotes mitochondrial biogenesis at or above VO_{2} \max.

Neural Adaptations (Early Strength Gains)

Responsible for early gains in strength (first 8–20 weeks), these include increased motor unit recruitment, altered motor neuron firing rates, enhanced motor unit synchronization, and removal of neural inhibition.

Hyperplasia

An increase in muscle fiber number. In humans, most muscle enlargement (90–95%) is due to hypertrophy, with limited evidence for hyperplasia.

Hypertrophy (Muscle Specifics)

Both type I and II fibers experience hypertrophy, with a greater degree in type II fibers. Increases in myofibrillar proteins lead to an increased number of cross-bridges and an increased ability to generate force.

mTOR (Mechanistic Target of Rapamycin)

A protein kinase that increases ribosome production and accelerates protein synthesis. Leucine and BCAA supplementation provide a small increase in mTOR activation but are ineffective in untrained individuals.

Muscle Fiber Type Shifts

Resistance training can cause a 5-11% shift from type IIx to type IIa fibers after 20 weeks. Small increases in type I fibers occur, but fast-to-slow shifts are less prominent compared to endurance training.

Concurrent Training Interference Potential

Strength training increases muscle fiber size, while endurance training does not. The degree of interference depends on the intensity, volume, and frequency of the training.

Impairment of Strength Gains (Concurrent Training)

Combining strength and endurance training can impair strength gains compared to strength training alone, with the extent of interference depending on workout specifics (intensity, volume, frequency).

Neural Factors Affecting Strength Development (Concurrent Training)

Impaired motor unit recruitment is a proposed factor, though evidence is limited. Low muscle glycogen content due to successive bouts of endurance exercise may result in impaired ability to perform subsequent resistance training bouts.

Overtraining (Concurrent Training)

There is no direct evidence that overtraining causes impairments in strength gains during concurrent training. However, depressed protein synthesis may occur due to endurance training adaptations that interfere (e.g., decreased mTOR).

Loss of Muscle Strength and Fiber Size (Detraining)

A slow decrease in strength of 31% occurs following 30 weeks of detraining, associated with small changes in fiber size (Type I: -2%, Type IIa: -10%, Type IIx: -14%). These changes are primarily attributed to nervous system changes.

Retraining Effects

Rapid regain of strength and muscle size can occur within 6 weeks after resuming training. Strength maintenance is possible with reduced training up to 12 weeks.

Decline in Strength After Age 50

Strength declines after age 50 due to loss of muscle mass (sarcopenia), loss of both type I and II fibers (including atrophy of type II fibers), loss of intramuscular fat and connective tissue, and loss/reorganization of motor units.

Progressive Resistance Training for Aging

Important for causing muscle hypertrophy and strength gains in older adults, which are vital for activities of daily living, maintaining balance, and reducing the risk of falls.

How does concurrent training impact gains in strength and endurance?

Combining strength and endurance training can impair strength gains compared to strength training alone, with the extent of interference depending on workout specifics (intensity, volume, frequency).

What is mTOR and why is it crucial in resistance training?

mTOR (Mechanistic target of rapamycin) is a protein kinase that increases ribosome production and accelerates protein synthesis, making it crucial for muscle growth and adaptation during resistance training.

How do muscle fibers adapt following resistance training?

Muscle fibers adapt through hypertrophy (especially type II fibers), increases in myofibrillar proteins, leading to an increased number of cross-bridges and force generation. There can also be fiber type shifts from type IIx to IIa.

What is the difference between hypertrophy and hyperplasia, and which is more relied upon during resistance training?

Hypertrophy is an increase in muscle fiber size, while hyperplasia is an increase in muscle fiber number. In humans, hypertrophy accounts for 90–95% of muscle enlargement during resistance training, with limited evidence for hyperplasia.

What physiological changes occur during the first 8 weeks of resistance training?

During the first 8–20 weeks of resistance training, neural adaptations are primarily responsible for early strength gains. These include increased ability to recruit motor units, altered motor neuron firing rates, enhanced motor unit synchronization, and removal of neural inhibition.

How do strength and muscle size change after periods of detraining, and how do they respond to retraining?

After detraining, there's a slow decrease in strength (e.g., 31% loss after 30 weeks) and small reductions in fiber size, primarily due to nervous system changes. Upon retraining, strength and muscle size can be rapidly regained within 6 weeks, and strength can be maintained with reduced training for up to 12 weeks.

How is strength affected by aging?

Strength declines after age 50 primarily due to sarcopenia (loss of muscle mass), which involves the loss of both type I and II fibers (including atrophy of type II fibers), loss of intramuscular fat and connective tissue, and loss/reorganization of motor units.