GW BGZ 2025 Case 6 - Train your muscles

What are the main intramuscular adaptations to resistance training?

The main intramuscular adaptations are:

1. Hypertrophy (primary adaptation in humans)

   • Increase in size of existing muscle fibers

   • Driven by increased myofibrillar proteins (actin, myosin)

2. Hyperplasia (minor/uncertain in humans)

   • Increase in number of muscle fibers

   • Evidence is weak in humans but stronger in animal studies

3. Structural remodeling

   • Increased sarcomere number (in series and parallel)

   • Increased connective tissue strength

   • Changes in fiber architecture (especially pennation angle)

4. Metabolic adaptations

   • Increased glycogen storage

   • Increased phosphocreatine (PCr)

   • Increased ATP turnover capacity

What is muscle hypertrophy?

Hypertrophy is the increase in cross-sectional area of muscle fibers due to:

• Increased synthesis of contractile proteins (actin and myosin)

• Expansion of myofibrils within each fiber

• Increased structural proteins and organelles

It results in:

• Greater force production

• Increased muscle size

• Improved mechanical strength

Hypertrophy primarily affects Type II (fast-twitch) fibers.

What is hyperplasia in muscle?

Hyperplasia is an increase in the number of muscle fibers.

Proposed mechanisms:

• Splitting of existing large fibers

• Activation of satellite cells forming new fibers

• Longitudinal fiber division

However:

• Strong evidence exists in animals

• Human evidence is weak and inconsistent

• It is considered a minor contributor compared to hypertrophy

What is the difference between myofibrillar and sarcoplasmic hypertrophy?

Myofibrillar hypertrophy:

• Increase in contractile proteins (actin, myosin)

• Increases strength and force output

• Dense, “hard” muscle appearance

• Driven by heavy resistance training

Sarcoplasmic hypertrophy:

• Increase in non-contractile components (glycogen, fluid, enzymes)

• Increases muscle size without proportional strength gains

• “Fuller” muscle appearance

• Common in higher-repetition bodybuilding training

What happens to muscle protein balance during resistance training?

Muscle mass is determined by the balance between:

• Muscle Protein Synthesis (MPS)

• Muscle Protein Breakdown (MPB)

If:

• MPS > MPB → muscle growth occurs

• MPB > MPS → muscle loss occurs

Resistance training increases MPS, especially when combined with:

• Adequate protein intake

• Energy availability

• Recovery

What role do satellite cells play in muscle hypertrophy?

Satellite cells are muscle stem cells located between the basal lamina and muscle fiber.

Functions:

1. Activated by mechanical stress/damage

2. Proliferate and differentiate

3. Fuse with muscle fibers

4. Donate additional nuclei

Importance:

• More nuclei increase capacity for protein synthesis

• Enables long-term muscle growth

• Supports repair and regeneration

What is the role of Z-discs in muscle damage?

Z-discs (Z-bands) anchor actin filaments and maintain sarcomere alignment.

During intense resistance training:

• Z-discs can become disrupted or torn

• Microdamage occurs in sarcomeres

• Contributes to inflammation and DOMS (delayed onset muscle soreness)

This structural damage triggers:

• Repair processes

• Remodeling

• Hypertrophy signaling

Why does eccentric training cause more muscle damage than concentric training?

Eccentric contractions involve force production while the muscle lengthens.

Reasons for greater damage:

• Higher mechanical tension per fiber

• Fewer active motor units sharing load

• Overstretching of sarcomeres

• Increased stress on Z-discs and cytoskeleton

Effects:

• More microtears

• Greater soreness (DOMS)

• Stronger remodeling stimulus

What is the difference between short-term and long-term muscle size changes?

Short-term (pump):

• Increased blood flow

• Fluid accumulation

• Glycogen and water storage

• Temporary swelling

Long-term (true hypertrophy):

• Structural protein growth

• Increased myofibril content

• Satellite cell contribution

• Permanent fiber enlargement

Why do beginners gain strength quickly in the first weeks of training?

Early strength gains are mostly due to neural adaptations, not muscle growth:

• Increased motor unit recruitment

• Improved coordination

• Reduced antagonist co-activation

• Better motor learning

• Increased neural drive from CNS

Muscle hypertrophy becomes dominant after several weeks.

What is a motor unit?

A motor unit consists of:

• One alpha motor neuron

• All muscle fibers it innervates

Types:

• Small units → slow, fatigue-resistant (Type I)

• Large units → fast, powerful (Type II)

Motor units follow the size principle:

• Small units recruited first

• Large units recruited as force demand increases

What neural adaptations occur in resistance training?

1. Greater efficiency in neural recruitment patterns

   1. Resistance training leads to the recruitment of more motor units within muscles, allowing for greater force production. Over time the nervous system becomes more efficient in activating the appropriate motor units needed for specific tasks → increasing strength

2. Increased motor neuron excitability

   1. Resistance training enhances the excitability of motor neurons, making them more responsive to stimuli

   2. This increased excitability allows for faster and more forceful muscle contractions, contributing to strength gains

3. Increased central nervous system activation

   1. The central nervous system becomes more activated during resistance training, leading to greater overall muscle activation. Increased activation results in a higher level of force production and improved coordination between muscles

4. Improved motor unit synchronization and increased firing rates

   1. Motor units are groups of muscle fibers controlled by a single motor neuron.

   2. Resistance training improves the synchronization of motor units, ensuring that they fire together more efficiently. Resistance training can increase the firing rates of motor neurons, allowing for more rapid and forceful muscle contractions

5. Lowering of neural inhibitory reflexes

   1. During resistance training, neural inhibitory reflexes that typically limit muscle contraction are reduced. This allows for greater activation of muscle fibers and increased strength output

6. Inhibiting of Golgi tendon organs

   1. Golgi tendon organs are sensory receptors located in tendons that detect changes in muscle tension.

   2. Resistance training can inhibit the Golgi tendon organs, allowing for greater force production without triggering protective reflexes that would otherwise limit muscle contraction

Result:

• Higher maximal force production

How are motor neurons recruited

Each motor neuron has a:

• membrane threshold

If excitatory input exceeds threshold:

• action potential fires

Heavy resistance training lowers the relative difficulty of activating high-threshold units.

So trained individuals can:

• recruit fast-twitch fibers earlier

• sustain high firing rates longer

How can the nervous system control force production?

• Motor Unit Recruitment: To produce maximal force, the brain recruits as many motor units as possible. It follows a specific order, starting with small, slow-twitch units and moving to large Type 2 (fast-twitch) units, which are capable of producing much greater power.

• Rate Coding (Frequency): Increasing the frequency of electrical impulses from motor neurons leads to summation, where individual twitches combine to produce more tension. At the highest frequencies, the muscle enters a state of complete tetanus, maintaining a smooth, maximal steady force plateau.

• Neural Drive: Training improves neural efficiency, including better synchronization of motor units and higher firing rates, allowing for higher force output even before muscle size increases.

How does the physical structure of the muscle influence its force capacity?

The physical structure of the muscle significantly influences its force capacity:

• Cross-Sectional Area: The force a muscle can produce is strongly connected to its physiological cross-sectional area. Larger muscle fibers (hypertrophy) and more fibers arranged in parallel (as seen in pennate muscles) allow for more cross-bridges to work in parallel, increasing absolute force.

• Fiber Types: Type 2b (or 2x) glycolytic fibers have the largest diameters and the fastest myosin ATPases, making them best suited for absolute peak force during short-term, intense movements.

What is the force-length and force-velocity relationship?

Peak force is highly dependent on the muscle's length and the speed of its contraction:

• Force-Length Relationship: Peak force is achieved at an optimal resting length (typically 2.0 to 2.3 μm per sarcomere). At this length, there is a maximal overlap between thick and thin filaments, allowing for the highest number of cross-bridge interactions.

• Force-Velocity Relationship: Muscles produce different levels of force based on movement speed. While force drops sharply as shortening velocity increases, it actually increases during eccentric (lengthening) actions. Eccentric contractions can be 50% more forceful than concentric ones, with force levels plateauing at approximately 150% of the maximum isometric tension.

What is rate coding in muscle contraction?

Rate coding refers to how often motor neurons fire.

Effects:

• Low frequency → twitch contractions

• High frequency → summation

• Very high frequency → tetanus (maximal force plateau)

Higher rate coding increases force even without more motor units.

What is motor unit synchronization?

Synchronization is when motor units fire more simultaneously.

Effects:

• Increases peak force output

• Improves explosive strength

• Reduces variability in force production

However:

• It is less important than recruitment and rate coding

What is autogenic inhibition?

Autogenic inhibition is a protective reflex mediated by the Golgi tendon organ (GTO).

Mechanism:

1. High tension detected in tendon

2. GTO activates Ib afferent fibers

3. Spinal inhibitory interneurons activated

4. Alpha motor neuron activity decreases

Result:

• Reduced muscle force output

• Protection against tendon damage

Training can reduce sensitivity of this reflex.

What determines absolute peak force production?

Peak force occurs when multiple factors align:

Neural:

• Full motor unit recruitment

• High firing frequency (tetanic contraction)

• Reduced inhibition

• High CNS drive

Muscle:

• Maximum cross-bridge formation

• Optimal sarcomere length

• High Type II fiber activation

Mechanical:

• Eccentric contractions produce highest force levels

How do resistance and endurance training differ neurally?

Resistance training:

• Maximal motor unit recruitment

• High firing frequency

• Increased synchronization

• Reduced inhibition

• Focus: force production

Endurance training:

• Efficient motor unit use

• Reduced unnecessary activation

• Improved fatigue resistance

• Focus: energy efficiency

What are the excitatory and inhibitory signals for muscles?

Movement depends on balance between:

Signal Type	Function
Excitatory	Promote motor neuron firing
Inhibitory	Suppress motor neuron firing

Force output depends on:

• descending cortical drive

• spinal facilitation

• inhibition thresholds

Excitatory Inputs

Sources:

• motor cortex

• brainstem pathways

• sensory feedback

Neurotransmitters:

• glutamate

• monoamines

These increase alpha motor neuron firing probability.

Inhibitory Inputs

Sources:

• Golgi tendon organs

• Renshaw cells

• inhibitory interneurons

• supraspinal protective circuits

Neurotransmitters:

• GABA

• glycine

These reduce excitation to prevent:

• tendon rupture

• joint damage

• loss of coordination

What is the cross-bridge cycle?

The cross-bridge cycle is the mechanism of muscle contraction:

1. Calcium binds troponin

2. Tropomyosin shifts

3. Myosin binds actin

4. Power stroke occurs

5. ATP detaches myosin

6. Cycle repeats

Force depends on:

• Number of active cross-bridges

• Frequency of cycling

What is cardiac adaptation to resistance training?

Main adaptation is concentric hypertrophy:

• Increased left ventricular wall thickness

• Minimal change in chamber size

• Adaptation to high blood pressure (afterload)

Purpose:

• Generate higher pressure during lifting

What is eccentric cardiac hypertrophy?

Occurs mainly in endurance training:

• Enlarged ventricular chamber

• Increased blood volume capacity

• Mild wall thickening

Purpose:

• Increased stroke volume and cardiac output

What changes in the cardiac muscle when doing resistance training?

• Thicker ventricular walls, resistance-trained athletes show thicker ventricular walls compared to endurance trained athletes. This adaptation helps the heart withstand the increased pressure and workload imposed by resistance training.

• Intraventricular septum thickness, resistance-trained individuals tend to have larger intraventricular septum thickness. This contributes to the overall strength of the heart.

• Increased ventricular mass, which means the heart muscle becomes stronger and more capable of pumping blood effectively

• Limited enlargement of the left ventricular cavity, unlike endurance training (which leads to enlargement of LV cavity), resistance training typically does not cause significant enlargement. This means that the increase in ventricular mass is achieved without affecting the size of the ventricular cavity

• Temporary myocardial stress, resistance training imposes temporary stress on the myocardium (heart muscle). Adequate rest periods allow for recuperation, ensuring that the heart can adapt to the demands of training without being overwhelmed

What is athlete’s heart?

Physiological enlargement of the heart due to training.

Types:

• Endurance → eccentric hypertrophy

• Strength → concentric hypertrophy

Characteristics:

• Reversible

• Non-pathological

• Adaptive to training demands

What are key cardiovascular adaptations to endurance training?

• Increased stroke volume

• Increased cardiac output

• Lower resting heart rate

• Increased plasma volume

• Increased capillary density

• Improved oxygen extraction (a-vO₂ difference)

What is the main determinant of increased VO₂max in trained individuals?

The primary factor is increased stroke volume, which increases maximal cardiac output.

How does resistance training affect the heart compared to endurance training?

Resistance training:

• Pressure overload

• Concentric hypertrophy

• Increased wall thickness

Endurance training:

• Volume overload

• Eccentric hypertrophy

• Increased chamber size

What is detraining (deconditioning)?

Detraining, also called deconditioning, is the partial or complete loss of physiological adaptations that occur when resistance training or physical activity is reduced or stopped.

It causes reversal of:

• Muscular adaptations

• Neural adaptations

• Metabolic adaptations

• Cardiovascular adaptations

The magnitude of detraining depends on:

• Duration of inactivity

• Previous training level

• Age

• Nutrition

• Overall physical activity

What happens if training frequency decreases from 3 times/week to 2 times/week?

Most individuals do not significantly lose strength when reducing training from 3 sessions per week to 2 sessions per week if:

• Training intensity remains relatively high

• Total workload is reasonably maintained

• Movement patterns continue to be practiced

Strength maintenance requires much less stimulus than initial strength development.

Research shows trained individuals can often maintain strength with:

• Approximately 1/3 to 1/2 of previous training volume

• As long as intensity remains sufficiently heavy

Example:

• Heavy squatting 2×/week instead of 3×/week usually preserves strength effectively.

Why is strength maintained when training frequency is slightly reduced?

Strength persists because several adaptations are relatively stable:

Neural adaptations persist

The nervous system retains:

• Motor learning

• Motor unit recruitment patterns

• Coordination efficiency

• Synchronization abilities

These adaptations do not disappear immediately.

Muscle tissue does not instantly atrophy

The body still maintains:

• Myofibrils

• Connective tissue

• Neural pathways

• Muscle protein synthesis capacity

especially if overall activity remains moderate.

Why does immobilization cause rapid strength loss?

Immobilization (such as wearing a cast) removes mechanical loading almost completely.

Consequences:

• Dramatic reduction in muscle activation

• Decreased neural drive

• Reduced muscle protein synthesis (MPS)

• Increased relative protein breakdown

• Loss of motor unit efficiency

The nervous system rapidly “downregulates” the unused limb.

Is it true that muscle strength decreases by 3–4% per day during immobilization?

Early strength loss can appear extremely rapid, but not all loss represents actual muscle tissue disappearance.

Early decline includes:

• Reduced neural activation

• Glycogen depletion

• Fluid loss

• Reduced muscle hydration

• Decreased motor unit efficiency

True contractile tissue atrophy develops progressively afterward.

So:

• Initial losses are partly neural and fluid-related

• Structural atrophy becomes more important later

Which muscle fibers atrophy fastest during detraining?

Type II (fast-twitch) fibers generally atrophy faster than Type I fibers.

Reasons:

• Type II fibers depend heavily on high mechanical loading

• They are more sensitive to inactivity

• They lose cross-sectional area more rapidly

Consequences:

• Loss of strength

• Reduced power output

• Reduced explosive performance

What neural changes occur during detraining?

Neural adaptations partially reverse during detraining.

Changes include:

• Reduced motor unit recruitment

• Decreased firing frequency

• Reduced synchronization

• Lower spinal excitability

• Reduced cortical drive

• Reduced neuromuscular coordination

As a result:

• Strength declines before major muscle atrophy becomes visible

What metabolic changes occur during detraining?

Detraining reduces metabolic efficiency within muscle.

Changes include:

• Reduced glycogen storage

• Lower mitochondrial efficiency

• Reduced oxidative enzyme activity

• Decreased ATP production capacity

Consequences:

• Reduced endurance

• Faster fatigue

• Lower exercise tolerance

What cardiovascular changes occur during detraining?

Prolonged inactivity causes cardiovascular deconditioning.

Effects include:

• Reduced stroke volume

• Reduced cardiac output during exercise

• Reduced plasma volume

• Lower aerobic capacity (VO₂max)

• Higher heart rate during submaximal exercise

These changes reduce endurance performance.

How long does recovery take after immobilization or detraining?

Recovery is usually faster than initial adaptation because:

• Neural pathways persist

• Myonuclei remain

• Motor learning is retained

Approximate estimates:

Time Inactive	Approximate Recovery
1–2 weeks	Days to weeks
3–6 weeks	Several weeks
Months	Potentially months

Recovery speed depends on:

• Severity of atrophy

• Age

• Nutrition

• Training history

Can muscle fiber types change with training?

No, but the fibers have more characteristics as other fibers.

Fiber type can change partially, but not completely.

Training mainly alters:

• Metabolic properties

• Enzyme profiles

• Fatigue resistance

• Contractile characteristics

Genetics still strongly determines:

• Baseline fiber distribution

• Sprint potential

What is muscle atrophy?

Atrophy is the decrease in muscle size due to loss of muscle tissue.

It occurs when:

• Protein breakdown exceeds protein synthesis

Consequences:

• Reduced muscle mass

• Reduced strength

• Impaired physical function

• Decreased motor control

What is disuse atrophy?

Disuse atrophy is muscle wasting caused by prolonged inactivity.

Examples:

• Bed rest

• Immobilization

• Sedentary lifestyle

Mechanism:

• Reduced muscle activation

• Lower mechanical tension

• Reduced MPS

• Increased protein breakdown

What is cachexia?

Cachexia is severe muscle wasting associated with chronic disease.

Common diseases:

• Cancer

• Heart failure

• Chronic kidney disease

Characteristics:

• Weight loss

• Inflammation

• Fatigue

• Severe muscle loss

Cachexia differs from simple starvation because inflammation strongly contributes.

What is sarcopenia?

Sarcopenia is the age-related loss of:

• Muscle mass

• Muscle strength

• Physical function

It is a progressive condition associated with aging.

• Little decline usually occurs before age 40

• After age 40:

  • Muscle loss accelerates progressively

  • Decline is generally greater in men

What causes sarcopenia?

Major causes include:

1. Reduced physical activity

• Less mechanical stimulation

• Reduced muscle maintenance

2. Reduced muscle protein synthesis

• Older adults show lower anabolic response

• MPS may be ~30% lower in elderly adults

3. Hormonal changes

Declines in:

• Growth hormone (GH)

• IGF-1

• Anabolic signaling pathways

4. Poor nutrition

• Inadequate protein intake

• Low caloric intake

5. Chronic inflammation/disease