L15: Energy systems, force generation, and fatigue

What 3 energy systems produce ATP?

1. ATP-PCr (phosphagen) system

2. Glycolytic (anaerobic) system

3. Oxidative (aerobic) system

What do system changes depend on?

Exercise intensity and duration

Why do muscles continuously regenerate ATP?

Because ATP stores are very limited

When does Phosphagen system dominate?

Short, maximal efforts (0-10 seconds)

What does Phosphagen system use in the muscle?

ATP and phosphocreatine (PCr)

Which systems are anaerobic?

Phosphagen and glycolytic

What does high power, low capacity mean?

Regenerates ATP quickly, but phosphocreatine stores are limited

What does low power, high capacity mean?

ATP production is slower than other systems, but can be sustained for long periods

What does moderate power, moderate capacity mean?

Generates ATP quickly, but can’t sustain maximal rates indefinitely

Which system is high power, low capacity

Phosphagen system

Which system is low power, high capacity?

Aerobic system

Which system is moderate power, moderate capacity?

Anaerobic system

When does anaerobic system dominate?

High intensity exercise, 15-60 seconds

What does the glycolytic system break down to regenerate ATP?

Glucose or muscle glycogen

What does breaking down glucose get?

2 ATP + lactate

What does breaking down glycogen get?

3 ATP + lactate

When does glycolysis occur?

When O2 is available

What does producing lactate do?

Regenerates NAD+ so glycolysis can continue

What contributes to muscle fatigue during intense exercise?

Accumulation of H+ and inorganic phosphate (Pi)

How does ATP-PCr system regenerate ATP?

PCr donates phosphate to ADP to regenerate ATP

How many ATP are produces per glucose?

30-32

When is the aerobic system dominant?

During prolonged, sustainable exercise

Where does oxidative system generate ATP from?

Carbs and fats

What does the oxidative system require to generate ATP?

Mitochondrial metabolism

What contributes to more prolonged, lower-intensity exercise?

Fat oxidation

What is the O2 binding protein found inside muscle fibers?

Myoglobin

What are the 4 roles of myoglobin?

1. Stores O2

2. Facilitates O2 movement

3. Releases O2

4. Supports aerobic ATP production

How does myoglobin help with storing O2?

Provides a small oxygen reserve inside muscle cells

How does myoglobin help with facilitating O2 movement?

Helps oxygen reach mitochondria

How does myoglobin help with releasing O2?

Myoglobin releases bound O2 as mitochondria consumes oxygen

How does myoglobin help with supporting aerobic ATP production?

Mitochondria uses O2 during oxidative phosphorylation

How do fibers rich in myoglobin appear?

Redder and are more fatigue resistant

What is VO2 max?

The maximum rate which the body can take in, transport, and use O2 during intense exercise

Which aspects of the body does VO2 max represent?

• Lungs

• Heart and blood

• Muscles

How does VO2 max represent lungs?

Brings O2 into the blood

How does VO2 max represent heart and blood?

Delivers O2 to working muscles

How does VO2 max represent muscles?

Extracts and uses O2 for aerobic ATP production

What does greater VO2 max indicate?

Greater aerobic capacity and supports endurance performance

What factors influence VO2 max?

• Genetics

• Cardiovascular function

• Endurance training

• Age and body composition

What is oxygen used by muscles (VO2) equation?

Oxygen delivered through circulation x proportion extracted by tissues

What is EPOC (Excess post exercise O2 consumption)?

Oxygen consumption remains elevated above resting levels after exercise as body recovers

How is oxygen consumption at the beginning of exercise?

Increases gradually

What 4 things does additional O2 during recovery support?

1. ATP-PCr restoration

2. Replenishing O2 bound to myoglobin

3. Lactate oxidation

4. Return of heart rate, ventilation, temperature, and hormone resting levels

What does EPOC reflect?

Metabolic cost of restoring homeostasis after exercise

What is muscle fatigue?

Exercise-induced reduction in ability to generate force or power

What happens with central fatigue?

Reduced neural drive to skeletal muscles

What can reduce force production?

Changes in motor neuron activation and central motor drive

What can influence performance?

Perceived effort and mental fatigue

What happens due to reduced neural drive?

CNS becomes less able to fully activate working muscles

What happens when there is peripheral fatigue?

Reduced ability of muscle fibers to generate force despite neural activation

What increases with peripheral fatigue?

Inorganic phosphate and other metabolic changes

What decreases with peripheral fatigue?

Ca2+ release from sarcoplasmic reticulum

What is impaired with peripheral fatigue?

Excitation-contraction coupling

What can contribute during prolonged exercise?

Glycogen depletion

What are the main things caused from central fatigue and peripheral fatigue?

Central fatigue= Muscle activation reduced

Peripheral fatigue= Muscle’s ability to respond/generate force reduced

What two characteristics influence fiber function?

1. Myosin ATPase activity

2. Metabolic characteristics

What does myosin ATPase activity deal with?

Contraction speed

What do metabolic characteristics deal with?

Fatigue resistance

What happens with slower myosin ATPaase?

Slower cross-bridge cycling

What are the two metabolic characteristics?

1. Greater oxidative capacity

2. Greater reliance on rapid glycolytic ATP production

What is caused by greater oxidative capacity?

Better ability to sustain ATP production

What is caused by greater reliance on rapid glycolytic ATP production?

Supports intensity activity but generally lower fatigue resistance

What are Type 1 muscle fiber characteristics?

Slow-twitch—> endurance

What are Type 2 muscle fiber characteristics?

Fast twitch—> rapid, powerful movements

What is the ATP metabolism for Type 1?

Primarily oxidative

What is the ATP metabolism for Type 2?

Oxidative + glycolytic

What is muscle tension?

The force generated by a muscle through active contraction or passive stretching

What is muscle tone?

Low-level muscle tension present even when a muscle is not voluntarily contracting

What is muscle contraction?

Muscle fibers actively generate force through actin-myosin cross-bridge cycling

What contributes to posture and joint stability?

Muscle tone

What length is contracting muscle?

It can shorten, stay the same, or lengthen

What is length-tension relationship?

Maximum active force depends on sarcomere’s initial length and degree of actin-myosin overlap

What is optimal length in skeletal muscle?

2.0-2.25 micrometers

What happens to actin and myosin when skeletal muscle is too stretched?

Less overlap between actin and myosin means that fewer cross-bridges can form

What is the result of stretched skeletal muscle?

Lower active tension

What happens with excessive thin-filament overlap?

Reduce effective force generation

What is the results of short skeletal muscles?

Lower active tension

What are the 2 main types of muscle contraction?

1. Isometric

2. Isotonic

What is isotonic muscle contraction?

Changing length of muscle while keeping tension constant

What is isometric contraction?

Keeping muscle length the same while contraction changes

What is concentric isotonic?

Muscle shortens

What is eccentric isotonic?

Muscle lengthens

What is a motor unit?

One motor neuron and all skeletal muscle fibers it innervates

What is each skeletal fiber innervated by?

One motor neuron

What happens when a motor neuron is fired?

All muscle fibgers in its motor unit are activated

What 3 things control muscle force?

1. Recruitment (adding more motor units)

2. Firing frequency

3. Asynchronous activation

What are small motor units innervated by?

Few muscle fibers

What increments of force do small motor units do?

Small increments of force that allow precise control

What are large motor units innervated by?

Many muscle fibers

What increments of force do large motor units do?

Large increments of force, supporting powerful movements

What is the size principle?

Motor units are usually recruited in an orderly sequence (small to large motor units—as more force is required, more motor units are recruited)

What is a muscle twitch?

A brief mechanical response of muscle fibers to a single action potential in their motor neuron

What happens in the latent period?

Action potential travels down sarcolemma/T-tubules, and Ca2+ is released from SR

What happens in the contraction phase?

Ca2+ binds troponin which exposes myosin-binding sites on actin. Cross0bridge cycling increases tension

What happens in the relaxation phase?

Ca2+ is pumped back into the SR and fewer cross-bridges are active, so tension decreases

What is temporal summation?

Another stimulus arrives before muscle completely relaxes

How is muscle tension in temporal summation?

Extra Ca2+ release allows tension to build on previous twitch

What is incomplete (unfused) tetanus?

Repeated stimuli produces sustained tension with partial relaxation between contractions

How is muscle tension in incomplete (unfused) tetanus?

Muscle tension fluctuates

What is complete (fused) tetanus?

Very frequent stimulation prevents noticeable relaxation between contractions