Exercise Physiology Flashcards
Quantifying performance in terms of power
Muscle fiber types
Slow twitch, fast twitch a and b
Slow-twitch muscle fibers
- Fatigue-resistant, for endurance (e.g. keeping your head elevated)
- Least powerful
Fast-twitch a fibers
- Intermediate power
- Fast fatigue-resistant
Fast twitch b fibers
- Most powerful (e.g. for sprints)
- Fast fatigable
- Twice as powerful as slow-twitch
Table showing classification of muscle fiber types
Force is equivalent to ___
Number of cross-bridges per second
Diameter of muscle fibers
Which fibers have the largest diameter and why?
- Fast twitch a
- It’s packing more things into one fiber (more cross-bridges and more mitochondria for aerobic fibers)
- Taking up space is a resource
Fast twitch b fibers are large but ___
Don’t have much oxidative capacity (mitochondrial and capillary densities are low, more space for actin and myosin)
Slow twitch fibers have a very ___ glycolytic capacity
- Low
- It only produces ATP at a certain level, so it can’t have too many cross-bridges
- If it has too many cross-bridges, it would use up ATP too quickly
Glycogen stores in fast-twitch fibers
- Very high
- Because glycolysis is featured
- A lot of ATP is produced
Why does it not make sense to store so much glycogen in slow-twitch fibers?
- The rate at which you can go through glycolysis and feed mitochondria is limited by the sluggishness of the aerobic pathways of the mitochondria
- So no reason to have a lot of glycolytic enzymes, would just cause a buildup of pyruvate
Stained muscle fibers
- Every cell has myosin, but only slow-twitch are dark because of the pH in the lab
- Fast twitch a are the largest
- The same muscle has a mixture of fiber types
Distribution of fiber types (within the same individual)
- Great variability between individuals
- Mostly determined genetically
Energy systems (source of ATP for cross-bridge formation)
What are the three energy systems that act as sources of ATP for cross-bridge formation?
- Phosphogen system
- Glycolytic system
- Oxidative (aerobic)
Phosphogen system
10 seconds
- Stored ATP- 3 secs
- Phosphocreatine - 7 secs
Glycolytic system
1-2 minutes, primarily described fast twitch b muscle fibers
- Glycolysis
- Inefficient (not getting a lot of ATP) but fast
- Buildup of lactic acid- must be buffered
Which type of muscle fiber is the glycolytic system associated with and why?
- Fast twitch b
- They have a lot of glycolytic enzymes in the cytosol
- A lot of carbohydrates to feed into glycolysis
- Not a lot of mitochondria to get in the way or capillaries to take up space
Downside of aerobic pathway for energy
- Stored ATP (phosphogen system) and glycolysis (glycolytic) enable more cross-bridge formation per minute than aerobic
- If you want to maintain cross-bridges using aerobic pathways, a smaller number will be able to be maintained
Capacity of energy systems vs. exercise duration
- Stored energy/phosphocreatine on green line gone quickly (but readily available)
- Glycolysis starts off low and when it turns on (takes a little time) but then spikes, stays there for minutes, then depletes
- For long-term aerobic pathways, it takes minutes for them to get up to full capacity, but can be maintained for hours/days
How do cells know to increase glycolytic pathway activity?
- All happens within the cell
Fiber type recruitment order
As the requirement for more power from muscle contraction increases the recruitment order is as follows:
- Slow-twitch (aerobic): FIRST
- Fast-twitch a (aerobic/anaerobic): SECOND
- Fast-twitch b (anaerobic): LAST
Anaerobic threshold (the old story)
As exercise intensity increases:
- Oxygen levels are insufficient to support aerobic pathways
- Pyruvate, the product of glycolysis, accumulates in cytosol
- At a higher cytosolic concentration of pyruvate the anaerobic enzyme pyruvate dehydrogenase now acts on substrate to increase anaerobic metabolism
- Lactate accumulates as “waste product” and enters blood
- Buffering of lactate by HCO3- results in increased VCO2
- VE increases disproportionately to VO2 due to ↓pH and ↑VCO2
- Many subsequent studies do not support this hypothesis.
- Lactate is not just a “waste product” but a resource that is shuttled between cells to be used as fuel, be converted into glucose (in liver) and serve as a signaling molecule (autocrine, paracrine, endocrine)
Comprehensive flowchart of metabolism
- Glycogen or glucose is used in glycolysis to create pyruvate and a little bit of ATP
- Pyruvate enters the Krebs cycle, producing more ATP (slow)
Sources of energy for aerobic pathways:
- Glucose and glycogen
- Free fatty acids (fat is the primary fuel source of the body)
Can fatty acids be used in the glycolytic pathway?
No, they are purely aerobic
Burning of what nutrient yields the slowest ATP production rate?
- Burning fat aerobically
- Burning fatty acids to produce ATP is a slower burn than using glycolysis as a source -> oxidative phosphorylation
How can you pick up your ATP rate of delivery but still stay aerobic?
- Use glycose and glycogen, fed through the aerobic pathways (a bit faster than fat metabolism)
- Slow-twitch muscle fibers will use fat as their primary fuel source when they’re not using a lot of energy. If they need to produce more power, they will shift to burning glucose/glycogen
What pathway do fast-twitch fibers lean more heavily upon than slow-twitch?
- Glycoslysis to produce ATP quickly
- Instead of pushing all the pyruvate down aerobic pathways, it goes down anaerobic pathways, producing lactic acid
End-product inhibition with lactate
- It slows down the enzyme that starts the pathway of pyruvate - acetyl CoA (end-product inhibition)
- This limits ATP production
How do carbohydrates being burned during exercise affect fat storage after?
Using carbohydrates during exercise results in less fat being stored after because glucose can be very easily formed into fat in fat cells (less glucose there to be converted into fat)
Phosphocreatine and ATP
- When ATP levels drop (as a result of contraction), phosphocreatine produces more ATP to replenish it
- ATP inhibits the breakdown of phosphocreatine
How does ATP affect aerobic pathways?
- When cross-bridges consume ATP, ATP levels drop
- ATP inhibits glycolytic enzymes in the glycolysis pathway
- If you disinhibit those enzymes (with decreased ATP levels), glycolytic pathways pick up (this takes a few seconds)
How would the flow-chart look different depending on the type of muscle fiber?
- If the muscle were slow-twitch, there would be a big emphasis on oxidative phosphorylation and less on glycolysis
- If it were fast-twitch b, glycolysis would be emphasized and oxidative phosphorylation would be a smaller box
Anaerobic threshold
- The x-axis shows oxygen consumption going up (proportion to power of muscles)
- As your VO2 goes up, there is an impact on blood lactate, arterial PCO2, and ventilation
- CO2 goes down as a respiratory compensation for metabolic acidosis (makes
pH more acidic)
-CO2 from anaerobic pathways and from buffering lactate
Problems with anoxia triggering lactate increase in blood
- PO2 does not drop to 2 mmHg which is theoretical limit at which aerobic pathways experience oxygen deficit
- Lactate is used to shuttle important molecules into and out of muscle cells
- Lactate is removed from the blood and “consumed” by other cells
- Trained athletes demonstrate a transient increase in lactate at their AT that diminishes over time
*N.B. at very intense levels of exercise, above AT, anoxia still plays a role
*N.B. Lactate Threshold is still an important measure for athletic training
Graph showing what happens to lactate when you exercise
Lactate levels go up within the first ~20 minutes, then as you continue, it comes back down (body adapts, maybe liver metabolizes it more quickly)
Percentage of CHO and Fat vs. Exercise Intensity
- You start with slow-twitch, and as the intensity of exercise increases, you recruit fast-twitch fibers
- The graph demonstrates the percentatge of calories derived from burning fat and carbohydrates
- The Y-axis is the PERCENTAGE of energy derived from fat vs. carbohydrates
- Overall, as intensity of exercise increases, the total total number of calories being burned goes up, but the % derived from fat drops and the % derived from carbs goes up (you still burn fats0
Why? (figure shows)
- Slow-twitch fibers switch from fat to carbs to generate more power
- Recruitment of fast-twitch fibers which use glycolysis exclusively
Glycogen depletion graph
- Shows how quickly you use up glycogen stores when the intensity of exercise increases
- At 31% of VOmax, you can go 180 mins and you’ve only depleted glycogen by ~50%
- If you’re above your VOmax, you’ll deplete your muscles of glycogen within minutes
Respiratory changes with increased exercise intensity
- You might extract more oxygen from the air you’re breathing in
- You’re certainly moving more air in and out of your lungs
- Minute ventilation goes up (consuming more oxygen)
What is VO max?
- Level of exercise where you’re consuming the maximum level of oxygen
- However, this is not 100% of your power output
Why is VOmax not maximum power output?
- At VOmax, you might still have some fast-twitch b muscle fibers in reserve
- You can generate more power than your VOmax power
Graph showing glycogen restoration with diet
- If you’re exercising at your VOmax for an extended period of time, consuming glucose is helpful (carboloading)
- The graph shows recovery of glycogen with different types of diet
- A person who only eats fat and protein has not replenished their glycogen even after 5 days
- Some people can lean on metabolizing fats more than carbs but it takes time to get to this point
Oxygen and myoglobin
- Myoglobin only binds one oxygen molecule (hemoglobin binds 4)
- Has a higher binding affinity for oxygen than hemoglobin does
- So when the partial pressure of oxygen drops from 100 to 40 mmHg, oxygen unbinds from hemoglobin (75% saturated)
- But at a PO2 of 40, it’s a bit lower that inside the muscle cell, and myoglobin is saturated with oxygen
- However, when the muscle becomes active, ATP levels drop, aerobic enzymes feeding glycolysis are disinhibited, more pyruvate goes through the Krebs cycle and electron transport chain
- Oxygen levels drop as it is consumed at a fast rate
- The first thing to respond is myoglobin
- As the PO2 drops below 40 mmHg as metabolism increases, the first thing that happens is that oxygen can be released from myoglobin to keep the aerobic pathways fed with oxygen
- It might take the cardiovascular and respiratory systems time to catch up and deliver oxygen
- In this way, myoglobin is a buffer
Fuel- glucose
- From blood (liver makes it from glycogen
- Stored in muscle (glycogen)
- Fast delivery rate
Fuel- fat
- Very abundant energy source
- Slow delivery rate
Fuel- protein
- Abundant but not ideal source
- Slow delivery rate
Graph of O2 consumption as a function of exercise time at a given level of effort
16:00
EPOC (excess post-exercise oxygen consumption)
- Increase in oxygen cosumption per minute that extends after an exercise
- The more intense and the longer the duration of the exercise, the larger the EPOC
- There are two phases: rapid component (mostly restocking component) and prolonged component (lasts 12 hours)
EPOC rapid component
- Restock Hb and myoglobin with O2
- Restore ATP and phosophocreatine
- Increased body temp
- Lactate removal
EPOC prolonged component (less is known)
- Shift from carb to fat metabolism?
- Increased protein synthesis?
- Micro-damage and inflammation in muscle that stimulate synthesis of new parts of muscle
What causes fatigue?
- Energy depletion
- Buildup of metabolic byproducts
- Nervous system
Energy depletion (cause of fatigue)
- Phosphocreatine (short, hard efforts)
- Glycogen depletion (longer efforts)
Buildup of metabolic byproducts (cause of fatigue)
- Lactic acid (current research does not support this as having a direct effect on weakening muscle contraction)
- Electrolyte changes (intracellular or extracellular):
-Increase in extracellular K+?
-Depolarization of Vm of muscle cells
-Less release of Ca++ from sarcoplasmic reticulum
Nervous system (cause of fatigue)
- Physiological (sensory neuron activity in muscle depolarizes motor neurons in spinal cord)
- Psychological (discomfort does not discourage)
What changes does endurance training lead to?
- Increased vascularization - more capillaries
- Increased myoglobin
- More mitochondria
- More oxidative enzymes
- Improved fat metabolism
Anaerobic training (increase anaerobic threshold and increase anaerobic capacity)
- More glycolytic enzymes
- More glycogen stored
- Increased capacity to utilize lactate:
-Less release from active muscle at a given power
-More uptake by other cells - lactate shuttling (e.g. some muscle metabolizes lactate; liver makes glucose from lactate) - Psychological training: deal with pain
Graph showing time in weeks vs. control
28:00
What causes muscle cramps?
- Overuse, muscle strain, holding the same position (prolonged contraction)
- K+, Mg++, or Ca++ deficiency
- Dehydration
- Symptoms of underlying conditions (nerve compression, inadequate blood supply)
- There are many theories but in most cases, the cause of muscle cramps is unknown
Recent theory about muscle cramps
- A recent theory implicates sensory nerve endings in muscle that are excited when muscle is fatigued or overused, resulting in a spinal cord reflex that activates motor neurons going to the same muscle
- In experimental animals, blocking the sensory axons coming out of a muscle or motor axons going into a muscle decreases cramping significantly
Diameter of muscle fibers
ST: small
FTa: largest
FTb: large
Mitochondrial density in muscle fibers
ST: high
FTa: high
FTb: low
Capillary density in muscle fibers
ST: High
FTa: Medium
FTb: Low
Myoglobin content in muscle fibers
ST: High
FTa: Medium
FTb: Low
Metabolic aspects?
At low levels of exercise, most ATP is generated from metabolizing fats. At high levels of exercise, most ATP is generated from metabolizing carbohydrates. List and briefly explain two reasons why there is an overall shift from burning fats to burning carbs when exercise level goes from low to high.
1) Higher level of exercise requires a higher delivery rate of ATP. Aerobic pathways shift from fat to carbohydrate metabolism when a greater rate of ATP delivery is required.
2) Higher level of exercise requires recruitment of more fast-twitch muscle fibers that metabolize carbohydrates more than fat.
In a muscle cell, a drop in the concentration of ATP will cause a (3 changes)
- decrease in glycogen synthesis
- increase in the production of CO2
- increase in the breakdown of phosphocreatine
