Sliding Filament Theory 9-24-07

FYI: This was a very hard class lecture to put into notes. There were a lot of pictures/dvd interactive things that I had to try to put into words. I spared you all my "paint" pictures of the NMJ and the sliding filament theory as you can find much much better accounts of these in the text or in the links I've included at the bottom. Many thanks.


Summary from last class:

EPOC: Recovery from exercise (anaerobic or aerobic) is aerobic in nature. You must get anaerobic athletes into some amount of aerobic shape. How much aerobic exercise they need depends on what sport, what position they play, what type of this sport they play. Hard aerobic training destroys explosiveness and strength. Test this by a vertical jump. Make sure their vertical jump stays the same week from week. (High level athletes).

EPOC can stay elevated for very long periods of time after exercise. EPOC is greater after anaerobic exercise. For the average out of shape American: they are still burning calories after exercise during EPOC. Sometimes can stay up for a week. Takes a lot of extra calories to keep EPOC going. Greater EPOC after resistance training. 2-3 resistance training sessions per week could help the average out of shape American lose weight right off the bat.

_____________________________________________________________________________________
STARTING MATERIAL FOR TEST 2:

Neuromuscular System:

Refers to the Nervous system and the muscular system interacting.

Muscular System:
1. Skeletal/ striated muscle
2. Cardiac muscle (heart)
3. Smooth- muscle that surrounds hollow organs and blood vessels.

Nervous system:

Two basic components:
1. Central
A. brain (two parts most related to movement)
* Motor cortex: part of the brain where we store how to perform very simple movementpatterns. i.e. for us: walking. Don’t have to think about doing. Automatic. Can call these patterns up immediately.
*Cerebellum: part of the brain where we store complex type movement patterns. Must focus and concentrate to do them. E.g. Professional dives off of a diving board during the learning phase. Ie Walking for babies.
Constant repetition transfers the storage from the Cerebellum to the Motor Cortex. This is the goal. When you have to use these movement patterns in real situations you will be able to call up the movements quickly.
B. Spinal cord: very thick nerve from base of scull down to sacrum.
2. Peripheral Nervous System: all the nerves that run into and out of the spinal cord, to and from muscles (and organs).
A. Sensory/afferent: feelings; pain, cold, heat, pressure: carries info from muscle to CNS.
B. Motor/ efferent Nerve: carries info from CNS to muscle (telling it to generate force or tension, contract).

Action Potential is how a nerve sends a message (electrical energy).
Fig. 1.
Neuron: smallest structural and functional unit of a nerve. A nerve cell Pg. 12

The NMJ is the Neuromuscular Junction (defined later).

The vast majority of motor neurons have myelin sheath insulation for the nerve to focus the electrical message. It is mostly made of fat and speeds the message. A message sent down a nerve with a mylonated sheath travels about 200 mph while a message sent down an unmylonated nerve only goes about 20 mph. Because the brain is made of mostly fat, it is impossible for humans to have 0% body fat.

Fig. 2

O= Na+ gate ion channels
X= Closed gate

At REST: the nerve is not sending a message. There is a difference in charge between the inside and outside of the nerve. + charged outside and - charged inside. High concentration of K+ (potassium) on the inside, high concentration of Na+ (sodium) on the outside. The charge difference comes from there being much more sodium on the outside than potassium on the inside.

Fig. 3

O= Na+ gate ion channels

X= Closed gate

Triangles= K+ gated ion channels

1st step of a nerve sending a message (action potential) is called depolarization. During depolarization Na+ rushes into the cell. This is a passive process: it does not take energy. These ions naturally move from high concentration to low concentration. At this point the charge is reversed to + on the inside and - on the outside.

Fig. 4 K+ rushes out of the cell from high concentration to low concentration.

If your muscle is picking up a light weight, your nerves don’t have to fire often. Maybe a couple of times to get it to contract. If you are picking up a heavy weight, your nerves will send many messages to:
1. Continue muscle activity
2. Strengthen the contraction of the muscle

To do this we must get the nerve back from re-polarization to Rest state. This takes energy in the form of ATP and requires use of the Na+/K+ pumps. For every 3 Na+ it pumps out, it pumps 2 K+ into the cell. It takes more energy to do this while we are learning a movement pattern than if we already know it very well.

This whole process happens as a chain reaction down the length of the nerve axon until the message (action potential) reaches the muscle.

(cliff’s notes of the other drawings)

Please see book or this link for a picture of the Neuromuscular Junction. http://en.wikipedia.org/wiki/Neuromuscular_junction

The Neuromuscular Junction:
The NMJ (neuromuscular junction) is the place where the motor neuron stimulates the muscle cell.
The muscle and the nerve do not touch. How do we get the message from the nerve to the muscle?

The action potential moves down the axon to the NMJ. This forces Ca+ to be taken into the end of the axon which triggers the vesicles to combine with the cell walls and release their Acetyl choline (ACH) into the gap. ACH binds to ion channels on the muscle causing an action potential.

The action potential runs down the length of the muscle and down the t-tubules (nerves that are located in the muscle). Inside the cell, the sarcoplasmic reticulum (that stores Ca+ ions) receives the action potential which triggers the SR to release its Ca+ into the muscle.

This Calcium then binds to troponin, changing the configuration of itself and tropomyosin, rolling the tropomyosin of of the active sites on actin. The cross bridges of the myosin are immediately attracted to the active sites. With the release of energy from ATP the cross bridges act spontaneously binding with actin’s active sites, and bending backward (power stroke) to pull actin along. This is the Sliding Filament Theory. ACTIN SLIDES OVER MYOSIN TO CAUSE MUSCLE CONTRACTION.

Sliding Filament theory: A muscle shortens and lengthens when the thin/actin filaments slide past the thick/myosin filaments without the filaments changing length themselves. See pages 5-7 of the class packet.

Actin: thin filament
Myosin: thick filament
Tropomyosin: located on actin and cover the active sites.
Troponin: located on the tropomyosin
Actin active sites: where myosin cross bridges connect to slide actin over it.

For more information outside your text, I have found these links to be helpful:

Sliding Filament Theory

http://www.miramar.sdccd.cc.ca.us/faculty/kpetti/WebImages/sliding_filament_theory.jpg

How Stuff Works link for How a muscle contracts (nice pictures)

http://health.howstuffworks.com/muscle2.htm

Just an interesting link

It's an article on active.com about how much protein you should take in after workouts.

http://www.active.com/nutrition/Articles/Protein__Pros__Cons_and_Confusion.htm

The Recovery Process 9-17-07

First Test is on October 1st and includes everything covered from the 1st day up to and including 9/17/07. Chapter 3 in the NSCA book is also included.

THE RECOVERY PROCESS
Y axis: VO2
X axis: Time

Graph 1. Pg. 4 of packet

Walking test of 3 mph on a treadmill for 30 min. The vast majority of energy for this activity comes from the Oxidative (oxygen system, aerobic system) System. During the test, the individual’s VO2 was tested (NOT VO2 max, but VO2 sub maximal value) i.e. How much oxygen is being consumed and utilized per minute. Average resting VO2 for most healthy humans is 3.5 ml/kg/min. This is referred to as 1 MET (metabolic equivalence). For example: if someone were to breath at 10 METs their VO2 would be 35 ml/kg/min. The kg in the equation is the individual’s body weight. So total oxygen consumed will be very different for different people but amount of oxygen consumed per kg will stay the same during rest.

1.As soon as the individual gets onto the treadmill their breathing rate will increase rapidly to meet increased oxygen needs in the body as activity goes from rest to exercise pace. This stage is referred to as “Oxygen deficit.” Oxygen deficit is when oxygen requirement is not being met at the beginning of exercise. (see yellow)

2.The breathing level will stop increasing and reach “steady state.” Steady State is when oxygen demands are being met by oxygen uptake. This is the feeling athletes get when they say “I feel like I could go forever.”

3.At this point the individual completely stops activity and returns to rest state. Theoretically, resting VO2 would be 1 MET. In this case (directly following exercise) the VO2 stays elevated.
This is called: EPOC (excessive post exercise oxygen consumption), O2 Debt, or Recovery O2 and is defined as that amount of oxygen uptake after exercise above that normally consumed during rest. (see blue)

There are two stages of Recovery O2/EPOC/O2 Debt for ANY activity:
Oxygen in these stages is used for completely different things to help your body recover after exercise.

1. Fast decrease or the fast component of Recovery O2. Aka Alactacid (without lactate)
In this stage, there is a rapid decrease in breathing rate. (Specific times vary by individual).
a. CP regeneration (re-synthesis) occurs (for anaerobic & aerobic athletes)
b. Myoglobin replenishment occurs.

CP Replenishment: Figure 2.

After ATP has been broken down and the energy released has been used for muscle contraction, CP is broken down for energy to be used to re-synthesis ATP (this is called a coupled reaction because the energy from one reaction is used to drive another). But how do we re-synthesis CP? We need energy. The energy needed is produced using the oxygen we take in during the fast component of recovery.
During the Aerobic System we break down CHO, Lipids, and Proteins in the presence of oxygen into CO2 + H2O + E. The oxygen needed for this system is the oxygen you breathe during recovery. This E CANNOT be used to re-synthesis CP. It is used to re-synthesis ATP. At this point ATP is restoring quickly and begins to surplus. CP begins to deplete so the body then breaks down some of the surplus ATP to re-synthesis CP.

Fast recovery can happen during a workout also. If you are running a long distance and perform a short sprint to catch up to someone then return to your previous pace, fast recovery is occurring to replenish the CP just used.
You can recover 50% of your CP just used within 30 seconds after an exercise.
Take 2-3 minute rest between 1 RM (heavy sets) to get back almost all of your creatine stores. For muscle hypertrophy take 1 min. rest, for strength a 2-3 min. rest, and for endurance under 30 second rest.


Myoglobin Replenishment
Hemoglobin – red blood cell – carries oxygen from the lungs to the muscles and carries iron. It is stored in the circulatory system.
Myoglobin – looks & acts like hemoglobin but is stored in the muscle. It is the muscle’s own personal store of hemoglobin – it attaches to & stores oxygen – it is an emergency store of oxygen for when the muscle needs it most – for a quick start into the Aerobic System (so the muscles don’t have to wait to receive hemoglobin from the circulatory system to start activity).
For every gram of muscle glycogen the muscle stores, it also stores 3g of water.
And for every gram of creatine stored, 2.5g of water is also stored.

2.Slow decrease or the slow component of Recovery O2. Aka Lactacid
a. Breakdown of lactate (and the clearing of H+ ions)
The oxygen taken in helps turn lactate back into pyruvate
Which can then be converted to Acetyl CoA to enter the Kreb’s Cycle.
b. Meet energy demands of increased: ventilation, heart rate (HR), sweating, and higher metabolism.
Metabolism: the sum of every reaction that occurs in the body (takes a lot of energy).
c. Takes longer to recover from intense anaerobic vs. aerobic workout.

Y axis: Blood lactate & H+ ions (mg %)
X axis: Recovery Time in minutes
Figure 3.

In this study, the individuals ran one mile as fast as they could then had their blood drawn as they recovered. After 20 min. of recovery, the individuals who stopped exercising completely got rid of 50% of their blood lactate and H+ ions. The individuals that used exercise recovery (either intermittent or continuous) got rid of much more. Getting rid of lactate and H+ ions sooner especially is important for athletes who plan to continue to workout soon after their previous workout.
Lactate is NOT what makes people sore one to two days after an exercise because it is gone 20min. To a few hours after a workout. Average time to get back to resting levels for athletes using exercise recovery is about 60 min. Average time to get back to resting levels for an athlete using rest recovery (stopping all activity) is about 120 min.

There are 4 things that happen to lactate during recovery
1. It is reconverted to pyruvate
70% of lactate cleared is reused to create ATP (goes through the systems-see packet pg.2)
2. Can be converted from pyruvate to glucose-muscle glycogen
3. Can be converted from pyruvate to liver glycogen
4. Can be converted to some non-essential amino-acids. (Non-essential: your body makes them and you don’t need to get them from your diet).

Recovery of the Most Important Energy Source in the Body (regardless of what activity):
Muscle Glycogen
How efficiently you restore it affects how much you can train.
Muscle Glycogen Recovery after Low Intensity High Duration Exercise
I.e. Aerobic workouts

1. Complete restoration of muscle glycogen requires high CHO diet
This is critical for active people who train day in and day out. What “high CHO” means is relative to every person. (High carb diet for a college student might be 45%; high carb for a vegetarian might be 65% of the diet).
2. Very poor restoration of muscle glycogen if little CHO in recovery diet.
3. Two days requirement for full restoration. Only if you don’t exercise in the interim. (However most athletes never take a full two days off after exercise)
4. Most rapid restoration occurs during the first 10 hours of recovery. You must eat frequently.
5. Two hour window for immediate optimal restoration immediately following your exercise. It doesn’t matter what type of CHO you take in. It could be candy or pasta-your body uses it the same. This does not take into consideration which is the healthy choice.
The most optimal diet combination is a 3:1 ratio of CHO to protein respectively. 3g of Carbs to 1g of protein. This is ideal for the two hour window. Low fat chocolate milk has this ratio. So do legumes-pinto, garbanzo, navy beans. Slim Fast canned drinks also contain this ratio.


Muscle Glycogen Recovery after High Intensity Low Duration Exercise
I.e. Explosive anaerobic workouts
Exact Same As Above
These requirements are for physically active people regardless of modality.

Muscle Glycogen Studies 9-10-07

The Graphs are now up!

3 Energy Systems Summary


Phosphagen

* Explosive movements- i.e. a 1 time throw, dive, jump, 8 RM heavy bench press (giving an all out effort for under 30 seconds).
It is the breakdown of CP to Creatine+Phosphate+Energy. This E is used to re-synthesis ATP from ADP. This E cannot be used directly for muscle contraction. We lack an enzyme to do this.


ENZYME: Protein catalyst (predominantly made of protein).
CATALYST: Causes a reaction to occur much faster than it would normally occur.
W/o enzymes it would take hours to get from one end of the class to another.




Glycolysis: breakdown of CHO to pyruvate.
Anaerobic/fast: w/o oxygen giving 100% for about 1-3 minutes. The ultimate example of using this system is the 800m sprint.
Pyruvate is further broken down to lactate. See Pg. 40 of packet to read article about the difference between lactic acid and lactate plus the hydrogen ions.

Aerobic/slow: w/oxygen. Pyruvate breaks down to acetyl CoA to go into the Kreb’s Cycle.


Oxidative Phosphorylation/Oxidative System:
Produces the greatest amount of ATP. Long Steady Distance (LSD)- i.e. spin class, aerobics class
98% of energy for a marathon comes from the oxidative system but the sprint at the end could make or break your time and the energy primarily comes from the Phosphagen System.
Vast Majority of energy during x activity comes from x system, however all three play a role in everything we do. This is called the energy continuum.




The Energy Continuum:


The y axis (vertical)= VO2 max (VO2 max: maximal oxygen consumption. It is the single best indicator of aerobic condition-heart, lungs, circulatory system).
X axis (horizontal)= (from left to right) High I/Low D to Low I/High D athletes, where D=duration of exercise and I=intensity of exercise.


Graph 1.



As you can see, this is a linear relationship showing the ability to use the aerobic energy system.
As intensity goes down and duration goes up VO2 max (maximal oxygen consumption) goes up.
Examples of High I/Low D athletes would be: Olympic lifters
Examples of Moderate I/Moderate D athletes would be: mostly team sports
Examples of Low I/High D athletes would be: Marathon runners


The higher your VO2 max, the better the shape you are in. It is the maximum O2 a person can breathe in AND utilize per min. The low for humans is about 10 which would be someone with chronic obstructive pulmonary disease. The highest ever recorded in a human was 90 which was a male cross country skier who uses both upper and lower body. Units are ml/kg/min. Altitude negatively affects VO2 max testing and corrections to the equation need to be made. (Fatigue sets in faster at higher altitudes)



The best way to test VO2 max is with a tread mill and a Douglas bag (to collect exhalation). Run to near exhaustion on a treadmill and breathe in room air and out through a one way valve into a Douglas bag. It is known that there is 20.9% oxygen and 2% CO2 in “room air“. The VO2 max measurement comes from analyzing the difference between the percentages in air inhaled and the percentages in the bag that you have exhaled. Percentage oxygen in the bag should get lower and lower with increased aerobic shape. CO2 should be higher in exhalation than in the “room air“.




How would you test the anaerobic glycolysis system? 800 m dash. Must measure time and H+ ions in blood. Must draw blood. If you are in good shape you would be able to tolerate high levels of H+ ions.


Y axis: H+ tolerance
X: Same as above


Graph 2.



The athletes who’s primary sport involves either VERY short D (high I) activity or VERY long (low I) activity struggle while the moderate athletes thrive.




What about the Phosphagen System? Really explosive activity for 30 sec. Must measure creatine levels. The best test for this is running up a flight of stairs. About 12 stairs. If you are in good shape, your blood will contain a LOT less creatine after your sprint than before it.
Y axis: Creatine usage
X: Same as above


Graph 3.





Olympic Lifters perform this test the best because they have trained this system well even though they don’t run. Quickness, Explosiveness.
However, as I decreases and D increases in athletes, so does tolerance in this system.


The energy continuum concept: energy system utilized during give activity depends on characteristics of that activity.
High intensity low duration = anaerobic
Low intensity high duration= aerobic system
Very simplified.




Energy Nutrients: can break these down to get E for physical activity. Aka “Food stuffs”
1. Protein
2. CHO (Carbohydrates)
3. Lipids/Fats


Protein does not play an important role in giving us energy for exercise. It is not a good idea to break down muscle for energy. Usually don’t get more than five percent or less energy from protein for any exercise. For long distance activities you can get 5-10% from protein. Ironman: 10-15% from protein. Other athletes that may start using more significant %s of protein are low cal dieters. They must increase their protein intake so as not to let their bodies us their own muscles as an energy source. For the athlete, carbs are by far the most important form of energy.


For every 1 mile you run you burn about 100 kcals. If CHO=X and Lipids=y and protein is not a factor as an energy source, then: x+y=100 kcals.




Factors affecting CHO vs. Fat utilization
1. Intensity/duration
2. Diet prior to activity


The left Y axis: % CHO used as fuel
The right Y axis: % Fat used as fuel (with 0 at top and 100 at bottom-inversely related to CHO)
X axis: Low to high intensity (left-right) and long to short duration(left-right)


Graph 4.



As you increase intensity and shorten duration from walking (left) to sprinting (right), the energy use of 50/50% CHO/Fat goes to almost 100 CHO and no Fat usage.
At rest you are getting 1/3 of energy from CHO, and 2/3 from fat. As intensity increases and duration decreases, your body prefers to use more and more CHO. To lose weight, you must be decreased intensity and increased duration.

Y axis: % of fuel supply
X axis: min of work

Graph 5.

During long duration exercise, CHO are the predominant energy source at first, over fat; However, fat usage slowly becomes predominant as the activity continues. It takes a while for the fat burning to kick in. For fat burning to kick in, fat has to go through beta oxidation, then the kreb’s cycle, and then the electron transport chain. That takes time. Must rely on CHO even at low intensities. Takes about 20 min for fat burning to kick in. If you are in great aerobic shape, it can take only minutes for fat burning to kick in. It could take 30 min. for the average out of shape American to get into fat burning (non-aerobically trained athlete).


Intensity is relative to your aerobic level, thus duration is also. Tom can run at 70% of his VO2 max for 2 hours and be burning mostly fat. However, if someone who doesn’t run, tries to run at 70% of their VO2, they would only be able to do this for 15 min and be burning mostly CHO.

3 major places we store CHO (forms)
1. Liver glycogen
2. Muscle glycogen
3. Blood glucose


All CHO need to be broken down or be converted to glucose to be used by the human body. The body is very strict about the levels of glucose it keeps in the blood. So it stores it as glycogen-a polymer or glucose molecules linked together-in the muscles or liver. Of the three stores, the most important by far for physical activity is muscle glycogen. Regardless of if you are an aerobically trained athlete or a resistance trainer.


Factors affecting muscle glycogen utilization:
1. Intensity of exercise
2. Duration of exercise
3. Mode of exercise- type of activity e.g. bike, run, swim, etc.
4. Fitness level- muscle glycogen use becomes more efficient for better trained athletes. Better trained athletes will use much less muscle glycogen.
5. Muscle fiber type-slow/fast twitch. How much slow/fast twitch you have is determined by genetics and cannot be changed.


Left Y axis: % CHO used as fuel 100-0 Right Y axis: % of fat used as fuel (inversely related to CHO-0 on top, 100 on bottom) X axis: Hours of running from 0-4 hours









Graph 6. The type of food, fats or carbohydrates, affect how we perform during exercise and what type of energy source is available to our body during that exercise. CHO is by far the most important food stuffs for active ppl. In this study, athletes performed at 70% of their VO2 max for as long as they could. Fatigue onset corresponded with muscle glycogen depletion. During long duration activity of constant intensity (LSD) fatigue sets in when muscle glycogen was depleted. Using predominantly fat during this test. Glycogen usage decreases quickly. Even though most of your energy comes from fat, muscle glycogen is the limiting factor and creates fatigue. Fat utilization is highly determined by CHO levels.




Graph 7.
Y axis: Muscle glycogen content (decreasing in downward direction)
X axis: Minutes on bike machine


This study was done with the athletes performing at 70% of their VO2 max as long as they could. Measured by biopsy (cutting) of muscle. Fatigue onset corresponded to muscle glycogen depletion. During long duration activity of constant intensity (LSD), fatigue onset occurs when muscle glycogen was depleted.


Graph 8.
Y axis: Muscle glycogen content of muscle (decreasing downward)
X axis: Day A, B, C, D with intensity increasing to the right


These athletes performed day A at 50% of VO2 max- low intensity for 2 hours-didn’t use much muscle glycogen.
Day B was at 60% for 2 hrs. and intensity was raised on each of the four test days (with rest days between). At 90% the athletes could not complete the 2 hrs. and muscle glycogen depletion was great. Muscle glycogen is still the limiting factor. The athletes still had enough fat to use but fatigue set in when muscle glycogen was depleted.

In conclusion to these studies: As intensity increases,; greater use of muscle glycogen to depletion/exhaustion occurs. As duration increases, greater use of muscle glycogen to exhaustion occurs.
DISPITE FAT BEING UNLIMITED.

Y axis: Muscle glycogen content (decreasing downward)
X axis: # of sprint bouts from 0-6

Graph 9.

As intensity of exercise increases, the amount of muscle glycogen utilized also increases. Although glycogen is a primary fuel during sprint bouts, plenty of glycogen remains at exhaustion. (gollnick and et all 1973)

These athletes had their VO2 max tested before this study. The athletes would sprint for 1 minute at 10% above their VO2 max with 10 min. breaks. (Supramax-above VO2 max)
Group averaged about 6 sprint bouts. At the end of the test, they had at least 40% of their muscle glycogen left at exhaustion. This study was testing the fast/anaerobic glycolysis system. Fatigue set in because of the accumulation of H+ ions, NOT because of depletion of muscle glycogen.
During VERY high intensity, short duration activity where anaerobic glycolysis is predominant, muscle glycogen levels are not depleted despite fatigue. High levels of H+ are the culprit of fatigue in this case.


In conclusion to these studies: Limiting factor for low to moderate intensity and long duration activity is muscle glycogen despite fat availability.


Limiting factor for very high intensity very low duration activities is the accumulation of H+ ions despite muscle glycogen and fat availability.


Fat storage can be high but muscle glycogen stores are limited. While you can store almost unlimited fat, you can only store enough muscle glycogen to jog 3 miles. You need muscle glycogen to kick start fat burning. Think of fat as a log and muscle glycogen as kindling. If you have used up all the kindling on the first log of fat and try to put another on the fire, the log will not be able to burn. It needs the kindling, I.e. more muscle glycogen. As you get into better shape your body will need less kindling to get into the fat burning.

Mode of Exercise:
In this study, the athletes were required to run on a treadmill at two resistances, level and uphill. Muscle biopsies were taken from three leg muscles, the gastrocnemius, the soleus, and the vastus lateralus and tested for muscle glycogen content for both the level and uphill sessions. The vastus lateralus used very little muscle glycogen on the level surface and significantly more on the uphill. The gastrocnemius used a lot of muscle glycogen on the level surface and somewhat more on the uphill. The Soleus used a moderate amount of muscle glycogen on the level surface and a bit more on the uphill. This shows the specificity of training certain muscles for certain activities. If you want to get ready for a marathon up Pike’s peak you will need to do a lot of uphill training. If you are training for the Boston Marathon you need to train on very flat surfaces.




There are two types of Skeletal Muscle fiber: ST(slow twitch) and FT (fast twitch).


SLOW TWITCH:
1.generate force or tension slowly, consistently, and for long distances
2. Aka red fiber, type I, slow oxidative
3. Doesn’t fatigue quickly, can last for long duration


FAST TWITCH:
1. Explosive movements
2. Aka type II, white fibers
3. Fatigue very quickly


All muscles are a combination of both. Genetics determines the percentage of both in each muscle.


Different muscles are mostly one or another e.g. posture muscles need to contain mostly slow twitch fibers as they are in use for very long durations.


Utilization of Fats
The major stores of fat are:
1.Viceral/internal fat around organs (apple/android/male type-belly & back, and pear-shaped/gynoid/female type-hips thighs & butt)
2. Subcutaneous- under the skin.
3. Blood-FFA (free fatty acids)
If you estimate someone’s subcutaneous fat you can estimate their overall internal fat storage. That is because 1/3 of our fat is subcutaneous and 2/3 is internal (this varies for different ages, ethnicities, etc.).
4. Muscle triglycerides
At Low/rest intensity- 2/3 fat burned is from blood then when that is used, the visceral fat supplies the blood with more.
At moderate I- we burn mostly visceral fat
And at High I- we burn mostly muscle triglycerides (75% of VO2 max)