I believe there was a chart posted in this forum that showed what fuel was being burned for energy (fat/glycogen) depending on what rep ranges were being used. Does anyone remember this or can point me in the right direction?
I can’t remember if coach posted it.
Thank you.
There are no normal rep ranges where fat is significantly used for fuel.
It’s either the phosphagens (ATP and creatine phosphate) or glucose.
Up to a 3 minutes of continuous intense effort, you don’t really use fat for fuel (unless you are completely deprived of glycogen).
This is a rough estimate of how much fuel can each energy system provides.
Phosphagens = 4-10 seconds (up to 15-20 with creatine supplementation)
Anaerobic alactic (making fuel without oxygen and without producing lactate) = 10-40 seconds… main energy sources are ATP-CP and muscle glycogen
Anaerobic lactic (making fuel without oxygen and with lactate production) = 40-120 seconds … using mostly glycogen
Combo anaerobic lactic and aerobic = 2-4 minutes … using glycogen and recycling lactate for fuel.
After the 3-4 minutes mark is when you start to use fat for fuel to some extent.
CAVEAT: the first factor that determines which energy source is being used is the intensity of effort/muscle contraction (the body doesn’t know in advance how long the effort will last).
To make things simple, the shorter duration an energy system can last (e.g.phosphagens is really short, anaerobic lactic is mid-duration, aerobic is long duration) the slower it produces energy.
First understand that the ONLY fuel the muscles can use is ATP. If cannot use fat, glycogen or amino acids… the energy systems will take the fat or the glucose (or ketones) and make ATP out of it.
The fast/powerful energy systems make ATP rapidly but don’t last long (they are thus the dominant systems during weight training) and the more resistant systems make ATP a lot more slowly (so they are not powerful enough to fuel the intense muscle contractions needed when lifting) but they last a lot longer.
For example the phosphagens and anerobic alactic systems produce fuel (ATP) SUPER FAST whereas the aerobic system takes a long time to make ATP BUT has almost an unlimited supply of it so that the muscles can use it for fuel.
The more intensely your muscles need to contract the more rapidly they need the fuel (ATP). So when you are weight training, the default energy systems are those that provide energy rapidly.
Even moderate weight lifting (like what you can lift for 15-20 reps or more) is intense enough to make the body use one of the anaerobic systems, which means that you do not rely on fat for fuel.
If you wanted to use weight lifting in such a way that fat is used to a significant extent, the intensity would be so low that it would not do anything for muscular development or strength. And your sets would need to be at least 4-5 minutes long. Might as well just do cardio.
This is very helpful, thank you.
What about between sets during weightlifting? Example: if you were performing giant sets. You’re burning mainly glycogen during the sets but your body is burning more fat for energy during your rest period between sets, correct?
Well yes. But technically that is true pretty much all the time except when doing high intensity exercises. For example, the activity that leads to the highest proportion of energy coming from fat is sleeping. Of course you don’t burn much energy while sleeping but most of it comes from using fat to make ATP. The lower the intensity of an activity is, the higher the proportion of fat use to make energy.
Now, between bouts of high-intensity activity, it is true that the aerobic system (which uses both fat and glycogen) is used to recover.
And I guess that doing stuff like circuits and giant sets can increase overall energy expenditure because it increases adrenaline and cortisol more (both increase energy mobilization and adrenaline increases the rate of energy use by various tissues).
BUT I want to mention a few things. Because from reading between the lines, I get the sense that you are trying to find a way to use weight lifting as a main fat loss tool, which I personally think is a mistake, especially if you are already using other strategies to lose fat (restricted diet, cardio).
In the grand scheme of things it doesn’t matter where the energy comes from when training. It’s the total energy expenditure that will determine how much fat you lose at the end of the day. Let me explain: when you use more energy than you ingest in a day you have to rely on your stored energy for fuel. If you “burn” 1000 calories in your workout, even if it all comes from glycogen it still largely contributes to establishing a caloric deficit, which will “force” the body to use its stored energy. And if you used up a lot of your muscle glycogen for fuel during your workout you won’t have a choice but to use fat during the rest of your day (anyway, during your day-to-day activities, fat is the primary fuel source). Not only that but if you empty up muscle glycogen stores during your workout, the carbs you eat are much less likely to be stored as fat as they will primarily be used to fill up muscle glycogen stores. Take the opposite scenario: you find a way to directly use a lot of fat during your workout but consume a caloric surplus during the day: you’ll still gain fat (or not lose any). So really, worrying about finding a way to directly use more fat when training is a moot point.
The main danger that comes from trying to design a lifting workout to be a fat-burning tool is that it will lead to excessive cortisol and adrenaline levels. This can have several side effects ranging from “training burnout” to losing muscle mass and even halting fat loss (too much cortisol, chronically, will reduce the conversion of the T4 thyroid hormone into the T3 hormone, leading to a decrease in metabolic rate). Excess cortisol will also lead to water retention, and while this is not truly fat, it looks just as bad.
The main reason people fail to transform their body is that they want to transform their body too much. They are emotional about it. They want the fat gone YESTERDAY. And that leads to being excessive. Too much caloric restriction, too much training, too much cardio. And hormonally speaking this can wreck havoc on your efforts. It will work for a few weeks, which gets you addicted and convinced that it’s the way to go. But once metabolic adaptations sets in, you’re screwed. Forget sleep, no libido, motivation drop, cravings galore, water retention, muscle loss, halting of fat loss, etc.
It is my personal belief that the role of a lifting workout when trying to lose fat is to preserve or increase muscle mass. Dieting and cardio/energy systems work are the main tools to increase fat loss.
That is not to say that the lifting workout will not have a positive effect on fat loss. It will. It increases insulin sensitivity, it does burn some calories and by preserving or increasing muscle mass you go a long way into preserving a higher metabolic rate.
This is tremendous info. Thanks.
I was most curious if lifting helps up-regulate fat mobilization enzymes and also helps you become a better fat oxidizer, just as steady state cardio. Is this the case?
Not really. It’s main effect are on improving muscle insulin sensitivity and the efficiency of the anaerobic pathways.
Even if you were to use a mixed regimen with a circuit-style training, glycogen would still be the primary system used and thus would receive most of the adaptations.
Also, weight training tends to focus on the mTOR pathway, which is antagonistic to the AMPK pathway (one of the main enzyme responsible for fat mobilization).
In the world of strength and conditioning there is a thing called “adaptation interference”: when you train aerobic and anaerobic energy systems in the same session, adaptation for both are much less than if you train each one individually.
Makes sense. Similar to interval training though, would a circuit style weight lifting make you better at oxidizing fat?
If I remember correctly that was the main benefit of interval trconditioning.
Oxidizing, maybe. But personally, I wouldn’t do it because it makes the workout less productive for the goals I use lifting for (building muscle and strength). I also think that circuit training tends to promote poor form (when using free-weight movements at least).
I learned so much. Thank you.