Fatigue and failure of bioenergetic systems

[jacob]

I know that we have three different energy systems, ATP-PC, glycolytic, and oxidative, which can supply high levels of energy w/o oxygen for 10-12 seconds, medium levels w/o oxygen for 15-60 seconds, and low levels w oxygen forever, respectively.

What I'm interested in what causes these systems to fail and how training influences this. For example, if we're lifting a heavy weight, we're using thr ATP-PC system until we run out of ATP and PC. What causes stronger people to be able to do more reps with the same weight before running out? Do they have more muscle fibers and thus more ATP/PC in total thus using a smaller fraction?

What happens if we lower the weight and enter the glycolytic system (strength-endurance). What's the limit here? Why can some people do 3000 pushups in an hour despite not being able to bench, say 250lbs? Is this the ability to clear lactic acid? I sometimes find myself in a position where I'm not out of breath (plenty of air), yet the muscle simply won't go anymore. This is not a case of ATP/PC since I might be 10+ minutes into the work. In a way it feels like bonking the oxidative system, but yet, I'm not 2000kcal into it so there's plenty of glycogen left.

So what's going on? Is the bottleneck at the chemical level or further up in the system. Heart rate is high enough, but breathing is not out of breath. Glycogen not getting into the cells fast enough? Something else (H+ acid) not getting out fast enough? Whatever muscle group is being worked on is just "exhausted" for lack of a better word.

[Alphaville]

Just stopping by quickly to say: beyond biochemistry look also at muscle fiber type distribution.
E.g.: https://www.bodybuilding.com/fun/kelly13.htm

[black_son_of_gray]

Some thoughts, hope they steer you in informative directions:
Years ago, before I had trimmed back dramatically the number of books on my shelf, I owned this book by Sebastian Coe. One figure from that book that has stuck with me was an incredible side-by-side comparison of muscle biopsies taken from the legs of untrained vs elite runners. The elite sample was jam-packed with capillaries and mitochondria to an almost ridiculous extent. Muscles can change dramatically over the long term with training. Quite eye-opening.

The reason I bring up that book, too, and books like it (specifically geared towards elite two-miler/5K/10K runners) is that they tend to go specifically into how to train different systems and the details of the biochemistry involved. IIRC, the average elite 5k runner runs their race at just above 100% of aerobic capacity. They literally run as fast as their cardiovascular systems will allow, with brief "kicks"--typically at the end--that take them as deep into lactate as they can on top of that. They preferentially target each system with different training intensities (e.g. Threshold, Interval, and Repetition paces.)

What I'm interested in what causes these systems to fail and how training influences this. For example, if we're lifting a heavy weight, we're using thr ATP-PC system until we run out of ATP and PC. What causes stronger people to be able to do more reps with the same weight before running out? Do they have more muscle fibers and thus more ATP/PC in total thus using a smaller fraction?

Amount and type of muscle fibers matters here, as does the specific body proportions (i.e. lever arms) that each person has, but two other important factors are: neural training and/or economy. There's what's called the "size principle." Neural training in part means getting more bang for your muscle buck simply through better/more effective coordination of muscular contraction. [Less "paradoxical contraction" where antagonistic muscle groups fire against each other, although as I google that term now, I get a lot of hits for something somewhat different... ] With respect to economy, once again from the realm of distance running, it turns out that highly trained runners become very efficient/economical at using the prime-mover muscles, and as little as possible to get the job done. More generalist athletes (even comparably conditioned ones, like e.g. triathletes) will show more "effort"/less neuromuscular efficiency during a run.

The flipped argument is also worth considering: What is the weakest link--or in this case, muscle--of the movement? As a personal example, I couldn't quite figure out why my barbell overhead press strength was so weak relative to other lifts ... until I started doing lots of dumbell overhead work. Just holding a weight overhead for a few seconds revealed how weak a bunch of tiny shoulder stabilizers were. Ditto with doing a bunch of hip abduction/adduction exercises. It only took about 10 seconds of Jane Fonda-style thigh-busters for me to feel an embarrassing burn. Not sure if this is your issue, but it could be worth exploring small but essential rate-limited muscle groups. The neuromuscular system has "checks" in it that won't allow you to apply strength that you actually have if it means risking injury. [This also happens with heat generation. Ever notice how badly you feel/perform when your body gets really hot? Your brain is literally not letting you perform as well as your muscles could do in order to prevent you from cooking yourself.]

All these energy systems are going on to some degree all the time, and during a bout of exercise the relative weight of each system is dynamically changing. And some people have vastly different capacities in each realm that bizarrely kinda add up to a similar performance. I've read about comparisons of world-class distance runners who had comparable PBs but had dramatically different VO2max and lactate capacities. [E.g. do a Google dive into Dean Karnazes' insane processing of lactate.] My point being, to improve performance at a given task, you might find yourself needing to expand capacity of one of these systems, and that might be a very different system than someone else of similar skill/conditioning.

What happens if we lower the weight and enter the glycolytic system (strength-endurance). What's the limit here? Why can some people do 3000 pushups in an hour despite not being able to bench, say 250lbs? Is this the ability to clear lactic acid? I sometimes find myself in a position where I'm not out of breath (plenty of air), yet the muscle simply won't go anymore. This is not a case of ATP/PC since I might be 10+ minutes into the work. In a way it feels like bonking the oxidative system, but yet, I'm not 2000kcal into it so there's plenty of glycogen left.

I'm out of my depth here, but it doesn't seem contradictory to me that you could have both 2000kcal in glycogen reserves and also not enough glycogen/ATP available in the relevant muscles. All you need is the rate of utilization in the muscle to be >> than the rate of supply (e.g. from the bloodstream) or regeneration locally within the muscle. In other words, you could be bonking the local oxidative system. Do you feel a burn in muscle that just won't go?

I am not an exercise physiologist, though, so grain of salt yadda yadda.

[IlliniDave]

It might be mental, and perhaps looking into experiences of graduates of programs like SEAL training or US Army Ranger School will provide some useful insight. I used to do Crossfit with a good contingent of the latter (including my first two coaches). What they consistently said was you can push your body much harder than you think, and that people who learn to tap into that mental toughness are the ones who succeed in such programs more so than the best conditioned/most athletic candidates. Exactly what the chemistry behind that is, I don't know. But if those anecdotal experiences have credibility, it might be that the brain signals, "OMG, stop before you kill yourself" stronger and faster for some of us than others.

My own experiments pushing beyond my "mental wall" didn't turn out well, but I was in my mid-late 40s, and mental toughness of that sort probably is not my strongest suit.

[Alphaville]

yep, the mental wall is there for a reason—uncle rhabdo will overhwelm your kidneys.

my #1 rule of fitness these days is *avoid injuries*. nothing hampers training like being bed ridden, medicated, paralyzed, and in pain.

[jacob]

To give an example, lets take a pair of small dumbbells, say 10lbs each. Sit on a chair and start doing shoulder presses keeping a constant cadence. I predict conking out after 5-10 mins of that without being out of breath. Unlike walking, it's not possible to do it all day. Was this just mental? Is there a small stabilizer muscle that finally gave out? A simpler example, just hold your arms level straight out to the sides finger tips point for time. (You're now an airplane!) Why is it not possible to do this forever? When lifting weights one can also do inverse pyramids (I think they're called IIRC) when one does weights like 100-90-80.... all the way down to the point when one can barely lift ones arms.

Back when I was trying to "get big" on standard 3x10 routines, I tried creatine. Creatine adds more C to the ATP-PC system. PC gets used up 1-2 seconds after ATP is depleted and creatine gives you an extra second or two for 1-2 added reps. Maxreps with a heavy weight seems clearly related to depleted ATP-PC reserves which needs to recharge on a timescale that is longer than the rep time. I did manage to google enough to learn that the ATP-PC system delivers more energy than the glycolytic which again delivers more than the oxidative, so maybe there's a "maxrep" for each system and a corresponding recharge time. That's what I'm getting at.

[basuragomi]

I'm no biologist, but commanding a muscle relies on the flow of chemical ions at the nerve-muscle junction. Since it's a chemical flow it presumably relies on differing chemical potentials. It makes sense to me that as exertion continues the system gradually reaches equilibrium and the signal drops below the response threshold (or saturates), causing the muscle to cease contracting. There's probably lots of other rate limiters like neurotransmitters and oxidative capacity but that should be one fundamental limit on the control system.

[Alphaville]

local muscle fatigue is independent of overall cardio load

see: https://en.m.wikipedia.org/wiki/Muscle_fatigue

[IlliniDave]

jacob, I don't know the answer to your question. Maybe had a person started doing the dumbell exercise as a toddler and did it every day for the same amount of time they walked each day, for their entire lives, adaptation to the motions would be similar? Or conversely, what happens when an incline is added to the walk?

How some people are able to display "superhuman" combinations of strength and endurance may not be terribly dissimilar to how humans sometimes "overachieve" physically when fear kicks in. There appears to be normally untapped potential in our physiology. Maybe looking at cases where people seem to get around the built-in limits would shed some light on the limits.

[fiby41]

Why is it not possible to do this forever?

Lactic acid? It's build up/accumulation.

[Alphaville]

This is just sorta like your brakes overheating while your engine is running at low rpm. It’s a local failure, not a total system overload. Muscle contractions only overwhelm the cardiovascular system when exerted in large groups, e.g. rowing or swimming or etc.

[Lemur]

@Jacob

You may like the training articles posted by Lyle Mcdonald http://www.bodyrecomposition.com . I think if you search around here you might find some answers. He can certainly explain it better than I can.

[white belt]

@Jacob

Training-induced fatigue has 3 primary proximate causes: substrate depletion, neuroendocrine alterations, and microtrauma. They each play a role in adding to fatigue, and must of course each be addressed if fatigue reduction is a goal.

When you train hard and heavy, you use up fuel (substrate) to do so. If you perform a heavy squat, your levels of ATP right after the squat are lower than they were before you began, which is the primary reason that the next rep will be harder. Resting for several seconds usually replenishes acute ATP stores. If your set lasts for multiple reps, your creatine phosphate (CP) levels will drop, and that will cause acute fatigue as well, this time taking several minutes to recover from, as levels of CP return to normal. Lastly, if you train for multiple sets and reps, glycogen is used to generate energy to recover your ATP and CP stores. Now, glycogen doesn’t replete in just minutes, and a voluminous workout can require up to several days of eating carbs to fully replete the glycogen used up.

See here: https://www.jtsstrength.com/fatigue-explained/

I also think you will find Stronger by Science to be a useful resource. They have a strength training guide that serves as a jumping off point for some of your questions: https://www.strongerbyscience.com/compl ... ing-guide/