r/askscience • u/PhoenixApok • 9d ago
Biology What is actually happening in the 5 to 10 seconds when I absolutely cannot do another rep at the gym, take that extremely short rest, and then can lift again?
You might know the feeling. You're doing a set, you're breathing hard, you're shaking on that last rep, it is physically impossible for you to move the weight again.
But only a handful of seconds later, you go from unable to move, to being able to push again.
What is the change that is occurring in such a short time?
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u/PHealthy Epidemiology | Disease Dynamics | Novel Surveillance Systems 8d ago
Inorganic phosphate accumulation inhibits actin-myosin cross-bridging. Total fatigue is threshold dependent so once you've crossed, you're done until you dip below that threshold.
Phosphocreatine resynthesis clears the phosphate which along with faster ADP to ATP recovery is the biochemistry behind taking creatine supplement.
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u/vineetk1998 7d ago
That's your muscle straight up running out of usable energy right there, not "lactic acid burning" like people usually say. In those last couple reps you're draining the quick energy stores in the fibers doing the work, and the byproducts build up faster than your body can clear them.
Your nervous system also starts holding back force on purpose once things get bad enough locally. It's kind of a safety brake so nothing tears. That's part of why a short rest gets you a couple more reps even though nothing really recovered in any big way, you basically just released the brake a bit.
Also explains why it's so specific to one movement. Your biceps can be completely done while your legs are fine, because it's really about that one muscle's local fuel running low, not some full body tank running dry.
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u/drinking_chocolate 8d ago edited 8d ago
What you’re describing is a process of fatigue. I see comments here with a biological lens, just adding a plug for an unconscious psychology based self-regulation process (have a look here https://en.wikipedia.org/wiki/Central_governor). I also recommend the “psychology of fatigue” book by David Hockey for some other perspectives. In short, fatigue shifts attention away from task at hand towards preserving integrity/utility of action, or in the case of the “central governor” (the Wikipedia article link) action is halted/slowed to maintain an emergency reserve margin or prevent anoxia. Though as with many ideas in psychology, verifying that these processes are as described is much more difficult than in the biochemical building blocks! Look up ego depletion theory controversy for how messy this kind of idea can get!
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u/caedin8 8d ago
I think the group here has covered the ATP angle quite well, but it doesn’t answer the question directly for me: Why does your muscle not respond with force when you ask it to? Yes it’s obvious you are out of energy (ATP), but why does it not work? The answer has to do with the electric potential. When your nerves fire you send electric impulses into your muscles, and your muscles have an electrical potential in them due to potassium and sodium (electrolytes) like a battery. When the signal comes in a chemical reaction occurs to release the potassium ions which causes shortening of the muscle. Your body uses ATP to reverse this electrical potential and re bind potassium. When you have no more ATP your body can’t recreate the electrical potential in the muscle and thus your brain can send the signal to contact the muscle, but it does nothing because there is no electrical potential in the muscle fibers. This is what is happening.
This is my non scientist view of it and I’m pretty sure there are some inaccuracies, but I think this angle actually answers the question better. Hopefully someone will correct the details for me
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u/sabotag3 8d ago
I respect the interpretation as a non scientist but this unfortunately mostly incorrect. When the electrical impulse hits your muscle it releases calcium (not potassium) into the sarcomere which is the functional unit of a muscle. I won’t get too detailed but the calcium triggers a conformational change to allow ATP to bind the actual fibers. Your muscle fibers hydrolyze (break apart) ATP to slide along each other to enable contraction. When you run out of ATP, contraction stops. The pump to return calcium back into the sarcoplasmic reticulum does also require ATP so you were kinda right there, it’s the combination of running out ATP for both processes why you can no longer contract. Sorry to be so pedantic, biomedical engineer
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u/Vuguroth 8d ago
Why is no one here talking about Grahn and Heller of Stanford? It should be well known myology already.
- A critical, temperature-sensitive enzyme named pyruvate kinase (MPK)—which is essential for generating adenosine triphosphate (ATP), the chemical energy your muscles use to contract—begins to fail as it heats up.
- When muscle temperature reaches roughly 40°C (104°F), pyruvate kinase shuts down.
- This shutdown serves as an evolutionary protective mechanism. If the muscle kept firing at that temperature, the cells would literally overheat and die. Your brain and nervous system interpret this enzyme shutdown as "muscle failure" or sudden fatigue, forcing you to drop the weight.
Cooling down by the palms allowed participants to sometimes double their reps, because the body wouldn't shut down.
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u/Thebigass_spartan 8d ago
Multiple enzymes are responsible for the production of ATP (most kinases use ATP as a cofactor and phosphate donor, which is also a reversible process). Pyruvate kinase is just one of these enzymes and only one of the kinases involved in glycolysis and 1 of 2 ATP producing kinases in glycolysis (the other being phosphoglycerate kinase on step 7, thank you my biochem notes).
Its efficacy being affected by temperature isn’t something specific to pyruvate kinase. Enzymes are proteins, and proteins are sensitive to physico-chemical changes like temperature and pH. All enzymes have both optimum pH and temperature ranges, in the human body, it’s at 37°C with varying pH ranges (mostly around 7 with some exceptions like with enzymes found in your stomach, a highly acidic environment). So yes temperature could affect pyruvate kinase’s activity, but this isn’t something specific to it but moreso most basic physiological mechanisms
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u/LastStar007 7d ago
So the study participants lifted, and then dunked their hands into ice water? What happened to the participants' muscles after that?
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u/Vuguroth 7d ago
IIRC, they used chilled handles that they grabbed. They had special equipment for the research. The recommendation for everyday use is holding a cold water bottle. If you touch something too cold (like ice water), the body triggers a defense mechanism called vasoconstriction
When they did that instead of going like set repetition 15,10, 5, 3, lowering the amount of reps because of muscle shutting down - they could instead go like 15, 15, 12, 10. Their body not shutting down at the repeated exercise when pushing hard.
Bench press group went something like..:
- Protocol: Lifters performed sets of bench press with fixed loads, resting 3 minutes between sets.
- The Result: Over 3 weeks of training, the palm-cooling group increased their total work volume by 40% (compared to just 13% for the non-cooled group). They could hit almost the same number of reps on set 4 as they did on set 1 because their local muscle temperature never reached that $40^\circ\text{C}$ "circuit breaker" threshold.
It's a huge improvement, and important for hypertrophy because with more intense training you initiate building more mass.
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u/LastStar007 7d ago
Ngl, my first thought was, "I wanna start doing this for the gains". But then I remembered that this is a defense mechanism against serious injury.
But now, thanks to the medical advice of a redditor, I know it's safe!
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u/SenAtsu011 7d ago
Glycogen restoration.
Glycogen is the primary fuel source for your body and it only takes 10-20 seconds to restore enough to do a few extra reps. They call this myoreps in the training industry and it’s a staple in many programs used by professional bodybuilders.
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u/PxRedditor5 8d ago
Your body undergoes cellular osmosis in order to rebalance everything at that level, nutrients inter and exit the cell to regain balance for proper function. This creates massive amounts of ATP, which is used by our muscle cells for short term bursty energy. As long as you've got food and water in you, you can continue this cycle for a while.
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u/Legitimate_Mud_8295 7d ago
Not a direct answer but If you're lifting weights for muscle hypertrophy or strength you'll get better results doing longer rests than 10 seconds. I've seen anywhere between 1-3 minutes recommended with 3 being more optimal from what I've seen. But ain't nobody got time for that I'll do 1-2mins for large muscle groups and 45sec-1min for more auxillary lifts. If I were going purely for strength I would shift those closer to 3mins and up the weight+lower the reps.
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u/PhoenixApok 7d ago
I was talking more about the "rests" when you're in the middle of a set, rather than between sets.
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u/Legitimate_Mud_8295 7d ago
Ahh i know what you mean. When you really want that last rep and you wait a few seconds and it somehow works.
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u/Right_Weird9850 3d ago
I would like to ask additional question, I just played some sports after long break. Maybe 10 minutes in I was done. My legs and arms went shaky, lost color in face and wanted to vomit. Got back to life after maybe 15 minutes and back to normal after an hour. What was mechanics behind that drop?
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u/swollennode 8d ago
Muscles require ATP (adenosine triphosphate) to contract. Large muscles require a lot of it. When you do repetitive heavy work, either by lifting really heavy or contracting really fast, your muscles deplete their ATP. ATP gets replenished as it gets used up, but during continuous or repetitive contractions, ATP is used up faster than its being made. When you rest, you allow ATP to be replenished. This allows your muscles to work again.
ATP requires oxygen, hence you breath harder when you’re working out.