# I exercise. My muscles become larger. But... when? **URL:** \ **Category:** Factual Questions\ **Created:** [September 27, 2002, 3:01am UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811 "2002-09-27T03:01:14Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [September 27, 2002, 3:01am UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/1 "2002-09-27T03:01:14Z")
I’m in a fitness program and it’s starting to show reults–with which I am very pleased–but tonight as I was using the leg-extension machine, I suddenly wondered: Precisely WHEN do muscles grow? During the actual exercise movement? During the relaxation? When I’m digesting food an hour later? When I’m asleep afterwards? ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [September 27, 2002, 3:09am UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/2 "2002-09-27T03:09:42Z")
Your muscles grow after you exercise, when you’re resting. The pain you experience when straining your muscles is caused by lactic acid building up in the area. Lactic acid is a byproduct of anerobic respiration (your muscle cells are using all the available oxygen.) This acid causes the muscle tissue to break down and tear. Later, when richly oxygenated blood is again flowing to your relaxed muscles, they heal, coming back bigger in anticipation of having to perform similar tasks the next day. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [September 27, 2002, 3:23am UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/3 "2002-09-27T03:23:50Z")
Neat! Thank you! ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [September 27, 2002, 4:25am UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/4 "2002-09-27T04:25:26Z")
Muscle growth occurs when you’re asleep (not sure if that was 100% clear from **friedo** ’s post). Be sure to get plenty of sleep after you lift. ---
**Author:** ![PosterChild](https://avatars.discourse-cdn.com/v4/letter/p/e274bd/32.png) [@PosterChild](https://boards.straightdope.com/u/PosterChild)\ **Post date:** [September 27, 2002, 4:53am UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/5 "2002-09-27T04:53:11Z")
> [@](#): > > \*Originally posted by friedo \* > \*\*Your muscles grow after you exercise, when you’re resting. > > The pain you experience when straining your muscles is caused by lactic acid building up in the area. Lactic acid is a byproduct of anerobic respiration (your muscle cells are using all the available oxygen.) This acid causes the muscle tissue to break down and tear. Later, when richly oxygenated blood is again flowing to your relaxed muscles, they heal, coming back bigger in anticipation of having to perform similar tasks the next day. \*\* I don’t know that it’s the lactic acid that causes “break down.” AFAIK, it’s tears in the muscle fibrils that occur from the physical strain. PC ---
**Author:** ![SenorBeef](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senorbeef/32/2220_2.png) [@SenorBeef](https://boards.straightdope.com/u/SenorBeef)\ **Post date:** [September 27, 2002, 10:06am UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/6 "2002-09-27T10:06:14Z")
> [@](#): > > \*Originally posted by friedo \* > \*\*Your muscles grow after you exercise, when you’re resting. > > The pain you experience when straining your muscles is caused by lactic acid building up in the area. Lactic acid is a byproduct of anerobic respiration (your muscle cells are using all the available oxygen.) This acid causes the muscle tissue to break down and tear. Later, when richly oxygenated blood is again flowing to your relaxed muscles, they heal, coming back bigger in anticipation of having to perform similar tasks the next day. \*\* This strikes me as unlikely, but my biology knowledge is limited. Lactic acid is a biproduct for using stored muscle sugars, if I recall - glycogen or something like that. I recall that when I was low-carbing, my glycogen stores were depleted because there were no sugars to replace them, and so my muscles used some other fuels that didn’t leave lactic acid. The result was that I was consistently less sore - but I definitely grew in strength and muscle size. Lactic acid is related to soreness, but it’s not the actual chemical that damages muscles, as far as I know. I was always under the impression that physical strain actually caused the microfibers to tear, and the body rebuilt ‘reinforced’ microfibers or more of them when it healed - kind of like when you break a bone, when it heals, that part of your bone is extra strong and thick. I might be way off, but it’d seem odd that an acid your body creates would damage the muscles, rather than the strain. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [September 27, 2002, 12:57pm UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/7 "2002-09-27T12:57:55Z")
Exercise = destroy muscle Rest = build muscle and replace with more Exercise = destroy muscle Rest = build muscle and replace with more Exercise your bigger muscles Rest = replace bigger with even bigger. ---
**Author:** ![KidCharlemagne](https://avatars.discourse-cdn.com/v4/letter/k/ed8c4c/32.png) [@KidCharlemagne](https://boards.straightdope.com/u/KidCharlemagne)\ **Post date:** [September 27, 2002, 1:29pm UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/8 "2002-09-27T13:29:51Z")
The idea that lactic acid causes DOMS is a myth. While the cause of DOMS is not specifically know it’s believed to be caused by the microtears themselves or the inflammation caused by the repair of those fibers (which is the one I believe). Lactic acid causes muscle fatigue during intense exercise but not DOMS. Lactic acid is removed/degraded within an hour of production. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [September 27, 2002, 2:46pm UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/9 "2002-09-27T14:46:55Z")
“DOMS”? ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [September 27, 2002, 2:49pm UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/10 "2002-09-27T14:49:47Z")
Delayed-Onset Muscle Soreness. ---
**Author:** ![KidCharlemagne](https://avatars.discourse-cdn.com/v4/letter/k/ed8c4c/32.png) [@KidCharlemagne](https://boards.straightdope.com/u/KidCharlemagne)\ **Post date:** [September 27, 2002, 3:22pm UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/11 "2002-09-27T15:22:53Z")
whoops, sorry. I could have sworn the term had been used in a previous post. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [September 27, 2002, 3:42pm UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/12 "2002-09-27T15:42:44Z")
Time to hit the library after work, I think, There;s a lot of ner terminology here… 🙂 ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [September 27, 2002, 3:44pm UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/13 "2002-09-27T15:44:16Z")
Grr. Preview, preview, preview! ---
**Author:** ![malaka](https://avatars.discourse-cdn.com/v4/letter/m/ecc23a/32.png) [@malaka](https://boards.straightdope.com/u/malaka)\ **Post date:** [September 27, 2002, 3:56pm UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/14 "2002-09-27T15:56:52Z")
Whether it’s DOMS or micro-fiber tears, I’m in pain. After a 2 year hiatus, I returned to the gym this week. Monday: chest and triceps, Tuesday: back and biceps, Wednesday: shoulders, Thursday: legs, Friday: I can barely move. I can’t even fully extend my arms. It hurt to type this. I guess I went at it a little too hard. _sigh_ Stupid, stupid, stupid! ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [September 27, 2002, 4:10pm UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/15 "2002-09-27T16:10:56Z")
[Required](http://www.howstuffworks.com/sports-physiology.htm) [reading](http://www.howstuffworks.com/muscle.htm) for those curious about the physiology behind exercise and muscle growth ---
**Author:** ![barbitu8](https://avatars.discourse-cdn.com/v4/letter/b/839c29/32.png) [@barbitu8](https://boards.straightdope.com/u/barbitu8)\ **Post date:** [September 27, 2002, 5:01pm UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/16 "2002-09-27T17:01:02Z")
I haven’t read **Ultrafilter’s** HSW links, so what I’m going to say may be in there, but in a nutshell I state the following. As already noted lactic acid is quickly removed from the body and does not cause DOMS. DOMS is caused by microtears in the muscle fibers. Lactic acid is a contributing factor (but not the only factor) in the “burn.” When you exercise the muscle PROPERLY a whole cascade of metabolic reactions occurs which causes the burn. (If you do the movement too fast, you will not feel the burn. The movement must be done slowly, preferably with a 2-count on the concentric (lifting), pause, and a 4-count on the eccentric (recovering to the starting position). At least pause at the end of the concentric. You gain strength during recovery periods. That is why it is recommended that you wait at least 36 hours before exercising the same muscle set again. The exercise tears down the muscle, but it is built up stronger than ever during the rest. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [September 27, 2002, 5:16pm UTC](https://boards.straightdope.com/t/i-exercise-my-muscles-become-larger-but-when/129811/17 "2002-09-27T17:16:46Z")
\ :eek: And I thought solid-state electronics was complicated!