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Why is it common to rest between sets of repetition? Could I, instead of resting, exercise another muscle group?

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Between what kind of repetitions? What kind of workout? How intensive is the workout? This questions is far too general –  Ivo Flipse Mar 5 '11 at 20:10
    
@Ivo: As detailed in the question, the two exercises are different enough that different muscle groups are exercised. If the answer depends on the intensity and type of workout, please write so in the answer -- I don't think it would be plausible to split this question up. –  user26 Mar 5 '11 at 20:18
    
Well you would want to rest, because it was intense. Either you can't do anything at all or the muscle doesn't have the energy to do it. As for what kind, there's a difference between taking sprints, bench pressing as much weight as possible or doing arm curls with a 10kg weight. Do you expect me to answer for every kind of workout + intensity? –  Ivo Flipse Mar 5 '11 at 20:35

4 Answers 4

up vote 12 down vote accepted

Exercise depends on several factors:

  • your muscles for doing the actual work and their local storage of energy;
  • your blood as a transfer system of O2/CO2 and energy;
  • your heart for making the blood flow;
  • your lungs as a transfer system of O2/CO2;
  • your liver as a supplier of energy;
  • your nerves for stimulating your muscles.

So depending on the intensity of your exercise, all of these systems will be taxed.

After a certain amount of reps, you have depleted all your muscles local storage of glycogen and they will need to recharge before they're able to perform work again. After the local storage is gone, your blood will have to resupply your muscles to deliver you new energy. This also means, that if you start working on another muscle group, that muscle group has to share its new supplies of energy with the muscles that are recovering.

Even if you were to rest, your body has a storage of lactate acid it needs to process and an oxygen deficit to compensate for. This requires oxygen, but more importantly produces a lot of CO2 your body needs to get rid off. This is called having an oxygen debt. If you switch muscle groups, you don't give your body time to recover from its oxygen deficit.

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If you're trying to work out on your maximum lung capacity, you have to subtract a part of your oxygen supply for the oxygen debt, so you're not working out maximally anymore: you're simply out of breath. Furthermore, if you can't supply your muscles with sufficient oxygen, it will anaerobically burn it's fuels. While they're a great fuel, it won't last you very long and it will most likely hamper the amount of repetitions you can make with the same fuel. Because as soon as you rest, then the fuel get's processed again, but you already moved on to another muscle group...

If your body feels tired, your central nervous system (CNS) might also inhibit any further exercise. It won't stop you from trying, but you probably won't be able to perform the amount of reps you could perform with sufficient rest.

So all in all, even if you were to switch muscle groups: your body needs some rest between repetitions. How much rest your body needs depends on: how heavy the exercise is, how fast your performing it and how many times you perform it per repetition.

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What's wrong with doing fewer reps? If you get to failure sooner, doesn't that still translate to improvement? –  Jeremy Stein Mar 11 '11 at 15:41
    
It depends on your workout @Jeremy, my answer is more a general statement, so yes in some cases in might not be 100% correct, but that goes for anything exercise related... –  Ivo Flipse Mar 11 '11 at 16:05

As Dan John would say, it all depends on your goals. In fact a lot of programming, or how you string your exercises together both throughout a session and throughout a cycle, has to do with your goals. There are a few approaches to programming:

  • Sets x Reps @ weight, with as much rest as needed in between
  • Supersetting: switch between 2-3 exercises one right after another and then rest before you start the next cycle
  • Timing: pause for seconds, slow down the concentric and/or eccentric phases, and limit rests between sets.

In programming for a lifting competition, a common practice is to have a main lift for the session. The main lift has plain old sets and reps at a set weight, with as much rest as you need to make sure you get the next set. The assistance work is where you get to have all the fun with the training variables.

Trade-offs

  • The more rest you give your body, the better it deals with fatigue
  • Fatigue is a component that can make the exercise harder, and may even bring you to a point of failure (i.e. not being able to complete one more rep). Depending on your goals this may be a good thing.
  • Some parts of your body are fully engaged on a number of exercises, so be careful of not hammering them too much
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I'm surprised no one has mentioned supersets, an ancient and honorable practice in which you do a set of one exercise then immediately do a set of another, rest and repeat. Supersetting 3 exercises is not uncommon. One kind of superset is the leg extension - lat pulldown type. It is done for time efficiency since you get to skip a rest period. Is your performance reduced? Probably. For sure if you're not in great shape or not using steroids. But if today's workout is chest and you do all your benching first, so what? The oddest kind of superset is the dumbbell curl - lat pulldown type. Here you're trying to take your biceps out of the back work, or paradoxically, you're hoping the pulldowns will extract the last bit of bicep strength. The most sophisticated superset is something I recently learned from Eric Cressey's Show and Go where an exercise is paired with a stretch or mobility exercise. As you can see, this is an advanced approach in every sense. All your energy systems need to be well developed and you need to know the primary and secondary muscles used by each exercise. If you're asking the question, you probably aren't ready, but the fact that you came up with it shows a creative mind that will take you far.

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I like the content of your answer, but better formatting and fleshing out the concepts would make it a lot better. –  Berin Loritsch Nov 10 '12 at 18:25
    
@BerinLoritsch. I did a search for 'superset' on this forum and found surprisingly little. Do you think this technique is old fashioned and no longer used? It is so second nature to me that when I start to describe it, I'm overwhelmed with all the ins and outs and variations. –  medmal Nov 12 '12 at 4:43
    
Supersets are very much used. They are a bread and butter of bodybuilding, and even power lifters use them with assistance work. The best approach is to provide links to an article that describes the word in more detail and just summarize the high points that pertain to your answer here. Bullets are a good way of organizing information for a readable answer. –  Berin Loritsch Nov 12 '12 at 13:17
    
it might be that the majority of people on this sight are so new to the concepts in lifting that they just don't know what a superset is, or how it can help them achieve their goals. –  Berin Loritsch Nov 12 '12 at 13:20

Without resting, you won't have enough energy to properly break down your muscle tissue so that it can build back up. If your goal is to build muscle, then to get the best results, you need to break down that tissue.

If you're tired from doing other exercises, you won't achieve the most repetitions in your sets.

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Regarding the last point, I the exercises would differ enough (e.g. leg curls and biceps lifts) so that they wouldn't affect performance. –  user26 Mar 5 '11 at 20:13
    
@Tim - When I exercise, even lat pulldowns, if I then try to do leg extensions, I'm still tired and can't do as many leg extension reps as I would if I waited 1 to 2 minutes. Thus, my leg muscles wouldn't break down as much. –  jmort253 Mar 5 '11 at 23:05

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