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My question is related to the following, I think:

But I would like to know how this works on muscle/biology level. The best thing I can imagine, is that the body switches very quickly inbetween using different muscle groups, so that the ones under stress get some rest. It does this as a sort of self-preservation so that certain muscle groups don't get damaged by being overstressed. Can anyone confirm this - or is my thinking wrong?

It's interesting for me, because it happens on all muscle groups when I'm tired, but always in my stomach area, and maybe I need to do a different exercize to train the concerned muscles individually on a smaller scale (if possible).

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up vote 6 down vote accepted

First there is nothing wrong with your muscles shaking. You are making them work and have good intensity. That is great.

Some reasons for the muscle shakes:

  • Balancing muscles (fixator) that are used to stabilize the lift cannot carry the load. Lifting weights is not overall natural. There are a lot of balancing muscles used that get little to no use in everyday life especially modern life. An example of this is the bench press. Your triceps are doing a lot of balancing. When you first starting benching it is natural that your chest is exponentially stronger. So your large muscle group will push but the balancing muscles are shaking.

  • Imbalance of power. When you have a muscle group that is superior to another and the inferior group tries to transfer load to the superior. Bench is a great example again. So you broke your left arm (which was already your weak arm because you are right handed). You start benching. Your left pec and deltoids are going to try to transfer the weight over to the right side. So your left side will start shaking because it is out of balance because it can't handle half the weight. Then your right gets out of balance because the pivot position for the weight has changed.

  • Not enough muscle power. When you start a new lift or if you are new to lifting the big muscles like to take over. Not only that but you probably don't use the smaller muscles much in everyday life because they are usually strengthened when the big muscles are tired or you perform a very specific movement to use them. An example for this is the squat. As your hips roll the weight starts off using the gluteus maximus (butt) and hamstrings (I am being very general here) and the weight is transferred to the quads - more weight if done improperly. Your quads say - hell no I can't carry that load - so the weight load transfers back and forth.

  • Fatigue - If you just worked out your triceps hard and go to bench there is a greater chance of them breaking down and shaking during the lift.

  • Lack of oxygen - This is really in the same category as fatigue but it is common for new lifters. You have to get oxygen to your muscles when working out. No matter how stupid you think you look when breathing heavy it is part of the process. Lack of oxygen can instantly fatigue the most seasoned lifter and start them shaking.

In general you should be able to take the shaking and use it for good purposes. First say to yourself am I drinking/eating enough before and am I breathing enough during my routine? If yes then you can start to work on the muscles when they are shaking. For example if your triceps shaked during bench I would have you bench first so that your triceps had the least amount of fatigue and then hit triceps hard after.

If you are a seasoned lifter and you are experiencing a lot of shaking (given good diet and breathing) then I would look at your routine. You may not be giving your body enough rest between sets. You may be trying to work out opposing muscle groups causing balance fatigue. Again there is nothing wrong with shaking as long as you can get your work done! Might be as simple as lowering the weight until your body is OK with it.

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I want to comment too that a new routine can cause you to shake and sometimes (relative) weight has little to do with it - especially during massive fatigue. With high school athletes I often do shock workouts to show them that they are not focusing on their athletic sport. Bench is a good example. Most athletes at that age are maxing out while not focusing on form, explosion, endurance. So I have them do sets of 12 at 1/3 of their max with 1 min. break in between. They are laughing at me until the 3rd-4th set and their whole upper body is shaking. Tiny weight, seasoned lifter... – DMoore Jul 30 '13 at 20:34
Great answer, thx! I do Yoga, and I start shaking when I hold certain positions, particularly, if they involve core-tension in my back and stomach. I can hold the positions fairly long, but I just start shaking uncontrollably, and I was wondering what caused that. I'll try to breath more, and identify the balancing muscles to work on them. – Rafael Cichocki Jul 31 '13 at 9:26
I am a very experienced lifter and athlete. I shake like hell after a few mins of yoga too. Mainly because I do it once a month and just don't use those balancing muscles enough. – DMoore Jul 31 '13 at 13:49

I'm assuming we are talking about normal exercise stress, and nothing related to a deeper physiological or neurological problem (like Parkinson's Disease). With that understanding, the most likely culprit is the normal way fast twitch muscles fatigue. The article "Why Do Muscles Shake During Exercise" has a few decent points to help understand what's going on:

  • Typically associated with intense exercise like sprinting or weight training
  • Fast twitch muscles only activate so many cells for contraction, quickly distributing the load to other muscle cells during exertion.
  • As the stress exceeds the energy and recovery available for the muscle, the distribution goes to other fatigued muscle cells and simply cannot keep up with the work.
  • If the shaking happens at the end of a workout, it's probably a good time to stop
  • If the shaking happens at the beginning of a workout, you are probably trying to do more than your body is capable of right now.

Some minor shaking is normal as you push heavier weights, but but as you move closer and closer to full exhaustion the biggest danger is inuring yourself due to your form breaking down.

The main reason the shaking occurs is because you are pushing yourself beyond your ability to recover. Recovery is multifaceted, and includes important topics like:

  • Sufficient hydration
  • Proper nutrition
  • Rest during exercises (i.e. rest between sets, sprints, etc)
  • Rest between training sessions
  • Sleep
  • Reducing life stresses

The topics were listed in order of importance. Bodybuilders will sometimes intentionally push to exhaustion to encourage the body to build more muscle size, but care must be taken to ensure good enough technique to avoid injury.

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It may be because of the build-up of Lactic acid and the formation of free radicals after an intense workout which alters the excitability of muscle tissue.

More about muscle twitching (Benign fasciculations):

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The reason you shake, especially if your a beginner is because of autogenic inhibition. For example on bench press, your arms will shake because as the tricep (agonist) is contracting the bicep(antagonist) is being stretched, therefore your muscles spindles are detecting this strength and sending signals to contract to protect the bicep from over stretching. So basically, both muscles are attempting to contract which causes shaking. As you continue to strength train you body will adapt and your antagonist will 'learn' to shut off as the golgi tendon relaxes, making you stronger and less shaky.

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You are going to have to put references in there. Currently this answer is just words with no evidence. – JJosaur May 11 at 12:06

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