Why do my muscles shake during a workout?

When you start to feel your muscles shake, you may feel like you can't finish your workout, but that doesn't mean you should stop. So what exactly happens when you start to shake?

Oct 14, 2021
Nicholas DiMeglio, CSCS
woman struggling to do a push up

I know you’ve felt it before, that feeling when you’re in the last few seconds of a plank, or that last set of a hard workout and you’re trying to squeak out those last few reps. All of a sudden, you feel your muscles start to tremble and shake — and you’re not sure if you can finish what you’ve started. You wonder to yourself, why am I shaking like this and what can I do to keep it from happening?

To fully understand why your body reacts this way when you push outside of your comfort zone, you must think beyond the physical exercise and muscles themselves. The brain remains the commander in chief of the body and will dictate how to best accomplish whatever task is thrown its way.

Activate your muscles

When doing any sort of exercise, the brain first has to send a signal through the nervous system to recruit the muscle fibers needed to complete the activity. You may have heard of “muscle activation” as part of a warmup before doing heavier or more intense training. If your plan is to do squats in your workout, you will likely do lower-level exercises, such as glute bridges or hamstring curls, to start the process of recruiting muscle motor units.

Think of muscle recruitment like your favorite restaurant. How do they provide staff for the restaurant? Does the manager schedule the staff on a Tuesday morning? Or do they schedule the majority of their staff on Friday and Saturday evening? My guess is the latter. They need to have enough staff on hand during peak hours and fewer staff on hand during the slower parts of the day and week.

All hands on deck!

This is the same way that muscle fibers get recruited: on an “as needed” basis. The more strenuous the activity, the more motor units are needed to complete the task. As the exercise becomes more fatiguing towards the end of the workout, more muscle fibers have to step in to take the place of the other depleted motor units. This is where the shaking and trembling comes into play.

So, you’re in the middle of your plank and so far, so good. You feel comfortable and capable of holding the position because you have yet to tap into those reserved muscle fibers. Only half the staff are working since it’s a Tuesday morning and the restaurant isn’t packed. And that’s when suddenly things ramp up. You’re starting to fatigue; the lunchtime rush is starting to make their way through the door. So, you call on more staff, more muscle fibers start to get recruited and give the other fibers a break so they can recover. Your body shakes because your muscles are constantly alternating between states of contraction and relaxation to give you the best chance of finishing that minute long plank or last set of bicep curls.

How do you prevent these muscle shakes from happening? You keep pushing yourself outside of your comfort zone and embrace the shaking because that’s the moment where you get stronger. Training never gets easier, and it’s not meant to. Training is meant to constantly challenge you to be better than you were yesterday, so if that 60 second plank starts to seem less daunting, keep pushing past that until you find yourself outside your comfort zone once again.


nicholas dimeglio

Nicholas DiMeglio

Nicholas DiMeglio, CSCS, is a personal trainer at our Greenwich and New Canaan locations. He has a passion for sports, which led him to become a trainer, and he now seeks to quarterback his clients' care in order to help them succeed.

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