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Why Do I Shake When I Work Out? 6 Likely Causes + Tips For Prevention

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If you have ever finished a set of heavy squats and found your legs trembling after you re-rack the weights, you’re not alone.

Shaking after a workout and muscle shaking during workouts are fairly common phenomena that are typically not a cause for concern. But you are probably asking yourself, why do I shake when I work out?

It is helpful to understand why your legs are shaking during workouts or trembling when you are done exercising because some causes are somewhat preventable. In addition, there are a few instances where shaking after a workout can be an indication that something is off and that you should speak with your doctor.

In this article, we will look at the potential reasons your muscles are shaking during a workout and how to prevent muscle shaking from working out.

We will cover: 

  • Why Do I Shake When I Work Out?
  • How to Prevent Shaking During a Workout

Let’s dive in! 

A person bent over at the gym, fatigued.

Why Do I Shake When I Work Out?

Feeling shaking after working out is fairly common, especially if your workout was vigorous, and is usually a benign response to pushing your body. However, tremors that continue well after you are done exercising are more concerning and should be addressed with your healthcare provider.

Trying to determine why your muscles are shaking while you work out or once you are finished can be difficult because there are multiple potential causes for muscle tremors during exercise.

It is often best to try and be mindful of other concurrent symptoms because this can help you narrow down the list of potential causes and pinpoint the likely culprit in your situation.

Here are some of the most common reasons why your muscles shake after working out:

#1: Muscle Fatigue

More often than not, you are experiencing muscle trembling after working out simply due to muscle fatigue. Both the muscle fibers themselves and the neuromuscular control can fatigue after prolonged use.

A person at the gym sighing in exhaustion.

When you perform any type of movement or exercise, the muscles responsible for the movement must contract to move your joints. These contractions are controlled by the central nervous system (CNS) through a system of motor units. 

Each motor unit consists of a motor neuron and multiple muscle fibers that the motor neuron innervates. Smaller muscles might have just a couple of motor units within them, but larger muscles are usually controlled by multiple motor units that each innervate a bundle of muscle fibers.

When the motor units fire, all of the muscle fibers innervated by that motor neuron contract. Essentially, there is an “all-or-nothing principle“ of muscle contraction that states that if a motor neuron successfully fires and sends a signal that exceeds a given threshold, termed the action potential, every single muscle fiber that the motor neuron innervates will contract. 

There are no partial contractions of a motor unit. 

The faster the motor neurons fire and the more synchronized the firing patterns of all the motor units in a given muscle are, the more forceful your muscle contractions will be. This is because a greater percentage of the entire muscle is contracting, and the contractions can be sustained for a longer period of time.

A person sitting on a gym bench wondering why their muscles are shaking.

However, the longer you exercise, the slower and less intense the neuromuscular firing rates become. If the action potentials sent down the motor neurons do not exceed the necessary threshold, none of the muscle fibers controlled by that motor neuron will contract. 

When your motor units are contracting, and the firing rates are slower, the force that you can produce when you move that muscle will be significantly reduced.

Additionally, as the firing rates slow down, there is a greater time interval between the contracting and relaxing pattern of the muscles, resulting in twitching or shaking.

Plus, as your muscles get tired, you will shake when you lift a weight because you have less control over the weight since less force is being produced and fewer muscle fibers are getting recruited.

Shaking due to muscle fatigue is most common after long or hard workouts that exceed your current training level or target the same muscle groups with minimal rest in between sets or exercises.

Other signs of muscle fatigue besides shaking or twitching include muscle weakness and soreness and overall body fatigue or low energy.

A person doing a wall-sit.

#2: Extended Contractions or Isometric Holds

If you hold a muscle in the same position or maintain a contraction for an extended period of time, you are more likely to experience muscle shaking during the exercise.

Examples of this scenario include when your abs start shaking while you are holding a plank or if your legs shake during a squat hold or wall sit.

When your muscles shake in these types of scenarios, it is again due to fatigue but also because the longer that you hold a contraction in a static position, the more motor units start firing. 

The neuromuscular system is designed to be efficient, so typically, only smaller motor units and fewer motor units in a given muscle are recruited at the start of a muscular action. 

If it becomes clear that more force is needed, additional and larger motor units are sequentially activated to increase the percentage of the muscle fibers in the muscles that are contracting to produce force. 

Therefore, as the duration of a contraction increases, more motor units start kicking in, which can lead to tremors or jerkiness since the level of force being produced is changing. 

Simultaneously, as smaller motor units that initiated the movement start to fatigue since they haven’t had a break, the firing rates decrease or become too weak to maintain the contraction. 

This, again, alters the consistency of the force being produced and can lead to muscle shaking during the exercise.

A person who is dehydrated holding on to a wall.

#3: Dehydration

Dehydration can potentially cause muscle shaking while working out, especially if you are significantly dehydrated.

Electrolytes are necessary to conduct normal neuromuscular signals and to contract and relax muscles. Therefore, if your electrolytes are imbalanced or become depleted during exercise, you may experience twitching and trembling. 

Exercising in hot and humid conditions, particularly if you sweat a lot, can increase the risk of dehydration. 

Other signs of dehydration after exercise include increased thirst, fatigue, dark urine, decreased urine output, dizziness, weakness, headaches, nausea, and confusion.

#4: Low Blood Sugar

Moving away from the neuromuscular control of your muscles, one of the most common causes of shaky muscles during a workout is hypoglycemia, or low blood sugar. 

Particularly for high-intensity exercise, such as strength training, your muscles rely on glucose (sugar) for fuel to produce the energy they need to contract.

Depending on your pre-workout meal or snack, your overall diet, and the length and intensity of your workout, your blood sugar might drop too low during your workout. This can make your muscles begin to shake because they are not able to receive the necessary fuel to contract as forcefully as you need them to. 

If you do fasted cardio or do not have a carbohydrate-rich snack before strength training and perform a vigorous, long workout, there’s a good chance that any muscle shaking during your workout is at least partially attributable to low blood sugar. 

Other concurrent signs of low blood sugar during exercise include headache, dizziness, weakness, fatigue, hunger, irritability, confusion, and an increased heart rate.

A cup of coffee and coffee beans.

#5: Excessive Caffeine Intake

Caffeine is a stimulant and can cause jitteriness or trembling. Many athletes drink coffee before working out or use a pre-workout supplement, which is particularly high in caffeine. 

If you are noticing an uptick in how shaky you are working out and have recently changed your caffeine habits or started taking a pre-workout supplement, this common side effect of excessive caffeine is likely to blame.

Other signs of caffeine overload include a rapid heartbeat, diarrhea, agitation, increased blood pressure, dizziness, nausea, and difficulty sleeping.

#6: Medical Causes

There are certain medical conditions that can cause tremors or trembling. 

If you experience prolonged shaking after working out that does not resolve rather quickly (especially after eating or drinking), or the tremors are accompanied by nausea, vomiting, changes in vision, fainting, seizures, or difficulty breathing, you should seek medical care immediately to rule out any more serious causes.

A person holding their hamstring muscle.

How to Prevent Shaking During a Workout

The best way to prevent muscle shaking during workouts will depend on the cause of the shaking.

Here are a few tips:

  • Increase the intensity and duration of your workouts gradually to give your muscles time to adapt.
  • Warm-up and cool down before and after your workouts.
  • Eat 30-60 grams of carbohydrates an hour before your workout to prevent low blood sugar, and consider an electrolyte sports drink with glucose.
  • Drink at least 4 to 8 ounces of water every 15 to 20 minutes, depending on your sweat rate. 
  • Limit caffeine intake before exercise.

Overall, if your legs are shaking during workouts, it’s a sign you’re pushing your body to the max. However, speak with your doctor if the shaking seems extreme or persists.

Now that we’ve covered shaking, what about dizziness? If you feel lightheaded when working out, check out our guide to Why You Might Be Dizzy After A Workout.

A person on a track, hands on knees.
Photo of author
Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a NASM-Certified Nutrition Coach and UESCA-certified running, endurance nutrition, and triathlon coach. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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