Shaking after your workout is often just a sign that you're pushing it hard in the gym, but it can indicate that your body is in danger. If your legs are tired it can throw your nervous system out of whack, causing your muscles to shake. You could also have low blood-sugar, which is more dangerous.
If you're simply fatigued from your workout, you won't have any extra symptoms. However, if it's something more serious like dehydration or hypoglycemia, then you'll experience additional issues. Either way, shakiness is a sign that you should take it easy and focus on recovering properly.
Shaking From Fatigue
Exercise is draining to the body, and the harder you push it in the gym the more you have to focus on recovery. Your muscles aren't the only part of your body that gets tired, either. Your nervous system can also fatigue from intense workouts.
Read More: I'm Shaking After a Workout
Nervous System Fatigue
When you do a leg exercise, like a squat, your brain sends signals at a rapid rate to your muscles. These electrical signals make your muscle contract and then relax. This happens in a fraction of a second, so fast that it's barely noticeable.
The reason that you don't look like a jittery mess when you use your muscles is that your nervous system only contracts the muscle as hard as it has to. It starts at a low level and works harder until it stops and you complete the repetition.
After a tough workout, your muscles are still primed and ready to contract. Your nervous system is hyped up and prepared to work. Even though you're done with the gym, your muscles don't know that.
Even if you're just walking around or climbing stairs, your muscle contractions that were smooth before might feel shaky, since your nervous system is overworked and over prepared. This is normal and doesn't mean you have any serious medical problem. The remedy is rest and relaxation.
Low Muscle Glycogen
Your muscle fibers don't contract on their own, they use stored energy to get the job done. One form of energy that your muscles use is glycogen, which is a variation of glucose.
Right after you eat, your body takes sugar out of your blood stream, processes it, and sends it to the muscles to form the glycogen. This is your most-used fuel source during a hard weight training workout. If this fuel source runs low, it can cause your muscle to go haywire and act abnormally, which can lead to shaking. In this case, you'll also feel very tired and the muscle will fatigue very quickly.
Read More: Trembling After Exercise
Low Blood Sugar
If your glycogen levels are low, it can either be from overuse or due to low blood sugar in your body. You can bring blood sugar levels back up by eating foods rich in carbohydrates, like fruit. You can also drink sugary beverages like fruit juice.
Low blood sugar is a bigger problem because it can come with more serious side effects, like fainting and unconsciousness. Exercise can cause low blood sugar because your whole body consumes more glucose as fuel. If you have a blood sugar disorder, like diabetes, or even if you simply aren't eating enough, low blood sugar can be the cause of your muscles shaking.
You'll also feel symptoms like hunger, irritability and lightheadedness. If you feel these other symptoms coming on -- and not just muscle fatigue -- it's a sign that you have low blood sugar. Stop exercising immediately and get some fuel in the form of a sugary drink or snack.
Start with 15 to 20 grams of carbohydrates, recommends the American Diabetes Association, and see how you feel. If you still feel shaky after 15 minutes, have another 15 to 20 grams of carbs. Repeat every 15 minutes until you feel better, then have another light snack an hour or two later to keep your blood sugar levels up.