Why Your Muscles Shake After Exercise

Ever wondered why your muscles shake after a period of exercise? Here's the answer.

Ever wondered why your muscles shake after a period of exercise? Here's the answer.

When your muscles shake after exercise, they do so involuntarily, and there is little you can do to stop them immediately. This lack of control of our body usually scares most people, but the truth is that muscle tremors are perfectly normal (after exercise), and it just means that you are working out really intensely.

But that’s the simple answer. To get a more in-depth view, as to why our body reacts in this way to strenuous exercise, we have to understand the concept of muscle fatigue.

Muscle fatigue is defined as “the decline in ability to generate force”, and although there are many diseases such as multiple sclerosis or myasthenia gravis that can cause muscle fatigue and tiredness, in our case, exercise is the factor that is causing the fatigue. The reasons why our muscles get fatigued could be due to; a lack of nutrients in our muscles, a build-up of metabolic waste, or finally an excessive strain on the CNS (central nervous system) in our body.

In terms of metabolic waste, lactic acid is the main waste product that is produced during exercise. Lactic acid, as many of you may know, is a by-product of anaerobic respiration, which occurs in our cells when there is a limited amount (or no amount) of oxygen. Anaerobic respiration results in glucose being broken down into lactic acid only. This form of respiration releases much less energy than aerobic respiration. Lactic acid, is (like it says in its name), an acid. This means that it releases hydrogen ions, which, in terms of our muscles, decrease their pH value, causing soreness, and thus a decrease in muscle deficiency.

Severe shaking is referred to as, physiological tremors.

Severe shaking is referred to as, physiological tremors.

Our muscles get ‘fatigued’, mainly because our body starts shutting them off, to prevent more lactic acid being produced, and damage to the muscles occurring. Finally, another myth is that ‘lactic acid is what causes my muscles to be swollen after a workout’. This is totally incorrect. Our bodies are efficient machines, so if something toxic (like lactic acid) needs to be removed from our bodies, our body will do so promptly. The real reason our muscles hurt is because they are inflamed/swollen, because they are damaged, and are starting to heal.

In our body, the central nervous system refers to our spinal cord and brain. When we do exercises that require more than one muscle group at a high intensity, there is a chance that it can cause severe shaking, which we call physiological tremors, this occurs when there are not enough chemicals to carry the electrical impulse across nerve cells (neurotransmitters). This happens because you are putting too much strain on your CNS, and motor units (see later), as the demand of neurotransmitters, exceeds the supply of them, greatly.

We now know what causes muscle fatigue, but the question is, how does muscle fatigue cause body shaking/tremors? Well, in our muscles, it would be stupid to have each muscle cell contract individually, seeing as there are hundreds of thousands of cells in each muscle. For this reason, our muscles contract in groups of 2000 or more, i.e. one motor nerve controls 2000 or more muscle cells. (an exception to this is in our voice box/larynx), where one motor neurone connects to only 2 or 3 muscle cells.

Anterior view of Muscles | Source: By CFCF (Own work) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

Anterior view of Muscles | Source: By CFCF (Own work) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

Also, when we contract our muscles, not all the groups contract simultaneously. In fact, they all contract at totally different times, although since there is so much overlap in the contractions, it just appears that the muscle is contracting smoothly. But when muscle fatigue kicks in, the motor units drop out at different points (either because not enough neurotransmitters or not enough nutrients). This means that there will be less overlap in the muscle unit contractions and the movements will seem more jerky, until it finally reaches the point where the muscle starts to shake. (as the gaps between motor unit contractions become really wide).

It takes around 15 minutes for the shaking to completely cease, and this is because it takes time for more nutrients/neurotransmitters to meet the demand, or it could be because it takes time for your body to remove the lactic acid and break it down.

But what can we do to prevent your muscles from shaking? The first step would be pay close attention to your nutrition. By eating slow-digesting carbs you will ensure that your blood glucose levels are level, during your workout, this will decrease the chance of you having tremors, but as mentioned above, the tremors could happen for other reasons as well. And although massage therapists may claim that they can ‘squeeze’ the lactic acid away from your muscles, this is utter rubbish that has no scientific evidence to back it.

Finally, although it may seem like such an easy question to answer, writing this truly made me think about how intricate, complex and interesting our bodies can be. For example, such a simple task, such as raising your arm, requires thousands of minute contractions at different times, and each contraction needs an input from a motor neurone, which needs a chemical neurotransmitter to carry the signal, you get what I mean, I could carry on for days.