Your exercise has become TOO extreme 

Yes, working out is an important part of a healthy lifestyle along with eating healthy, getting enough sleep, drinking enough water, and taking good care of your body in recovery. However, there is such a thing as working out too much and too hard. Intense workouts 7 days a week with no recovery days can do more damage than good and you’ll notice that shaking coming full force more often. If your workouts continue to go up in intensity without rest time between intervals or proper recovery time after, you’ll likely experience this too. If you’re working out for several hours a day with high intensity during most of that time, that’s simply too much for most people. Life is about balance. You can’t make up for a primarily sedentary life by suddenly pushing yourself to the extremes. Make a workout schedule that works for you and be sure you give your body plenty of rest and recovery time around those workouts. Intense workouts are great for stress relief and your health, as long as you’re listening to your body and not overdoing it. Those muscle shakes aren’t necessarily a bad thing in and of themselves, but they are a sign that you’re on the brink of pushing yourself too far. 

You’re doing too much too soon 

So, you’ve decided you want to get healthy. As with most people you will likely go from nothing to a lot of something overnight. Many people feel as though they need to go all in as soon as they make the decision to start working out again. They may think they need to overcompensate for all of the lost time. But the most important thing to remember is that your body needs time to catch up with your mind. If you go from doing nothing to working out for an hour a day, 5 days a week, your body is going to revolt because it’s simply not prepared for it. Your muscles aren’t used to that sort of thing so they will start to talk to you, and by talk to you, I mean shake like crazy. When you decide to take the leap into working out more, make a realistic schedule so your body can slowly ease into this new lifestyle. So, instead of working out 5 days a week for an hour, start with 3 days a week for 30 minutes with a rest day between each. Instead of starting out with jogging or running, start out with a brisk walk and work your way up. Remember this schedule is not set in stone. Once you and your body get used to it you can increase the time and frequency of those workouts. The more you control your introduction to exercise, the easier it is to build up a habit as well as take care of your muscles on the way up.  

When those muscles start shaking, it’s a good sign that you’re at your current limit and it’s time to start winding down that particular exercise. With each day you’ll grow stronger and those muscles can handle more and more. Just give them time.