I'm assuming we are talking about normal exercise stress, and nothing related to a deeper physiological or neurological problem (like Parkinson's Disease). With that understanding, the most likely culprit is the normal way fast twitch muscles fatigue. The article "Why Do Muscles Shake During Exercise" has a few decent points to help understand what's going on:
- Typically associated with intense exercise like sprinting or weight training
- Fast twitch muscles only activate so many cells for contraction, quickly distributing the load to other muscle cells during exertion.
- As the stress exceeds the energy and recovery available for the muscle, the distribution goes to other fatigued muscle cells and simply cannot keep up with the work.
- If the shaking happens at the end of a workout, it's probably a good time to stop
- If the shaking happens at the beginning of a workout, you are probably trying to do more than your body is capable of right now.
Some minor shaking is normal as you push heavier weights, but but as you move closer and closer to full exhaustion the biggest danger is inuring yourself due to your form breaking down.
The main reason the shaking occurs is because you are pushing yourself beyond your ability to recover. Recovery is multifaceted, and includes important topics like:
- Sufficient hydration
- Proper nutrition
- Rest during exercises (i.e. rest between sets, sprints, etc)
- Rest between training sessions
- Reducing life stresses
The topics were listed in order of importance. Bodybuilders will sometimes intentionally push to exhaustion to encourage the body to build more muscle size, but care must be taken to ensure good enough technique to avoid injury.