"If your muscles are not adapted to stress, they contract inefficiently and aren’t able to work in a coordinated manner, which usually leads to such shaking," explains Michelle Olson, a physician and professor at the University of Physical Science in Alabama.
Muscle tremors can also mean that you started running too fast.
"When you start too fast, the oxidation system in the body doesn’t have enough time to get into the job properly," says Dr. McDaniel. "It's like pushing the gas pedal hard into the floor - you're just prematurely depleting your energy supplies."
After a rapid start, muscles may start suffering from a lack of electrolytes or glycogen – their natural fuel. In such a case shaking muscles are just the way your body says: "Hey, I need refueling."
Running tip: Warm-up the muscles well before the race. Start running slowly, gradually approaching your planned pace.
If you are going to run for a long time at the limit of your abilities, drink isotonic 20 minutes before the start. Also, do not forget to load with some carbs.
If your muscles are still shaking after the finish, walk for a while, gently stretch and drink some isotonic.