Shakiness in a muscle while it is being stressed is a normal sign of fatigue or weakness. That weakness may be due to insufficient conditioning or, more rarely, a disease process. If the shaking is taking place during an exercise like riding a bike, and stops when you stop riding, it is likely a symptom of fatigue in the affected muscles.
Muscle Fatigue and Shaking
When you begin an exercise, your muscles draw on the energy reserves stored as glycogen. Muscles require this to maintain a contraction. When your muscles have to overcome a lot of resistance, like in weightlifting, or when they have to hold a contraction for an extended period, like riding a bike, they will use up their available energy. When that happens, they can no longer hold the contraction. As the muscle loses it's ability to hold the contraction, it will begin to shake or tremble. This is a temporary change until the muscle has a chance to rest.
Conditioning for Biking
When you are biking, you are using your muscles to hold one position for a prolonged period of time. The arm muscles you are using to hold yourself over the handlebars will fatigue and possibly cause shaking. You can reduce the shaking by training those muscle for more endurance. Examples of training exercises for the upper body endurance would be pushups, tricep extensions, wrist curls and extensions and shoulder presses.
Adaptations to your Bike
Sometimes the position you are in on your bike is a factor. With racing handlebars, your upper body is extended forward over the handlebars and more of that weight is carried by your arms. Raising your hand position on the bars or changing to higher bars will reduce the demand on those muscles. Consider changes in your bike to allow a more upright riding position.
Other Considerations
Muscle shaking while exercising can also be due to low blood sugar or hypoglycemia. This is more of an overall shakiness than just your arms. Additionally, more serious conditions, although rare, should be ruled out. Diseases like myasthenia gravis, ALS, and even Parkinson's cause trembling or shaking. If your shaking persists after exercise, if it occurs without exercise, or if it worsens, you will need to talk to your doctor.
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