During the first few minutes of exercise: Glycogen in the muscles is broken down anaerobically and used to fuel the muscles. As the exercise continues, oxygen becomes available for the aerobic break down of carbohydrates, fats and proteins. In addition to the glycogenolysis, muscle takes up the glucose in the circulation. After 5–10 minutes of activity: Muscle glycogen break down decreases. The decrease in the blood glucose suppresses insulin and stimulates glucagon which in turn causes hepatic glycogenolysis. Glycogen is broken down in the liver to glucose and is released into the blood stream and is taken up by the muscles as fuel. This glucose becomes the major source of fuel (hepatic glycogenolysis). Your blood glucose will rise since your body doesn’t make enough insulin to increase muscle uptake of the excess glucose.
Using your muscles helps burn glucose and improves the way insulin works. That’s why blood glucose levels usually come down during exercise. But you might see blood glucose go up after exercise, too. Some workouts, such as heavy weightlifting, sprints, and competitive sports, cause you to produce stress hormones (such as adrenaline). Adrenaline raises blood glucose levels by stimulating your liver to release glucose.
The food you eat before or during a workout may also contribute to a glucose rise. Eat too many carbs before exercising, and your sweat session may not be enough to keep your blood glucose within your goal range.