Great question!!
A BG spike after exercise is common. It is more pronounced if the exercise is anaerobic…like weight lifting. Yes running is not usually considered anaerobic but it does use the large muscle groups.
Muscles store glucose for immediate use when required. Anaerobic exercise taps the muscle stores and consumes the stored glucose. After exercise is over those stores are replenished. The liver, which stores energy as glycogen, will initiate conversion of glycogen to glucose. This “converted” glucose is secreted into the bloodstream, hence, a BG spike after exercise. A work colleague of mine first brought this to my attention when he borrowed a CGM from me. He is non-diabetic but an extreme keto fanatic. He wanted to know how his BG varied as a result of his keto-diet and exercise. He logged his CGM data with Tidepool and noted his own BG spike after an intense weight lifting session. This peaked my interest so I did my own trial. I intentionally took no calories before weight lifting one evening(before dinner). My pre-workout BG was 87. My BG dropped a bit at the beginning of the workout as my heart rate increased. By the middle of my workout, my BG had stabilized to the mid nineties. Afterward, with no meal, my BG gradually rose to 169(glucogenesis via stored glycogen) over a two hour period. After the 169 peak, my BG began to drop as the muscle groups recouped their stores from the blood and there was essentially no new calories since lunch time. The liver began using the present insulin from the pump to convert the BG to its glycogen stores. This of course go on in perpetuity…I did have to eat eventually, but, it was an eye opening event for me. I wish I had understood this before 36 years as a T1D…I would have used it to better regulate my activity to calories over the years…not that I am overweight or underweight…better regulation of calories translates to better BG control.
The five minutes of elliptical is a quick burst of aerobic exercise that increases heart rate quickly and yes it uses large muscle groups but with less resistance than propelling ones own body across the ground. The heart also has to use glucose for energy and it is always beating…the heart cannot store glucose like the other muscle groups…it is never at rest