I think the eating sugar part has nothing to do with it. Try this again, without eating sugar, and I'm guessing you'll get the same result: the person pushing up won't be as strong the second time. Or try it again, but have the person on the ground eat sugar and then use their other arm. The person pushing down has the force of gravity on their side. They can lean into the push with their weight, and so it doesn't involve as much muscle strength as the poor person lying down pushing up.

Muscles get tired, and as they get tired they get weak. Leg muscles are very strong since they support your weight all day, but arm muscles are not as strong. If you repeat this trick, the arm and back muscles of the person pushing up will get weaker every time, no matter what they eat or drink or don't eat or drink.

Muscles recover their strength after a rest, so you can try an experiment to see how much rest the person pushing up needs to recover their full strength. If you want to try this, a more accurate test would be to have one person lying down, with their palm up. Have the second person place a weight on it (5 lbs, maybe, or a heavy book), and then time how long the person lying down can hold the weight off the floor. Be careful not to use too heavy a weight or object, as muscles and joints (e.g. the shoulder) can get injured when they are overworked.

Are you sure your arm weakened, or did you just think your arm would weaken because that's what they told you would happen? Or do you think maybe the other person pushed harder on your arm the 2nd time? Or maybe your arm was weaker from being pushed on the first time. 20 years ago, the 'experiment' was to just hold a junk food in your hand, and you were supposed to get weaker. I didn't get weaker.

Another 'experiment' I saw was this: A really tall guy stuck his arm out and a tiny woman held onto it and lifted her feet off the ground to try to pull his arm down. Then we all booed at him, and when she could pull his arm farther down. Afterward he told someone that he just did what he was supposed to, and he could have held his arm up just as well.

It's good to have doubts about things sometimes. That's a good scientific practice. Or maybe I am wrong, and your arm did get weaker after eating sugar. This is a question you can keep working on. Make your own observations and experiments, and see what you come up with.

I am really not sure, but at a guess, the person's blood is going to the stomach to intake the sugar, meaning less blood (less sugar and less oxygen) to the arm.