Muscles store energy now?

“I wanted to talk on the subject of science with you.”

OK, this is kind of weird. I just took a walk and this time walked up two long, steep hills, one atop the other. My pulse stayed around 120 for much of it and only reached 130 near the top of each hill.

I know this is not really international news, but there are reasons for my surprise. Only a couple weeks ago I crossed the first of these hills and slowed down to an amble because my pulse reached 135. That is around the upper limit before I trigger my lifelong exercise asthma. Also because of that asthma, I never did sports as a kid, and my lung capacity never developed fully. A couple years ago it was around 2/3 of normal for a man my age (50 years old at that time). And back in 2005, before the illness that changed me, I would stop twice in a hill shorter and less steep than this. I felt like my heart just couldn’t take the strain of climbing it all in one go.

I am so old that I have to warm up before my warm-ups. And yet for each passing month – if not week – my pulse seems to get lower and lower. That is a bit bizarre, I think.


Or perhaps not. After an hour’s walk, I came home and wrote the previous part, then set off again. This time my pulse was normal, and went all the way to 135 before I rounded the first hill. So it is not my heart. Somehow my muscles seem to store energy for the expected challenge, but when I then throw an unexpected challenge at them, they need the help of the rest of the body.

I wonder how the muscles can store up energy like that. There are probably books about it, but I don’t even know what to look for. I know all energy in the human body comes from burning the four food groups: Sugar, fat, protein and alcohol. But I was under the impression that they have to be burned within seconds of the actual energy use, not used to “charge up” muscles in advance.

I guess this explains why I have to walk longer and longer to burn the same number of calories. My body charges up the muscles beforehand (perhaps while I sleep?) and then releases this energy during the first hour of walking. I wonder how they do that. Actually, I wondered so much that I asked Google: How do muscles store energy? It provided links to sites about ATP and glycogen, but they were pretty random. I don’t think Google really understood the question…

My best guess would be glycogen, since ATP only lasts for a few seconds at best. I know glycogen (“animal starch”) is stored in muscles and broken down to glucose during exercise. But that does not really explain it to me: Glucose still needs oxygen to burn, and that oxygen must come through the blood. But the blood already contains glucose. That is what my doctor is worried about, the 6.1 mmol of glucose that is always in my blood, even 12 hours after eating.  Why then would the muscles need to store energy in the form of something that becomes glucose?

I mean, if glucose is a scarce resource in muscles,  if it is the bottleneck and not oxygen, then diabetics should be world champions in sports. There is no sign of that, to put it mildly.

I suppose it would make sense if muscles have some bottleneck in how fast they can absorb glucose from outside. Sugar molecules are not all that big, but they are a lot bigger than oxygen, so it may be that absorbing glucose is slower. So while the supply of glycogen lasts, the muscles need only import oxygen, but afterwards they need to import both oxygen (fast) and glucose (slow). But there is no mention of that in any of the articles I have read, this time or before when I read about physiology. It is as if no one has ever asked themselves why the pulse is low during the first part of exercise. That just cannot be: Humans are too curious for their own good, much of the time. So that leaves me with the notion that the answer is totally obvious to anyone except me.

Please tell me, since Google won’t.