Still not sure about the muscles and energy thing. After I came home on Saturday, my pulse remained 10-15 beats above normal for at least a couple hours. I was also warmer than normal. According to the “muscles store energy” theory, this probably came from my leg muscles recharging, drawing energy from elsewhere in the body. But as I said, Wikipedia and a couple more pages of highly respected websites don’t believe that muscles can store energy like a battery, just a substance known as glycogen, basically very densely packed glucose (the simplest form of sugar).

If that is the case, then presumably my leg muscles were storing up glycogen by drawing glucose from the blood (where there seems to be no shortage of it, based on recent blood tests – I am not diabetic, but the blood sugar is at or just over the upper boundary of normal). There must have been rather a shortage of glycogen if the mucles had to increase the flow of blood to get enough sugar. You’d think since the heart is beating anyway, the muscles could just pick up sugar as it passed.

Yesterday it rained like a bathroom shower, and my jogging shoes soon became soaked all the way through. They are not going to last long if I use them for wading, so I returned home after only about half an hour. That put an end to my plan to check whether the energy reserves had regenerated fully.

Today I took a 1.25 hour walk. I carried my umbrella, even though I only needed it for a short while. I am not sure if carrying something had an effect, but my heart rate was somewhat higher and I burned more calories than on the first trip Saturday, although less than the second. I guess it may count that I also jogged down the other side of the hill and a few other places where my pulse started getting too low to count as training. Burned about 800 calories (kcal), which is pretty decent for that length.

However, I did not trigger a forced recharge this time. There must have been some reserves left, because my pulse almost immediately fell back to normal range. So evidently if I don’t discharge past a certain point, recharge will happen entirely in the background. That is what I thought, since I normally don’t experience any disruption of heart rate or breathing after I took up the habit of walking briskly for an hour.

Then again, according to what I learned before, the body should be burning mostly fat during so moderate exercise. OK, I guess climbing those hills might need some faster energy, but still. In theory the body should use mostly fat, and I have enough of that to walk for weeks.

I wish I had meters in my body so I can see what it is doing. It would be cool to be able to monitor my blood sugar, blood fat, remaining sugar storage and fat storage. Clearly the body does know these things, because it adjusts to them on the fly. But unless I take things to extremes, I cannot actually feel the variations. Then again, thanks to the glucose syrup tests, I seem to have achieved the ability to feel my blood sugar being higher than necessary. It is a kind of acute “fed” feeling, if that makes sense. It is different from the long-term “fed” feeling that comes from having my fat stores reasonably filled. (I am still not fat, thanks for asking, but I have more than I realistically need in peacetime.) So perhaps with the right experience, I may become able to feel other statuses in my body too. If I live that long… Just to be safe, I don’t think I will be running any marathons anytime soon!

***

Edit to add: Found it! The answer from nycgirl here is loaded with useful facts about glycogen.  Evidently even a walker like me has several hundred grams of the stuff, and it is always used in the beginning of any exercise (thus the low pulse during the first quarter, I guess). With slow and steady exercise, fat is burned but together with glycogen. This is probably which happens after I am fully warmed up.  I still don’t know what the trigger threshold is that causes my pulse to rise to the next level (after an hour or so, in my case). I also don’t know where the trigger is for the forced “recharge” after exercise.  Saturday I spent 1300 calories, and did get the recharge effect. Today 800 and nothing.  So more studies are in order.

“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.