This is a series of blog posts that talks about the different aspects of an exercise app and how we're approaching them in BatWerk (Intro, How do muscles work, Pains, Maintaining the routine, How to play). Interested? How would you like to improve it?
The goal of the BatWerk exercise app (Android / iOS) is to keep you healthy, happy and productive without requiring you to overhaul your entire life.
So, of course I ended up wandering down some researchy rabbit holes to figure out how to do that. One of these was figuring out what actually happens when you do exercise. Why does your body move in the first place and how does it go about achieving that. Here are some confused notes on the whole process. Comments and corrections welcome, I'm out of my depth here!
What's a Muscle Anyway
Hey, your muscles are bags of string-like proteins that slide across each other with the help of ATP molecules. The proteins start moving and using ATP after calcium ions released by motor nerve activation open a lock molecule. Each of the sliding proteins generates force on the piconewton scale. To lift your arm, trillions of proteins need to slide in unison.
The three ways to make your muscles work better are to increase the number of muscle proteins, increase the amount of ATP available, and improve the motor nerve firing.
Moving a muscle activates it and makes it grab oxygen, sugars and fat from your bloodstream for ATP generation. The more capillaries you've got around the muscle, the more ATP it has available. Activating muscles also drives growth hormone secretion, which makes the muscles grab proteins from the bloodstream for conversion into muscle protein, and stimulates the growth of capillaries. Conversely, inactive muscles go into power saving mode where they don't pull in fuel and proteins, leading to lower metabolic rate and loss of muscle proteins.
Activating muscles to perform motions also trains your motor nerves to fire at the right time. Motor nerves activate only a part of the available muscle fibriles to contract, cycling through these activation groups over the contraction period. Motor nerves need to be trained through repeated use. Moves also require technique training to accomplish them through muscle contractions.
The ATP used by the proteins comes in four ways. First is the existing reserves inside the muscle. These are used up in about 3 seconds. Then come the muscle's creatine phosphate reserves. These are converted to ATP and last for a bit more than 5 seconds. Next up is anaerobic conversion of sugars to ATP, which can be maintained for a minute or two. After that, the muscle switches to aerobic ATP conversion from sugars if available, falling back to fats and even proteins if not.
Anaerobics
The thing about anaerobic and aerobic conversion is that aerobic conversion of sugars is around 15 times more efficient than anaerobic (30 aerobic ATP per glucose molecule vs 2 anaerobic ATP per glucose molecule). If you operate aerobically, you'll have to do 15 times the work to process the same amount of sugar. In other words, a 800m sprint needs to process as much sugar as a 10k run.
Sounds wasteful, right? But your body has a trick up its sleeve, called the Cori cycle. The anaerobic splitting of glucose also yields two molecules of pyruvate that get fermented to lactate. In the Cori cycle, the lactate is taken to the liver and converted back to glucose. However, this conversion uses up 6 ATP molecules, so it's not a perpetual motion sugar-recycling machine. In aerobic ATP generation, the pyruvate is taken through the Krebs cycle in mitochondria instead.
So, while on the whole, the Cori cycle loses 4 ATP, the liver's ATP molecules can come from aerobic generation of ATP before or after the anaerobic exercise. If you go through the Cori cycle once before switching to aerobic generation of ATP, you'll generate 26 ATP with the Cori cycle vs 30 ATP when fully aerobic. Less efficient, sure, but not 2 vs 30.
This creation of glucose from other molecules is called gluconeogenesis and it's also how your body maintains your blood sugar levels during intense exercise, starvation or a low-carb diet. In a way, your body is a factory dedicated to converting stuff to glucose, storing it, transporting it, and creating ATP out of it. Which makes sense on a cellular level, cells are all about breaking down complex molecules for energy and using the energy to build other complex molecules.
ATP ADP GTP GDP NAD NADH FAD FADH2 WHAT
ATP molecules aren't so much created from sugar, as they are "recharged". When a muscle protein uses ATP to do its tiny sliding walk, the ATP is converted into ADP by breaking off one phosphate group. This ADP then gets turned back into ATP by attaching the phosphate group back to it.
ADP stands for adenosine diphosphate, and ATP stands for adenosine triphosphate, so you can see how the addition of a phosphate turns a diphosphate into a triphosphate and vice versa. And yes, there's also AMP, or adenosine monophosphate.
Similarly, the NAD+ and FAD+ ions (nicotinamide adenine dinucleotide and flavin adenine dinucleotide, respectively. Those adenines keep popping up everywhere, huh?) are recharged to NADH and FADH2 molecules in the Krebs cycle.
The Krebs cycle can generate either ATP or GTP (guanosine triphosphate). GTP can be converted to ATP by swapping the phosphate chain of an ADP for the GTP's one. This is done by a molecular machine called the nucleoside-diphosphate kinase that turns the ADP into an ATP and the GTP into a GDP (guanosine diphosphate). This swap can be done in the other direction as well. Muscle cells tend to generate ATP in the Krebs cycle as they use a lot of ATP. Other tissues like the liver can generate more GTP, as it's useful in protein synthesis.[Citric Acid Cycle]
Right. To wrap up, your muscle cells break down sugar to convert ADP to ATP by adding one phosphate group to it. The ATP is used by the proteins in your muscles to help them climb past each other, which breaks off the phosphate group and creates ADP. The sugar can be processed anaerobically or aerobically. The aerobic processing is an extra phase added after the anaerobic process, and can be delayed for a later date with the help of lactic acid.
This sugar processing cycle is happening right now inside each of the 37 trillion cells of your body. Every move you make requires coordinating trillions of proteins to react in the right way at the right time.
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