Ok, I think I’ll talk about the bodies energy systems. From where does our energy come , what energy system does the body use for various activities, how is it stored?
This is going to be another thumbnail sketch of my understanding of it.
Swimming movement comes from muscle contraction. All energy for muscle comes from inputted energy that is derived from food.
Energy is the body is stored in five different ways:
ATP (Adenosine triphosphate)
CP (Creatine phosphate)
ATP is actually used for muscle contraction. It’s stored in the muscles (and liver) and once it’s used it has to be reconstituted or replaced, but there is a large store and it is readily replaced.
CP’s main function is the repair of the used ATP. There’s only enough in the body to last for a few seconds of all-out effort such as a sprint.
Once the CP is used ATP has to be repaired or replaced from other sources. The main sources are carbohydrates.
Glycogen is also called blood sugar but is stored in both the blood and liver and used in the muscles. It is derived from glucose which in turn is derived from carbohydrates (sugars/starch) and to be used it turns back to glucose .
If you have a pretty good diet, you should have sufficient muscle glycogen to provide energy for two to three hours. However regular heavy training depletes this store, which is why serious training, like Channel training requires a serious increase in food intake. (for example jumping my normal approximately 2 and a half thousand Calories per day to anywhere from 6 to 8 thousand daily last year, as for most Channel swimmers. Blood and liver glycogen takes time to get into the muscles, so endurance athletes have to keep the store up by a constant supply of glucose. For cyclists, running out of glycogen is called the bonk (or the knock in my racing days).
Fat also supplies ATP. (White) fat is a very dense energy store. It’s the bodies emergency energy store. However it requires oxygen to be converted and it’s slower than carbohydrates (which can be converted both with and without oxygen, aerobic and anaerobic). It’s not useful for sudden energy demands so can only be used for lower energy rate requirements. (This is one of the reasons why swimming often isn’t a good exercise for losing weight, since even poor swimmers will get a high cardiac rate due to poor breathing or technique). Fat is used by endurance athletes by staying well below the anaerobic threshold.
A primary reason of the developed world weight issue is because humans have excess food for the first time ever. Extra food metabolised is stored as fat against an emergency future requirement.
So Channel and marathon swimmers will start to metabolise fat once the glycogen is used, and since the body can only metabolise a certain amount of carbohydrates per hour. This the reason we lose a lot of weight over a short period (4 kgs for me in 2 days). Carbohydrate metabolism is improved by a 4:1 carb:protein ratio.
Protein however isn’t stored as a free resource in the body but is all used (in muscles etc). Protein is used mainly therefore as a muscle repair energy source. But if someone trains with low glycogen stores too often, muscle is used leading to muscle loss.
Any corrections delightedly accepted.