Cardio Sample

Cardio in the real world: explaining all your worst feelings

I say “mile for time” and you heart rate jumps right away…it’s almost as if your heart knows what’s coming. Not only does your body prepare for the coming race by upping the cardiac output, it even opens up capillaries to muscles fibers it knows it will need, like slow twitch for distance races, and fast twitch for sprint races (that’s a little freaky to me…how does it know)? What your body is trying to do is up your O2 delivery to the muscles by maximizing cardiac output (and all its components we spoke of earlier). Let's see how this works during a race…


As soon as the muscles start working, they make all kinds of nasty stuff like lactic acid and urea (yeah…pee). These directly signal the capillary sphincters to open up and increase blood flow to the muscle fibers that are working. That’s good for the muscle, but the blood pressure drops because now there are more pipes open with the same amount of blood in the system, otherwise known as a drop in after-load. Your heart instantly reacts by increasing heart rate and contractility to maintain blood pressure.

Tangent: same thing happens when you stand up quickly and your heart beats real fast for a few seconds.

After a few minutes of working hard, some more things start to change in order to improve cardiac output and O2 delivery to the muscles. Remember how we said most of the body’s blood is sitting in the veins? This blood starts to get pumped forward by the contraction of muscles. This “adds” to the blood returning to the heart, increasing pre-load and increasing the stroke volume (kind of like being super hydrated). This is also why your veins start popping like it’s hot when you’re working out. Yet another reason why you need to warm up, because blood that is pooling in your veins gets put to work only when the muscles are working.


As workout starts getting harder, the body runs into a problem. There is only so much blood to go around, and the body needs to start choosing where to send it. The first thing that happens is organs that don’t involve swimming start getting less blood. Mainly these are the stomach, intestines, and kidneys. That’s not a problem if your bowels don’t have much to do, but if you ate a big meal just before working out it causes a big problem. Now, you just sent a whole lot of blood flow to your stomach to help digest food, and there isn’t enough left over for the muscles to use. That is why you feel so crappy and weak if you try to swim hard right after a meal, there isn’t enough blood to go to both your stomach and your muscles, at least…not if you want to kill it at practice.


Skin is another organ that can be responsible for “stealing” blood, especially in a hot pool. Your body doesn’t like to send blood to the skin during workout. Adrenaline, and hormones like it, do a good job of causing the sphincters around capillaries to squeeze shut and cut off blood supply to the skin during workout in order to force more blood into circulation, increasing pre-load. But, if your body temperature gets too high (which will melt your brain btw… literally), the capillaries will open back up and the body will choose to supply the skin more than then muscles, because to the body: a slow swimmer is better than a dead one.


Muscles that are not involved in the swimming set also get cut off, that would be arms during kick sets, legs while pulling, and calves pretty much all the time. This is why shifting from pulling sets to kicking sets is so hard. Those muscles were not getting any blood, and now you are asking them to work overtime without “warming up”. Without blood and O2, the muscles now have to rely on glycolysis for energy, producing a lot of lactic acid which can’t even be shipped out because there is no blood flow (that’s a lot of pain for little gain). That is one of the reasons for doing a short “re-warmup” of specific muscles before changing sets.


So how much can blood flow change based on what muscles you are using? An experiment showed that even working out one leg vs. both legs at one time will increase the flow to the one leg almost twice as much as when you are using two legs. In theory, that means the more focused your training, the better you are able to use that muscle and the more it will adapt since you can train it at a higher level. At the same time, you are not really challenging your heart because now it only has to send blood one place instead of everywhere (which is what is going to happen in a swimming race). The more places your heart is forced to send blood (arms and legs and abs… during IM sets for instance), the better your cardio will be trained and the better your VO2 max will be. This is the trade-off: Train muscles one at a time or train the heart. Understanding this concept can have a big impact on how your train.


Tangent: While this hasn’t been tested, if I had full control of my team's environment, I would train certain sets in a hot pool (endurance IM or combos of kicking, free and IM) to focus on developing the heart (since I’m forcing the heart to pump to all muscles and skin), and other sets in a cold pool (sprint, pure kick or pull) in order to focus on those muscle groups. In theory, that would maximize the training of both muscle and heart.

The hotter the pool and the more muscles you use at the same time will increase this effect, and your body adapts to that too. After starting a new season and training again, your body will “hang on” to more water and your blood volume will jump up to 10% in one week! This is actually the fastest and a very effective way to improve endurance ability. By having more blood in the pipes, it is easier for the heart to send it to multiple locations at the same time. This is why you feel so much better even after just a week of training (even though little has changed in your muscles or heart). This is also why you feel so bad after taking a few days off. As quick as your blood volume goes up, it goes down.

Tangent: Next time you start swimming after being off for a few weeks, take note that the first few days you will be drinking a lot more, and peeing a lot less. You might even see a jump in your body weight (a good increase).


Being dehydrated will have the opposite effect by decreasing the amount of blood in the pipes and forcing the heart to pump faster and harder to maintain the same cardiac output. This happens to you every single day. As workout goes on and on, and you don’t drink enough water to replace it, your blood volume slowly goes down, and by the end of workout your heart rate is way higher even if you are doing the same intensity workout. Want to measure how well you do at staying hydrated during practice? Measure your body weight right before and after a workout. The weight you lose is sweat.


The extra blood volume at the beginning of training is all water which dilutes out the red blood cells (the main cause of “athlete’s anemia”). But over a few weeks, the extra water gets filled in with new blood cells that are bigger and hold more hemoglobin. This helps to improve VO2 even more! Increases in blood volume are related to how much endurance training you do, and how many much you challenge your cardio system. So next time you’re working out in a hot pool and you feel like dying, instead of complaining, try to understand you are getting your heart in better shape than someone who is living the life of luxury in a cold pool.

Tangent: another idea I would like tested is whether taper would be better in a hot pool vs. a cold one, or at least slow stuff in the hot pool and high intensity in the cold one (a diving well near a competition pool would be ideal). This way you can maintain blood volume even though your work load has gone down, improving pre-load during tapered races.

Ok…that was a lot, but it is the foundation of understanding how all of this improves with training, which we will talk about later. For now I leave you with one more Tangent:

Tangent: About half of the entire world’s population undergoes these changes at least once in their life without training at all. The increased blood volume, the larger heart, the growing of red blood cells… can you guess who? Pregnant woman! Women undergo the same adaptations during pregnancy that a training athlete undergoes. This makes sense since being pregnant and giving birth is kind of like its own competition.