The heart, though made of muscle tissue, is not a muscle, but a pump; cardiac tissue looks different from muscle under a microscope, functions differently and uses different fuel. How it responds to training is also different. One would think that the heart should grow in response to training, so it can pump more blood per heartbeat - that an athlete's heart rate slows at rest supports the idea - but things are more complicated. The average sedentary person's resting heart rate is 68-72 beats per minute. Mine this morning was 34 (almost a record!), but my heart is only about 25-30% larger than average. People with congestive heart failure have enlarged hearts, but are obviously unhealthy. If the heart grows too large, it takes up space needed by the lungs, so there's a limit to how large a healthy heart can get. After reaching this maximum size, the lungs, muscles and blood all adapt to more efficiently transport oxygen to exercising muscles.
This is what training for maximal oxygen uptake is all about.
The heart doesn't immediately respond fully to intense exercise, but there's a time lag. The reason for this is that oxygen is toxic in doses just slightly greater than usual; the body has much better methods for dealing with temporary lack of oxygen than with excess oxygen, so it errs on the side of safety. This lag results in the body having less oxygen than it needs, the so-called oxygen debt. When one starts running, or excelerates, the heart responds by slowly increasing its rate of beating and this continues until the oxygen debt is repaid or the heart reaches its maximum rate.
There is nothing one can do to increase one's maximum heart rate. [How I've tried!] One can only make everything else in the oxygen delivery system more efficient. The way to do this is by interval running: run very hard, let your heart rate get to maximum (or near it) and then let it drop back to resting, or a rate close to "normal" training rate. Then do it again. And again. Repeat until one starts to feel fatigue; beyond this is lactic acid tolerance training.
Slightly more practical stuff
If you have a heart rate monitor, it's easier to figure out how to do this, but the beauty of the method is that one doesn't really need to be precise. It's as simple as alternating hard and easy paces, which is why one doesn't need to run intervals on a track and measure everything accurately (though that too can be useful). The free-form alternating of fast and slow is known by the Swedish term "fartlek," pronounced "fahrt-lake," and universally called "speed play" by coaches of easily amused high school students.
The way I do it is to work on taking steps as quickly as possible, getting my cadence to about 190-195 steps per minute (I manage 160 at 8 minutes per mile, 180 at 6 min./mile), just to have something to focus on. I set my monitor to measure how long my heart stays between 171-177 beats per minute (95-98% max) and note the time - only after finishing the workout. Generally, 5 repeats of 2-5 minutes hard alternated with 3-4 minutes easy (or until heart rate drops to 125-135)... But, that's just me.
For beginners, it's best to do these workouts on a track, because one can keep track of improvements by doing the same workout each time.
Fisher's Big Wheel
1 week ago