Three years ago, I was still running in the 16 minute range for 5K and now probably am just under 20 (I'm not running 5K's, so it's hard to say). That's a dramatic drop in speed, almost entirely due to a drop in maximal oxygen uptake, as that's the main factor in races of that length. I haven't had my VO2(max) measured in 25 years, because it wasn't useful information - the test was faulty - and one's 5K race times correlate very well with the number (3K works even better, but no one races that distance). If my oxygen uptake is lower, both the lab test and racing can measure by how much, but not why.
Oxygen uptake is a complicated thing and one can measure the different components separately; that should point out where the problem lies.
- Weight. Oxygen uptake is measured as milliliters per minute per kilogram body weight, so the less one weighs, the better. When I ran my fastest, I weighed 130-135 lbs and now am at 155-160. That's the majority of the difference right there. Right now, I need the strength that comes with the muscle mass (though not the fat!) for hills, but next year...
- Stroke volume. The larger one's heart, the more blood it pumps per beat. The more blood per beat, the slower the resting heart rate. Resting heart rate tends to vary with workloads and other factors, but mine's been as low as 34 recently. That's world class.
- Maximal heart rate. One needs a heart rate monitor for this, as one can't measure a pulse accurately when very fast. Running as hard as possible up a steep hill will stress the heart to maximum (if the hill's too short, it may take more than one time going up). Maximum heart rate drops with age due to slower nerve conduction and mine's dropped from 192 to 181. Pretty good for my age, but a definite drop. One can't increase this number, though. My measurement went from 178 to 179 last year to 180 to181 this year, but not because it was increasing, but because I was finally able to push myself to maximum.
- Duration at maximum heart rate. This is my downfall. The length of time one is able to maintain one's maximum heart rate is an important measure of one's racing ability at short distances. I used to be able to manage 10 minutes (unusually long) and now only a second or two. This comes with practice and means I'll need to do hard intervals and race a lot. This isn't actually used in VO2(max) measurement, but it's an important test. This can be improved with practice.
- Lung capacity. This influences how much oxygen one can get into one's blood. One can make a respirometer by filling a container with water and inverting it into a filled basin, then running a tube into the container. One simply blows through the tube, forcing water out of the container; the volume of air in the container is the lung capacity. I used to be just over 4 liters (you need a big conainer for this); 5 with some diving tricks like buccal pumping. I haven't done this in a long time, as I don't think it's very telling. One can increase lung capacity with breathing exercises, but I think hard running does it just as well.
- Transport efficiency. This is also a complex thing, dealing with red blood cell counts, enzyme levels (2,3-bisphosphoglycerate phosphatase, among others, for the scientists), capillarization... The simplest way of testing this is holding one's breath. [I have to point out that breath-holding is always a dangerous thing. Consider yourself warned.] One's lung capacity partly determines how long one can hold one's breath, but the rest is dependent upon how effciently the body can use the little that's left after a minute. I used to be able to hold my breath 4 1/2 minutes (more than 5 with some tricks), but am down to under 2 1/2. This can be improved through hard running or by practicing breath-holding. A fun note: Emil Zatopek used to do sprints while holding his breath; he also used to run carrying his wife on his shoulders!
Other factors can come into play, like temperature, elevation, nutritional status, hydration and electrolyte balance, medications, stress, etc. These have to be dealt with, but aren't trainable.
Looks like I'll be doing some pretty hard workouts and eating less next year.