One of the best measures of fitness change is your output-input ratio. That may sound quite technical but you use this a lot when talking about your car. You call it “miles per gallon” or “kilometers per liter.” That’s a nice way of measuring how economical your automobile is. In this case, high numbers are good and low numbers are bad. Your wallet likes it when mpg/kpl is a high number. Of course, “high” and “low” are relative terms. High or low compared to what? If your old car got 18 mpg and you buy a new one that gets 25 mpg you’re happy. But if the old one got 32 then 25 isn’t so great.
It’s the same for your fitness. For performance, one way of using output-input ratio is to express how fast you are going relative to how much effort it takes to go that fast. Output can be measured with speed using a GPS device or power using a power meter. Input can be measured with a heart rate monitor.
So, for example, in running if you average 10 miles per hour (6 minutes per mile) in a 5km race and your heart rate averaged 160 then your output-input ratio is 0.063 (10 ÷ 160 = 0.0625). If in a 40km time trial in a bike race your average power was 280 watts and your heart rate was 150 then your output-input ratio was 1.87 (280 ÷ 150 = 1.8667). In either case what you’d like to see happen in future races is that you run faster or your power goes up but there is little change in your heart rate. Aside from the faster times, that would be an indicator of increasing fitness.
But it doesn’t have to be a race to use O-I ratio. This can be done with any standard workout, but works best with steady, as opposed to highly variable, outputs. And it also works best with relatively long efforts due to the lag in heart rate change when output increases. With intervals, for example, it works best when the work interval is around 5 minutes or longer.
Let me describe how I’ve used the O-I ratio myself recently.
You may recall that on April 9 while at a camp in Spain I crashed on my bike and fractured my pelvis. I commented on this in an earlier post. I was lucky in that the bone was not displaced so it just took some time for everything to heal. For the first 11 days I pretty much did nothing other than whine and moan so my wife would feel sorry for me. Then on April 20 I decided the pitty party was over and got back on my bike, albeit on a trainer in the garage. Basically, all I did was turn the pedals with no concern for output or input. This was for my head, not by body. On May 3 I started using my power meter and heart rate monitor again. It had been 23 days since the crash so a lot of fitness was lost.
The first order of business was to rebuild aerobic endurance, the most basic ability for endurance athletes. Beginning on May 3 I rode with as much time as I could manage at my aerobic threshold heart rate of 120-125 bpm. For the most part it was an hour a day, although some were as short as 15 minutes and others, more recently, up to 90 minutes. Every day I observed what happened to power at that HR range and recorded the daily output-input ratio. The accompanying chart illustrates the changes that happened to the ratio over a 17 day period.
On April 3, 6 days before the crash, I had done a 98-minute aerobic threshold (AeT) maintenance ride. My average power then was 175 watts with an average HR of 120 for an output-input ratio of 1.50. After 23 days of doing almost nothing (other than issuing proclamations to my wife-nurse) my first AeT O-I ratio was 0.68 (84w and 123 bpm) – a 55% decrease. Pathetic. It probably wasn’t all fitness loss, however, at least in the traditional sense. When there is an injury the muscles in that area of the body are inhibited from working normally. This is a protective measure. There is no doubt, however, that there was a lot of aerobic fitness lost in addition to the muscle inhibition.
During this period I rode every day doing the same AeT workout except for one (day #7 was a complete day of rest). Notice the steady climb in the ratio on the chart since my first AeT ride. This is an extreme example of fitness change given my situation. Someone who is in moderately good shape will see much less change over the course of several weeks as training ramps up, and there will likely be a large number of downturns due to fatigue and other confounding variables.
If you only measure input using a heart rate monitor and have no gauge of output then you are limited in how to determine changes in fitness other than by racing, which isn’t always an option. Races also don’t tell you much about specific intensities such as aerobic threshold. Of course, you could pay to have a VO2max test done every month. Or, lacking GPS or a power meter you could run on a track or ride on a velodrome. You could also do a workout on a measured section of road or hill at given heart rates comparing time with HR, but this certainly limits your training options. (Speed isn't a good measure of performance on a bike due to the exponentially increasing wind drag as speed increases linearly. This affects running to a lesser extent since the speeds are so much lower. Excessive, head- and tail-winds also affect both considerably.)
Having output-measuring devices gives you a lot of options for workout venues and, essentially, makes every workout a test of your fitness progress.