Alternatives to CTL, ATL and TRIMP for running?

Question

I'm an avid runner and like so many other runners, I like to get a measurement or number for my current fitness. I use the measurements for two purposes: to see how the different types of exercises affects the fitness and to see how much I can challenge myself for a specific race.

But how exactly can I measure fitness?

For the last 4 years, I have used SportTracks and in particular the Training Load plugin to calculate CTL, ATL and Training Stress Balance and have used these numbers as a measurement for my current fitness.

Historically, these numbers comes from work by Dr. Coggan and others (as described in this article by Hunter Allen) for use by cyclists and was later adapted for use by runners. Originally, CTL and ATL was based on a power meter (TSS - Training Stress Score), but for runners the concept of TRIMP (TRaining IMPulse) was used instead as it is rather difficult to directly measure the power used by a runner. As TRIMP is based on a simple accumulation of your heart-rate over the course of the run, it makes good sense as an alternative to TSS.

But... I curious, and I wonder whether there are other similar (well-founded) numbers that can be used to calculate your fitness?

I forgot: One such number is rTSS (running TSS) by Dr. Steve Macgregor, but it seems to be nearly impossibly to use for your daily runs as it requires many subjective numbers...

Answer 1

Another variable to track training (and life) stresses is heart rate variability (HRV). It will go up as fitness improves, but will go down as life stresses and overtraining impact the ability of your body to cope/recover. Here is an article by Dr. Phil Maffetone that discusses it.
I use the ithlete app mentioned to track my HRV score. There may be other apps by now that work similarly.

Answer 2

First Beat (see White papers) has a concept they call "Training Effect" that guides your training based on the history of the last couple of months and your last traning (besides weight, age, sex,..)

In their system, fitness is measured through two parameters: Activity Class and Training level. Both are based on the activity for the last couple of months and they are used to determine how hard you should train for the next couple of weeks.

I have been using it for the last 5 years and it has helped me to train at the right level of effort. My main issue before the using the system was that I trained too hard and got injured.

A drawback of the system is that it always tries to guide you to higher training levels, I would like it to have an explicit "maintenance mood" since I want to vary my training over the seasons (harder in the spring/summer, easier in the fall/winter).