How the training load works?

[afarnedi]

Hi everyone, I've never taken too much into account this information (last 7 days training load) but today I've noticed a strange thing and probably it's related to the way this training load is calculated...yesterday my training load was 613, after a run of 45 minutes. Today I made the same run, around 47 minutes and the training load has decreased to 491. How it is possible that after a run the training load decreases? If I look into the previous 7 days my activities are almost the same, so I'm really confused on how this training load is calculated.
Anyone? nothing I can't live without, but just curious

[WillNorthYork]

Training load is based on intensity/heart rate and duration. Maybe the run you did 7 days ago was more intense then today's run. Even if they were the same distance and speed, maybe your HR was higher last week because of higher temperatures? Unfortunately, GC doesn't give you a way to see the "individual training load" for an activity, so the best you can do is infer it a week later, from how your 7-day training load changes day to day.

[tombom]

The way Garmin calculates training load is actually a bit silly. I have made the same observation as you. The training impulse from an activity seem to be counted 100% for 7 days and then the impact goes suddenly to 0%. The reasonable way to do it would be to apply exponential decay to old activities.

[WillNorthYork]

Yeah, sorry I didn't make that clear. The training load number that's displayed does seem to be the sum the individual training loads of your activities for the past 7 days (including today), without any kind of weighting.

So if like most of us, you don't train every single day of the week, then exactly 1 week after a rest day, your training load could suddenly go down, even if you have a run of normal intensity on that day. The easiest way to see this would probably be to train 6 days a week, but having a different rest day on consecutive weeks.

Some other sites will calculate a kind of training load (sometimes called "fitness") with exponential weighting, which may give you more sensible numbers. There's a few ways to see this for your Garmin data:
- Use runalyze.com, which is free and can automatically sync with your Garmin account
- Pay for Strava Summit to get "fitness and freshness" data. (Only available on the website, not the app)
- Use Strava for free, with the desktop Chrome extension/app Stravistix, which gives you "fitness and freshness" for free. You have to use the website on a computer, though

OTOH, Strava Summit has a "weekly relative effort" metric which is even worse - it adds up all your intensities starting on the first day of the week (as opposed to a rolling 7-day total), meaning that the number is meaningless 6 out of 7 days. (It will literally tell you "this week has been easier than previous weeks", right up until that final day of the week).

So I guess we should count our blessings.

[mcalista]

Some good comments on this thread already. I'd add a couple more points.

1. Activities from 7 days ago drop out of the Training Load number at midnight. So if you run every day, but haven't yet gone for a run today, there will only be TL from 6 runs in the system.
2. Garmin (or rather Firstbeat, whose software Garmin licence) base their TL numbers on EPOC (Excess Post-Exercise Oxygen Consumption). This tends to "reward" higher intensities, rather than longer durations compared with other measures such as Training Stress Score. For example, a 2 hour zone 2 run will give you a relatively high TSS, but low EPOC TL. A 40 minute run with some hard zone 4 and 5 intervals will give you a much higher EPOC TL, but a lower TSS.
3. TL does take into account your fitness level (as measured by VO2Max). If your VO2Max has increased during the week, then a run today will result in a lower TL than exactly the same run a week ago.

[afarnedi]

that's a way too tricky for me!!! As I said, that's nothing I can't live without so I rather go with my personal feelings than rely on this kind of information...anyway, thanks folks for your answers

[WillNorthYork]

No problem. Like mcalista said, you may just be getting in better shape. Does your training status show "Improving" or "Peaking"? This would happen if your fitness is increasing, even if your VO2Max looks like it isn't changing (because it's rounded to the nearest whole number).

But yeah, I wouldn't blame you for ignoring it. Personally I just like to look at the VO2Max trends to get a general idea of where I am compared to the past.