Can someone explain how a 20 minute threshold run is enough stimulus to trigger adaptation?

Think about it like this: you need about 10 days to recover from a hard 1 hour race.

In that time you could do and recover from 3-4 20 minute tempos, likely getting a better cumulative improvement.

Thanks, that makes sense, although I still wonder about the minimum dose required to improve, for a single session.

3-4 20 minutes tempos are better, if they are stressful enough to force the body to adapt, which again is the piece I'm hoping for more info on, not that I doubt its accurate.

The biggest adaption is a longer stride for the same effort. Running is a skill. That's the point your missing. This fact is overlooked by most runners, coaches and exercise physiologists.

To develop the skill takes times as well as effort.

I think the “evidence” is more experiential than scientific, or at least I’m not aware of and would love to see studies showing that 20 mins of tempo (once per week?) is optimal or the point of diminishing returns. I suspect the optimal number would have to vary with individual physiology and goal race.

I think a lot of the evidence points to more volume being beneficial. Daniels recommendation was 20-30 mins. 30mins is a lot more training than 20. Or things like 6-8*1 mile. And most people recommend a lot more than 1x week for good chunks of the season. There is always been some debate about intensity versus volume. Is 30mins at HM pace better or worse than say 50m at MP?

Keep in mind that threshold pace is defined as "60 minutes all out," like during experimental conditions in a lab or a race. At the end you're tapped out, dead and fried. You need several days to recover.

Sure, that's in theory a "better stimulus" than running that same pace for 20 minutes, but with higher risk. Alternately, running that pace for 20 minutes, gives you just enough stimulus without the fatigue and exhaustion that would take you out of commission for longer.

Therefore, you can do the 20 minutes tempo many, many more times than the all out 60 minutes at the same pace, and in the long run get a better cumulative stimulus, with less risk of injury, fatigue and over training. With repeated 20 minute tempos, there's no taper or significant recovery period needed. WIth a 60 min all out effort, i.e. race effort, you would best taper and recover after. That lowers the overall mileage and overall aerobic training load you're capable of.

It isn't. I've seen runners doing 20 minutes at 6:00 and then running something like 18:24. I've seen 5 miles at 5:30 or cutting down down to 5:10 and barely going sub 16. The tempo doesn't work for most.

In general, you need about one day per mile to recover from a race. So, you'd need about 10-13 days to recover from it depending on how fast you are.

However, if you can hit more mileage at threshold over a couple of training sessions with less strain on your body, you'll be training more. So, think of it like you're gaining a stimulus of "4" 3 times instead of gaining a stimulus of "10" 1 time. These are just arbitrary numbers, but if you're able to stack lighter workouts back to back you'll keep improving more, and it lessens the importance of any one workout. If you're having one very hard workout per week and the rest is recovery, if you have a bad day you've lost all training for the week.

I also believe that the damage / tiredness grows exponentially. What I mean is that running (for example) 2 miles at 10 mile pace will barely tire you out - maybe it's 10% of the energy needed for the entire 10 mile race, and running 4 miles could only be 25% (so you can do another workout faster), but you're getting 40% of the stimulus.

You have answered your question in your own post.

The tempo isn't stressful enough to produce adaptation for most people, it's just a watered down race effort with no solid science behind it.

Intervals will work for you and 95% of runners.

Why do a 20 min tempo and not 45 minutes of cumulative interval training at treshold?

They weren't running tempo were they? They were time trialling, almost racing in training.

And to get more cumulative training stimulus, you can slow down a bit and extend duration a lot.

For example: Train like a marathon runner without racing a marathon.

Right, that makes sense. Sounds like some others are saying 20 is on the low end, even if its still used. I guess then it comes down to whether or not the 45 cumulative is detracting from subsequent workouts, even if there's some rest.

Side note, anyone have any opinions on the interval aspect of the original post?

Lactate Threshold or "Tempo Runs" should require a bit of pain. Like 85% overall effort.

If you don't think 20-min in a zone over 85% max HR can cause adaptation then what about a workout like 4 x 400m or 5 x 1km only? Even 10-15min at Vo2max (3km-5km pace) on short intervals can cause a lot of adaptations. It's not just about flooding the system with lactate, but also recruiting fast twitch muscle fibers to improve Running Economy (efficiency) at faster than Marathon Race Pace.

But it isn't one specific session but a mix of sessions over weeks and months (as well as repeating that 20-min Tempo Run multiple times) that yields changes over time. Mixing in shorter, faster intervals with Strides and Long Runs with progressively higher mileage.

And think if someone wasn't running at all vs someone who ran (Easy) 3 times a week a 20-min a time. The latter would be a runner that would have all sorts of health benefits....even from just jogging 20-min at a time.

Sure, if you're running over 80mpw and have a huge base (And are a pretty fast runner), then maybe you would do something bigger like 2 x 20-min or 4 x 2-miles (10-min) or even 3 x 3 miles in a single Tempo/LT session.

Not going to go into a ton of detail now, as I’m on my phone, but you’re looking at stimuli in the wrong way. The objective with any workout is to disturb homeostasis enough to stimulate AN adaptation or to prevent loss of an existing adaptation. Big stimuli are only necessary when small stimuli stop working. If a small stimuli will produce an adaptation, then going beyond that generally doesn’t give you (much) more adaptation. It does wear you down, meaning you can’t workout again as soon and making it more likely that you will be unable to sustain training over the weeks necessary to achieve a properly timed peak. The place for “big workouts” is pretty late in the cycle.

Is there any research that shows that increased volume is what elicits an aerobic endurance adaptation? What exactly is/are the stimulus(i) that makes one adapt? I'm genuinely curious because a lot of running advice, just like lifting advice, comes from opinions and hearsay without much scientific validity.

I think another factor that's overlooked is that you are also looking to "turn on" certain genes that have a signalling effect in the metabolic process. As far as that's concerned all it needs is the amount to be sufficient stress, so that will happen.

20 min at 60 min pace would be considered the minimum effective dose for most people. You could do 30 minutes at 60 minute race pace, but it would only trigger slightly more adaptations.

Is 60 mins max pace the baseline assumption for threshold?