HIIT what?

High-Intensity Interval training (HIIT) is a popular aspect of many training programmes, essentially hard intervals followed by rest, then repeat (I'm guessing you already know that but if you are not sure this is a nice summary and buffed.com takes a look at the best HIIT workouts here including those for endurance) HIIT is what makes you want to cry more often than anything else - that's a fact I believe.

So understandably, there has been a lot of coverage that watchOS 4 Workouts supports HIIT.

But what does "support" mean? It is not, as I first supposed, a guided structured workout, or even a timer for the intervals. In fact it is much simpler in it's ambition. Here's how Apple describes it:

So it is an optimised algorithm to calculate calorie burn and measure HR accurately, taking advantage of the accelerometer as needed.

So that's clear then.

But how does it perform?

One of my readers asked if I could test HR performance during HIIT using Apple Watch vs a Garmin Forerunner 935 with a HR band. Your wish is my 30mins of agony.

However it didn't turn out quite as I expected. My plan was:

Start an Outdoor Run with the Workout app and run a bit through the woods nice and easy
Get to a triangular bit of land I like to do intervals on, stop the "Outdoor Run" and switch to HIIT using the new multi-sport feature in watchOS 4
Do a few intervals running up the gentle incline to raise my heart rate then take it easy back down
Switch back to Outdoor run and run home.
All the time record the run on my Garmin 935 paired to a Bluetooth HR band (Tickr X)

Here's how the run looked on the 935 exported to Strava.

On Apple Watch I logged the first part of the run (up to that triangle at the top of the Strava image) as an Outdoor Run, then switched to HIIT, then back to Outdoor Run. Here's a terrible quality video of me running up the hill showing my HR at the top and bottom on Apple Watch:

Blurry video showing HR going from 138bpm to 179bpm for an overweight 52 yr old running up a hill in the dark (not speeded up at all - honest guv)

The main misunderstanding on my part here is that the HIIT session did not record any GPS data, rather it just recorded the HR. Here's how that HR looked though on the Workout app, for all three segments

Summary

Run there

HIIT

and run home

I actually messed up the first run up the hill and paused the Apple Watch which is the blank HR area above. You can also see this in the comparison to between AW and the 935 + Tickr X where there is a flat part of the HR data below at about 42 mins. But apart from that they track very well.

If you want the original data you can download from here.

So what have we learnt

I've picked up a few things from this:

HIIT workouts on Apple Watch Workout app are really meant for Indoor use and not outdoor, which sorta makes sense really.
They don't offer any structured workout guidance or timers, but instead promise better HR tracking when you are working hard and your HR is changing quickly.
In this test the the HR data from the optical sensor on Apple Watch tracked very closely to the HR band connected to a Garmin 935 which is very positive.

In the end, this is probably not the best test for HIIT. An indoor HIIT session would be a better use case, especially one with much quicker and harder intervals.

However I'm not convinced you really need to bother with HIIT workouts. In my case I have been perfectly satisfied with the HR data from all the sessions I have done indoor and out without ever choosing the HIIT workout type.

It feels a little over-hyped, like it would be great to have HIIT workouts in the marketing material for watchOS 4, and many of the reviews I have read seem to make a big deal of it, without really explaining why, other than in a fairly shallow "this is good for those who do HIIT workouts" type of way. I think many people reading some of those review would have expected something like a workout timer, or structured plans (I certainly did), but that is not the case.

Still I am happy to be concede that if you are doing an indoor workout that doesn't fit into any other category such as indoor running or cycling and involves Intervals then you lose nothing in choosing this, and potentially gain more accurate data, which can't be a bad thing.