Four By Four

I'm often asked for turbo sessions so I thought I'd detail one of my favourites - the Four By Four. If I had to choose just one session it'd be this one. It's a total of 25 minutes plus warmup, but it can be so hard I have to get a friend in for moral support.

Simply put:

• Warm up (I usually take about 15 minutes, with some bursts)
• 4 minutes on
• 3 minutes off (some people take 4 but I find it's too much)
• Repeat for a total of 4

How hard to go

These are supposed to be paced at your VO2Max pace. It's really common to use VO2Max in scientific studies, but it's a bit more difficult for those of us sat on a turbo in the garage. It should be somewhere around the maximum pace you could sustain for 6-10 minutes.
If you're a power geek and know your FTP, it should be about 110-120% depending on how fit you are (fitter people tend to have pushed their FTP closer to their VO2Max so will need a lower percentage).
Otherwise, the way I describe it is - at the end of the 4 you should feel that with a gun to your head you could manage another, but probably not two!
Within each interval the speed/power should be roughly constant; and each interval should be at broadly the same speed/power too. If your last is much slower than your first, you probably need to back off next time. Conversely, if you get much quicker over the intervals, you could probably give more.
It's quite difficult to use heart rate on these intervals as they're so short and heart rate has quite a lag - but you should be up at 95%+ of your maximum at the end of each one.

Evidence

There are quite a few studies of this and similar workouts. Seiler et al. [1] tested a few variations (4x4 at approx VO2max, 4x16 at approx FTP, 4x8 somewhere in between) over 7 weeks - they found improvements in all groups but 4x8 gave the best results. There are a few studies by Laursen et al. [2 and others] looking at shorter, harder intervals individualised by cyclist - 4 intervals at just over 2 minutes on average, showing some good improvements. Stoggl and Sperlich [3] tested almost my exact workout as part of a polarised or HIIT training program, and found that the programs including it were much better than those including only threshold sessions or just high volume.
I think the fundamental message from most of these is that intensity is important, and it should be above threshold - but how far is debatable, so long as it's hard!

Alternatives

In line with Seiler et al. [1] you could consider dialing down the intensity slightly and going longer (4x8 just above threshold) - but it's important to stay above FTP. Or you could pick Laursen [2] and go shorter and harder (with longer rest).
If you're a Sufferfest fan, A Very Dark Place is pretty similar to this session. Nine Hammers incorporates similar ideas, but is slightly less intense with a lot more intervals. I've not tried Fight Club but it looks like it might just fit (but with extra sprints!)

References

1.  Seiler S, Jøranson K, Olesen BV, Hetlelid KJ. Adaptations to aerobic interval training: interactive effects of exercise intensity and total work duration. Scandinavian Journal of Medicine & Science in Sports. 2013 Feb 1;23(1):74–83.
2.  Laursen PB, Shing CM, Peake JM, Coombes JS, Jenkins DG. Interval training program optimization in highly trained endurance cyclists. Medicine and Science in Sports and Exercise. 2002;34(11):1801–7.
3. Stöggl T, Sperlich B. Polarized training has greater impact on key endurance variables than threshold, high intensity, or high volume training. Front Physiol. 2014 Feb 4