By Dr. Jack Daniels
The ideal intensity of Threshold (T) training is spending time at the individual runner’s lactate threshold without losing the body’s capability of clearing blood lactate at the rate at which it is produced; and without an accumulation above the individual’s blood-lactate threshold.
I was lucky enough to spend a year studying in Stockholm, Sweden under Per Olaf Åstrand, and got to meet Bertil Sjödin on a few occasions. I also did a little research with Bengt Saltin. Interestingly, I think it was Sjödin who talked me into using the 20-minute duration for steady threshold runs. As I remember it, they found that 20-minutes provided the most benefit relative to the time spent running at that intensity. It seems to my recollection that 30-minutes ended up accumulating too much blood lactate, whereas 20-minutes was about the longest time that wouldn’t produce that overload response.
I came up with “Cruise Intervals,” from swim coaches (my original sport), who had swimmers go for repeated 5 or 6 minutes at normal “Threshold” intensity, with just 1 minute recovery between workbouts. The intensity was the same as used during a steady 20-minute T run. A runner can do a number of 6-minute T runs with 1-minute rests and accumulate a good amount of time running at T pace without producing more blood lactate than is desired.
Some elite runners can run at T speed that is 5-minutes per mile pace (or faster) and that means they can run 4-mile T runs in 20 minutes. Some could run 5-mile T runs and it was not the smartest thing for me to assume if some could do it, others should give it a try. The key issue here, and with lots of workouts, is that time spent at various intensities is more important than distance covered.
What I found to work very well during a season of cross country training was to have one Threshold session each week, with a steady 20-minute T run one week and 4 or more 5-minute workbouts (with 1-minute rests) the next week, and continue alternating the type of T runs that are performed each week. Cruise Intervals allow a runner to total more time at T pace and steady 20-minute T runs improve a runner’s mental attitude about running at T pace, so both types are ideal to include in a season of training.