“Why these short rests?”

October 31, 2016

“So what’s the idea behind these short rest intervals?” This is the question posed to me by an athlete at my running training group after a particularly tough set of interval training. It’s a good question, which,  the half garbled answer I gave as I sucked air through a straw during the warm down did not fully address. As a coach and former teacher I hate to feel I have not given a good explanation. So Andy, hopefully this will give you a little more clarity on how we manipulate interval training to meet various aims.

When designing interval training sets there are three variables a coach or athlete can manipulate; pace, rest interval or distance (both per interval and in total). By altering the ratio of these three variables the intensity of the session can be changed and a different aspect of running performance developed.

Now in general terms the average pace dictates the intensity of the session. So, if you run at 95% of your mile P.B pace the intensity of each interval will be high but you will need very long rest periods to sustain the pace and will unlikely be able to sustain the effort for very many reps so the total distance will be low. If you run at 85% of your best mile pace then each interval won’t be so tough but you will need less recovery to sustain the pace and cover more distance. See below example for how recovery intervals can affect the intensity of the session for an athlete with a 5min mile P.B:

3x1600m @ 5.15/mile (95% max mile effort)  with 3min recovery = 8.15min/mile avg

3x1600m@ 5.45mile (85% max mile pace) with 1min  recovery = 6.45min/mile avg

A quick glance at this highlights the potential for covering greater distances in the second example. So in simple logistical terms by keeping the rest period shorter we can increase the volume of running so it is closer to race distance and increase the intensity of the session. Low recovery times in conjunction with a moderately fast interval pace mean most athletes can run between 6-10km of intervals at just a little quicker than their 10km pace. This is a great way to develop speed endurance. The key to achieving high intensity interval sessions is keeping the intervals to between 20-30% of the work interval. This way recovery is always incomplete leading to the development of greater aerobic endurance than if the recovery was longer (50-100% of the work interval).

Training at this pace means that athletes are able to spend long periods running at just above threshold (a proven way to improve race times) whilst the short rest intervals mean that they will be working at high percentages of V02 max (time spent running at V02 max is strongly linked to performance improvement). In addition, as the body fatigues due to the relatively high intensity of the session, fatigue occurs in the muscle motor units that are well adapted to this type of running. This forces less well adapted (fast twitch) fibres to be recruited and over time they become more efficient and develop greater endurance to this type of work. This means on race day your body has a broader base of muscle fibres to call on which will likely improve your times and give decent finishing kick!

But this is not just for super speedy runners looking to shave seconds off their 10km p.b’s. It is true that most runners spend most of their intervals running well above even their 3km pace whilst not building sufficient speed endurance to really bring down their race times. Most runners have more than enough ‘speed’. In other words their mile pace is a lot faster than the splits they can achieve in say a 5km.

Take this example.  I can run a mile in about 4.45min but struggle to hold 5.40min pace throughout a 5km. Speed is clearly not my problem. Endurance is! Running very fast, short reps, will unlikely improve my 5km time because it will not sufficiently improve my speed endurance. So, if I want to get quicker over 5km I need to run at lower percentages of my mile pace in each interval but at higher average speeds for the total session. To achieve this I have to increase the volume covered in each session and decrease the rest period. 

Of course there is a place for high rest to work ratios in the same way that we all need some long slow runs, when it comes to running training or indeed any training a variety of training stimulus is the best way to improve. That said, if you’re a 5-10km runner looking to crush your P.B then short rest long intervals are going to be the bread and butter of your training!