The Science of Speed Work

Why do we have so many different types of speed work? Isn’t one the best? Why don’t we just do that all the time to get faster? Also if that’s the case can it please not be hills? I’ve been doing speed work in a dedicated fashion for like 5 yers now. I know it’s supposed to make me faster but I don’t understand why. Does it do anything else because it seems really freakin’ hard? If it’s on the training plan schedule I just do it. I’ve never thought deeply about speed work, until now. Does it even work? So let’s find out together shall we? That and science starts now!

What is speed work

Speed work is a specific workout you do (usually once a week) in training where you specifically go faster or work harder with the overall aim of getting faster. It also turns out that speed work is an efficient way to go further boosting your endurance. Be very careful about training plans that suggest doing more than one of these types of workouts a week. The commonly held belief is that it’s best to limit these sorts of workouts to about once a week to minimize injury risks. Doing too many hard workouts each week can also just zap your energy for the rest of your life too. These workouts often center around going faster than usual ( Tempo runs, fartleks and interval runs) or just being harder (hills and two a days). 

Speed workouts in general are geared to increasing your lactate threshold which has long been shown to increase your running performance over all (Allen et al, 1985). As you exercise your muscles produce lactate which turns into lactic acid. At some point that amount of lactate produced starts to take off exponentially which is to say really, really fast. Now we get into the threshold stuff. Your muscles can clear that too a point but eventually you are producing more than your muscles can clear. When levels get too high you just gotta stop or at least slow down and let those muscles catch up. So athletes are always trying to move that point forward so that their muscles can clear all that lactic acid they can push harder for longer. Speed work promises to help move that needle. The idea is that in training you repeatedly practice at that high level of exertion and then your muscles ‘learn’ to deal with more lactate.

It looks like you have at least 5 options for getting faster then. You might enjoy some more or less than others or hate one the very most. In my running life I find all the kinds of speed work hard and my natural predilection is to do literally none of them unless told to. I’m probably not alone there either. There’s also evidence that when you do what you do matters.

Hill repeats:

This is exactly what it sounds like and that is running up a hill over and over. You can be that team serious about it or you can just run a particularly hilly route if your more low key about it. Hill running is unique among speed workouts because it doesn’t ask you to run faster than you are used to. Also running up hills generally forces you to improve your form and though it hurts is unlikely to lead to injury. That’s only the case if you walk down those same hills though because running down hills can lead to injury if your form isn’t on point. Running hills is thought to increase lactate production due to the added stress of gravity and therefore push your lactate threshold further. 

Studies on hill running were harder to find suggesting it’s been studied less. One great one I did find compared runners who ran on the flat to those that did long slogs uphill and those that ran hill intervals (which is as terrible as it sounds by the way.) All those that ran hills showed increases in lactate thresholds however those that ran intervals had higher gains than the slow hill runners even though they spent less than half the time on the incline (Ferley et al, 2006). 

Tempo runs:

This was the last sort of speed work that I became aware of along my running journey. During a basic type of tempo run you start at a certain pace, after a while go faster, then speed up a bit and then push even harder. Basically you just keep going faster and faster through the same run. It might also include a cool down portion at the end. Tempo runs are great for increasing lactate production because in theory at least you run past your lactate threshold once then again at a higher level which you maintain over and over again during one workout. In my experience at least tempo runs feel exactly as unpleasant as that all sounds.

It turns out tempo runs and interval runs have lots of overlap in the research. Tempo runs are one of the two best options for increasing blood lactate levels from running (Saraslanidis et al, 2009) especially during the training phase of race preparation (Kurz et al, 2000). Tempo runs again one of the best predictors of performance again along with intervals (Casado et al, 2019) when compared to all of the options.


During an interval run you are meant to go faster for short periods of time interspersed between bouts of more moderate effort. You’ll have a time or distance goal for each interval and probably a target speed. As a plan continues you’ll usually be asked to make those intervals faster and longer week after week. So this means at several points during the same workout you are performing past your lactate threshold for fairly short periods of time and increasing your threshold several times during the workout typically to a greater extent week by week.

Interval runs are probably the best studied of all the speed work options and have constantly been shown to cause a big spike in lactate production at similar levels to tempo runs (Saraslanidis et al, 2009). Other studies however have shown that intervals are more effective then tempo runs in terms of the amount of lactate produced (Helgured et al, 2007). Intervals might not be the perfect solution for every speed work scenario. When runners ran intervals during the taper phase they did worse on race day but when run in training led to better performance (Kurz et al, 2000).


Fartlek is Swedish for speed play and is basically a more fun free and easy version of an interval run. Classically as you run you pick an object down the road and then sprint to it before  returning to a more conservative pace. Essentially Fartleks are supposed to have all the same benefits of interval runs and work the same way. Except, you’re in control of the interval and intensity so it should feel way less sucky and in theory you’ll let up some as you get more and more tiered. I like to do fartleks by the tempo of music that I listen to one slow one fast song which is still really fun rather than sprinting to a series of distant trees.

Fartleks were shown to lead to slower performance during base building stages and taper phases for cross country athletes (Kurz et al, 2000). Other than that after doing a deep-ish dive I really couldn’t find well done studies on fartleks.

Two a days: I’ve done this exactly once and it was unpleasant to say the least. It is what it sounds like, running twice a day. It’s most often suggested that you do an easy recovery run later in the day after a speed workout earlier in the day but really if you’re running twice a day no matter the effort, it counts. This sort of training looks to increase your lactate production in a different sort of way by getting you to run on muscles that are already tiered. This is meant to make your lactic acid cycling more efficient and overall train you for those fast endurance races. Some claim that it lessens the time taken to recover after a tough run.

Running twice a day was shown to be detrimental to performance between training cycles (Kurz et al, 2000). When athletes train the same amount per day either divided into one session or two at least some studies show that there is no difference in performance (Mostardi and Campbell, 1981, Willmott et al, 2016) 

What does it all mean?

Well taken together it means that speed work does work. That is great news if you want to get faster but not such great news if you, like me, hate doing speed work. You will get results if you do the run faster thing with interval runs and tempo runs but the jury is still out on fartleks which is personally insulting to me as these are my favorites. Maybe save these for speed days when you just can’t face intervals or a tempo run. If pushing your pace isn’t your thing hills will work, slow or even better if you run up them fast. Splitting your workout into two sessions probably isn’t magical.

Here’s my thing with speedwork. Do some of each type you gotta find out what you like after all. Since I don’t love it in my off season I only go for speed work when I feel fast or like I’m up for it that run. Which is to say not often. Durning training I pepper in all the different types during the cycle each week but as the week milage creeps up it starts to feel like more of a chore. If I can’t face the 14 km run as a series of intervals I do a fartlek and call it a day. Though I’ll be less quick to make that call after doing this research.

I believe that the nitty gritty on the science here is only for the elite to follow at least obsessively. Take the science for what it is worth for you. If you love running two a days, you’re weird, but go for it. If tempo runs make you feel terrible swap them out for something else. For us mere mortals it’s probably not something that we need to freak out over at all. But remember over all speed work not only is a key tool in getting faster it’s also important for building endurance and conditioning for race day. So probably do some.

How do you feel about speed work? Love it or hate it? What is your least favorite and favorite type to do? Have you found including it makes you faster? I have!

Read what I read:

Allen et al, 1985. Lactate threshold and distance-running performance in young and older endurance athletes. Journal of applied physiology.

Casado et al, 2019. World-Class Long-Distance Running Performances Are Best Predicted by Volume of Easy Runs and Deliberate Practice of Short-Interval and Tempo Runs. Journal of strength and conditioning research.

Ferley et al, 2006. Incline Treadmill Interval Training: Short vs. Long Bouts and the Effects on Distance Running Performance. International journal of sports medicine.

Helgured et al, 2007. Aerobic High-Intensity Intervals Improve VO2max More Than Moderate Training. Journal of the American College of Sports Medicine.

Kurz et al, 2000. The relationship of training methods in NCAA division 1 cross-country runners and 10,000-meter performance. Journal of strength and conditioning research.

Mostardi and Campbell, 1981. Effects of Training Once vs. Twice Per Day and Improvement in Maximal Aerobic Power. Ohio Journal of Science.

Saraslanidis et al, 2009. Biochemical Evaluation of Running Workouts Used in Training for the 400-m Sprint. Journal of strength and conditioning research.

Willmott et al, 2016. The effects of single versus twice daily short term heat acclimation on heat strain and 3000 m running performance in hot, humid conditions. Journal of thermal biology.

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