4 min read

Internal vs External Load and some weekend thoughts on Marathon training.

Internal vs External Load and some weekend thoughts on Marathon training.

When you’re doing a run workout, a useful concept is to consider the load (the stress being applied to the system) in terms of whether it is an internal or an external load. Another way to conceptualize this is to think about whether you’re simply looking for a metabolic stimulus, or whether you are looking to dial in an exact pace.

I’m currently coaching a group of local marathoners, most of whom are targeting CIM in early December. They are all fit and accomplished athletes, and are coming to their marathon training in pretty good shape from a summer of trail running. Marathon coaching is quite different from trail run coaching, and if I’m being honest, it’s definitely a bigger challenge to get right. It’s a mathematical problem, and one that is quite different to solve than the solution for trail or mountain ultras! When I think of marathon training, I see three different issues:

(1) Fuelling and substrate use.
2) Musculoskeletal strain
(3) Speed and specifically speed over a long distance

There’s a lot to unwrap here! I’ll be brief and won ’t go into too much detail, but feel free to message me or add your thoughts in the comments below.

Firstly fuelling. We have enough glycogen stores in our muscles and liver to last around an hour to an 90 mins if we were working hard. We have enough fat stores to last us all day. We need to be able to tap into our fat stores, and preserve glycogen, or we’ll ‘bonk’. This is the proverbial ‘hitting the wall’ that marathoners talk about.

Musculoskeletal strain. Our joints, muscles and connective tissues need to be able to withstand several hours of hard pavement pounding. Throw in a few hills and this is a big challenge.

Speed. Most marathoners will have a goal time they are looking at. They use charts and tables to figure out their pacing strategy. Many of you who have run a marathon will be well aware that whilst it is easy to hold a prescribed pace for 30 or 35km, it can be a real challenge at the end of your marathon to hold that desired pace for the final few kms. To hit your goal time, however, you need to be able to run at that pace for 42.2km. Data shows that most marathon finishers run a positive split. The second half is typically run slower than the first half. To hit your goals, the aim should be to run as close to an even split as possible, and that’s tough.

So what has all this got to do with the original concept of internal vs external load, and how can we use this idea in our workouts?

Well firstly, in the early stages of a training block, we should be more concerned with internal load. Paces are less important. We do more easy paced running, and the goal here is to improve our fat utilization, promote better glycogen sparing, and to condition our skeletal system to the rigours of hard road running. I consider this the internal load. So if a long run is being done early on in a marathon block, it is likely that the actual pace you run at is a secondary issue. We just need to put in the miles and an easy to moderate intensity. These adaptations are slow to develop, but there are long training ‘residuals’, meaning that the adaptations hang around for a long time, and are easy to maintain. Strength training, including hill workouts, as well as longer slower efforts rule during this phase. Internal load is what matters here.

At the end of our training blocks, as we approach our goal race, however, we really need to start to dial in our pacing strategy. We should be spending more time at ‘race pace’. Initially race pace may feel hard, but with careful planning, we extend the number of miles we run at race pace and it starts to feel easier. Note I don ’t say ‘easy’, as it’s definitely not easy, but as the number of miles spent running at that goal pace increases, those long runs should be less daunting. When we are targeting a specific pace, this can be conceptualized as the external load. If your goal is to run a marathon in 3 hours, then you really need to be spending a lot of time running at around 4:15/km. The work you did in the earlier phases of training, where the focus was on internal load, should help you to achieve this.

So, when I am coaching athletes in that final run up to a race, paces really are important, and in fact, if you have a race-pace workout to do close to your event, and you’re just not able to hit those km splits, due to stress, poor sleep, too many beers the night before, etc, then it may be best to simply call it a day, and come back tomorrow in better condition, and give it another shot. BUT, in the early, more generalized phase of our training block, pacing really isn’t nearly as important. The goal is to get that metabolic stimulus, and to stress the joints and muscles. If you’re struggling to hold a certain pace during this phase, then just back off, but keep plugging away, grit your teeth and just get it done; you will still get most of the training benefits and adaptations. The internal load is still being applied.

That’s all I’m going to say - and of course there are lots if nuances, but I thought this was an interesting topic, and something from which we can all benefit! Send me your comments.