Know when to pull the plug
For any given workout, we should be trying to get a specific result. We may be working on our lactate threshold, our coordination, our top end speed, or the ability to buffer our blood pH, whatever. We’re also often working multiple things into the same workout. From a theoretical perspective, the more time we spend working hard enough to go over the minimum threshold required to improve, the more improvement we’re going to see. This is why miles matter. The more miles we’re putting in, the more time we’re spending working on the fundamental aspects of running (biomechanics, cardiovascular development, etc.). The hard part is recognizing when we’ve hit the point in the workout where we’ve hit the point where we are not able to keep operating at a level that is above that threshold. When do we pull the plug on a workout? It depends on the workout.
The short answer is that we need to pull the plug on a workout when we recognize that we’re not going to get any more benefit out of it. The perfect example of this is the classic 400 meter repeat workout. Depending on what stimulus we’re working on, we’ll be targeting a specific time to complete each rep in with a specified amount of rest. If we’re looking to push our VO2-max, then we’ll probably rest for an amount of time roughly equal to our time spent running. Once we are no longer able to run fast enough to demand more oxygen from our cardiovascular system, it’s time to pull the plug on the workout. We could struggle through more reps at a slower pace, or we could take more rest between reps and run the right pace (but at a higher relative effort level), but we’re not going to be able to tax our oxygen delivery system enough to stimulate further development. If we have it in us to work hard enough to get any real gain from it, we’ll be working on a different system than the one we’re targeting (and some workouts take that into account and target both). But most likely we won’t be running at a high enough level to really get any real benefit out of it besides building our ability to mentally suffer while we’re in the hole.
The most common mistake that folks make is they push too hard on the front end of a workout, putting themselves in oxygen debt for the rest of it. For example, if we are trying to work on our lactate threshold, then we need to stay just under the crossover point where carbohydrates become a bigger part of the fuel mix than fat. For most folks this is somewhere between 80-85% of our maximum heart rate. The real objective of the workout is to enable us to run as fast as possible without stepping over that red line where we’ll run out of glycogen. So the more time we can spend just below that 80-85% maximum heart rate (i.e. our Lactate Threshold), the more benefit we’ll get. If we push too hard, then we training our bodies to burn that precious glycogen as fast as it can to run faster (not good for any race over an hour long). Even worse, it makes it even harder to actually metabolize fat as a fuel and get rid of the metabolic waste products of the workout, resulting in less time spent above that all critical stimulus threshold. And if the goal of training is to be able to get to the finish line faster, or run for longer, then we need to spend as much time as possible at the right training zone. You’ll still get the same stimulus to be able to burn fat faster to provide more fuel at the higher intensities, but you won’t be able to get as much time spent in training working on it. Simply put, if that Lactate threshold translates to 8:00 pace and you run at 7:45 pace, you’ll still get that same amount of improvement for the time you put in. But you’ll be able to put in more time if you run at 8:00 pace than if you run at 7:45 pace.
So what do we do if we notice we’re working too hard, or we can’t maintain a high enough effort level to reach that critical threshold for whatever stimulus we’re working on? One thing we can do is to reduce the duration of the work. If we’re doing true speed work that requires us to be operating at 100% of maximum effort and we simply can’t do that for the whole time, we can do shorter repeats. The goal for that kind of work is really to spend as much time in the sweet spot where it’s a challenge to stay coordinated and moving at top speed, so we’re only really getting anything out of the part of the rep where we can still do it, but it’s a struggle to do so. The other thing we can do is increase the amount of rest we’re getting. If we’re accidentally spiking our heart rate too high and going anaerobic on a workout that’s targeting our aerobic system, then taking a little extra rest can allow us to clear any oxygen debt we have, allowing us to work hard enough to get the stimulus we’re looking for without being limited by a different factor.
The important thing to remember is that the harder we push in any workout, the longer it’s going to take the targeted system to recover. If we’re doing a really hard workout, it’s going to be a few days before we’re able to hit it hard again. The best rule of thumb I’ve ever come across is the two more rule. If you don’t feel like you can do two more, it’s time to pull the plug. You should finish every workout with just a little gas left in the tank so that you’ll be recovered in time for the next workout. If you go to the well in every single workout, you either a) won’t be able to do another quality workout until you’ve lost any benefit you gained from the hard work, or b) you won’t have the gas required for the next workout (so you get anything out of that one, even if it keeps you from going backwards). Don’t get me wrong, the tail end of a quality session is probably going to be hard. It has to be hard if it’s going to be enough to get your body to go “well crap, I guess I need to rebuild myself better”. But no workout should impair your ability to train tomorrow. You’ll see more improvement over the long term if you work just hard enough to get the maximum benefit without putting yourself in the hole for the next workout.