Updated: Jun 10, 2019
One of the key ways that we can improve endurance is by increasing the total pool of muscle fibers that we can recruit. When your muscles fire, not all of the fibers in the muscle fire simultaneously. The nervous system rather cycles through firing different fibers in order to let the others recover, so the more fibers we have available, the longer the recovery intervals for each fiber. This combination of increased recovery per fiber, and the ability to spread load across more fibers increases endurance. Thankfully, increasing this pool of fibers is very trainable. Considering the extreme endurance demands of ultra running, this training is important, as we’d like to keep our muscles as fresh as possible for as long as possible. Here, I will argue how running fast in states of glycogen depletion accomplishes this task.
Reviewing Muscle Fiber Recruitment
Let’s first review the types of muscle fibers and how they are recruited. Muscle fibers exist along a spectrum from slow-twitch fibers, those having characteristics geared towards aerobic metabolism through a higher mitochondrial density and higher concentrations of aerobic enzymes, to fast-twitch fibers, those having characteristics geared towards anaerobic metabolism and power. Fibers in the middle can have combinations of these characteristics – for simplicity, we will call these intermediate fibers. While the details are slightly controversial, over time, some of these intermediate fibers can shift along the scale towards slow-twitch or fast-twitch, depending on the training stimulus.
Without diving into a full review of exercise metabolism, let’s remember that aerobic metabolism (the type favored by slow-twitch fibers) is approximately 16 times more efficient than anaerobic metabolism. This efficient metabolism requires a drop of sugar (glycogen) to burn several drops of fat, resulting in a bucket of power output, whereas anaerobic metabolism requires a bucket of sugar to yield a bucket of power (horrendous analogy, I know).
During endurance exercise, the slow-twitch fibers are preferred at first due to their efficiency. As glycogen is depleted, the contribution of other fibers increases. This is because although aerobic metabolism primarily uses fat as its substrate, glucose is still required upstream of that reaction, so glycogen depletion will in fact inhibit aerobic metabolism. Studies have confirmed that fast-twitch fibers are spared early in exercise and recruited on par with slow-twitch fibers after glycogen depletion. In other words, we can think of fiber recruitment as progressing from predominantly slow-twitch to mixed slow/fast-twitch over the course of exercise and subsequent glycogen depletion. However, in high-power activities such as sprinting, fast-twitch fibers are almost exclusively recruited.
Enlarging the Pool of Recruitable Fibers for Endurance
From the above mechanisms, we can infer routes for manipulating fiber recruitment for endurance. The first is to force the nervous system to recruit fibers that it normally wouldn’t recruit, because these fibers will share the load with slow-twitch fibers during glycogen depletion. The second is to stimulate and thus reinforce aerobic metabolism in the intermediate fibers, which will increase the efficiency of those fibers and effectively give you a multiplier on the number of slow-twitch fibers you have at your disposal.
We will next see how manipulating glycogen depletion in combination with fast running gives us training instructions!
Strides and hill sprints are short, high-power bursts of speed. While these have a number of beneficial training stimuli, from improving form to improving connective tissue elasticity, in this context, we are interested in muscle fiber recruitment.
Luckily, strides and hill sprints are useful no matter the glycogen state. If we do them early in a run, we are fresh, have topped-up glycogen, and no central nervous system (CNS) fatigue. Thus, we will be able to recruit a large number of intermediate and fast-twitch fibers and the form and speed benefits will be maximized since the nervous system is fresh. That being said, the effort will need to be near-maximum to really recruit more fibers than you normally would. This is because you have a full pool of fresh fibers.
However, what if we do strides after a moderate to high volume of aerobic work and thus glycogen depletion? Well we know from above that the fraction of fast-twitch fiber recruitment increases. This means that we are dealing with a depleted muscle fiber pool, with a significant number of slow and intermediate fibers out of the mix. Now if we perform this work, we exclusively target these fibers and force the muscle to fire involve different fibers than normal to generate the same power output. This means that since the nervous system has been conditioned to recruit these fibers, over time, they will be added to the pool of available fibers called upon to divide the load during glycogen depletion.
How to use this in training? After moderate-length aerobic or even intense work, you can throw in a handful of strides or hill sprints with full recovery. I don’t recommend doing these after your long run, as muscle stiffness and form breakdown can raise your injury risk.
Sustained Fast Running after Glycogen Depletion
For our second training mechanism, we can force the body to rely on intermediate fibers, and then force those fibers to perform aerobic metabolism, stimulating these intermediate fibers to adopt more slow-twitch characteristics. This stimulus is easily achieved by sustained, faster aerobic running in a state of glycogen depletion.
After a moderate to high volume of aerobic work to deplete glycogen in slow-twitch fibers, a higher fraction of intermediate fibers will be recruited to perform the work. Moreover, running closer to threshold pace will further call upon these fibers. Thus, in this state, we have to some extent isolated these fibers. Once isolated, we now sustain some fast running, requiring these isolated intermediate fibers to perform aerobic metabolism, picking up the slack of the depleted slow-twitch fibers. This can be a powerful stimulus for muscle fiber conversion and forcing aerobic metabolism.
How to use this in training? This sort of work is the mainstay of advanced marathon training. In essence, all you need to do is fast running into the second half of your long run. For example, if you were in a marathon build, you could do 20 miles, with the second half progressing from easy pace down to half marathon race pace. Alternatively, you could break up the fast running and do something like 20 miles, with alternating miles marathon race pace/easy after 10 miles. You can even incorporate it into easy days mid-week. During many of these days, you are already glycogen depleted from the previous day or two of training, so even a moderate volume run can put you into a state of significant glycogen depletion. Picking up the pace towards the end of these runs, if you’re feeling good, can be a nice aerobic boost.
Coaching legend Renato Canova has used workouts that nicely blend all of these principles together. I’ll describe an example workout and it’s effects to illustrate how these stimuli can be seamlessly and rather artistically integrated together, to induce a whole range of adaptations from a single workout.
The workout: 6-10x1km repeats at 10k pace with 2 min jog rest; after every 3 reps, perform 6x10-20sec hill sprints with full recovery.
What is this sorcery and why do it? If you ignored the hill sprints, this workout would look like a classic lactate threshold to critical velocity workout, effective but pretty standard. So what’s the deal with the hill sprints?
Let’s start at the beginning. In the first set, the athlete will be maxing out their aerobic system and slow twitch fibers, because the intensity is so high. Lactate will be produced as this effort requires a high degree of anaerobic metabolism. Accordingly, glycogen utilization will be high.
The athlete then recovers and moves to the first set of hill sprints. At this point, they are still fairly fresh, but the high power required recruits a larger number of intermediate and fast twitch fibers than the 1km reps. The athlete then moves into the next set of 1km repeats, but now, a larger set of intermediate and fast-twitch fibers are being recruited due to the hill sprints. This is the first mechanism that we explored above.
Since these additional fibers are in play, but a high degree of aerobic metabolism is being called for given the threshold repeats, the intermediate fibers will be stimulated to perform aerobically.
Progressing to the last set of hill sprints, the degree of glycogen depletion is very high from the threshold work, meaning that intermediate and fast-twitch fibers are now emphasized. The hill sprints at this stage thus force a high power output from a deplete fiber pool, simulating increased fiber recruitment.
In this way, Canova designed a workout to play different stimuli in succession against each other, kicking off a large number of independent adaptations. Genius.
In conclusion, I hope that I have illustrated how the manipulation of glycogen depletion can inform workouts that stimulate increased muscle fiber recruitment, a key adaptation conferring endurance. As a final note, many of these adaptations will occur naturally even without targeting these mechanisms specifically. For instance, if you are running hills late in your long run, this will enhance muscle fiber recruitment due to requiring a high power output in a depleted state. However, strategic deployment of glycogen depletion and fast running can heighten these adaptations without significant additional work.