Fiber is important—and not just for digestion. To keep your running progression regular, you should know about your muscle fiber.
The anatomy of a muscle fiber could fill dozens of pages, but the basics are simple: Muscles are composed of individual cellular units, which contract to generate the force needed to run. These fibers are bundled in groups, to form larger muscles.
Muscle fibers fall into three categories with different properties, and their composition and recruitment plays a large role in determining the design and execution of running training. Here’s a primer on how different kinds of muscle fibers interact with your training plan.
What are the types of muscle fibers?
There are three types of muscle fibers. Slow-twitch fibers (Type I) use oxygen, relying on aerobic energy processes to do work. They contract less forcefully, but are filled with capillaries and mitochondria that make them more resistant to fatigue.
Fast-twitch fibers are broken down into two subgroups—Type IIa and Type IIx. Type IIa have properties of both ST and FT, use aerobic energy processes and have high capillary and mitochondrial density, but are capable of producing more power than ST fibers. Type IIx fibers are all power, and use anaerobic energy processes.
Hey, I want to be fast. Shouldn’t I want fast-twitch muscles?
Trail running is a slow-twitch sport, even for the pro runner competing in a trail 10K. Those type IIx FT fibers fatigue too rapidly to be of much use outside of sprints. Meanwhile, ST and Type IIa fibers can go long, they can go strong and they’re down to get the friction on.
Thus, most of your training should be focused on optimizing ST and Type IIa fiber recruitment. But it goes beyond that, because there is evidence that muscle fibers can actually change their type (or act as if they change) over time. In other words, with consistent aerobic training, type IIx fibers can act more like Type IIa or ST fibers.
My development is a case in point. In high school, I was a 200-pound sprinter, full of Type IIx fibers and souped-up type IIa fibers, in addition to gross protein shakes. After a decade of endurance sports, I’m 140 pounds of pasta and probably a fair bit more ST and ST-like type II fibers. An important long-term adaptation from aerobic training is to grow more slow-twitch-like fibers than your body may be genetically predisposed to do.
So where does critical velocity come in?
Top coach Tom Schwartz is famous for critical-velocity (CV) workouts, which are designed to recruit type IIa muscle fibers. The theory is that if you want to be slow-twitch but also want to go fast, type IIa fibers are the gateway to better performance, since they’re recruited at almost all distances from 800 meters and up. Make them stronger, and you’ll get stronger at everything.
Based on empirical data and experience, Schwartz estimates that CV is 90 percent of VO2 max for most runners, which equates to a pace most runners could hold for 30-35 minutes in a race. For fast runners, this is around 10K pace; for some it might be 2- or 3-mile pace. It lies between VO2 max (which is faster) and Lactate Threshold (which is slower).
Schwartz bases his training system around CV workouts because type IIa fibers are highly adaptable, becoming better at using oxygen with moderate intensity.
What do CV workouts look like?
A typical CV week-by-week progression would be 12 x 400 meters in week one, 6 x 800 meters in week two, and 5 x 1K in week three, all with 60 to 90 seconds of recovery between the intervals (which are all CV pace), varying distance and recovery as an athlete develops over time.
The rest interval is pretty short, the intervals pretty controlled and the overall workload not exhausting. After the CV work, Schwartz will often have an athlete do slightly faster, controlled strides, like 4 x 30 seconds at mile to 5K pace (or even faster, depending on an athlete’s background) to keep speed up without the need to hammer workouts (and avoid recruiting tons of Type IIx fibers).
That’s all well and good, but what does CV training look like for a trail runner?
Improving type IIa fiber recruitment should improve running economy for people racing longer distances or on more gnarly terrain than Schwartz’s track runners. In addition, learning to do intervals in a controlled, sustainable fashion is a good lesson regardless of your training or race goals, to avoid burnout and injury, and overdevelopment of type IIx muscle fibers (though there is a place for improving recruitment of those muscle fibers for some athletes).
One way to approach training-plan development is with early season intervals structured around CV fartlek runs (after a base period of easy running and some strides), before getting into more event-specific work closer to races. Fartleks are essentially just time-based intervals, meaning you can do them on variable terrain like trails without worrying so much about specific pace or distance, but rather an effort you could sustain for a specific amount of time. For CV intervals, this is around a 30- to 35-minute effort. An example progression for an advanced athlete might be:
Week 1: 10 x 1 minute at CV effort/2 minutes easy
Week 2: 15 x 1 minute at CV effort/1 minute easy
Week 3: 12 x 2 minutes at CV effort/1 minute easy
Week 4: 8 x 3 minutes at CV effort/1 minute easy
For a less experienced athlete doing lower volume, it’s ideal to do 15 to 40 percent fewer intervals in each set to avoid over-stressing the musculoskeletal system. These intervals can be done on roads or trails, with the general rule to keep the footing good so that you don’t have to slow down too much to watch their stride.
How does a CV workout fit into a training week?
Most athletes shouldn’t be doing more than one structured workout in a week, possibly two for very advanced athletes. Simply combine CV intervals with some faster strides to improve power output and running economy, lots of easy running to power up those ST fibers and a longer run to work ST fibers (and FT fibers as you fatigue). Here’s a sample week, with CV workouts built in:
Monday: rest and recovery
Tuesday: 75 minutes easy with 4 to 8 x 20 seconds fast/2 minutes easy strides
Wednesday: 20 minute easy warm-up, CV workout, 20 minute easy cool-down
Thursday: 60-80 minutes easy
Friday: 40-60 minutes easy or rest
Saturday: 2 hours easy/moderate (with option to add surges at CV effort)
Sunday: 60-80 minutes easy with 4 to 8 x 30 second hill strides
CV training is one of many possible approaches to developing a well-rounded, burnout-proof, super-charged training plan.
David Roche runs for HOKA One One and NATHAN, and works with runners of all abilities through his coaching service, Some Work, All Play.