By Tom Schwartz
February 18, 2016
Editor’s note: On February 4, LetsRun.com ran a story written by Jonathan Gault on Drew Hunter, America’s top high school distance runner, and his coach, Tom “Tinman” Schwartz. In the article, Gault mistakenly discussed the role of Type IIb muscle fibers in Schwartz’s training philosophy, specifically during the following paragraph
“In humans, there are two types of muscle fibers, Type I (slow-twitch) and Type II (fast-twitch). Type II fibers can be subdivided into two categories: Type IIa (aka intermediate or fast-twitch oxidative) and Type IIb (fast-twitch glycolytic). Type I fibers use oxygen to produce energy and take a long time to fatigue. Type IIb fibers do not use oxygen, relying solely on glycogen to produce energy. These muscle fibers contract faster and more powerfully than Type I fibers but fatigue quickly.”
Schwartz reached out to clarify his philosophy and we have published his response below. The original article has been updated; LetsRun regrets the error.
Due to the challenge of understanding technical elements of conversation during a phone interview, portions of the article written and posted on LetsRun.com were a bit off-target. This response attempts to clarify information.
Type IIb muscle fibers are found only in rodents, not humans. That aside, type IIb, like their human counterpart, type IIx fibers, are commonly defined as fast-glycolytic muscle fibers in exercise physiology textbooks. (I prefer to call them fast-explosive fibers.)
I briefly talked about type IIx fibers: I did not describe a conversion process to type IIa – the fast intermediate fiber group. However, I did say that type IIa muscle fibers are the main target of my training system.
In my opinion, type IIa fibers are critical to a runner’s success. These fibers are highly malleable. Using high intensity training, these fibers become proficient at regenerating ATP (the substrate energy used in muscle contraction) without oxygen via the “anaerobic” or non-oxidative metabolic pathway. Using moderately intense training, such as Critical Velocity workouts, these fibers become proficient at processing available oxygen inside muscle mitochondria, the organelles inside of muscle fibers (cells) that use oxygen to generate ATP (energy for work).
On page 153 of Exercise Physiology: Theory and Application to Fitness and Performance, Sixth Edition textbook, Dr. Scott K. Power and Dr. Edward T. Howley state that type IIa (intermediate) muscle fibers, or fast-oxidative glycolytic fibers, “…contain biochemical and fatigue characteristics that are between type IIx and type I fibers. Therefore, conceptually, type IIa fibers can be viewed as a mixture of both type I and type IIx fiber characteristics. However, note, that type IIa fibers are extremely adaptable. That is, with endurance training, they can increase their oxidative capacity to levels equal with type I.”
My training system targets type IIa muscle fibers. Specifically, based on both experience and empirical data, I believe that 90% of V.O2 max is the overall best intensity for meeting that goal. In 1989-90, as a volunteer coach and graduate assistant at the University of Wisconsin- La Crosse, I created the CV concept and workouts for my athletes. I combined feedback from runners who were doing workouts that I assigned, lab data, and studies on lactate dynamics. Further, I included what I learned while training for summer road races. I learned that when I trained at a pace a few seconds per mile faster than my 10k intensity (I ran about 34:00-35:00 at that time), my race times improved steadily, week after week. When I ran intervals at 5k pace or faster, however, I seemed to improve for only three or four weeks before struggling in races.
When I assigned threshold workouts, athletes tended to run too fast — by 8-12 seconds per mile. Rather than telling them to slow down, I asked them how they felt. They said, “Great!” Conversely, when they ran at threshold pace, they often said it felt too slow or uncomfortable. During the days that followed workouts in which they ran a bit faster than threshold pace, they recovered quickly – as long as they did not run too fast. When runners covered ground at 5k pace or faster, their ability to recover was lengthened substantially. And, like my experience with the timeline of performance enhancement, they tended to improve in races for just three or four weeks when training in large volume at faster paces.
I learned quickly that athletes improved a lot, felt good, and seemed to have a lot of enthusiasm for training at 5k pace plus 12 to 20 seconds per mile, with approximately 16 seconds per mile the most typical best sensation, as reported by them. Later, I created math formulas to equalize the intensity across a spectrum of runners: i.e. the relative time was adjusted to each runner’s reference race performance level.
During that same time-frame of coaching, I tested runners for V.O2 max (maximum oxygen consumption) and lactate production in the Human Performance Lab at UW-La Crosse. I noticed that the average lactate for our team’s runners at the end of a V.O2 test, taken three to five minutes post-exercise, was ~11 mmol. During the test, athletes were asked to point to a number of the Borg Scale (perceived exertion), positioned in front of the treadmill. Typically they pointed to somewhat hard exertion level approximating 50% of their peak V.O2 lactate level, or ~5.5 mmol. That was 1.5 mmol above the highly regarded anaerobic/lactate threshold value at the time – as described often in endurance sports magazine articles.
I computed the oxygen cost of the ~5.5 mmol level, and it matched up with ~88-91.5% of V.O2 max. (By the way, I used an American College of Sports Medicine formula to convert oxygen cost to meters per minute, and later time per kilometer and mile.)
By the way, CV originally meant Critical Value, a term that defined statistical significance level. Later, I changed it from Critical Value to Critical Velocity, simply to make it a more clear and practical term. (Yes, I know others have, over the years, used a similar CV term in the literature. I never heard of the term at the time I labeled it as 90% of V.O2 max, however. Yet, just like threshold as a term has many meanings to many people, the CV doesn’t mean the same thing to everybody.)
I’m often asked how that I set up CV pace (90% of V.O2 max). I use several mathematical steps, outlined as follows below
- I calculate relative oxygen cost for running at 100% of V.O2 max pace, which is the pace a runner can handle in an all-out 7-minute test, using my method.
- I then multiple the V.O2 cost by 90%.
- I then convert the 90% oxygen cost to meters per minute.
- Finally, I convert the 90% meters in per minute to kilometers per mile and minutes per mile.
A little background about V.O2 max testing: Over the years, researchers have used a variety of methods to test and identify the average duration that runners can sustain at V.O2 max velocity/pace. Typically, the time range is four to eight minutes. It’s important to note the method used for assessing the initial V.O2 max speed influences the follow-up test, which measures sustainability of the pace.
Due to the size and expense of equipment, most researchers test runners for V.O2 max in a Human Performance lab using a treadmill and metabolic cart, rather than transport the bulky equipment to the track or other location. Here’s a visual:
A small number of researchers test runners exclusively using the Douglas Bag collection method, either in a lab or at a track (a Douglas Bag is expands when the runner breathes in it, and the air is retained for later extraction). Here’s a visual:
More recently, portable technology for testing V.O2 max has been used, especially in Europe. Here’s a visual.
My observations: runners who train their type IIa fibers to process oxygen effectively may be able to sustain CV pace (90% of V.O2 max) for up to 40 or 45 minutes. Runners who have a lot of speed and train using CV-type workouts tend to sustain 90% of V.O2 max for 20-25 minutes. On average, serious runners can sustain 90% of V.O2 max intensity for 30-35 minutes. As a matter of practicality, I tell my runners to ask themselves a simple question while running assigned CV workouts: Can I hold this pace for half an hour? If so, they are close to the correct CV pace.
Note: I’ve observed good aerobic benefits from training at plus or minus six seconds per kilometer or 10 seconds per mile from the exact CV pace. Farther away from that range of paces, it seems that expansion of stamina — the ability to sustain a high fraction of V.O2 max during racing — becomes limited.
LRC Note: LetsRun.com’s Wejo wrote “Tinman” to find out more specifically what a CV pace type workout is in layman’s terms. Tinman wrote back:
“Think of it this way, Weldon, in your prime, you could run 7-8 times a kilometer @ your 10K pace, jog 200 m between reps, and conclude with 5 x 200 at mile pace every week of the year. It just so happens that your 10k pace was close to your true CRITICAL VELOCITY pace.