Muscle Fibers and Overreaching
This week Alex Hutchinson shared an article he had written for Outside magazine, which featured a study that examined the effect of athletes’ muscle fiber types on their proclivity to overreaching (heavy fatigue that precedes overtraining and is accompanied by a decline in performance) when training volume was increased. The 24 male and female, middle-distance runners in the study were very fit with aerobic capacities that would typically put them in or near an “elite” category. They trained for 7 weeks during the time they were observed by the scientists. For the first 3 weeks they trained at their normal volume. For the next 3 weeks they gradually increased the training load by 10% in week 1, 20% in week 2, and 30% in week 3. Then they tapered for a week (a 55% reduction from week 6—the highest volume week). That followed with testing for time to exhaustion at a high effort. At this point 12 of the runners showed signs of overreaching as their times to exhaustion at high intensity decreased. They fatigued earlier than they had in the pre-study testing. The other 12 showed signs of definite fatigue, but their times to exhaustion did not decrease.
Then the key question was asked: Why did some of the athletes wind up overreached while others weren’t? Several physiological metrics were examined. The only one that was significantly different between the two groups was their muscle fiber types. The athletes who did not show signs of overreaching had proportionately more slow-twitch (type I) muscle fibers than the other runners. And that kind of makes sense as type I muscles are the pure endurance muscles. They tend to handle training load duration increases better than fast-twitch muscles (type II).
So, all well and good: Endurance athletes with predominantly type I muscles seem to handle high volume training better than those who have a higher percentage of type II muscle fibers. The type II endurance athletes may, therefore, do better with high intensity interval training rather than long duration workouts and, especially, high volume. And the latter apparently also must have frequent and regular recovery breaks. Enough said. Time to move on.
But not so fast.
Slow-Twitch and Fast-Twitch Muscle Fibers
After I retweeted Alex’s piece from Outside, Scott Francis (@scutrsft) asked me on Twitter: “Is there a good ‘field test’ or other way for an individual to sort out their personal FT or ST profile?” Good question. I wish I had a good answer. I don’t know of any. But I expect we could answer that question by looking at what we know about ourselves and our unique responses to training.
For example, a rather gross “field test” would be one’s race results. If you tend to do better in short, fast races and workouts rather than long events or high duration workouts relative to a similar group of athletes, then you probably have more type II or fast-twitch muscles. The opposite would also appear to be true. So, if you are considered a “sprinter” in your sport (very powerful for short durations) then you are also probably a type II athlete. Other possible (and somewhat weak) indicators of predominant type I—slow twitch—include frequent delayed onset muscle soreness (DOMS) from weightlifting. And along this same line of thinking, a propensity to bulk up with weightlifting may indicate type II. Type I athletes are also probably more efficient in their sport-specific movements for long durations than the type IIs who will likely become sloppier in their sport-specific movement patterns late in a long workout.
But there are several other factors that could have an effect on all of this. For example, there’s always a mental aspect when it comes to training and racing. A type I athlete may have a real disdain for long distance workouts for any number of reasons. So although this person may be made for long-distance sessions, they actually hate doing them and may prefer short, fast interval-type sessions. Body mass index could also play a role in one’s performance. A high BMI athlete may have a lot of type I muscles but performs much better in short, powerful workouts. Then there’s age. As we get older we typically lose type II muscle fibers and so have a greater proportion of type I.
The bottom line is that it’s very difficult to accurately determine someone’s predominant muscle type and therefore how their training should be shaped. It’s pretty much a guessing game. This is one of the many challenges coaches face in trying to optimize training for an athlete. If the coach looks back at the athlete’s history of training and racing, he or she may get a rather biased view. About the only way to find out, at least based on such field testing, is trial and error. Try something and see what happens. Then make adjustments and again see what happens.
I’m curious how coaches and athletes handle this dilemma. Have you found field tests or other markers of muscle type that help you point your athletes in the right direction for training?