A similar story played out in Ponferrada this year where Kwiatkowski launched an attack with 7 km to go and rode away from the field after more than 6 hours of racing. Thanks to Strava, we can take a look at what it took to win the 2014 World Championship. Kwiatkowski averaged 370W for those final 8 minutes of racing - not a particularly impressive figure considering he weighs in at 68 kg. Then again, considering this came after 6:20:00 of racing at an average of 240W, may help to put it into perspective.
My point being, fatigue happens - but some can resist fatigue better than others.
There are many strategies athletes can employ on race day to delay fatigue. For example: drafting decreases the work required from the athlete; proper carbohydrate supplementation decreases reliance on muscle glycogen, preserving it for later in the race; and proper hydration limits or prevents dehydration, maintaining stroke volume and cardiac output. But how might an athlete's training prevent fatigue late in a race?
Enter Progressive Recruitment
I want to address the training component of fatigue resistance in this post with the concepts of progressive recruitment and the VO2slow component in mind. Progressive recruitment occurs as slower twitching motor units become fatigued or depleted and faster twitching motor units begin to be recruited to maintain force/power output.
With progressive recruitment, your EMG over time with fatigue looks like this:
|Eight motor units were recruited for the first contraction. |
By the 10th contraction, 12 motor units were recruited to maintain the same force output (Adam and De Luca, 2003).
While progressive recruitment has been linked to the slow component of VO2 (Saunders et al., 2000), the relationship between the two has been questioned and debated many times (Zoladz et al., 2008; Borrani et al., 2009). Some debate whether progressive recruitment occurs, but the phenomena has been documented in many studies (Adam & De Luca, 2005). Also, couldn't reports of increased blood lactate in the final stages of a marathon indicate that faster twitching fibers are being recruited (Billat et al., 2002)?
Training with the Concept of Progressive Recruitment
No ground breaking science here. Essentially, our understanding of progressive recruitment reinforces the practices many endurance athletes and coaches have relied on for decades: hard, fatigue inducing work. There's no way around it. If you want to get better, you have to induce fatigue (hopefully with sport/event specificity in mind) and this is not going to be comfortable. But, knowing that faster twitching motor units are recruited after fatigue has been induced gives us a window to target those motor units specifically.
With progressive recruitment in mind, we can theoretically train to both delay recruitment of the faster twitching motor units and to improve the oxidative capacity of those faster twitching fibers when they are recruited so that they are more efficient and fatigue resistant.
What about time trials?
Time trials are not paced in the same way. They are typically a hard sustained effort from the get go. Or what about those athletes that just want to run a personal best marathon? While these sustained efforts don't necessarily require sprints at the finish to break opponents, the athlete will still recruit faster twitching fibers to get the job done once fatigue sets in. In the last few miles of a marathon, an athlete will recruit faster twitching motor units (Borrani et al., 2001).
There is some evidence indicating that these faster twitching motor units have the ability to become more fatigue resistant and take on characteristics of slower twitching motor units. In a review, Kubukeli et al. (2002) note that several studies have documented shifts in muscle fiber types from the faster type IIb fibers to slower type IIa or type I. While this could, in theory, be helpful - Kubukeli et al. also point out that much of the literature on fiber type conversion has shown inconsistent results. To take a theme from my previous post - what are we training for here? Increased type IIa MHC or increased fatigue resistance/increased power output? Should the goal of training be to convert fiber types or to maximize performance?
I know I want to maximize performance, regardless of fiber type.
Understanding the concept of progressive recruitment helps enforce the need to make those faster twitching motor units fatigue resistant. There are a few different ways this can be done - but the common theme is the recruitment those faster twitching motor units. To recruit those motor units, you have to either demand a lot of force, demand high velocities, or both. As mentioned above, inducing fatigue will also recruit those motor units.
Here are some example training techniques that would recruit faster twitching motor units, potentially increasing fatigue resistance:
- Lifting - moderate to heavy loads
- Power training/plyometrics
- Hill sprints
- High intensity interval training (induce fatigue and demand high force/velocity)
- Extensive endurance training (going long)
- Cumulative fatigue? (doubles, multiple days/weeks of intensified training)
Recruitment and fatiguing of faster twitching motor units will stimulate PGC-1a through glycogen depletion, oxidative stress, ADP/AMP accumulation, calcium release, epinephrine, Lactate/NAD+, etc. PGC-1a promotes mitochondrial and capillary growth - which, in theory, makes muscles more efficient at a given workrate and more oxidative/fatigue resistant. In the simplest sense; chronic recruitment of motor units triggers adaptation, making them more fatigue resistant.
Here, if an athlete performs high intensity work the goal will not be to improve VO2max or to improve lactate/H+ production and clearance, but to make those faster twitching fibers more oxidative (efficient) and fatigue resistant.
Another strategy the athlete could employ would be strength training with the goal of increasing a muscles maximal force. If the athlete can increase the strength of those slower type I fibers, they will operate at a lower percentage of their max during submaximal exercise, potentially extending their ability to complete work before recruiting the less efficient type II fibers. Additionally, through resistance training - recruiting the faster twitching motor units again and again may increase their resistance to fatigue, potentially shifting their characteristics from faster twitching (type IIa) to slower twitching (type IIx). There are many other benefits to resistance training as described by Ronnestad and Mujika here.
Progressive recruitment occurs when slower twitching muscle fibers become fatigued. This results in faster twitching motor units being recruited to maintain force/power output. These faster twitching motor units are not as efficient or fatigue resistant. There are many paths to improving performance, and some of those paths may involve training to prevent progressive recruitment and/or training to improve the endurance and efficiency of those faster twitching motor units for when they are recruited.