Training for Endurance: Progressive Recruitment

I read an article on VeloNews a while back describing how many cycling races are won and lost in the final hour or minutes of racing. And this is generally true, it often comes down to who can sustain the highest power output in the final push to the finish after three, four, or five+ hours in the saddle. Rationally, it makes sense that being able to delay fatigue and enter that last hour of racing with a greater capacity for work will enable an athlete to finish faster. Many of this spring's one day classics have served prime examples - those that produce the greatest amount of power in the end will prevail. Take this years's Milan-San Remo for example: after 6 hours of riding, the race hits a series of small climbs before a sprint to the line. This year, it was Alexander Kristoff in the final sprint (after nearly 7 hours on the bike) who took the win - out-sprinting the likes of Mark Cavendish and Fabian Cancellara. When asked about the finish, Kristoff acknowledged that a sprint after 300km is not the same as a sprint after 200km and stated that he usually does not "lose much power" late in a race.

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).
Typically, when an athlete begins sub-maximal exercise like an easy run or ride, he recruits slower twitching motor units (once a steady state is reached). But as time passes and exercise progresses, even at a constant sustained workrate or pace, the athlete begins to fatigue and faster twitching motor units are recruited to do the work to maintain the workrate. In short, these faster twitching fibers are not as efficient and do not have the same oxidative capacity and fatigue resistance as slower twitching fibers (Jones et al., 2011). This may lead to an increase in O2 uptake at a constant workrate over time.

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.

To summarize:
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.


  1. The slow component of VO2 is basically what forced me to keep aside my Stryd powermeter that I had for a couple of years. They claim to have invented a power metric that is directly proportional to metabolic rate and a mechanical efficiency of 21-22%. The reality of the situation is exactly as you describe - due to progressive recruitment, it seems mechanical efficiency should decrease over time and that aligns with what most runners feel as a 'increased effort' to maintain the same pace. This pie in the sky power metric for running doesn't seem to make sense at all. And it doesn't even account for the fact that humans recycle energy on the eccentric phases of propulsion, thereby making a closed-loop mechanical efficiency on the order of upwards of 50%.


Post a Comment

Popular posts from this blog

Don't believe everything that you read.

Lactate, Lactic Acid and Acidosis

Death March Revival ITT Ride Report