Why Ample Rest Is Key to Building More Fast-Twitch Muscle Fiber

If you're interested in becoming the fastest athlete possible, muscle fiber composition is something to keep in mind.

Athletes are always interested in "getting more fast twitch."

Everyone has two general types of muscle—slow-twitch muscle fibers (also known as type 1 fibers) and fast-twitch muscle fibers (type 2 fibers). Although slow-twitch muscle fibers do the majority of the work during slow and moderate aerobic workouts, fast-twitch muscle fibers are used for shorter, more explosive movements—like sprinting and jumping.

However, most athletes are confused on how to develop fast-twitch fibers. Some think it comes from Vertimax workouts, increased strength, or any explosive-looking plyometric. These all can certainly play a role in the equation, but athletes often overlook the simplest factor that affects the function of their fast-twitch muscle fibers: Rest.

With aspirations of jumping higher and sprinting faster, many athletes undertake heavy weight training. Sprinting fast and jumping high are dependent on power production. In physics, Power = Force x Velocity. Heavy weight training benefits power performance because it can enhance an athlete's capacity to produce force. However, without rest, these gains may not be realized.

A 2017 study published in the Scandinavian Journal of Medicine & Science in Sports looked at the effect of heavy lifting on the gene for fast twitch fibers. The study involved sedentary people performing two consecutive days of strength training on the same leg, with their other leg acting as a control. In the four days following the training, the gene for the fastest-twitch fibers (known as myosin heavy chain IIX) turned off.

Researchers found that glycogen was depleted inside these muscles during the heavy lifting, so while heavy strength training does recruit these fibers, the genes for these fibers were still "silenced' in the days that followed the training.

Was this study just a fluke, or can strength training actually cause a temporary switch-off of fast twitch fibers? The next study answers that question.

In another study by the same group of researchers, they examined a group of adult sedentary men and looked at fiber composition before and after three months of heavy resistance training and after three months of detraining. Myosin heavy chain IIX (the isoform for the fastest twitch fibers) content was tested throughout the study.

The MHC IIX content started out at 9.3 +/- 2.1%.

After the three-month period of resistance training, it decreased to 2.0 +/- 0.8%.

Following the three-month of detraining, it increased to 17.2 +/- 3.2%.

After training, the fastest-twitch fibers shifted to the less-fast twitch (IIA) but had an "overshoot" after a long period of rest (three months). This study confirms the drop in fast twitch fibers after weight training. But it offers a simple solution: Rest. The next study explains why.

In a 1984 study published in The Journal of Histochemistry and Cytochemistry, researchers looked at the muscle fiber composition of 5 Controls, 7 Weightlifters and 8 Runners. Here's what they found:

  • Type I (slow twitch) was greatest in distance runners (68.2%)
  • Type IIA (fast twitch) was greatest in lifters (39.7%)
  • However, type IIX (fastest twitch) was greatest in sedentary control subjects (43.2%)

Sedentary people do not exercise and therefore have no need for endurance, so their body "defaults" toward being fast twitch. But for anyone who exercises, the body seems to adapt to exercise in any form by increasing composition of slower twitch fibers. This does not mean that sedentary people are more explosive or athletic than trained people by any means, but it just goes to show the importance of rest in building fast twitch fibers.

The book Supertraining by Yuri Verkhoshansky confirms the findings of the previous studies. "If an athlete wishes to increase the relative amount of fast twitch muscle fibre isoforms, a logical method would be to decrease the training load and allow the faster fibres to express themselves a few weeks later," Verhoshansky writes. "Immobilisation causes the normally slow-twitching soleus muscle to become fast twitching...The 'default' option for muscles seems the be the fast myosin chain."

If you're interested in becoming the fastest athlete possible, muscle fiber composition is something to keep in mind. It's not all about increasing strength and performing a ton of jumping and sprinting work year-round. Taking time off (even multiple weeks) where you perform significantly less work can help you reach new levels of explosiveness and allow you training time to manifest in gains.

To reach your potential for explosiveness, you need to put in the work. But after a long period of training, experiment with rest and watch your sprinting and jumping ability soar. In The Vertical Jump Protocol, athletes report the best gains in jumping ability after an easy week of training or even after a week of no training.

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