FAST TWITCH FOLLOW UP

Endurance training causes muscle fibers to become more slow twitch. This is a well established, uncontested fact. While this may seem like vital knowledge at first glance, it seems to be often forgotten or ignored, and it’s actually not difficult to see why. For one, the effects of fiber type changes may not actually be noticeable depending on the context. Secondly, when a decline in explosive performance does occur, there are numerous alternative theories available. And lastly, athletes and coaches can’t actually measure fiber type distribution, so during the training process it is hard to make firm statements about what is happening at that level.

Fiber type testing can only be done in a lab using a muscle biopsy, so it’s exclusively a research thing, and even in research it is not common. I will admit that there is not tons of practical data on which to base claims made on this topic. However I think we have enough to understand how and why muscle fibers respond to different forms of training. I laid this out in my first fast twitch article.

There are some current trends in the training field promoting conditioning. Concurrent training in which all physical qualities are trained simultaneously is now the norm; there is a big emphasis on the importance of cardiovascular health; and some coaches have claimed that interference between different types of training does not exist. My aim is not necessarily to war against these trends (except that last one). Instead I just want to give a friendly reminder. We should definitely consider fiber type changes in the training process. It’s highly relevant for athletes training to get more explosive. 

To be clear, I am not jumping to this extreme conclusion that all explosive athletes should stop any form of conditioning or endurance training. I am just saying we need to keep the effects in mind. I actually think we need a lot of nuance on this topic when it comes to different athletes, different sports, different training periods, different goals, etc. And it’s definitely not a black and white issue. 

I cannot speak on every context, but let’s cover a few examples. 

FAST TWITCH MONSTERS

First, the fast twitch monsters. These are typically males who are naturally muscular and naturally explosive. These athletes tend to thrive on low training volume. They will do well training short sprints, jumps, and strength 2-3 days per week with lots of off days. If they train enough to get a bit fatigued, they can take 4+ days off in a row, come back and break records. 

Speaking to these athletes specifically, if you play a sport, you will have to get in shape for that sport. I’m not saying you should try to avoid that. You need the work capacity. But I want you to understand the impact. Conditioning is going to mess with your fast twitch talent a bit. If you have an off season when you are doing quality, low volume training, you will probably have some great outputs in short sprints, jumps, and lifts during that period. Some of those outputs are almost certainly going to be difficult to replicate during a different part of the year when you have a higher activity level and more conditioning. So just understand that and don’t get frustrated by it. 

EXPLOSIVELY TRAINED

Now consider a more modestly gifted athlete, someone who is not a fast twitch freak but has at least some explosive talent and has a history of doing good explosive training and strength training. Again, even for this athlete, introducing endurance into the mix is going to have some negative impact on muscular RFD and therefore on explosive performance.

I can use myself as an example of how that shows up. I have a long history of sprint, jump, and strength training with modest volume. I have had success with that. In that context, if I add some endurance training, even a small amount, it makes me worse at acceleration, jumping, and strength. I have run this experiment over and over. A 10-minute jog twice per week is all it takes. I get worse at everything else. It’s not fatigue. A short jog twice per week is not hard enough to create lasting fatigue. It’s negative adaptation. 

Another example would be someone who is running track and doing a fair amount of conditioning. He could be performing well in the 200m and 400m but still be less explosive in other ways. He is slower in the first 20m and his vertical jump is down a bit. He has gained some ability from all the running, namely improved elasticity and sprinting efficiency. But he has also become a little less fast twitch, and that shows up in some other metrics. 

After track season is over this athlete should definitely rest and train in a more fast twitch friendly way. That is true even if track is the only sport. It is especially true if American football is coming up in the fall, and he needs to maximize acceleration, agility, and power on the field. 

OPPOSING EXAMPLES

On the other hand, are there plenty of anecdotes of athletes improving speed, strength, and endurance all at the same time? Yes, of course. For example, teenagers with bodies that are growing and maturing and adapting rapidly can easily have a strong response to training that outweighs any negative influence from conditioning.

Similarly, people with more of a background in endurance or just general fitness may not notice negative effects. They have never been fast twitch in the first place. If a fitness enthusiast decides to pick up sprint training, can she see some improvement while still training endurance? Yes. Can a cross country runner add strength training and make progress over time while still running a ton? Of course.

These examples do not mean that we should just ignore fiber types altogether. Instead we should consider all anecdotes, try to make sense of them, and let that understanding shape our approach to training in different situations. A fitness enthusiast aiming to merely participate in sprinting or jumping is not the same thing as a competitive track athlete with a decade of speed training experience trying to sprint faster than she ever has.

Speaking of speed…

THE DUALITY OF SPEED

The previous 200/400m runner example brings to mind an important concept, the two somewhat distinct components of speed. One is the ability to generate muscular force fast to contract the muscles and propel the body. The sprint start is heavily reliant on this. Then there is stiffness/elasticity, the ability to quickly deflect off the ground, which we rely on more for top speed and speed maintenance. This of course also involves generating muscular force fast, but the action is quasi-isometric. The muscle just resists lengthening, and the tendons (mostly the Achilles) do the lengthening and shortening during ground contact.

I think we have a long way to go in understanding the anatomy and physiology behind these abilities, but the distinction is clear when we observe different athletes and their training over time. Some athletes are better at fast acceleration, others better at top speed. In response to a certain style of training, an athlete can get better at acceleration but worse at top speed or vice versa. And of course one athlete could respond to a program much differently than someone else.