A common belief in the field of physical training is that “You’re born a lifter. Either you’re strong and explosive, or you’re not.”
While there is definitely truth in this statement – especially looking at it from a genetic point of view – recent literature has shown that hybrid muscle fibers (a combination of different types of fibers) as well as slow-twitch fibers can indeed be transformed into fast-twitch muscle fibers.
Say you were to become a marathon runner, your ratio of fast-twitch to slow-twitch fibers would drop as a result of frequent usage of myosin heavy chain (MHC) type I, a form of muscle protein responsible for slow contractions with relatively low force output, but long duration work capacity.
What about the opposite? Can we convert slow-twitch into fast-twitch muscle fibers with training?
Science has shown us that this is indeed possible with the right type of training.
In a study by German researchers (1), 24 young male resistance-trained adults were split into two groups: 12 subjects performed heavy strength training in the bench press (5 sets of 3 reps @90%RM) Monday/Wednesday/Friday for 6 weeks.
The other half of the group performed the same heavy bench workout on a Monday, 10 reps of the bench throw exercise (a ballistic upper body exercise) at 30% bench press 1RM on Wednesday, as well as 10 repetitions of an explosive press-up on Friday – all for six weeks.
Here is what the researchers found:
Heavy strength training led to an increase in type IIa fast-twitch fibers, while type IIx (super-explosive fibers) dropped, and slow-twitch type I fibers remained the same after a biopsy of the triceps brachii muscle (picture A).
The combination group (heavy strength training + ballistic movements) increased type IIa content equal to the heavy strength training group. However, in this group the content of super-explosive type IIx fibers remained unaltered, while the amount of slow-twitch type I dropped (picture B).
This leads us to conclude that heavy strength training combined with power exercises result in a transfer of slow-twitch to fast-twitch type IIa muscle fibers.
From an adaptational point of view, this can be explained as follows: the body responds to the demands it is exposed to with its appropriate responses. Train slow, and you become slow. Train explosively and “heavy”, and you become powerful and strong.
As the results showed, strength training by itself can lead to a slightly slower profile, whereas the use of strength work IN COMBINATION with explosive movements increased the conversion of slow-twitch to fast-twitch fibers.
In terms of practical application, exercises such as the power clean and power snatch are a great way to develop explosive power and a more favourable fast-twitch profile.
One or two days per week of this type of training, with 3-5 sets of 2-5 repetitions and a rest of 4-6 minutes between sets develops this quality.
A simple practical application of both heavy strength training and explosive work can look like this:
- Squats 5x5
- Bench 5x5
- Deadlift 3x2
- Overhead press 3x5
- Chin-ups 3x AMRAP
- Squats 3x3
- Bench 3x3
- Power clean 3x3 @80%
- Power snatch 3x5 @70%
Three sessions of those (Mon/Wed/Fri) are heavy strength workouts, whereas once a week (Sat) we practice explosive power.
Depending on our goals (we may even partake in a sport that requires explosive actions), we can increase training frequency of explosive power work to two times per week, substituting one strength training session with it.