Not all muscle fibres are the same. Some are more powerful but, fatigue quickly (fast twitch), while others are less powerful but are very fatigue resistant (slow twitch). The range of fibre types runs across a broad spectrum. Within this spectrum they are categorised into one of three types. These are:
Type 1 – Slow twitch. These are smaller and do not grow as much in response to training. They are activated first when doing light weights or aerobic activity.
Type 2A – These are your intermediate type fibres. They produce more force than type 1, but not as much as type 2B. They are less fatigue resistant than type 1 but more than type 2B.
Type 2B – Only activated when enough force is required (e.g. heavy weights or explosive movement). These fibres can produce a lot of force but fatigue quickly. They are large and grow the most when stimulated.
So, in order of growth potential they go as follows:
1. Type 2B
2. Type 2A
3. Type 1
Consequently, it makes sense to focus most of your efforts on type 2B and 2A to grow. However, Type 1 fibres do have some growth potential. In fact, one of the main reasons bodybuilders look more jacked than powerlifters is that they have greater type 1 growth. To maximise your growth, you should train all muscle fibres for growth.
This probably shouldn’t be done on an even 1:1:1 ratio though!
Biasing your efforts towards type 2 fibres will still give you the most bang for your buck. Sprinkle in some type 1 focused work to build your best physique. Doing this tactically will magnify the results.
Each muscle has a mix of fibre types within it. The ratio is different from muscle to muscle though. Your soleus, for example, is very slow twitch dominant. As a result, it is probably a good candidate for training with lighter weights and higher reps. This will challenge the type 1 fibres that are dominant in the muscle and stimulate growth in them. Since that is what the muscle is primarily made up of it makes sense to try and grow these fibres.
Other muscles which are more mixed or fast twitch dominant might need very small doses of high rep work to challenge the few slow twitch fibres in the muscle.
To challenge the fast twitch fibres there are three basic approaches:
1. Lift heavy weights. 80-85% of your 1-rep max activates fast twitch fibres.
2. Lift explosively. Fast movements with lighter loads can still recruit your fast twitch fibres. Or this reason, I suggest you do the lifting phase of exercises powerfully but, maintain a slow controlled tempo on the lowering phase. Be wary of trying to be “too” explosive and throwing weights around. The risk of injury increases if you get sloppy doing this.
3. Training to failure. Even with lighter loads you will activate the fast twitch fibres (but only on the final few reps) if you go to failure. Henneman’s size principle states that, muscle fibres are recruited in order of size (from smallest to largest) on a needs basis. Has fatigue kicks in on a light set to failure you will begin recruiting larger and larger fast twitch fibres to complete the set.
Note. Training to failure can severely diminish your recovery so, it is probably prudent not to train to failure on all your sets, all the time as it will limit your overall training capacity and actually slow your gains. Instead train to failure for brief periods of time &/or just on some exercises/sets within a session. The final set for a muscle group using a machine or isolation lift is a good rule of thumb in my opinion.
Slow Twitch Fibres can be recruited by performing high rep sets (15-30) or taking short rest periods between lower reps sets (e.g. 30-60ss rest between 10-15 rep sets).