Muscle Building Part V: Muscle Fiber Recruitment

March 15, 2010

in How Muscle is Built, Strength Training

I’ve now arrived at one particular neuro-muscular adaptation that once again leads us to understand why the 5-8 rep range is so effective for building muscle: muscle fiber recruitment. Before I proceed, let me recap:

The two most important factors that go into gaining muscle is tension and mechanical work.
- Tension is created by lifting weights. The more weight lifted, the more tension created in the muscle.
- Mechanical work, also referred to as volume or fatigue, leads to muscle protein breakdown. This breakdown leads to protein synthesis and permits the muscle to be repaired in such a way that will leave it bigger and stronger. However, too few reps doesn’t result in much breakdown, but neither does too many reps.
- The muscle building sweet spot is the intersection of heavy weights and sufficient reps.

Muscle fiber recruitment is the process by which the CNS recruits muscle fibers to accomplish a given task. It enables the CNS to adjust its power output according to the external load. For example, try performing a biceps curl with a pencil. Now curl a 20 pound dumbbell. The CNS recruited more muscle fibers to lift the 20 pound dumbbell than it did the pencil. It would be silly for the body to only be able to produce one level of force for all its activities. The body uses muscle fiber recruitment to match the external resistance. It’s one more wonder of the human body to be appreciated.

As always, nothing is as simple as it seems. The human body has two types of muscle fibers, slow twitch and fast twitch. To be complete, there are two types of fast twitch fibers, but that is mostly irrelevant for this discussion.
-Slow twitch fibers “fire” more slowly, are more endurance based, have less potential for growth, and don’t produce as much force.
-Fast twitch fibers “fire” more quickly, fatigue more quickly, have more potential for growth and produce much more force than slow twitch.

Think of slow twitch fibers as marathon runners and fast twitch fibers as sprinters. The marathon runner goes for hours without tiring, whereas the sprinter goes unbelievably fast for a very limited amount of time and looks completely exhausted after a ten second race. The same holds true for the muscle fibers and that is exactly how they work.

The CNS recruits muscle fibers according to the size principle. This means that the smaller, slow twitch fibers will be recruited first. Once the slow twitch fibers fatigue it will recruit the bigger fast twitch fibers. By doing so, the CNS effectively regulates the amount of force that the muscles produce. After all, how much force is required to curl a pencil? BUT……..

If an external load is so heavy that the slow twitch fibers can’t possibly handle the task of lifting it, the CNS will immediately look to recruit the fast twitch fibers.

Muscle fiber recruitment has a number of practical implications.
1. A load that is light enough to permit the completion of 15+ reps will not optimally recruit the fast twitch fibers. The body will recruit and exhaust all available slow twitch fibers and only at the very end of the set will it be forced to recruit the fast twitch fibers. By using a light weight you aren’t recruiting the muscle fibers that have the greatest strength and size potential until the very end.

2. Even though the fast twitch fibers will be recruited eventually, the weight is so light that not enough tension will be produced within the muscle to spur growth. Fatigue does not equal tension. Working out with a weight that would enable you to do 15-20 reps would be equal to roughly 60-30% (depending on the individual) of your 1-rep max! This is very light. Recall that high amounts of tension created within the muscle is the most important factor in promoting strength and growth. There isn’t much tension being produced in this scenario.

3. The heavier the weight, the bigger contribution made by the fast twitch fibers. In fact, if the weight is heavy enough the fast twitch fibers will be recruited from the beginning of the set.

4. Muscle fiber recruitment is responsible for roughly the first 80-85% of strength production. Rate coding is responsible for the rest. Performing a set with a weight that will permit you to lift it for 80-85% of your maximum works out to be…………5-8 reps. Lifting a weight equal to your 5-8 rep max will give you full recruitment of the fast twitch fibers with high tension from the beginning of the set!

Using a weight equal to your 5-8 rep max will permit you to lift the heaviest weights possible via immediate fast twitch muscle fiber recruitment while also accumulating a significant amount of volume (mechanical work). This is how the muscle building sweet spot was determined and this is why I advocate doing most of your work in this rep range.

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