Crash Course in Muscle Fiber Types & Recruitment

Today I would like to spend just a bit of time clarifying a few things for endurance athletes regarding the training of muscle strength, endurance, and/power and I would like to make a case for strength training and plyometrics in your routine.  The nature of this article is not exhaustive but it will make a few points critical to your performance and your injury risk.

First let's look at muscle fiber types.  Some of you may be familiar with the idea that every skeletal muscle in our bodies have a mix of muscle fiber types.  Some fibers are more aerobic in nature and demonstrate better endurance (type I) while others are more anaerobic or glycolytic and help us with maximum power and strength movements (type II).

Simplistic summary of the main muscle-fiber types.

Now it's important to realize how our nervous system actually recruits our muscle fibers when we move.  The brain will always recruit it's type I or "slow twitch" fibers first because they exhibit the greatest endurance.  If the contraction needs to be stronger, the brain will then recruit more and more type II or "fast twitch" fibers until it can no longer recruit more fibers.  Interestingly it is believed that our brains cannot recruit every muscle fiber in a muscle all at once and it's true in the laboratory we always see only a percentage of muscle being recruited during a maximum effort.  This is partially modifiable as one adaptation to training is teaching the nervous system to recruit more of what we already have.  This is like taking the speed governor on a school bus from 55-mph to 75-mph.

The extreme example of full muscle recruitment would be a person showing superhuman strength in a crisis situation (such as lifting a car).  One theory is that the brain might decide in a life or death situation that it is willing to risk personal injury (muscle/tendon ruptures or even broken bones) in order to survive the situation at hand.  In other words, the brain removes the governor completely and the school bus is now traveling at maximum speed!  This hearkens back to the 1994 action-thriller SPEED with Keanu Reeves.

"Tell me again Harry, why did I take this job?"

Back to reality, we spend most of our time performing sub-maximal muscle contractions, and if you're an endurance athlete, you deal with muscle fatigue the most.  In that case, your brain gets a lot of practice cycling through your muscle fibers to maintain the correct amount of force over a sustained or repetitive effort.  In other words, as fatigue increases, your brain will recruit more type-II, fast-twitch muscle fibers.  Of course much of this is still very theoretical.  We can observe certain patterns in various athletes and note that certain styles of training tend to produce certain results.  However having a framework, despite its over-simplification, can still be of value when determining what a holistic and sustainable training plan might look like.

The primary issue that I see with endurance athletes is a heavy focus on training the nervous system to activate predominantly slow twitch fibers.  On one hand, this is a necessary part of endurance training.  We are all aware how effective base miles at an aerobic effort is to our long-term training goals each year.  Don't skip your base.  The issue arises when we actually need to recruit beyond the slow twitch fibers but we haven't practiced.  Running situations in which you would need to recruit beyond the slow-twitch include speed-work, running uphill or on difficult terrain, running in a state of fatigue, and racing.  Here we find ourselves once again staring the use it or lose it principle in the face.  It's important to note however that it is impossble to maximize your recruitment potential by running alone.  Yes it is good to vary your running workouts but our human bodies are better equiped to run when we challenge our muscles in novel ways.  This was less of an issue before the running boom in the 1970's and when most people had more physically demanding lives.

We are kind of meant to use our bodies all our lives and drop dead suddenly.

"Tell me again which color thera-band you wanted me to use?"

Unless you're doing something specific to target your other muscle fibers (or one could argue, to train the nervous system to properly recruit all types of fibers efficiently), you're clearly not taking advantage of your full potential and you will be at a higher risk for injury.  We need our nervous systems and muscles to be well-trained because it is not a question of if things will go bad but merely a question of when.  You will eventually run fatigued or need a bit more speed and power in a situation and it's better you have an excellent buffer of strength and power before that time comes.

Many roads lead to Rome but I have found that in general, most people benefit from 3-4 days a week of resistance training in their non-racing season where the goal is to build strength, speed, and power and 2-days a week to maintain when their running workouts become more intense and/or they're racing.  The easiest and safest way to strength train is to chose movements which you are more familiar with and which you have excellent body control performing.  A person can start with lower weight and higher repetition training and progress to higher weights as their form and technique allow.  The primary goal in building strength is to safely recruit as many of your muscle fibers as possible.  To build strength I personally do moderate-weight/moderate-repetitions most of the time and I move very slowly until I hit a fatigue point in which I cannot maintain excellent form.  By moving slowly I practice complete control of my exercises and minimize my injury risk.  That said, cycling the types of strength training is known to be highly effective as well.  Find a good strength coach if you need one (Colorado Springs is home to the National Strength and Conditioning Association) and be sure that any discomfort you feel in exercise is muscle burn, not joint pain.  Discomfort should not linger if you're doing things correctly.

Sorry Peter but don't be Peter...

Plyometrics, agility, and running drills that include skipping or hopping are also excellent ways to train your nervous system to better recruit its muscle fibers.  Here we are talking about more than overall strength (i.e. the total number of fibers you can recruit).  Plyometric drills target the speed at which we can recruit our muscles.  By increasing either the number of fibers recruited and/or the speed of recruitment, you increase your muscle power.  Power is the secret neuromuscular ingredient which helps us run faster with less effort.  For endurance athletes, power also helps us maintain our speed and good form when fatigue is a factor (and when is fatigue not a factor?).  These recruitment drills are a common missing ingredient in many runner's recipes for success but improving muscle recruitment is really the gift that keeps on giving.  Plyometric, agility, and running drills are ideally performed throughout the year in a smart and safe manner to help maintain all of the aforementioned benefits.  If you have no experience with this type of training, it may be smart to get professional assistance in the beginning in order to learn the basics.  Because plyometrics are inherently higher-force than most people are used to, the likelihood of sustaining an injury temporarily increases.

Olympic lifting (left-side) versus Power lifting (right-side).
Olympic lifts actually require a lot of power, not just strength because the speed of movement is very important in a successful lift.  Power lifting requires power but essentially one needs to be VERY strong as these are maximum lifts and the speed of movement is relatively slower.

While this crash course in muscle fiber types and recruitment won't make you an expert today, perhaps your brain is beginning to ponder how you might improve the effectiveness of your own training methods.  If you are attempting to be a competitive runner or if you would like to make yourself more resistant to injury, intelligent strength training and plyometric programs are a must.  The key is to start where you're at...we want to slowly re-train your governor, not have the bus speeding off on a draw-bridge.

Oops, should have thought this through...