Muscle Fiber Type/Recruitment and the Implications for Strength Training: You Need to Understand the Difference!

In past blog entries I have talked about overload and adaptation.  Most people are familiar with the principle.  You work a muscle or energy system above and beyond the normal workload and it will adapt.  The time for it to adapt is different with individuals and varies widely.  Some individuals adapt much faster than others.  Part of this adaptation is based on the type of muscle fiber you are given at birth.  Some individuals have a higher percentage of Fast Twitch muscle fiber and some have more Slow Twitch fiber.   So be patient, because the type of overload and what you have been given genetically will influence your individual adaptation. 

Overloads can come in different forms.  You can increase resistance or needed force production necessary to move an object.  You can overload the number of repetitions per set.  You can also increase the number of sets.  Overloads can also come with contraction velocity.  Movement velocity can also be where overloads occur.    Understanding these overloads and muscle fiber recruitment is important to determine how to most effectively impact a particular athlete (athletic maturity) and is used in supporting a strategy that addresses the particular needs of a sport.  Muscles will adapt differently based on the type of overload.

These overloads require an understanding of muscle fiber and muscle fiber recruitment to better design a strength and conditioning strategy.

Types of Muscle Fibers:

Slow Twitch Fiber:  A simple definition is muscle fibers that have slow contraction time and a high resistance to fatigue.  They have a smaller structure with higher mitochondria and capillary density.  They have much of the enzymes to support oxidative (endurance) pathways and are used for low force/high aerobic activities.  Most of your daily movement is supported by these types of fibers. This would be a 24 hours of Le Mans car.  Made to go long but will not be the quickest in the quarter mile.

Fast Twitch Fiber:   Quick contraction time, and low resistance to fatigue.  Increase in the speed of release of calcium and breakdown of ATP.  Fast Twitch can be broken down again to Fast Twitch A or IIA and Fast Twitch B or IIB fibers.

Fast twitch A fibers would be seen as a long sprint motor and Type B as a top fuel dragster, one run and done. 

There is a condition known as the size principle.   It is an efficient way your body has evolved in the recruitment of muscle fiber.  If the demand for force production is large then your body will increasingly recruit larger motor units to satisfy the need.  The largest motor units (Fast Twitch Fiber B) are recruited last and have the highest threshold.   Have you ever lifted something thinking it was heavy and then are surprised when the weight is much lighter?  This is a good way of seeing your body recruiting an unnecessary amount of muscle fiber for a perceived needed production of force.  Your brain was prepared for the higher perceived effort and therefore higher force production.  If we did not recruit in an order of force production needed, we would constantly be unable to regulate the amount of force being produced.   It makes a lot of evolutionary sense.

So the order of recruitment is Slow Twitch, followed by Type A fast twitch, and Type B fast twitch when needed.

With this understanding you can start to see how the strategy and tactics of program design is very important when creating a workout to support the muscle fiber needs of a particular sport.

So many athletes wonder if they can change the percentage of fast twitch to slow twitch.  Most of the research shows that this cannot really change.  However, you can match training to more effectively increase the performance of the fibers you are born with.  Through training, the cross sectional area of the muscle fiber will change.  This will help athletes in sports where rapid production of force is of great importance.  Conversely, selective training can add to capillary and mitochondria density.   See my recent blog entry Strength vs Endurance Training: How do you mix oil and water?   

Strength and Power training will result in increased glycolytic enzymes and hypertrophy of IIB Fibers.  It will not add more IIB fibers but they will become larger.  Also you will see better speed of muscle contraction. 

Aerobic training will add hypertrophy to Type 1 and increased capabilities of IIB fibers (Long sprints).  Also, increase in capillary density, aerobic enzymes, and mitochondria.  These adaptations support the need for less force and greater endurance.  

Now how does this translate into training?  At Sirens and Titans we focus on the value or return on and exercise and the risk associated with this return.  Most of our athletes are not competitive weight lifters so some exercises may incrementally increase strength, but are just not worth the risk of the potential injury.  An example of this would be with many overhead lifts and throwing athletes.  There are some exercises we use and some we are more reluctant to use.    If you are not looking at this risk/return you may eventually come up short.

Next focus on effort of the exercise in the particular area you are trying to stress and not just the force being produced.   Your goal should be to get overloads to improve performance in the muscle fibers that will most effectively increase the performance in a particular sport.  This matched with the type of athlete you are training is the optimum strategy.   There is a sweet spot of training where you are getting the greatest effort with the lowest amount of risk and the highest specificity to your sport.  One thing to remember is that there are legitimate correlations in training that may enhance your primary objective.   Pay attention to ways of supporting these positive correlations.  When evaluating sprinters the best sprinters are usually the best vertical jumpers not the best squatters.  However squatting will improve your vertical jump.  So look for correlations that will enhance your primary objective.  Allocate your time accordingly.

We rarely do one rep max lifts with any athlete.  We try to find a weight where the effort increases dramatically at 3-6 reps and motor recruitment is still maximized.   Remember, the last 2 reps are of great importance because the effort becomes the greatest and the overload is maximized. A good example of the importance of effort and the last reps can be demonstrated in a plank.  When you first start a plank the force production is X to hold your body up.  When you first establish the plank the effort is not hard.  However, as you hold the plank longer and longer your effort starts to increase dramatically even though the force produced is the same to hold your body up.   Greater amounts of muscle fiber are being recruited to support the same force production to maintain the integrity of the plank.  The initial hold was supported by a greater percentage of slow twitch fibers, but as fatigue increases a larger number of muscle fibers are recruited to pick up the load and effort increases.   The greatest amount of recruitment is in the last seconds of the hold before you drop.  That is why it is important to challenge yourself in your lifts and pay attention to effort and weight.  

The sweet spot is a lift where effort is highest and risk of injury is low.  So find your sweet spot and focus on effort as well as weight being lifted.

I have spoken in the past about Power times an X factor.  Each sport has a particular requirement for sport and position.  Once this has been established then the requirements for absolute force, power, maximum sustained power, and average power all come into play.  Understanding strength is a great place to start in developing a strategy that matches the needs of your sport.   


Truth in Fitness

Jacques DeVore, CSCS, Certified Primal Health Coach