This week’s article is designed to generate discussion by giving some general facts on muscle fiber types and how it relates to SUF training.
There is a huge variability between individuals in their muscle fiber make up due to genetics, circulating hormone levels and training. Muscle fibers will adapt to the training stress placed upon them and can change characteristics as a result of consistent training. Training that involves recruiting more muscle fibers of all types and training them to be more efficient is crucial in improving fitness and performance. For example, training at low force, low velocity will only recruit type I fibers. A better option is to train our Type I fibers to be more efficient by performing steady state workouts with surges above Vo2 to activate the type IIa fibers and dump a load of lactate which the type I fibers have to clear. We can also train our type IIa fibers to be more like type I by increasing mitochondrial density and capillarization from Vo2 workouts. Repeated MAP and AC workouts which activate type IIa fibers trains tolerance to lactate and thus quicker recovery between intervals.
Skeletal muscle is composed of three types of fibers: They each have unique characteristics:
Slow Twitch (Type I) :
● Fatigue resistant
● Used for endurance activities
● Low force production
● High concentration of mitochondria and utilize oxygen to produce energy
● High capillary density to transport nutrients to muscles and remove waste products ● High oxidative capacity
● High myoglobin concentration- myoglobin is the pigment found in muscle cell that aids hemoglobin in producing oxygen for aerobic metabolism
● Low glycolytic capacity - the ability to store and breakdown glycogen for use as a high intensity source of energy
Fast Twitch (Type IIA):
● Moderate resistance to fatigue
● Used for short duration (up to 2minutes)
● High force production
● Moderate concentration of mitochondria
● Moderate capillary density
● Moderate oxidative capacity
● Moderate myoglobin concentration
● High glycolytic capacity- can breakdown glycogen and use for high intensity efforts
Fast Twitch (Type IIx or IIB):
● Low resistance to fatigue
● Used for very short efforts up to 8 second at maximal intensity
● Very high force production
● Low number of mitochondria
● Low capillary density
● Low oxidative capacity
All muscles have a mix of the three fiber types. Most Type I fibers are located deep in the muscle and most Type IIs are on the surface. The lower body generally has more Type I than the upper body. Muscle fiber type structure may shift in response to training or injury. Any shift in structural change is a long term training adaptation. Endurance athletes have a higher proportion of Type I than Type II in trained muscles. Strength trained athletes have higher proportions of Type IIA fibers and power athletes have a higher proportion of Type IIx/b fibers.
What does this mean for your training:
Neuromuscular power and coordination can be improved through all out sprints and cadence drills which teaches our muscles to turn on/off more efficiently.The higher the recruitment, the stronger the muscle contraction will be. Motor units (a group of muscle fibers and a motor neuron) are generally recruited in order of smallest to largest (slow twitch to fast twitch). And also realize any path to fatigue/failure will lead to recruitment of Type IIa and IIx fibers like at the end of an ironman.
The 4DP model is geared for each individual to ensure that the workout is specific in generating the stimulus to enhance fitness. During a Full Frontal 4DP fitness test the, NM 8 sec sprint will be predominantly type IIx muscle fibers. The 5 min MAP and 20 min FTP segments will recruit all the fiber types due to the fatigue generated by the tests. The 1 min AC test is unique. It does activate predominantly type IIa fibers, but it really tests the ability to produce repeated efforts due to the heavy fatigue load already experienced. It is by no means not an all out fresh 1 min effort.
The goal of our training plans is to improve fitness by increasing density of mitochondria, recruiting more muscle to stave off fatigue, have more matches to burn without draining the battery during high intensity efforts and improving our maximum power in the race to the finish line.