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Skeletal Muscle Fiber Types
Skeletal Muscle Fiber Types
Presented by Catherine Val
Introduction
Introduction
There two general categories of muscle fibers:
- Slow, type I
- Fast, type II
Most Muscles contain a mixture of both skeletal muscle fiber types.
The percentage of muscle fiber types in an individual can be influenced by genetics, hormone levels in blood, and exercise habits.
Graph
The graph below shows how different types of exercise can effect the muscle fiber composition.
Graph
Slow vs. fast twitch muscle fibers
Graph
(click here)
(Widrick, Stelzer, Shoepe, & Garner, 2002)
Biomechanical properties of muslce
The three primary biomechanical charachertistics of muscle that are important to muscle funtion are:
- The oxidative capacity
- The type of myosin isoform
- The abundance of contractile protein within the fiber
Biomechanical Properties of Muscle
Oxidative capacity
The oxidative capacity of a muscle fiber is determined by the following three things:
- The number of mitochondria
- A large number of mitochondria = greater capcity to produce ATP aerobically
- The number of capillaries surrounding the fiber
- High number of capillaries = adequate oxygen supply
- The amount of myoglobin within the fiber
- High myoglobin concenctration = improved delivery of oxygen to mitochondria
- Different myosin isoforms differ in their myosin ATPase activity.
- Isoforms differ in the speed tat they break down ATP
- High ATPase activity = degraded ATP activity, resulting in a high speed of muscle shortening
- Low ATPase activity = slow speed of muscle shortening
Type of myosin isoform
Actin and myosin are the two main contractile proteins in skeletal muscle.
Fibers that contain large amounts of actin and myosin generate more force than fibers with low levels.
More actin and myosin = more force
Abundance of Contractile protein
In comparing the contractile properties of muscle fiber types, the following characteristics are important to consider:
- Maximal force production
- Speed of contraction
- Maximal power output
- Efficiency of contraction
Contractile Properties of Muscle
Large muscle fibers produce more force than smaller fibers.
Fiber specific tension is the maximal force that a cross-section of muscle fiber can produce.
Fiber specific tension = force production/size of the fiber
Maximal force production
Speed of contraction
The contraction speed of the muscle fiber is compared by measuring the maximal shortening velocity (Vmax) of the the individual fiber.
Vmax = the highest speed at which a fiber can shorten
High myosin ATPase activty (fast fibers) = high Vmax
Low myosin ATPase activity (slow fibers) = low Vmax
Maximal power output = force x shortneing velocity
Muscles with high force and high shortening velocity produce high power output
Muscles with low force and low shortening velocity produce low power output
Maximal power output
An efficient muscle fiber requires less energy to perform a certain amount of work compared to a less efficient fiber.
Efficiency = ATP used/force produced
Efficiency of contraction
There are three major muscle fiber types in human skeletal muscle:
- Slow (type I) fibers
- Fast (type IIa) fibers
- Fast (type IIx) fibers
Table 8.1 shows the characteristics of human skeletal muscle fibers
Characteristics of muscle types
(Powers, 2015)
Type 1
(Slow)
Type I fibers are called slow twitch or slow oxidative fibers.
Characteristics of type I slow twitch fibers:
- contain large numbers of oxidative enzymes (high mitchondrial volume)
- surrounded by more capillaries
- contain higher concentrations of myoglobin
- large capacity for aerobic metabolism and high resistance fatigue
- possess slower Vmax
- produce lower specific tension
- more efficient
Type IIa
(Fast)
Characteristics of type IIa (intermediate fibers):
- viewed as a mixture of type I and IIx
- regular exercise training can result in the conversion of fast fibers into slow fibers
Characteristics of type IIx (fast twitch) fibers:
- small number of mitochondria
- limited capacity for aerobic metabolism
- less resistance to fatigue
- rich in glycolytic enzymes
- fasteset skeletal muscle in humans
- highest Vmax of all fibers
- generates the highest power output
Type IIx
(Fast)
Muscle contraction = muscle force generation
Isometric action = static exercises
Dynamic exercise = involves movement of body parts
- Concentric action results in muscle shortening
- Eccentric action results in muscle lengthening
Muscle Actions
Concentric vs Eccentric Demo video
Speed of muscle action and relaxation
Muscle twitch movement phases:
- immediately after the stimulus there is a brief latent period lasting a few milliseconds
- contraction phase lasts about 40 milliseconds
- muscle then returns to the original length (relaxation period) and lasts about 50 milliseconds
The speed of shortening is greater in fast fibers than in slow fibers
Higher ATPase activity results in a more rapid splitting of ATP and quicker release of energy.
QUestions:
1. explain the difference between the three types of muscle fibers.
2. Give an example of an isometric action, concentric exercise, and eccentric exercise
3. Which type of muscle fiber would a marathon sprinter most commonly possess and why?
Questions and Refernces
References:
(2014, August 26). Retrieved April 07, 2018, from https://www.youtube.com/watch?v=5r-F7DODMTg
Powers, S. K., & Howley, E. T. (2015). Exercise physiology: theory and application to fitness and performance (9th ed.). New York, NY: McGraw-Hill Education.
Widrick, J. J., Stelzer, J. E., Shoepe, T. C., & Garner, D. P. (2002). Functional properties of human muscle fibers after short-term resistance exercise training. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 283(2). doi:10.1152/ajpregu.00120.2002