Muscle Fiber Types PDF

1   A motor unit contains only one specific muscle fiber type (type I or type II) or a subdivision of the type II fiber with the same metabolic profile  Motor units are classified based on three physiologic and mechanical properties of the muscle fibers...

1   A motor unit contains only one specific muscle fiber type (type I or type II) or a subdivision of the type II fiber with the same metabolic profile  Motor units are classified based on three physiologic and mechanical properties of the muscle fibers they innervate: Twitch characteristics Tension characteristics Fatigability 22   Major characteristics for common motor unit categories:  Type I: Slow twitch, low force, & fatigue resistant  Type IIx: Fast twitch, high force, & fatigues quickly  Type IIa: Fast twitch, moderate force, & fatigue resistant  Motor neurons have a stimulating effect on the muscle fibers they innervate in a way that modulates the fibers’ properties and adaptive response to stimuli 3 3 3  Type IIx Type IIa Type I 44 Fig. 19.11. McArdle et al. 2010. Exercise Physiology… LWW   The proportions of each type of muscle fiber vary from muscle to muscle and from person to person  On average, 50:50 slow and fast twitch per person  Proportion varies considerably among different muscles For example, soleus is ~80% Type I for everyone  Skeletal muscle fibers differ in:  Biochemical properties Oxidative capacity  Number of capillaries, mitochondria, amount of myoglobin Glycolytic capacity – Amount of sarcoplasmic enzymes Muscle fiber efficiency 5 5   Skeletal muscle fibers differ in:  Contractile properties  Maximal force production  Force per unit of CSA  Speed of contraction (Vmax)  Type of myosin ATPase  Speed of ATP hydrolysis  SR development  Speed of calcium handing  SERCA pump ATPase speed  Both slow and fast muscle fiber types contribute during near‐maximum Fig. 1.12. Kenney et al. 2015. aerobic and anaerobic Physiology of Sport… Hum.Kin. exercise 6 6   Generate energy for ATP resynthesis predominantly through the aerobic system of energy transfer Slow‐oxidative fibers (SO) with slow shortening speed and rely on oxidative metabolism  Their six distinguishing characteristics include: Low myosin ATPase activity Slow calcium handling ability and shortening speed Less well‐developed glycolytic capacity Large and numerous mitochondria High capillary density and myoglobin Red in color 77   Exhibit the following characteristics:  High capability for electrochemical transmission of action potentials  High rate of crossbridge turnover High myosin ATPase activity Rapid Ca2+ release and uptake by an efficient sarcoplasmic reticulum  High glycolytic enzyme level Very fatigable  White in color  These factors contribute to this fiber’s rapid energy generation for quick, powerful muscle actions 8   Speed of shortening and tension development is 3‐5x faster than slow‐twitch fibers Rely on a well‐developed, short‐term glycolytic system for energy transfer  Predominates in anaerobic‐type sprint activities and other forceful muscle actions that rely almost entirely on anaerobic energy metabolism Important role in the stop‐and‐go or change‐of‐pace sports 99   Type II fibers distribute in subtypes: Type IIx: Possess the greatest anaerobic potential and most rapid shortening velocity “True” fast–glycolytic fiber (FG) Type IIa: Represent the fast–oxidative–glycolytic fibers (FOG) 10 10  Tab. 1.1. Haff et al. 2016. Essentials… Hum.Kin. 11 11  Determinants of Fiber Type:  Genetic factors Determine which ‐motor neurons innervate fibers Fibers differentiate based on ‐motor neuron  Training factors Endurance versus strength training, detraining Can induce small (10%) change in type  Aging muscles lose type II motor units  Fiber type not sole predictor of success  Cardiovascular function  Motivation  Training habits  Muscle size Tab. 1.3. Kenney et al. 2015. Physiology of 12 12 Sport… Hum.Kin.   Low‐threshold motor units are recruited first and have lower force capabilities than higher‐threshold motor units.  Typically, to get to the high‐threshold motor units, the body must first recruit the lower‐threshold motor units. 13 13 Fig. 5.2. Haff et al. 2016. Essentials of Strength… Human Kinetics 13   This orderly recruitment of specific motor units to produce a smooth muscle action allows the CNS to fine tune skeletal muscle activity to meet demands of the motor task.  Exceptions exist, especially with respect to explosive, ballistic contractions that can selectively recruit high‐ threshold units to rapidly achieve more force and power. (Selective recruitment) 14 14 Fig. 5.2. Haff et al. 2016. Essentials of Strength… Human Kinetics 14   Compare and contrast the types of fast twitch and slow twitch fibers according to their metabolic and mechanical properties Be able to explain what structural and physiological properties give them their performance / applied characteristics  Apply the Size Principle to lifting weights and explain how different fiber types are recruited Explain how Selective Recruitment influences fiber type recruitment 15 15   Haff & Triplett. Essentials of Strength Training & Conditioning, 4th edition. Human Kinetics, 2016.  Kenney, Wilmore, Costill. Physiology of Sport & Exercise, 6th edition. Human Kinetics, 2015. 16 16