Motor Units PDF

1   The motor unit consists of the anterior motor neuron and the specific muscle fibers it innervates  Each muscle fiber generally receives input from only one neuron  Yet a motor neuron may innervate many muscle fibers ...

1   The motor unit consists of the anterior motor neuron and the specific muscle fibers it innervates  Each muscle fiber generally receives input from only one neuron  Yet a motor neuron may innervate many muscle fibers  The number of muscle fibers per motor neuron generally relates to a muscle’s particular movement function  Muscles that require less precision may have several hundred fibers served by one motor neuron.  Muscles that function with great precision may have as few as one muscle fiber per motor neuron. Fig. 1.6. Kenney et al. 2015. Physiology of 22 Sport & Exercise. Hum.Kin.   A stimulus strong enough to trigger an action potential in the motor neuron activates all of the accompanying muscle fibers in the motor unit to contract synchronously  There is no such thing as a motor neuron stimulus that causes only some of the fibers to contract.  A motor unit does not exert a force gradation; either the impulse elicits an action or it does not  Similarly, a stronger action potential cannot produce a stronger contraction. Fig. 1.6. Kenney et al. 2015. Physiology of 33 Sport & Exercise. Hum.Kin.   Amount of force generated in a single muscle fiber is dependent upon the # of crossbridges  Force generation of a group of muscles depends on five factors: 1) Number of motor units recruited A muscle generates considerable force when activated by all of its motor units 2) Frequency of motor unit discharge Repetitive stimuli that reach a muscle before it relaxes increases the total tension 3) Type of motor unit recruited Larger / Type II motor units will generate greater force 4) Preloading the muscle (activating the stretch reflex) 5) Speed of contraction Fig. 19.7. McArdle et al. 2015. Exercise 44 Physiology… LWW   Normal body movements are not made up of simple twitches, but sustained contractions.  ↑frequency of stimuli increases > fibers do not have time to relax > “summation” and ↑ force production Fig. 1.12,13. Kenney et al. 2015. Physiology of Sport & 5 Exercise. Hum.Kin.   Static (isometric) muscle action (“contraction”)  Muscle produces force but does not change length  Joint angle does not change  Myosin cross‐bridges form and recycle, no sliding  Isotonic contractions (same force)  Isokinetic contractions  Speed of movement does not change  Dynamic muscle action  Muscle length & joint angle change  Includes both isotonic & isokinetic actions  Concentric contraction Muscle shortens while producing force Sarcomere shortens, filaments slide toward center  Eccentric muscle action Muscle lengthens while producing force Cross‐bridges form but sarcomere lengthens 66 Fig. 1.14. Kenney et al. 2012. Physiology 7 of Sport… Hum.Kin. Fig. 1.14. Kenney et al. 2012. Physiology 8 of Sport… Hum.Kin.   Discuss the 5 ways in which a muscle can acutely increase force output  Distinguish among the major types of muscle actions and be able to provide examples of each  Explain the relationship between force and velocity of contraction At what general concentric speeds can you generate the most force? What about power? How does your ability generate force compare for concentric, eccentric, and isometric muscle actions? 9   Haff & Triplett. Essentials of Strength Training & Conditioning, 4th edition. Human Kinetics, 2016.  Kenney, Wilmore, Costill. Physiology of Sport & Exercise, 6th edition. Human Kinetics, 2015.  McCardle, Katch, Katch. Exercise Physiology: Nutrition, Energy, and Human Performance, 8th edition. Wolters Kluwer Health, 2015. 10