Motor Unit Recruitment - Slides PDF

Motor Units  Motor Units 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 T...

Motor Units  Motor Units 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 2 Sport & Exercise. Hum.Kin.  All-or-None Principle 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 3 Sport & Exercise. Hum.Kin.  Gradation of Force (Acute) 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 4 Fig. 19.7. McArdle et al. 2015. Exercise Physiology… LWW  Summation of Force 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 & Exercise. Hum.Kin.  Muscle Actions 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 Dynamic muscle action  Muscle produces force and changes length Joint angle changes  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  Isotonic contractions (same force)  Isokinetic contractions Speed of movement does not change Can be either concentric or eccentric 6  Force-Velocity Curve Power = Work Time Fig. 1.14. Kenney et al. 2012. Physiology of Sport… Hum.Kin.  Force-Velocity Curve Fig. 1.14. Kenney et al. 2012. Physiology of Sport… Hum.Kin.  Size Principle  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.  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) 9 Fig. 5.2. Haff et al. 2016. Essentials of Strength… Human Kinetics Review Questions Discuss the 5 ways in which a muscle fiber 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? Figure & Notes References Kenney, Wilmore, Costill. Physiology of Sport and Exercise (5th ed). Human Kinetics, 2011. McCardle, Katch, Katch. Exercise Physiology: Nutrition, Energy, and Human Performance, 8th Edition. Wolters Kluwer Health, 2014. Baechle & Earle. Essentials of Strength & Conditioning, 3rd Editiion. Human Kinetics, 2009.