Muscle Mechanics Claudia Stanescu Ph D Office Hours in Gittings 108 Tue 10 11am and Thurs 1 2pm or by appointment Objectives 1 List the three phases of a muscle twitch 2 Compare and contrast the characteristics of a muscle twitch and tetanic contraction Describe the development of tetanus in terms of temporal summation 3 Define a motor unit and discuss it in the context of spatial summation 4 Describe the size principle in the context of development of increased tension during a voluntary muscle contraction 5 Describe isometric and isotonic contractions 6 Describe the strategies used to increase force in the muscle including increasing motor unit recruitment frequency of activation and the length of muscle at the beginning of a contraction Muscle Mechanics 1 The Muscle Twitch contraction arising from a single electrical stimulus does not exist in body when we contract always train or e g an action potential series of AP or signals never just one more like a thought process 3 Periods 1 Latent slight delay in change of muscle 2 Contraction 3 Relaxation can be done experimentally hook muscle to electrodes and zap it see what happens pattern of response to one action potential 1 Latent Period 2 msec or less Action potential sweeps over the sarcolemma Calcium ions released from the sarcoplasmic reticulum No change in tension delay between zapping muscle and crossbridge formations stimulus applied to muscle but no change in tension delay Ca pumps back into the SR as soon as it is released this is why lifespan of contraction is short time for contraction and relaxation depends on the fiber types 2 Contraction Period 10 100 msec Calcium binds to troponin Myosin binding sites on actin are exposed Crossbridges form and swivel Peak tension develops Ca release important to generate tension cross bridge needed to generate tension 3 Relaxation Period 10 100 msec Calcium ions transported back into the sarcoplasmic reticulum Myosin binding sites on actin are covered by tropomyosin Myosin heads detach from actin Tension decreases The Time Course of a Twitch varies for different muscles fast twitch vs slow twitch AP always the same from nervous system doesnt range in strength Fast twitch Slow twitch eye more fast twitch fibers has faster time frame gastroc Tension eye on foot slower to contract higher endurance gastroc calf muscle involved with posture fast fibers to push off slow to stay standing soleus Time there is a difference in time frames depending on the type of fiber involved Muscle Mechanics 2 Tetanus or tetanic contraction summation over time Temporal Summation of individual twitches AP comes so fast there stimulating muscle more than once series of electrical signals twitches add up temporal summation don t fully relax is no chance for relaxation stimulate fibers so Ca release just keeps increasing doesn t normally happen in muscles ie lock jaw contraction that doesn t let up external stimulation taze one stimulus more stimuli before relaxation SR releases more Ca wave summation at faster rate happens in most muscles Tetanus temporal summation is a mechanism for increasing produces more crossbridges increase in AP rate increase tension in muscle tension tetanus nothing to do with more stimuli more Ca released faster stimulation more force produced bacterium name for contraction requiring series of action potentials more than one stimulus Tension Production and the Motor Unit Motor Unit a motor neuron and all the muscle fibers cells it innervates fibers contract together depending on what unit they belong to not all clumped together spread out and intertwined throughout muscle whatever happens to that unit it affects all fibers within that unit only slow or only fast fibers in one unit not mixed Muscles typically contain many motor units recruited based in order for how much force is required activating more fibers more force by activating recruiting increasing numbers of motor units tension can be increased Spatial Summation activate 1st neuron activate 2nd neuron activating more fibers more force if you want to increase force and you measure inside the muscle stick probe inside muscle tell them to increase force and measure neuron and how fast it reacts see how many motor neurons are firing like gears of a car 1st starts gets faster then 2nd starts and gets faster then 3rd etc Motor Units and the Size Principle activated in order of size start with smallest The first motor units recruited are small innervate few fibers allows us to have fine carefully controlled units the result is fine carefully controlled increase in tension As more force is required larger motor units are recruited Small SO slow most endurance SO Slow Oxidative Medium FFR Large FF Small SO Intermediate FOG Large FF ability for some endurance and anerobic activity FOG Intermediate oxidative glycolytic fatigue resistant Small SO Medium FOG Large FG produce most force fatigue quicker FG fast glycolytic fatiguable Size Principle 100 maximal force Jump Sprint 60 Run 20 add FF Walk Stand add FR 25 50 75 100 of motor units recruited smallest MN s largest MN s Isometric vs Isotonic Contractions 1 Isometric same length contractions in which muscle doesn t change in length just generates tension not shortening holding object muscles don t shorten but they generate tension so you don t drop it muscle generates tension but does not shorten pre lifting object tension in beginning like dead lift before you bring the bar off the ground every contraction begins isometrically 2 Isotonic same tension when you get ready to move load muscles start to shorten refers to load not changing but the muscle changing isotonic contrations in both shortening and lengthening after sufficient tension is generated to move the load the muscle changes length Two types 1 concentric 2 elongating eccentric shortening Concentric vs Eccentric contractions 1 When developed tension is sufficient to move the load the muscle shortens lifting concentric contraction Effort Load 2 As tension decreases and the load exceeds tension the muscle lengthens lowering eccentric contraction muscles contract in pairs Load Effort bicep contracts concentriclly while tricep contracts eccentrically Note All muscle actions are based on the concentric basing on concentric contraction contraction of a muscle always IMPORTANT Strategies to increase force 1 Increase motor unit recruitment spatial summation 2 Increase frequency of stimulation temporal summation 3 Optimal muscle