HOW MUSLES WORK Readings: H (h. 3,13), T (h 2,3)* How Muscle Force is Generated:! Muscles convert chemical energy into! Muscles are internal motors of human body responsible for all movements of skeletal system! Muscles can only ; they cannot! Muscles must cross a joint to create motion! Our focus--skeletal (striated) muscle! Active vs. passive muscle tissues... Excitable ( ) muscle tissues -- muscle fibers: actin/ myosin elements and their linkage (cross bridges) Non-excitable ( ) tissues -- connective tissue that organizes muscle fibers and binds them together! Actin/Myosin arrangement: Myosin - filaments or myofibrils Actin - filaments Basic muscle characteristics:! --sensitive to stimulation! --capacity to shorten! --capacity to lengthen! --ability to return to original state after lengthening or shortening Basic muscle structure:! There is considerable variation in size, shape, structure depending on function, e.g., need for power, ROM, fast movement, postural control, or precise movement. * See syllabus for textbook abbreviations 1 2
Sliding Filament Theory Steps: 1. Myosin filament forms x-bridge to actin 2. Myosin pulls actin 3. x-bridge releases 4. Myosin ready for another x-bridge formation Muscle actions:! oncentric -- active muscle that against resistance e.g., biceps brachii during elbow flexion in a standing biceps curl! Eccentric -- active muscle that against a resistance e.g., biceps brachii during elbow extension in a standing biceps curl Myosin Actin 3 4
! Isometric ("same length") or static -- essentially in overall muscle length during activation - rate at which MUs are stimulated. To increase force, NS recruits MUs more frequently (e.g., from 10 Hz to 25 Hz).! It is important to remember that when a muscle is activated, the only thing it can try to do is (i.e., tries to bring the origin and insertion closer together) Relates to directionality of cross-bridge function. Factors affecting muscle force production:! Muscle architecture Physiological area: greater area, greater force producing ability Fiber orientation...! Excitation factors - role of nervous system Motor unit recruitment (MU) = a motor neuron and all muscle fibers it innervates. When a motor neuron is activated, all of its muscle fibers are activated and contribute to force production. To increase force, nervous system (NS) recruits. Lower SA Higher Force Shortening 5 6
! Muscle Mechanics Force - Velocity Relationship Force production is a function of the number of formed at any given time. Force-Length Relationship active and passive components... Active component - related to overlap: # cross-bridges formed at any given time Passive response - related to response: a very stiff rubber band oncentric -, lower potential to form cross-bridge attachments, lower F producing potential Eccentric - greater forces can be sustained by muscle under conditions compared to concentric and isometric "Directionality" of cross-bridge function - takes force to break attachment under eccentric conditions. 7 8
Muscle Temperature Factors affecting muscle torque production! Muscle force! Muscle moment arm - angle of pull Value of warm-up: primarily affects ; has the effect of increasing force and power potential at any given velocity 9 10
Roles of Muscles! Movers or. Muscles that are directly responsible for effecting a given movement. There are both prime movers and assistant movers, depending on the % of the total responsibility.!. Muscles which oppose the desired movement. For example: Elbow flexion: biceps are agonist, triceps are antagonist. Elbow extension:! & supporting muscles. Muscles that act statically to steady or support some part of the body against the pull of other active muscles, pull of gravity, or any other force that interferes with the desired movement.!. Muscles that act to prevent undesired action of one of the movers. (Note: Both of the above roles depict muscles acting as Synergists to other muscles.) 11 12
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