Lecture 3 (revised).docx

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McGill University
Kinesiology&Physical Education
EDKP 206
David J Pearsall

2/18/2013 3:38:00 PM Function of muscle  Movement o Produce force o Create movement/maintain posture o Provide joint stability o Transmit forces  PASSIVLEY  i.e. to tendons/bones  Other non-movement related functions o Support and protection of visceral organs o Alter/control pressure within cavities o Help maintain body temperature o Control entrance/exits to the body  40% of body mass is MUSCLE Properties  Irritable (inflammation = -itis)  Contractile (unique) up to 50-70%  Extensibility  elastic Structure  Largest – smallest o Fiber filaments (largest = myofibril)  sacromeres  myofilaments (actin/myosin) Movement: Sliding filament theory (A Huxley- 1957)  Was able to determine where the contractile portion of muscle was o Before in ~1940’s we knew muscles were making forces, we just didn’t know how o  Theory: muscle generates force by the sliding of actin and myosin myofilaments over one another o A band: DOES NOT CHANGE LENGTH during contractions I band: DO change length when muscle is stimulated  Together they form the functional components of the contractile portion of the muscle  Influencing factors of muscle’s ability to produce motion  Length of fibers o Initial length will dictate how much force a muscle can produce o Muscle can shorten to the length of the myosin myofibrils. Since sacromeres are in series with myofibrils the amount of shortening a muscle can produce is the sum of the shortening of all sacromeres  Total shortening of a muscle fiber depends on the number of sacromeres arranged in series within each myofibril  The more sacromeres, the longer the muscle, the more force it can generate  Longer fibers = more force produced o Note: Orientation of fibers  Parallel > pennate fibers in terms of force generation  Length of muscle’s moment arm o Moment arm = perpendicular distance from the muscle and point of rotation  Depends on the attachement of the bone and the anlge b/w the line of pull of muscle and the limb it attaches to (i.e. angle of application) o Shorter moment arm = larger angular exertion (muscle generates more force b/c it needs to work harder) Note: o Long fibers & short moment arm = large joint excursion (amplified by long fibers) o Long fibers & long moment arm = long fibers can produce a lot of force, but it is minimized by the long moment arm (muscle don’t need to produce a lot of force to get desired movements) Muscle force generation variables  Amount of muscle & its architecture o INFLUENCED BY  Fiber arrangement (pennate vs parallel)  Physiological corss sectional area o Muscle cross sectional area (CSA)- doesn’t tell us how many fibers are within muscle o Anatomical (physiological) cross sectional area – BETTER MEASUREMENT (PCSA)  Cross sectional area at widest point, perpendicular to the length of the whole muscle  Better measurement b/c it will account for girth of straight fibers AND the angled fibers of pennate muscel (which CSA typically forgets)  Pennate > parallel for PSCA Note: hypertrophy- increased SIZE of fibers Hyperplasia- increased NUMBER of fibers Excursion = instant movement o Architectural factors  Muscle fiber length  Arrangement of fibers relative to resultant axis of force  Parallel – 2 types  Fusiform: tendon at each end o Has a pennation angle at insertion onto tendon -> will experience some changes in shape depending on type of muscle contraction  Strap: less visiple tendons  typically has all long fibers: good at producing excursion, but b/c low PCSA you can’t fit many fibers into one area so it won’t generate much force  Pennate- 3 kinds (depends on number of tendons penetrating muscle) angled insertion to tendon  Unipennate, bipennate, multipennate  Increase in pennate angle, o tensile force of contraction decreased o increased PCSA- can fit more fibers into a smaller place  THUS specialized for force production but b/c fibers are typically short it won’t produce a large excursion  Pennation angle increase with level of muscle activation (not all muscles will extend to the same amount so you will get a formation of an angle)  Muscle shape changes promote rotation of fibers during low forces, and resist rotation of fibers at high forces  Muscle-shape changes act as an automatic transmission system allowing a pennate muscle to shift from a high gear during rapid contractions to a low gear during forceful contraction o Pennate muscles can provide a mechanism to modulate muscle performance during mechanically diverse functions  Favor velocity output during low-load contractions  Less pennation = greater deformation  Favor force output during high load contractions  More pennation = greater force  Transmission system: o Relaxed state: easier for muscle to extend its length o Contracted state: easier for muscle to change thickness, much harder to change length  main difference b/w the two: pennate have more tendon than muscle belly  Type of muscle fiber o Slow twitch (type 1- red/aerobic) o Fast twitch (type 2- white/anaerobic)  Reaches peak force quickly and returns to resting levels quickly Fast > slow in terms of tensile/contractile force production  Muscles with more type 2 fibers will have a higher rate of force production and a high contractile force  Length of muscle o Resting length- optimal amount of cross bidge formations b/w actin and myosin = optimal conditions for max force production  Anything much shorter/longer than this will decrease amount of possible cross bridges and decrease force production  Inverted U shape of graph o Active insufficiency: when you contract your muscle it will only shorten to a given length. At a certain point, your muscle will be unable to get any short and therefore cannot
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