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University of Toronto Scarborough
Biological Sciences
Jason Brown

SLIDE 2Cells of muscles are part of a functional level systemLevers have 3 componentsoLoad fulcrum effortWe can arrange these 3 components in levers in a number of ways to generate different kinds of leversFirst class lever has a fulcrum on the centre and the effort and load on the sidesoA nonbiological example of this is scissors A biological example of a first class lever is the head where the load is your head the fulcrum is the joint between the skull and the first vertebra and the effort is exerted by the trapezius muscle which pulls on the region at the base of your skullSecond class lever has a fulcrum on one end effort on the other and load in the middleoNonbiological example is the wheel barrel A biological example is standing up on your tiptoes or jumping The fulcrum is at the joint between your toes and foot the load is every part of the body and the effort is the muscle on the base of the heelThird class levers have the fulcrum at one end load in the other end and effort in the middleoNonbiological example is tweezers Biologically third class levers are the most common example a bicep muscleSLIDE 3Mechanical advantage is defined mathematically as the length of the effort arm divided by the length of the load armThe effort arm is the length of the fulcrum to the effort The load arm is the length of the fulcrum to the loadIf you know the lengths of these 2 arms you can calculate mechanical advantage which tells us how efficiently muscular force effort being exerted is translated into leverage in other words how effectively its used to lift that loadIf you look at the first class lever for scissors the effort arm and load arm seem to be relatively equal so the mechanical advantage is 1How can you modify the system so you can use muscular force more efficiently with the same amount of effortoYou either decrease the load arm or increase the length of the effort arm Example a crowbar if the load is very close to the fulcrum its much easier to lift the load or you can get a crowbar with a longer armTake a cheetah being chased by a lion as a biological example One animal is built for power and one is built for speed Mechanical advantage tells us how efficiently muscular force is translated into leverage but the opposite of mechanical advantage which is mechanical disadvantage is how fast that lever can operateIn third class levers the load arm is much greater than the effort arm So in these third class levers which are common in biological systems have very low mechanical advantage but they have the advantage of being speed leversIn a third class lever arrangement you can move a load over a distance than in first class levers So when you have a mechanical disadvantage you have a speed advantageGoing back to the cheetah and lion we look here at the Teres Major muscle which originates at the scapula and inserts in the humorous The lion uses the effort arm for power the cheetah is using the same muscle to move its limb as fast as possibleIn these 2 systems which have third class levers the picture on the left has the lower mechanical advantage therefore its built for speed We know this because the effort arm is much shorter here By changing the insertion of the Teres Major muscle to make it closer to the shoulder you can move this limb over a greater distance by compromising the power you can generate Similarly for the lion by moving the insertion of this muscle away from the fulcrum you compromise the ability for this limb to go over a distance but you produce more powerSLIDE 4Prior to 1954 Archibald Hill proposed the Hill Model of muscle anatomy before we had an understanding how muscle contraction workedHe proposed muscles can be thought of consisting of 3 componentsoContractile component the actin and myosin myofilaments The part of the muscle that generates force during muscle contractionoSeries elastic components largely made of tendons Muscles are attached to bones with tendons The series elastic component means an elastic component of the muscle is directly in line with the contractile component where the sarcomeres
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