Levers in Musculoskeletal SystemLever - simple machine consisting of a rigid, barlike body that can be made to rotate around an axisComponents - Fulcrum-pivot or where rotation occurs (axis of rotation) (ex. joints)Force arm- distance between the fulcrum to the point where a motive force is applied (ex. tendon attachment of agonist muscle)Resistance arm - distance between the fulcrum to the point where the resistance is applied (weight, [and/or inertia] of body segments or outside loads)

Lever

First Class LeverExamples: see-saw, scissors, crowbarDFA = DRADFA > DRADFA < DRA

Second Class LeverExamples: wheelbarrow, wrench, nutcracker

DFA > DRA

Third Class LeverExamples: paddling boat, most muscle-joint systems

DFA < DRA

Lever Examples1st class2nd class3rd class

Musculoskeletal Levers - 1st ClassTriceps brachii

FtricepsFweightshoulderelbowAlso: neck extension, plantar flexion (foot on gas pedal)

Musculoskeletal Levers - 1st ClassNeck extension

Musculoskeletal Levers - 1st ClassGastrocnemius,Soleus

Musculoskeletal Levers - 1st ClassAgonist/antagonist muscle pairs act as 1st Class Levers!!

FagonistFantagonist

Musculoskeletal Levers - 2nd ClassGastrocnemius can be a second class lever when jumping or doing toe raises.

fulcrumDRADFARFM

Gastrocnemius - 2nd Class Lever

aqFm wheelbarrowTOESBWAxisANKLEGastrocnemius can be a second class lever when jumping or doing toe raises.

Musculoskeletal Levers - 3rd ClassPrimarily third class levers

Mechanical AdvantageMechanical Advantage: effectiveness of a lever system in moving a resistanceRatio: DFA/DRA

DFA > DRAThe applied force to needed to successfully move a resistance is lowerCan move a great resistance using a small applied force (large torque production - leverage)Crowbar, wrenchLarge torque, sacrifice speed at the end of resistance arm

rockDRADFAHard place (fulcrum)crowbar

Mechanical AdvantageMechanical Advantage: DFA/DRADFA < DRATakes more applied force to move a resistanceResistance can be moved through a larger distancesmall torque, advantage--> speed at the end of resistance arm

dumbellDRADFATypical DFA/DRA = 1/8In most muscle/joint systems!!! Fro

Mechanical AdvantageDFA/DRA = 1/8FR = 20 lbsFind: FRO for isometric contractionTR = TmFR DRA = Fro DFA FRO = F DRA/DFA FRO = 20 lbs 8 = 160 lbsdumbellDRADFAFroNote:Since a usually < 90, then Fm even greater than 160 lbs!!!!

Why use 3rd Class Levers?Increased ROM over 2nd Class leversAnd 1st class where dRA > dFA

Increased velocity at the end of the limb

Increased joint stability

2nd Class Levers and Skeletal MuscleMuscles have only a limited ability to shorten (40%); 2nd class levers would result in limited shortening, reducing tension, ROM.q2nd class leverAttach to wrist

Torque vs. Speed and Limb LengthThe longer the body segment, the greater the velocity at the end ( r)

v = d/td = AL = qrad rv = (qrad r)/tqr

Torque vs. Speed and Limb Length r d = AL = qrad r v = d/t v = (qrad r)/t

r2axisr1

3rd Class Levers and Stability static mechanics ST = 0 SFy = 0

elbow20 lbsFro = 160 lbsTRTmTm + TR = 0

3rd Class Levers and Stability static mechanics ST = 0 SFy = 0

elbow20 lbs160 lbsJRF = 140 lbs160 + (-20) + x = 0x = -140 lbsJoint reaction force