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Levers in Musculoskeletal System

Levers in Musculoskeletal System. Lever - simple machine consisting of a rigid, barlike body that can be made to rotate around an axis Components - Fulcrum- pivot or where rotation occurs (axis of rotation) (ex. joints)

christian-kennedy
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Levers in Musculoskeletal System

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  1. Levers in Musculoskeletal System • Lever - simple machine consisting of a rigid, barlike body that can be made to rotate around an axis • Components - • 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)

  2. Lever motive force resistance force arm resistance arm fulcrum

  3. First Class Lever applied force resistance force arm resistance arm fulcrum DFA = DRA DFA > DRA DFA < DRA Examples: see-saw, scissors, crowbar

  4. force arm resistance arm resistance applied force fulcrum Second Class Lever DFA > DRA Examples: wheelbarrow, wrench, nutcracker

  5. resistance arm force arm resistance applied force fulcrum Third Class Lever DFA < DRA Examples: paddling boat, most muscle-joint systems

  6. Lever Examples 1st class 2nd class 3rd class

  7. Musculoskeletal Levers - 1st Class • Triceps brachii Ftriceps elbow Fweight shoulder Also: neck extension, plantar flexion (foot on gas pedal)

  8. Musculoskeletal Levers - 1st Class • Neck extension

  9. Musculoskeletal Levers - 1st Class • Gastrocnemius, • Soleus

  10. Musculoskeletal Levers - 1st Class • Agonist/antagonist muscle pairs act as 1st Class Levers!! Fagonist Fantagonist

  11. Musculoskeletal Levers - 2nd Class • Gastrocnemius can be a second class lever when jumping or doing toe raises. R FM DRA DFA fulcrum

  12. Gastrocnemius - 2nd Class Lever BW Fm Gastrocnemius can be a second class lever when jumping or doing toe raises. a q ANKLE TOES • wheelbarrow Axis

  13. FM FRO DFA R DRA Musculoskeletal Levers - 3rd Class • Primarily third class levers

  14. Mechanical Advantage • Mechanical Advantage: effectiveness of a lever system in moving a resistance • Ratio: DFA/DRA • DFA > DRA • The applied force to needed to successfully move a resistance is lower • Can move a great resistance using a small applied force (large torque production - “leverage”) • Crowbar, wrench • Large torque, sacrifice speed at the end of resistance arm DFA crowbar rock Hard place (fulcrum) DRA

  15. Mechanical Advantage • Mechanical Advantage: DFA/DRA • DFA < DRA • Takes more applied force to move a resistance • Resistance can be moved through a larger distance • small torque, advantage--> speed at the end of resistance arm Fro Typical DFA/DRA = 1/8 In most muscle/joint systems!!! DRA dumbell DFA

  16. Mechanical Advantage • DFA/DRA = 1/8 • FR = 20 lbs • Find: FRO for isometric contraction • TR = Tm • FR • DRA = Fro • DFA • FRO = F • DRA/DFA • FRO = 20 lbs • 8 = 160 lbs Note: Since a usually < 90°, then Fm even greater than 160 lbs!!!! Fro DRA dumbell DFA

  17. Why use 3rd Class Levers? • Increased ROM over 2nd Class levers • And 1st class where dRA > dFA • Increased velocity at the end of the limb • Increased joint stability

  18. 2nd Class Levers and Skeletal Muscle 2nd class lever Attach to wrist • Muscles have only a limited ability to shorten (≤40%); 2nd class levers would result in limited shortening, reducing tension, ROM. q

  19. Torque vs. Speed and Limb Length • The longer the body segment, the greater the velocity at the end • ( r) • v = d/∆t • d = AL = ∆qrad • r • v = (∆qrad • r)/∆t q r

  20. Torque vs. Speed and Limb Length • r • d = AL = ∆qrad • r • v = d/∆t • v = (∆qrad • r)/∆t axis r1 r2

  21. 3rd Class Levers and Stability • static mechanics • ST = 0 • SFy = 0 Fro = 160 lbs 20 lbs elbow Tm TR Tm + TR = 0

  22. 3rd Class Levers and Stability Joint reaction force • static mechanics • ST = 0 • SFy = 0 160 lbs JRF = 140 lbs 20 lbs elbow 160 + (-20) + x = 0 x = -140 lbs

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