Mechanics in Exercise

Levers

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Levers Movement in the body is produced by a system of levers. These series of levers work together to produce coordinated action, some by actual movement (dynamic) and others by stabilization (static). Definitions Lever: Rigid bar that turns about an axis of rotation or a fulcrum (A) Motive Force (F): effort or exertion applied to cause movement against resistance or weight Resistive Force (R): opposes motive force First Class Lever axis is placed between force and resistance examples: crowbar, seesaw, scissors examples in body: elbow extension triceps applying force to olecranon (F) in extending the non-supported forearm (R) at the elbow (A) flexing muscle agonist (F) and antagonist (R) muscle groups are simultaneously contracting on either side of a joint axis (A). lever characteristics balanced movement axis is midway between force and resistance e.g.: seesaw speed and range of motion axis is close to force e.g.: elbow extension force axis is close to resistance Second Class Lever resistance is between axis and force classic examples: wheelbarrow, nutcracker complex example: rowing paddle in water acts as slipping axis (A) boat resistance is resistive force (R) rower is motive force (F) relatively few examples in body planter flexion of foot to raise body up on toes ball of foot (A) serves fulcrum as ankle plantar flexors apply force to calcaneus (F) to lift resistance of body at tibial articulation (R) with foot. entire body during push-up foot is axis of rotation (A) when reaction force of ground pushing against hands (F) lifts weight of body's center of gravity (R). lever characteristics produces force: large resistance can be moved by a relatively small force weight machines: more resistance needed, lower inertia, smoother feel.

Third Class Lever force is placed between the axis and resistance examples: tongs: food (R) is supported by grip on handles (F) while axis is on opposite end. shovelling: dirt on shovel (R) is lifted by force to handle by hand (F) while upper hand on end of shovel handle serves as axis (A) rowing: oar is moved through water (R) by pulling on middle of oar (F) while holding end of oar with opposite hand (A). Note: shovelling and rowing actions can also be first class lever systems if the hand closes to the force remains stationary (A) and the hand on the far end of the shovel or oar is moved (F). batting: ball is hit (R) by moving bat toward ball with hand of far arm (F) while supporting lower portion of bat with hand of near arm (A). example in body most levers in body are third class elbow flexion Biceps and brachialis pull ulna (F) lifting the forearm, hand, and any load (R) at the elbow (A). knee flexion hamstring contract (F) to flex the lower leg (R) at the knee (A). lever characteristics produces speed and range of motion requires relatively great force to move even small resistances weight machines: less resistance required, greater inertia harder to start and stop movement Lever Arm Length Definitions Resistance Arm: distance between axis and point of resistance application. Force Arm: distance between axis and point of force. Formula F x FA = R x RA Force x Force Arm = Resistance x Resistance Arm Example: Initial Example   Change insertion 1 cm   Change point of resistance 1 cm  F x 2 cm = 10 kg x 9 cm 2 F = 90 kg F = 45 kg   F x 3 cm = 10 kg x 9 cm 3 F = 90 kg F = 30 kg   F x 2 cm = 10 kg x 8 cm 2 F = 80 kg F = 40 kg Lever characteristics Long resistance arm: speed and range of movement Short resistance arm: force Mechanical advantage Motive force arm length / Resistive force arm length No mechanical advantage if quotient = 1 (same length): If quotient >1: mechanical advantage in force If quotient <1: mechanical advantage in speed and range of motion Variable Resistance Levers Example Universal gym equipment Resistive force (R) is initially relatively short [close to fulcrum (A)]. As motive force (F) acts on lever, resistive arm becomes longer requiring progressively greater motive forces throughout movement.