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If muscle works with force on joint and joint works with exact force on muscle(newton III law) how can muscle drag much heavier weight than itself?

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Qmechanic
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hygrok
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    The short answer actually is because you weigh more than the weight you're lifting – Jim Aug 09 '14 at 19:28
  • Some athletes lift more than their weight – hygrok Aug 09 '14 at 19:45
  • Not the way the diagram depicts it. But nonetheless, in those cases, it's because they use all they're muscles to fix themselves in place. Then since the weight is free to move, the net force from the muscles moves the weight – Jim Aug 09 '14 at 19:49
  • joint is a pivot, muscles force is providing a torque around the pivot. Torque from muscle force is greater than torque from weight of arm and dumbell. Arm and dumbell rotate around pivot in an "upwards" direction – Jim Aug 09 '14 at 19:51
  • Newton's III law means a body acted upon acts similarly on the other body. The forearm feels the muscle-force and reacts by imposing the muscle force on the muscle in reverse. But the reaction force is not applied to the forearm. If the muscle force is greater than the weight of the arm and the weight, then the forearm will accelerate – Jim Aug 09 '14 at 19:55
  • possible duplicate of With Newton's third law, why are things capable of moving? – Jim Aug 09 '14 at 19:58
  • So you mean when muscle force move forearm upward the reaction force move entire body downward (into the ground)? If yes does it increase weight for a brief moment? – hygrok Aug 09 '14 at 21:00
  • other muscles are employed to counteract downwards motion of body. It does not increase weight any more than simply holding the extra weight does – Jim Aug 09 '14 at 22:32

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Short answer:

It is a combination of the lever/pulley principle plus the transformation of chemical energy into mechanical energy which makes the muscle/weight system work like this.

Longer Answer:

The arm joints are small levers or pulleys, meaning the amount of force can be multiplied by the relative lenghts of arm joints.

The chemical/thermal energy of food is transformed into mechanical motion by specific muscle cells which expand and contract.

These two combined make the effect of lifting a weight which can be (relatively) bigger than the arm.

An additional element is the initial inertia (or initial static friction) of a load, muscles can do an explosive action which can generate a large amount of energy in small amount of time to overcome initial inertia of a load, then less energy/force is needed to keep the weight moving or lifting.

This is usually exploited (among other things) in heavy-weights lifting. If one can see olympic games weight-lifting, one can see great athletes do this explosive action and then lift easier the weight.

Plus in heavy-weight-lifting, athletes lift the weight with their feet and not with their hands. They do an explosive action, lift a weight just high enough so they can get underneath and use their (stronger) feet to lift the weight.

Feet are stronger than arms (both longer thus the lever principle can be stronger, plus have more muscles), however same principles apply in different measure

Nikos M.
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