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upper body v lower body rotations, chicken or egg?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by markjohconley, Apr 4, 2018.

  1. markjohconley

    markjohconley Well-Known Member


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    Are upper body rotations always compensating for lower body rotations or can it be the other way?
     
  2. efuller

    efuller MVP

    I don't like the term compensation. Rotations are caused by moments, or torques. With inverse dynamics we can know the moments at a joint. If multiple structures can create a moment at a joint, we cannot know, from inverse dynamics, which of those structures is causing a moment.

    Another aspect of your question is whether the upper body can move the lower body. The upper body only has its inertia to cause the leg to move. The upper body also has the problem of Newton's third law. (For every action there is an equal and opposite reaction.) When a muscle that would cause the femur to internally rotate relative to the pelvis contracts there will be a moment from pelvis trying to move the femur and at the same time there will be a moment from the femur trying move the pelvis. Which one will move? It depends on the inertia of both parts and other forces acting on both of those parts. In walking, when the leg is on the ground, ground reaction force will tend to prevent rotation of the leg. Unless the arms or teeth are grabbing on to something there will not be any external forces acting on the trunk other than the forces from the leg acting through the hip joint. Inertia from the trunk is probably significant, but I would guess has less of an effect than ground reaction forces.
     
  3. markjohconley

    markjohconley Well-Known Member

    Thanks Eric, great lesson. My query arose after I watched a physio podcast during which this claim was made.
     
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