23.33 Forces observed at the gleno-humeral joint

• Any muscular contraction at the shoulder level causes compressive forces (S) on the glenoid and shear parallel to the joint.

• The forces have a resultant (R) dependent on

  • The position of the upper limb including rotation

  • The weight of the upper limb and the moment arm (including elbow flexion)

  • The contraction force of the muscles (maximum depends on muscle volume)

• Although not a weight bearing joint, large forces occur

  • Elevation to 90\(^{\circ}\) results in a force equal to body weight

  • 5 kg weight lifted with a straight arm leads to 2.5 \(\times\) body weight

  • Values are comparable to a hip but with a smaller contact area

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