Quote:
Originally Posted by davsur08
If i consider PB in isolation the ground reaction forces which act at 90 degrees to the body will exert a vertically directed force on to the medial side of the foot. This vertical GRF creates a supinatory moment about the Subtlar joint axis (because it is a triplanar joint). For example in a partially compensated forefoot/rearfoot varus the pronation moment created by PB would allow the 2nd and 3rd metatarsal head to weightbear then would this not cause dorsiflexion of the forefoot (as the pronatory moment arm is small or/and due to lack of available eversion motion at the subtalar joint) ?. Is it fair to say that the resultant moment is subject to available motion in the involved joints and may or may not create an equal and opposit moment?
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By definition, in the partially compensated rearfoot varus foot will not get the medial forefoot to the ground. If it got to the ground it no longer would be defined as a partially compensated varus foot. Yes, with peroneus brevis tension, it will be trying to get to the ground, but it won't have range of motion to do so (assuming tibia stays at same angle to ground.) What will happen with peroneus brevis contraction, in the partially compensated varus foot, is increased compression in the floor of the sinus tarsi. In fact with sinus tarsi sydrome you can see decreased peroneal activation and an increased peroneal reaction time. The muscle may not be weak but it is inhibited.
So, in the partially compensated varus foot, with peroneus brevis contraction, you would not see a medial shift in the center of pressure, because there is not range of motion to create the STJ supination to get the medial shift.
Quote:
Originally Posted by davsur08
However, if not in isolation, The posterior Tibial Muscle contraction would create a supinatory moment when the medial side of the foot contacts the ground?
i ask this because i think Muscle timing is as important as muscle strength in the eitiology of foot pathology.
Thanks in advance
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Have you read Kevin's rotational equilibrium paper. He goes into this scenario perfectly. With nice pictures and everything. The posterior tibial muscle will create a supination moment at the STJ whether or not the foot is weight bearing. If there is an internal supination moment, say at the floor of the sinus tarsi there will be a pronation moment from the ground that is balanced, when at equilibrium, by the supination moment from the floor of the sinus tarsi. As the posterior tibial muscle increases tension, there will be a gradual increase in supination moment. Since the floor of the sinus tarsi can't push the talus into supination, as the tension in the tendon increases the compression in the sinus tarsi will decrease. When the force in the sinus tarsi is zero, any additional tension in the tendon will create a net supination moment and the STJ will supinate. After the supination and if the muscle tension stays constant, ground reaction force will shift a little more lateral creating a pronation moment that will be equal to the supination moment from the tendon.
Eric
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