When a joint doesn't move?

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Is there a term for the potential energy stored or created when a joint is forced but doesn't move beyond its range of motion.

Is it the "joint torque"?

Example: an ankle cannot move beyond 90 degrees but because of activity in gait the tibia attempts to move forward.

I don't want to measure this; I want to name it.

Paul R. Scherer, D.P.M.

 

Paul:

Discussing the energy of a joint is complicated since energy has contributions from potential energy due to gravity, deformational energy due to spring deformation (i.e. spring stiffness), and kinetic energy due to translation and rotation (Nigg BM, MacIntosh BR, Mester J (eds.): Bomechanics and Biology of Movement. Human Kinetics, Champaign, IL, 2000, pp. 8-11). Because of the complexity of discussing the energy of ankle joint sagittal plane motion, I believe the preferred method to discuss this concept would be to describe the limitation of ankle joint dorsiflexion in terms of ankle joint moments (moment=torque).

The way I would describe the clinical observation that the ankle joint will first dorsiflex, then stop dorsiflexion motion, and next plantarflex is by using a description of the concept of rotational equilbrium or counterbalancing of ankle joint moments. In other words, when the magnitude of ankle joint dorsiflexion moment from ground reaction force (GRF) is greater than the magnitude of ankle joint plantarflexion moment from all sources (i.e. ankle joint plantarflexor muscles, ankle joint capsular ligaments and/or interosseous compression forces between the tibia and talus) then ankle joint dorsiflexion acceleration will occur due to the net ankle joint dorsiflexion moment. When the magnitude of ankle joint dorsiflexion moment is less than the magnitude of ankle joint plantarflexion moment, then the net ankle joint plantarflexion moment will cause the ankle joint to either decelerate its dorsiflexion motion or accelerate its plantarflexion motion. The above description is specific for the midstance and propulsive phases of walking gait.

Of course, somewhere in between decelerating dorsiflexion motion and accelerating plantarflexion motion of the ankle, there will be an instant in time when the ankle joint becomes motionless. At the time the ankle is motionless, the ankle joint dorsiflexion and plantarflexion moments may be unequal in magnitude, with a net ankle joint plantarflexion moment, since the moment of inertia of the tibia tends to "prevent the ankle from stopping dorsiflexion immediately" due to inertia of the tibia tending to continue ankle joint dorsiflexion.

These concepts could be demonstrated fairly nicely to podiatry students or podiatrists with a piece of 2x4" wood hinged to a piece of plywood with a rubber band (or bungee cord) attached to the upper end of the 2x4 and the plywood. Then the 2x4 could be set in rotational motion away from the rubber band to show how the "tibia first dorsiflexes at the ankle and then plantarflexes at the ankle under the contant plantarflexion moment from the rubber band". Adding additional weights to the top of the 2x4 could also demonstrate the concept of moment of inertia. In addition, adding more rubber bands from the 2x4 to the plywood could demonstrate the effects of increased "ankle joint plantarflexoin moment" on the "movement of the tibia".

Hope this helps.

 

Kevin:

Thanks for your response. You should know that the description of moments and rotational equilibrium is used to describe ankle joint motion, as well as other joints already.

I was hoping we could take the next step and talk about the instant in time when a joint is motionless and the moments are equal. I want to describe developing joint pathology in this context.

As an example: if we knew that in the average and "non pathological" joint, at the instant in time when motion stops both moments are both equal at 10 foot pounds. What if we found an individual whose instant in time the moments are both equal at 50 foot pounds?

My question is what do we call this increased performing energy in the joint? Is it joint torque?

Selfishly, I want people to agree on the term before I use it rather than have the debate after I use it.

I also thought of term excessive intra-articular torque.

Paul R. Scherer, D.P.M.

 

Paul:

I understand what you are trying to describe here, but in strict biomechanical terms, a moment (i.e. torque) is not a measure of energy but a measure of rotational force at the joint in question. Therefore, I would try to avoid using the concept of energy in this context since this may add confusion to the discussion.

I would not use the term "excessive intra-articular torque" or "excessive intra-articular moment" to describe this condition you describe since this implies that it is a joint problem alone. A more accurate and biomechanically accepted term would be "excessive internal ankle joint plantarflexion moment" that could be then defined as being that internal ankle joint moment that comes from the structures within the body that resist ankle joint dorsiflexion moments from ground reaction force.

For example, the anatomical structures/internal forces that could contribute to an "excessive internal ankle joint plantarflexion moment" are as follows:

1. Achilles tendon tensile force
2. Flexor hallucis, peroneus longus, peroneus brevis, flexor digitorum longus and posterior tibial muscles tendon tensile force
3. Posterior ankle capsular ligament tensile force
4. Abnormal interosseous compression forces acting anterior to the axis of rotation of the ankle at the anterior aspect of talo-tibial joint

The combination of these structures/forces can generate sufficient internal ankle joint plantarflexion moment to resist even very large magnitudes of external ankle joint dorsiflexion moment from the mechanical effects of ground reaction force acting anterior to the ankle joint axis during the late midstance and propulsive phases of walking gait.

I believe using the above terms would be consistent with the international biomechanical literature and would be an excellent method by which to introduce your very interesting topic in a concise and clear manner both to practicing clinicians and biomechanics researchers.

 

Causes of Joint Stress

I would like to suggest high stress in the joint. For example, in a functional hallux limitus foot during propulsion, there is a high dorsiflexion moment from the ground and the joint is not moving. Therefore there must be a high plantar flexion moment from the strucutres within the foot. The structures that create the force couple that prevents dorsiflexion will be placed under high stress (force/ cross sectional area). Tension in ligament between the sesamoids and the base of the phalanx and compression of the joint surface will create a force couple that will cause a plantar flexion moment that would resist a dorsiflexion moment from ground reactive force. It has been shown that cartilege is more likely to be damaged when movement is attempted with high compressive forces. The structures proximal to the sesamoids (e.g. medial slip of plantar fascia) can also sustain high tension in this situation.

You could say high joint forces and ignore the size of the structure and convey a similar meaning.

I hope this helps

Eric Fuller