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How significant is the Peroneus longus as a Plantarflexor of the 1st Ray?
Re: How significant is the Peroneus longus as a Plantarflexor of the 1st Ray?
Quote:
Originally Posted by Simon Spooner
You sound very sure Kevin. If we look at the insertional point of the EHL, is it not closer to the foot's supporting surface (i.e. inferior) than, for example, an anterior projection of the STJ axis?
OK. If we look at the projections of joint axes, and not the axes that exist within the joint themselves, then you would be correct.
Quote:
If we assume a rectus foot type, the insertion of EHL is inferior to the 1st met-cuneiform joint. If we assume an instant centre of rotation to be within that joint passing 90 degrees to the sagittal plane, then the EHL tendon insertion is inferior to this axis, i.e the 1st metatarsal-cuneiform joint axis is further from the ground than the EHL tendon in the sagittal plane. When the EHL contracts, compression will occur between the proximal phalanx of the hallux and the 1st metatarsal head- agreed? So long as the centre of pressure produced by this compression between the proximal phalanx and the head of the first metatarsal and, moreover, the line of action of the force vector is inferior to the axis of rotation between the articulation at the base of the 1st metatarsal and the medial cuneiform, a plantarflexion moment of the first metatarsal will be produced- right? In other words, for your conjecture to withstand, the base of the first metatarsal should be lower to the ground than it's head. - right? Or, at least the force vector produced by inter-osseous compression at the first MTPJ has to pass superiorly to the axis at the first metatarsal-cuneiform joint- does it?
It is not the insertion point of the EHL tendon that is important but where the tendon path is in relation to the joint axis of the 1st MC joint, which is always superior to the horizontal axis of the 1st MC and the always superior to the horizontal axis of the first ray.
I do agree that the EHL has the capacity to indirectly generate a first ray plantarflexion moment if it acts to dorsiflex the hallux, causing, in turn, increased tensile force in the plantar fascia, causing in turn, increased 1st MPJ compression force, which, in turn, causes a plantarflexion moment at the first ray. However, this analysis falls apart when the hallux doesn't dorsiflex (e.g. hallux rigidus, 1st MPJ arthrodesis, functional hallux limitus, co-contraction with anterior tibial muscle) since the slightly superiorly directed tensile vector of the EHL tendon on the hallux would cause a mild first ray dorsiflexion moment, not a first ray plantarflexion moment.
Therefore, I would agree that, in some circumstances, the EHL has the capacity to indirectly cause a first ray plantarflexion moment. However, it also, in some circumstances, has the capacity to indirectly cause a first ray dorsiflexion moment.
It is my contention there is no muscle in the body that has the ability to cause a direct first ray plantarflexion moment other than the peroneus longus muscle. I would define an indirect first ray plantarflexion moment as being caused by the muscle increasing the posteriorly directed joint compression forces at the first metatarsal head due to increased 1st MPJ compression force.
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
Re: How significant is the Peroneus longus as a Plantarflexor of the 1st Ray?
Quote:
Originally Posted by Kevin Kirby
I do agree that the EHL has the capacity to indirectly generate a first ray plantarflexion moment
Kevin, I'll come back more to you last post tomorrow, if I may. For tonight, what is the difference between an "indirectly" and therefore "directly" generated moment in terms of the joint kinetics and the ultimate influence of such moment upon the restraining tissues of said joint? In other words, why do we need to segregate "direct" from "indirect" internal joint moments?
It might be helpful if you could define "indirect moment" and "direct moment".
P.S are we talking about a foot with an arthrodesis of the 1st MTPJ or not? In the case of a fixed first MTPJ, then the hallux flexors should all create "direct" plantarflexion moment of the first metatarsal.
Re: How significant is the Peroneus longus as a Plantarflexor of the 1st Ray?
I know that I may have opened a can of worms here and that this is something of a grey area due to the complexity of the moments acting on the human foot from muscle action and ground reaction force and joint compression forces. Maybe this direct vs indirect muscle action should be discussed in another thread.
However, for the purposes of this discussion, I would initially define a direct muscle action (moment) as being that action/moment caused solely by the increase in tensile force of the muscle/tendon unit across a joint axis that is separate and distinct from the external forces or internal forces caused by ground reaction force or by movements of other adjacent joints that may also affect the kinetic and kinematic action/moment of the original muscle/tendon tensile force.
For example, the direct muscular action of the gastrocnemius muscle in open kinetic chain is an increase in STJ supination moment. However, in the medially deviated STJ axis foot during weightbearing activities, the indirect muscular action of the gastrocnemius may be an increase in STJ pronation moment due to the increase in ground reaction force (GRF) on the forefoot (which is also, in turn, due to gastrocnemius activity) and that GRF being being far lateral to the STJ axis.
I may need your help with this one Simon, Eric and others to make sure this all makes sense and is coherent and unambiguous. We should separate this discussion as a separate thread from peroneus longus function.
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
Re: How significant is the Peroneus longus as a Plantarflexor of the 1st Ray?
Quote:
Originally Posted by Simon Spooner
You sound very sure Kevin. If we look at the insertional point of the EHL, is it not closer to the foot's supporting surface, viz. the ground (i.e. inferior) than, for example, an anterior projection of the STJ axis?
If we assume a rectus foot type in static stance and a sagittal plane view, the insertion of EHL is inferior to the 1st met-cuneiform joint due to the declination of the metatarsal to the ground. If we assume an instant centre of rotation to be within the 1st met-cuneiform joint and passing 90 degrees to the sagittal plane, then the EHL tendon insertion is inferior to this axis, i.e the 1st metatarsal-cuneiform joint axis is further from the ground than the insertion of the EHL tendon in the sagittal plane. When the EHL contracts, compression will occur between the proximal phalanx of the hallux and the 1st metatarsal head- agreed? So long as the centre of pressure produced by this compression between the proximal phalanx and the head of the first metatarsal and, moreover, the line of action of the force vector is inferior to the axis of rotation between the articulation at the base of the 1st metatarsal and the medial cuneiform, a plantarflexion moment of the first metatarsal will be produced- right? In other words, for your conjecture to withstand, the base of the first metatarsal should be lower to the ground than it's head. - right? Or, at least the force vector produced by inter-osseous compression at the first MTPJ has to pass superiorly to the axis at the first metatarsal-cuneiform joint- does it?
But in a fusion can we have a centre of pressure between the proximal phalanx and the met head? This all comes down to the vectoring influence of the extensor hood/ sling.
In which case, doesn't it depend on the angle of fusion between the hallux and it's metatarsal?
I'd agree with Kevin's answer to this, but I would like to add a little more.
To calculate a moment about a joint axis you have to know the location of the axis, the point of application of the force, and the line of action of the force. The lever arm of the force is perpendicular distance from the line of action of the force to the axis of rotation. So, the line of action of the EHL tendon will always be dorsal to the first ray. The hood/sling may lower the point of application of force to some extent, but the line of action of the force will still be dorsal to the joint.
Getting back to peroneus longus.. Even though the insertion is at the base fo the first met, in terms of what the STJ sees, the insertion is essentially where the tendon curls around the cuboid.
Re: How significant is the Peroneus longus as a Plantarflexor of the 1st Ray?
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