How the forces of a P/A pressure are distributed : an example with numbers

[Frédéric]

Assessment of Lumbar Spine Kinematics
Using Dynamic MRI: A Proposed
Mechanism of Sagittal Plane Motion
Induced by Manual Posterior-to-Anterior
Mobilization

I decided to extrapolate from this article what percentage of a P/A strenght is actually applied the the intended level of the technic.

I used the average number of degrees mesured on every specific levels on the MRI with the P/A on L4 as an exemple. The picture of the motion mesured is attached below.

They mesured degrees of rotation of each segmentary levels in the saggital plane when a P/A was applied to L4. Here are the results (I averaged the results from the graph to simplyfy)

L1-2 : 2°
L2-3 : 2,5°
L3-4 : 3°
L4-5 : 5°
L5-1 : 3°
D12-L1 : 1° (not mesured, but I estimated it was logic there was mvt there also)

That makes a total of : 16,5 ° of total angular motion combined in the spine
We could suppose some nutation motion is also taken in both SI joints and some extension in both hip joints. But lets not considerer it to keep it simple.

So L4-5 gets 5° out of the 16,5° of total motion : thats makes 30 % of the total mvt.

Now, if we assume that the relation between the angular mvts and the Force applied is linear, we can say this : Only 30% of the Force that actually gets to L4-5 is really producing a mvts at L4-5.

Now to know what amount of the Force applied to the skin actually reaches L4-5, we'd need to know it's coefficient of absorption and the one of the underlying muscles. Since we don't know it, we can only assume. So just for fun lets say it's 20% of absorption (it's probably more but let's give the OMT logic a chance).

That means 80% of the applied force to the skin reaches L4. Out of this 80%, only 30% is really producing mvt at L4-5. Now if someone is applying 50 lbs (a lot more than the idealized pressure, see lower) of pressure to L4 we get this

80% of 50 lbs : 40 lbs actually manages to get to L4
30% of 40 lbs : 12 lbs actually manages to AFFECT the L4-5 level

If we were to also consider the SI and Hip mvts, mvts futher than T12, absorption from the cushion of the table, we would probably reduce this number even more, maybe under 10 lbs.

Now if we go back to Threkeld' connective tissu article, we know this

Lee et a129
reported that an average peak force
reached during an idealized posteriorto-
anterior (P-A) mobilization of the L3

vertebra was approximately 33.3 N

(3.4 kg).

CT
would begin to experience microfailure
at around 224 to 1,136 N (24-115
kg).

24 kg is the minimal (not that of a z-joint capsule) weight required to produce microfailure in human tissu (wich one ?)
24 kg = 52.8 lbs

So 10 to 12 lbs of pressure reaches and affect tissus at L4-5, there are many tissus aside the z-joint capsule at L4-5 and there is 2 z-joints, not one. So a certain amount of this pressure is really applied to the CT of the Z-joint capsules. Whatever this amount is, in a central, pure L4-5 pressure it is definitively not enough to produce a microfailure at any of L4-5 CT. Is it even enough for creep to occur ?


Threkled also says that about grade V thrusts on the thoracic spine

Figure 10. Grade V thoracic mobili.
zation pt?rfomd by two manual therapists.
nerapist I: peak force=578.2 N

(59 kg, rime from beginning of highvelocity
thmt to the peak force= 100 ms.
%rapist 2: peak force=476 N (48.57
kg), time /ram beginning of high-uelocity
thmt to the peak force=120 ms.

With 59 kg we would have about : 14.16 kg (same math here) affecting the targeted segment, again there's a need to divide by all the tissu stressed at that segment. It still falls short to the minimal 24 kg needed for microtrauma.

Now this is a very simplistic way to put numbers on what is really happenning here but I think it is rather easy to say that no permanent change in CT can happen with a pure P/A done in the neutral spine.

The next interesting step, would be to do the same kind of experience with PIVM's done on a «Locked» spine with only the intended level left unlock and a long lever used to apply the force. Since the rest of the spine is locked it is only fair to assume the technic will be more specific. This is why a thrust done in this fashion will make the memorable POP sound. Nevertheless, I think we can assume that it might not be enough to match the necessary amount of pressure to produce any microfailure. Maybe creep, but that is all.

Now, many of you here already know/think that, just though it could be a educative post for others.

Frédéric, PT

PS : Feel free to correct my logic if it is wrong in some ways!

References:
Lumbar Kinematics and MRI from 2004, found here.
Threlkeld's landmark work found here.

[bernard]

That makes a total of : 16,5 ° of total angular motion combined in the spine

It would be true for a bare spine.
It would be true if your spine works like you thought but...

If you have a stiff spot: Do you suppose that the applied force will remove it or diffuse in the less stiff ones?
At the spine level the displacement is very small... And it becomes very important at skin level. So, the brain is informed sooner when something goes wrong...

So you really suppose that a brain will let you go, normally, beyond its physiological limits?

[bernard]

Originally Posted by bernard

So you really suppose that a brain will let you go, normally, beyond its physiological limits?

Incidents may happen with PTs that use too much force or are going beyond the normal resistance of tissues.

[Frédéric]

Myself

Now this is a very simplistic way to put numbers on what is really happenning here but I think it is rather easy to say that no permanent change in CT can happen with a pure P/A done in the neutral spine.

Just wanted to demonstrate with numbers that even if this was happening, CT would still probably not be possible.

Obviously it's way more complicated than that.

[Frédéric]

Bernard, just not sure what I should understand from your attached drawing, this is not what i meant with the total 16,5°.

[bernard]

Originally Posted by Kharma44

Bernard, just not sure what I should understand from your attached drawing, this is not what i meant with the total 16,5°.

If the total angle is 16.5° then the total displacement is greater as you go far from the center of rotation. Thus, the skin moves more than the vertebra itself, even if the rotation is the same. This means that the sensitive parts are more involved in skin than in bones.

[Diane]

I have always considered this amplification of normal movement of the body and from this deduced the uselessness of thinking too far in advance about joints or about moving them. This video of a karate chop breaking a brick clearly shows high velocity accessory movement of the distal region of a limb.

In the trunk you could think of a washing machine agitator. The spine is the agitator, all the "stuff" attached to the spine waggles from side to side during locomotion, Put some skin around the laundry load in the machine and you could see it as a human trunk subject to simple washing machine physics.

Then consider how that skin is innervated and consider the forces on the nerves. That's how I think about this. The trunk as more a verb than a noun.

A large percentage of the total length of 72 km. of peripheral nerves in a human body end up out in the skin, at least a third if not more. One should consider what they have to put up with to reach the level of skin - a lot, winding outward through lots of tough layers, being bent clear backwards sometimes once they get there (e.g., segmental cutaneous nerves of the trunk).

Compared to all the relative motion nerves endure joints move very little. I think that was the point. Correct me if I'm wrong Bernard.

[bernard]

Originally Posted by Diane

Compared to all the relative motion nerves endure joints move very little. I think that was the point. Correct me if I'm wrong Bernard.

That is my point of view.

[pht3k]

but skin dont have to move as much as the joint (in term of degrees of rotation). there is gliding surfaces... think scapula

[Diane]

but skin dont have to move as much as the joint (in term of degrees of rotation). there is gliding surfaces... think scapula

Active movement at large joint is one thing, but passive motion of dependent tissue during an activity like running is something else again. In the second case I'd argue that skin moves more. Way more. It jiggles. Especially on women and especially if the woman is ample.

[Jon Newman]

Diane that is interesting. I have experienced tactile allodynia along the PSIS after a vigorous exercise such as running. I'd place my hands on my hips while walking around after the run and would feel a sudden superficial stinging sensation and the area feels cold despite the fact that I'm sweating and trying to catch my breath from the effort.

[Diane]

I realized I had not added the link to my post higher up, of the video of a guy's hand breaking a brick in slow motion. Note the accessory movement, and this is in a guy's hand, presumably strong, tough, sinewy etc, not flabby or weak.