Teach-in #6 The Heel-strike Transient: What people said...

Hello Everyone! My comment concerns the question posed by Rachel Beyer.

My suggestion is that the first alternative is the correct one: "A subject who digs his heels backward at heel strike".

This phenomenon has in fact been investigated by myself and Lennart Strandberg quite a few years ago, and presented in Lanshammar H and Strandberg L: Horizontal floor reaction forces and heel movements during the initial stance phase. Biomechanics VIII-B. eds. H. Matsui, K. Kobayashi, Human Kinetic Publishers, Champain, Illinois, pp 1123-1128, 1982.

To briefly describe this work, 111 steps were investigated, and it was found that this peak existed in 82 of these steps. The velocity of the heel was calculated from three markers located on the foot, and it could be shown that in the majority of these cases (69 out of the 82) the heel had a backward velocity at heel strike. Further, In the majority of the cases where the force peak was not present, the heel was not moving backward during heel strike. A statistical analysis showed that without any doubt (P=0.00001) the existence of the peak horizontal force was related to a backward movement of the foot during heel strike.

I think that this analysis is pretty convincing, since the force data and kinemtic data were collected with independed (although syncronised) devices: a Kistler force plate and a Selspot system. Also, I do not think that it is surprising that the peak does not always occur, since it is probably optimal for the heel to land vertically, without any horizontal motion. Since we humans are no robots, we cannot succeed, so it would be about 50% probability that the foot has a small backward velocity during landing.

Thank you for your time! - Håkan

Håkan Lanshammar Systems and Control Group, Uppsala University hl@SysCon.uu.se P.O. Box 27, S-751 03 Uppsala, SWEDEN Tel:+46-18-471 30 33 Fax: +46-18-50 36 11 http://www.syscon.uu.se

Dear CGA members,

The phenomenon of backward movement of foot giving a backward horizontal force during heelstrike was also noted in my study of post-op gait fx of TKR subjects. I used Kistler FP with sampling rate of 50 Hz. at that time and out of ~ 36 subjects, my impression was that most of them showed a backward "clink" during heelstrike. I doubt why the AMTI don't record it, Chris, if you are using 50 Hz. too (I presume).

My haunch to such phenomenon is that if you look at the kinematic curve, the hip is actually overswing and come down to land on the ground during terminal swing, and this gives the backward horizontal force. Perphaps we are not exact in the movement we want! I would like to suggestions from others.

Regards, Dora

Ms. Dora M.Y. Poon Assistant Professor Physiotherapy Section Department of Rehabilitation Science Hong Kong Polytechnic University Hung Hom Hong Kong E-mail: RSDPOON@POLYU.EDU.HK Fax: 852-23308656

Ask to Richie Gill richie.gill@ox.ac.uk He wrote a PhD Thesis on heel-strike!

Alberto Leardini

Movement Analysis Lab. Department of Orthopaedic Surgery - Istituto Ortopedico Rizzoli Via Di Barbiano, 1/10 40136 Bologna, ITALY tel: ++39 51 6366520 (secretary) ++39 51 6366571 (direct) fax: ++39 51 583789 email: VI6BOQ71@ICINECA.CINECA.IT

We had a patient in recently who was experiencing significant anterior tibial pain. On analysis she had unusually large external plantarflexion moment during loading response. It would seem a bit strange if a muscle of this size would have a prominent role in shock absorbancy - try walking fast with long strides and limited knee flexion and you will probably feel it after a relatively short time. I have long felt that the firing of Tibialis Anterior in early stance has more to do with the control on pronation - any comments?

Jeremy Linskell Manager, Gait Analysis Laboratory Co-Ordinator, Electronic Assistive Tehcnology Service Dundee Limb Fitting Centre Dundee, DD5 1AG, Scotland tel +1382-730104, fax +1382-480194 email: j.r.linskell@dth.scot.nhs.uk

Dear Chris

I am sending you data of a Kistler plate where the heelstrike is visible.

Of particular importance: the A/P force has also a clearly visible backwards heelstrike.

Tab separated ASCII data.

Best Regards Christian Calame

Christian Calame, Product Manager Biomechanics Kistler Instrumente AG Winterthur, P.O.Box 304, CH-8408 Winterthur, Switzerland Tel: +41 52 224 11 11, Fax: +41 52 224 14 14 E-Mail: cl@kistler.ch, WWW:http://www.kistler.com/biomech

A recent posting on October 30, by Dr. Chris Kirtley, inquired as to the detrimental affects of impact shocks and whether these impacts can be changed by different types of footwear. I have included some basic responses and appended each with a few citations. I thought it would be better to include the citations rather than ramble on about each issue. In this way, those people interested in the topic can read the publications and this ‘letter’ will not occupy significant amounts of everyone’s time.

Indeed, impacts and the associated shock wave have been implicated in degenerative disease of the knees, hips and spine. These occur over time and may manifest themselves in soft tissue or hard tissue damage. Analysis of these shock waves have also been used to determine characteristics of pathologies of the lower extremity. The pathologies have been described in humans and generated in sheep and rabbit lower extremities. Two citations that come to mind are:

Simon, S., E.L. Radin and I.L. Paul. The response of joints to impact loading-II: in-vivo behavior of sub-chondral bone. J. Biomech. 5:267-272, 1972.

Voloshin, A.S., C.P. Burger, J. Wosk and M. Arcan. An in-vivo evaluation of the leg’s shock absorbing capacity. In: Biomechanics IX-B, D. Winter, R. Norman, R. Wells, K. Hayes and A. Patla (Eds.). Champaign, IL: Human Kinetics, 1985, pp. 112-116.

There have been recent inquiries on this list server regarding the effects of balance and head carriage during the gait cycle. There has been much work regarding the affect of impacts on motor tasks. It seems possible that the attenuation, or dampening of the shock wave as it travels through the body, is of primary importance to limit the impact registered at the head to maintain a stable reference and field of view.

Hamill, J., T.R. Derrick and K.G. Holt. Shock attenuation and stride frequency during running. Hum. Mov. Sci. 14:45-60, 1995.

Pozzo, T., A. Berthoz and L. Lefort. Head kinematics during various motor tasks in humans. Prog. Brain Res. 80:377-383, 1990. -

Pozzo, T., A. Berthoz, L. Lefort and E. Vitte. Head stabilization during various locomotor tasks in humans. I. Patients with bilateral vestibular deficits. Exp. Brain Res. 85:208-217, 1991.

As for footwear, there has been a lot of work done on various parameters that could influence impact shock. These parameters include, but are not limited to; footwear, walking/running speed, grade (on a treadmill), stride frequency and the type of movement being performed. Basically, each of these parameters can affect the resultant impact shock to some degree, whether by increasing or decreasing it.

Clarke, T.E., L.B. Cooper, D.E. Clark and C.L. Hamill. The effect of increased running speed upon peak shank deceleration during ground contact. In: Biomechanics IX-B, D. Winter, R. Norman, R. Wells, K. Hayes and A. Patla (Eds.). Champaign, IL: Human Kinetics, 1985, pp. 101-105.

Frederick, E.C., T.E. Clarke, J.L. Larsen and L.B. Cooper. The effects of shoe cushioning on the oxygen demands of running. In: Biomechanial Aspects of Sport Shoes and Playing Surfaces. B. Nigg and B. Kerr (Eds.). Calgary: University of Calgary. 1983. Pp. 107-114.

Light, L.H., G.E. McLellan and L. Klenerman. Skeletal transients on heel strike in normal walking with different footwear. J. Biomech. 13:477-480, 1980.

Shorten, M.R. and D.S. Winslow. Spectral analysis of impact shock during running. Int. J. Sports Biomech. 8:288-304.

Mahar, A.T., T.R. Derrick, J. Hamill and G.E. Caldwell. Impact shock and attenuation during in-line skating. Med. Sci. Sports Exerc. Vol. 29, No. 8, pp. 1069-1075, 1997.

I do realize that in the large scope of clinical gait, impact shock and attenuation will not be primary concerns to the specialist. However, I think that these parameters are important when a healthy individual is recovering from some lower extremity injury or surgery. people from time to time. I look forward to any replies or further inquiries. I hope that some people have found this helpful, and others interesting.

Sincerely,

Andrew Mahar, M.S. Research Biomechanist QUALISYS, INC.

Thanks to Håkan, Jeremy, Dora and Alberto for the comments so far. Christian Calame at Kistler has been kind enough to send me a sample file from one of their platforms, which I've substituted for the previous graph on the page at:

/teach-in/transient

As I mentioned, I rarely see the transient on my AMTI plate. I suspect due to a combination of low-pass filtering (there's a 10Hz filter on the bridge amp, which we always use - the traces are quite noisy without it) and perhaps the frequency response of the platform. We all know, Christian, that Kistler is best in this regard (free promotion!).

Now, if you look at this recording, you will see a definite small spike in the AP shear trace simultaneous with the load spike. Notice that the spike is in the anterior direction, i.e. the same direction as during push-off. Håkan & Dora believe that this represents a backward movement of the foot at the instant of heel-strike. However, I confess I'm still confused. None of David Winter's graphs show a negative velocity - on page 21 of his book Biomechanics & Motor Control of Human Gait you can see that he records the horizontal velocity of the heel at about +0.8 m/s during normal gait. The velocity doesn't fall to zero until about 10% into stance. However, on p. 20, he says:

"Horizontal velocity builds up gradually after heel-off and reaches a maximum late in swing and drops rapidly to near zero just prior to HC."

Why are we talking about velocity, anyway? We all know that F=m.a, and so its acceleration that gives rise to force. At the instant we're discussing, I would guess that the proximal segments are exerting little influence, so we're really talking about acceleration of the foot. Now, in order to get an anterior force spike, there would have to be an anterior acceleration (compare with the situation at push-off).

Now, an anterior acceleration can also be a posterior deceleration, and I think that's what's happening. The foot must be moving backwards at the moment of contact, at which time it's velocity is arrested, giving a posterior deceleration.

But why do Winter's graphs not show this posterior velocity of the foot? Does anyone else have any data on heel velocity. Until my Vicon arrives here, I'm afraid I don't have the wherewithall to investigate myself. Can someone help?

Chris -- Dr. Chris Kirtley (Kwok Kei Chi) MD PhD Assistant Professor Department of Rehabilitation Sciences The Hong Kong Polytechnic University

My suggested explanation to the horisontal peak in the ground reaction force was that the heel is moving backward around heel strike. As Chris points out, forces are not related to velocity, but to acceleration. My attempt to explain the phenomenon by the velocity implicitly means that the backward velocity must approach zero, since the foot is not moving during most of the stance phase. Therefore, the backward velocity must be related with a forward acceleration (=backward deceleration). This phenomenon has been measured by us, as I previosly pointed out, and an example is given in the enclosed diagram. I cannot explain why Winter did not see this negative velocity. I can see two simple reasons: 1) Excessive low pass filtering removes the negative component, since the velocity is positive prior to heel strike, and zero shortly after heel strike. 2) It is essential to estimate the velocity for the rear end of the heel. Markers located at other locations on the foot may have a positive velocity since the foot is plantarflexing shortly after heel strike. We estimated the velocity of the rear end of the foot by applying a rigid body model on 3 foot markers.

Yours - Håkan --

Håkan Lanshammar Systems and Control Group, Uppsala University hl@SysCon.uu.se P.O. Box 27, S-751 03 Uppsala, SWEDEN Tel:+46-18-471 30 33 Fax: +46-18-50 36 11 http://www.syscon.uu.se

I am attempting to build up a normative database of heel-strike transient in human walking. I am using 1000 Hz to collect analog data ( from force platform, EMG, accelerometers ) by a Vicon system with seven cameras.

Among 12 subjects which I have finished data processing, only 4 subjects exhibit an anterior impulse force in 25 ms immediately after heelstrike before posterior force reaches the first peak. It is not necessary to be bilaterally involved, because one subject showed this anterior spike only when he stepped on force platform with his right foot instead of left foot. Another subject exhibiting anterior spike bilaterally happened to have quite tight iliotibial bands. It also occurred on the longer leg in a subject who had operation for congenital hip dislocation when she was a baby.

As a physiotherapist, I am wondering if the explanation is something to do with alignment (e.g. leg alignment, leg length discrepency ) and motor control, since this anterior impulse force doesn't occur to everybody during normal walking. I am still working on data processing for 150 more subjects. I am looking forward any comments and possible explanations.

Yours sincerely,

Wen-ling Chen

Wen-ling Chen, M.S., P.T. D.Phil. student, Oxford Orthopaedic Engineering Centre University of Oxford Email: spet0196@sable.ox.ac.uk

Richard Baker Musgrave Park Hospital Belfast <baker@unite.co.uk>

I agree with Håkan's interpretation of the backwards velocity of the foot which creates a backwards force peak when decelerated by ground contact. I suggest a simple measurement with an accelerometer mounted vertically on the heel and synchroneus data acquisition with the force plate. This would tell us whether it is an inertial impact effect (which would be visible, what I expect) or a muscle action (which would not be visible by the accelerometer). I just can't find the time to do it....

Regards Christian Calame

Christian Calame, Product Manager Biomechanics Kistler Instrumente AG Winterthur, P.O.Box 304, CH-8408 Winterthur, Switzerland Tel: +41 52 224 11 11, Fax: +41 52 224 14 14 E-Mail: cl@kistler.ch, WWW:http://www.kistler.com/biomech

Actually, it was the first time I ever used an accelerometer, and I was quite impressed. The whole experiment took less than half an hour.

I used a Kistler model 8302B2 taped on the back of my heel (os calcis). In this position gravity has almost no effect when the heel is vertical. I collected Load and AP-Shear during my own (normal?) gait. I remembered to turn of the 10 Hz filter I normally use on the GRF data, and I didn't filter the accelerometer data either. The results are at:

/teach-in/transient/heelacc.gif

They certainly confirm that there is a large positive spike at heel-strike, which could either be a forwards acceleration or, of course more likely, a backwards deceleration. Interestingly, you'll also note that, as I mentioned previously, my AMTI plate did not pick up the load transient. So this all seems to confirm Håkan's findings.

I've also had a private communication suggesting that the phenomena might have something to do with compression of the heel fat-pad. While this is an interesting suggestion - I've always thought of the events around heel-strike as being somewhat visco-elastic, I don't think it fits in this case. If the fat-pad were to move, it would surely go backwards wrt the os calcis, so this would give a forward velocity and thus backward shear force. Unless the foot accelerated away from a stationary fat pad... no I think that's getting a bit implausible!

No, I think we have to conclude that the foot is driven backwards at the moment of heel contact, presumably by the K4 power burst in the hamstrings during late swing.

Thanks all for another interesting Teach-In!

Chris -- Dr. Chris Kirtley (Kwok Kei Chi) MD PhD Assistant Professor Department of Rehabilitation Sciences The Hong Kong Polytechnic University

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