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Thinking Like an Engineer

Discussion in 'Biomechanics, Sports and Foot orthoses' started by admin, Nov 20, 2005.

  1. admin

    admin Administrator Staff Member

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    I am grateful to Kevin Kirby and Precision Intricast for permission to reproduce this Newsletter (you can buy the 2 books of newsletters off Precision Intricast):


    In the past few years, I have been interested in reading literature outside the podiatric profession. Some of the most enlightening information comes from the engineering profession. Structural engineers spend their careers analyzing loading forces on structures such as buildings and bridges and how these loading forces are converted into tensile, compression or torsional forces within the components of the structure. It is the detailed analyses of these internal forces within the components of structures which allows engineers to determine the type and size of the structural components necessary to prevent structural failure.

    For engineers, it is relatively easy for them to improve their designs of structures by performing tests on the component materials of the structure to determine the failure points of each specific material under increasing load. For example, engineers know the exact loading force which is required to break or bend a beam of wood or steel. They also know the tensile force at which a steel cable will at first reach its elastic limit and then, with increasing tensile force, the force at which the cable will break.

    Another important method which an engineer can use to determine whether a structure will or will not fail under the its expected loading forces is to build a model of the structure and analyze its response to various loading conditions. This "trial and error" method of structural testing is very important since even the most sophisticated computer analysis of a structure may have faults in it due to the inherent complexity of the interaction of the components with each other.

    As podiatrists, we could all benefit from thinking like structural engineers at times. Unfortunately, for many of us, our podiatric biomechanics education instructed us to simply measure externally apparent deformities of the foot and lower extremities (i.e. tibial varum, rearfoot varus, forefoot to rearfoot relationship) to determine the correct orthosis prescription for the patient. Our podiatric biomechanics education involved very little instruction in regards to the internal tensile, compression and torsional forces which occur within the structural components of the human foot and lower extremity. I believe that analysis of externally measurable deformities of the foot and lower extremity does not give us near enough information to predict the mechanical behavior of the foot and lower extremities during weightbearing activities and therefore is insufficient to prescribe the best orthoses for our patients.

    As mentioned in my last newsletter in which I discussed the internal compression and distraction forces within the midfoot, weightbearing loads on the foot cause compression forces between the dorsal joint surfaces of the bones of the midfoot and tensile forces in the plantar ligaments and plantar fascia. The magnitude and exact location of pathological tensile, compression or torsional forces on the internal structures are very difficult to predict using the externally measurable parameters such as tibial varum, rearfoot varus, and forefoot to rearfoot relationship. In other words, it would be very hard to predict which bone, joint, ligament, tendon or muscle in the foot would become painful just by doing the standard biomechanical examination.

    If podiatric biomechanics could somehow give us enough information so that we could predict, in each type of foot, which specific structure of the foot or lower extremity would become painful during weight bearing activities, then we would also know the best way to treat these painful conditions with foot orthoses or shoe modifications. I feel we are a very long way from attaining these goals since we are unable to do enough "material tests" on the bones, ligaments, tendons or muscles in feet. In other words, we don't have any simple way of determining the exact size, strength and shape of the bones, ligaments, tendons and muscles of each person's feet which would give us a better idea of their failure point.

    However, what we do have is a fairly good knowledge of normal and abnormal gait cycles (i.e. through video analysis), the sequential activity of the lower extremity muscles during gait (i.e. through electromyography), the structure of the foot (i.e. through detailed anatomic dissections) and the magnitude and direction of the ground reaction forces acting on the plantar surface of the foot during gait (i.e. through force plate analysis). Using these known variables, we can create basic models of the foot and lower extremities. With these crude models, an intelligent prediction can be made whether one of the structural components (i.e. bone, ligament, joint, tendon or muscle) of the foot is under tensile, compression or torsional loading stresses during gait.

    It is with this knowledge and the experimental observations made by a few researchers in the past 50 years that we can be very certain of the various types of stresses which the structural components of the foot and lower extremity are subjected to during different weight bearing activities. And once we are certain of the various stresses on the structural components then we can design our mechanical therapy specifically to reduce or eliminate those stresses. It is in this way that podiatrist, by thinking like engineers, can improve our decision making processes so that our patients will benefit by more specific and efficient orthosis prescription and design.

    [Reprinted with permission from: Kirby KA.: Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc., Payson, Arizona, 1997, pp. 267-268.]
  2. This is the Precision Intricast Newsletter from March 1992 when I was starting to explore the ideas that we now call "Tissue Stress Theory"... even though the term "tissue stress theory" had yet to be coined by McPoil and Hunt until three years later (McPoil TG, Hunt GC: Evaluation and management of foot and ankle disorders: Present problems and future directions. JOSPT, 21:381-388, 1995).

    In reading this newsletter again, it is amazing to me how far we have come in the past 13 years but also how far behind many podiatrists still are that still think that foot orthoses should be designed to "hold the subtalar joint in neutral position" and "prevent compensation for forefoot and rearfoot deformities".
  3. C Bain

    C Bain Active Member

    Understanding the Engineering of the Foot?

    Hi All,

    As I consider myself a layman in bio-mechanics as you will see when reading this? It was near beyond belief in my initial training that people were instructing in anatomical foot stresses and were considering the longitudinal arch as a fixed arched in their modelling? And not expected it to move under load, (Or so it appeared?). Turning moments of foot/leg movement in walking for instance left in favour of the arch being considered a near fixed bridge support in their modelling?

    Any bow type support arch such as for example over the top of a river bridge must move like a bow under tension and stress. Without this movement eventually it would give and break down through fatigue-stress. Ligaments were virtually ignored in the understanding in favour of arch stability from the tendons of their muscles! The Fascia appeared to be totally ignored in the stability of the foot.

    None of this took into the account the turning moments of the Longitudinal arch or the Semi-horizontal arch across the Metatarsal joints. Granted we may consider ligaments as stabilising anchor type ties around the joints and bones as shown in the model designed for load and stress made by Kevin in another thread not to far away from here. But the element of load stress became obvious and a good visual aid as I imagined movement under load. Would a series of photographs showing load variants and effects be possible here Kevin? I really did appreciate that one, thank you for showing us it! Hope you have a patent pending. From little acorns you know?

    That was as far as I got in my initial training, for I soon realised to develop this one would need a massive computer to calculate stress in tissue. Also in the modelling of the working parts of the foot together in concert, moments and viscosity in soft tissue, bearing in mind I was in Electrical Engineering and Road Runner does not appear to fit the human foot!).

    Can orthotic's actually be produced from pure or applied engineering I wonder, and should they be made with natural give with respect to the foot? It reminds me of putting the fracture in plaster, the muscles become inactive and waste away. Does the foot in a fixed Orthoses also have it's inactive muscles waste away thorough lack of use I wonder? You know the old rule if you don't use it you loose it? Or will it as it appears always be by rule of thumb in the making with assumptions like water of unknown variants everywhere as you row your boat on the sea of life whilst making them, (Orthoses I mean!)?


    Colin. (Well you must admit it's different!).
    Last edited: Nov 21, 2005
  4. Precisely answering a seemingly simple question such as "what makes the arch of the foot not flatten" requires using engineering and modelling principles that for most podiatrists are beyond their educational skills. However, when this question is answered in general terms with clarity and by an individual that understands the concepts well, then all podiatrists will understand the reason that the arch of the foot doesn't flatten.

    Why aren't the building blocks of basic mechanics hammered into freshman podiatry students around the world? I believe that the main reason is that so few of their "biomechanics" professors in podiatry school can precisely define and explain basic biomechanical terms such as moment, stress, strain, elastic modulus, stiffness, compliance, moment arm and moment of inertia so that they can effectively teach their students. However, they could probably all teach the students how to "bisect a calcaneus", "measure forefoot to rearfoot relationship" and "make a neutral cast of the foot".

    "Think like an engineer!!" should be the battlecry for the future of podiatric biomechanics in the podiatric medical institutions around the world to ensure that podiatrists stay at the forefront of treatment of foot and lower extremity biomechanical pathology. I realize this fact much more now that when I originally wrote this newletter over 13 years ago.
  5. efuller

    efuller MVP

    physics for poets

    One does not need massive computers to use stress analysis in desinging an orthosis using mechanical principles. If you understand which structure is in pain and then reduce the forces that act on that structure you should theoretically reduce the pain. You don't have to actually measure the forces. If your second metatarsal has a stress fracture and you put Korex under Mets 1, 3-5 then you are using mechanical principles without all the computers and calculations.

    In undergrad there were three levels of physics. Physics for physicists and chemists, physics for biologists and physics for poets. You can treat feet if you understand physics at the poetic level. However, teaching biomechanics may require a deeper understanding.

  6. David Smith

    David Smith Well-Known Member

    Dear Colin

    The practice of assuming that a linked segment model is entirely rigid about its joints at the point of analysis is quite usuall. This enables convenient analysis of forces and moments which at the point in time of analysis are all in equilibrium. This is convenient since if one finds that the external moments are greater than the internal moments for instance then either there is a mistake in calculation or another muscle/tissue and its force must be considered to add into the equation until equilibrium is achieved.
    This does not mean that the engineer assumes that the structure is rigid at all times. External forces can be calculated from observing and recording the motion and calculating the accelerations. Once the internal stresses have been calculated from the external forces acting on the model/body then stiffness and deviation/deflection of any segment can be obtained providing (as Kevin said) we know the properties of the materials of construction.

    Cheers Dave Smith

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