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Maintenance of Balance in Relaxed Bipedal Standing

Discussion in 'Biomechanics, Sports and Foot orthoses' started by admin, Dec 6, 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):

    Maintenance of Balance in Relaxed Bipedal Standing

    The mechanisms by which an individual maintains balance during relaxed bipedal stance is a very important subject for podiatrists and other health professionals who treat patients with mechanically based foot or lower extremity pathology. The basic mechanical interactions that exist between the foot and the remainder of the body need to be understood before one can appreciate the more complex biomechanics of how an individual walks, runs or performs any weightbearing activity. Since relaxed bipedal stance is one of the least complex weightbearing activities that humans perform, it is this activity that should be first explored in detail in order to form a basis for comprehending the biomechanics of more complex weightbearing activities.


    [​IMG]
    Figure 1. During relaxed bipedal stance, the center of mass (CoM) is normally positioned anterior to the ankle joint axis so that ankle plantarflexion moment from one or a combination of the ankle joint plantarflexors is necessary to counteract the dorsiflexion moment caused by gravity acting on the CoM (left). The center of gravity (CoG) is the vertical projection of the CoM on the ground. As long as the CoG is positioned within the base of support of the feet, balance can be maintained (right).

    During bipedal standing, the feet support the body weight of the individual so that upright standing can be maintained. The base of support in bipedal standing is made up of the area bounded by the outline of the plantar aspects of the feet and the area directly between the two feet (Fig. 1). In order for the individual to maintain balance, the center of mass (CoM) of the body must be positioned somewhere over the base of support of the body. As long as the CoM of the body remains within the area bounded by the plantar feet and the area between the feet, stable equilibrium will be maintained since the individual can alter the magnitude and location of the ground reaction force (GRF) between the plantar foot and the ground to force the CoM to stay over the base of support (Winter, David A.: A.B.C. (Anatomy, Biome-chanics and Control) of Balance During Standing and Walking. Waterloo Biome-chanics, Waterloo, Ontario, Canada, 1995).


    In order to more fully understand how the body maintains stable equilibrium, or balance, in bipedal standing, it is important to review some terms and concepts commonly used in biomechanics research and in force plate and plantar pressure measurement systems. When an individual is standing on two feet, each foot has varying magnitudes and locations of GRF depending on the structure of the feet and lower extremity, the contractile activity of the muscles of the foot and lower extremity and the location of the CoM in relation to the plantar feet. One method that a researcher may objectively measure the relative location of the GRF acting on the plantar feet when an individual is in bipedal stance is with a device called a force plate.


    [​IMG]
    Figure 2. Force plates can measure the center of pressure (CoP). CoP is defined as the point location of the resultant ground reaction force vector acting on the plantar foot or feet. When a person stands on just the right foot on a force plate, the CoP will be under the plantar aspect of the right foot (A). When a person stands on a single force plate with feet equally weighted, the CoP is located between the two feet (B). When a person stands on just the left foot, the CoP moves to the plantar aspect of the left foot (C).


    A force plate can measure the location, direction and magnitude of the resultant force acting on the plantar feet. In bipedal standing, the force plate will indicate a resultant force vector acting in a vertical direction with a magnitude equal to body weight and with the line of action of the resultant vector passing between the two feet (Fig. 2). The point at which this resultant vector intersects the force plate is defined as the center of force (Nigg, Benno M. and Walter Herzog (eds.): Biomechanics of the Musculoskeletal System. John Wiley and Sons, New York, 1994, pp. 226-227). Center of force is also more commonly known as center of pressure (CoP), which is probably the more commonly used term in force plate and plantar pressure analysis systems (Winter, p. 4).


    Center of pressure is defined as the point location of all the vertical ground reaction force vectors acting on the plantar foot. CoP takes into account the location and magnitudes of all of the GRF vectors and is measured relative to a location on either the plantar foot or on the ground. If only one foot is in contact with the ground, then the CoP will be plantar to that foot. If an individual is standing with both feet on one force plate, then only one CoP can be measured and it will be located between the two feet, assuming approximately equal weight on each foot (Fig. 2). If two feet are on the ground, with each foot on a separate force plate, then each foot will have its own individual CoP (Winter, p. 4).


    Another term commonly used in human balance studies is called center of gravity (CoG). Contrary to common usage in which the term center of gravity and center of mass are thought to be synonymous, in the biomechanics community CoG is often defined as the vertical projection of the CoM on the ground (Fig. 1). Since both CoP and CoG are located on the ground, then their relative positions to each other allows one to make assumptions about how alterations in the CoP and CoG to each other allows an individual to maintain their CoM in a balanced position during relaxed bipedal stance. The method of control by which the individual maintains balance with the feet side-by-side to each other is known as the “ankle strategy” since the CoM of the body can be maintained essentially over the feet by altering the rotational forces, or moments, acting across the ankle joint axis (Winter, p. 5).


    One other important concept that is central to this discussion is that the human body may be modeled as an “inverted pendulum” during both standing and during walking activities (Winter, p. 5). A simple pendulum consists of an object that is attached to a fixed axis of rotation so that when the object is set into motion, the acceleration of gravity on the CoM of the object causes it to swing back and forth repeatedly under the axis of rotation (Cutnell, J.D., Johnson, K.W.: Physics. (3rd ed). John Wiley & Sons, New York, 1995, pp. 298-299). In the inverted pendulum model of bipedal standing, however, the CoM of the body sways back and forth about the ankle joint axis which is both under the influence of the acceleration of gravity acting on the CoM and under the influence of the muscles which directly cause plantarflexion and dorsiflexion moments across the ankle joint axis.


    When an individual stands with their CoM positioned anterior to the ankle joint axis, the acceleration of gravity acting on the CoM of the body causes an ankle joint dorsiflexion moment (Fig. 1). When the CoM is positioned posterior to the ankle joint, gravity causes an ankle joint plantarflexion moment. It is only when an individual is standing so that the CoM is directly over the ankle joint axis that gravity causes neither a plantarflexion moment or a dorsiflexion moment across the ankle joint axis (Hicks, J.H.: The Three Weight Bearing Mechanisms of the Foot. Pages 161-191 in F.G. Evans (ed): Biomechanical Studies of the Musculoskeletal System. C.C. Thomas Co., Springfield, Ill. 1961).


    [​IMG]
    Figure 3. When the center of mass (CoM) of the body is anterior to the ankle joint and the center of pressure (CoP) from ground reaction force (GRF) is positioned anterior to the center of gravity (CoG), there will be a net posterior acceleration of the CoM.


    If the CoM is anterior to the ankle joint, then in order to maintain balance, an ankle joint plantarflexion moment must be produced by contractile activity of one of the muscles capable of producing an ankle joint plantarflexion moment (i.e. gastrocnemius, soleus, deep posterior compartment or peroneal muscles). The increased contractile activity from one or a combination of these muscles increases the ankle joint plantarflexion moment, which, in turn, causes the CoP to move anteriorly on the plantar foot. Therefore, the increase in ankle joint plantarflexor contractile activity causes the CoP to move anteriorly which acts to counterbalance the tendency for the individual to lean further forward due to the CoM being anterior to the ankle joint.
    If the CoM is posterior to the ankle joint, then in order to maintain balance, an ankle joint dorsiflexion moment must be produced by contractile activity of one of the muscles capable of producing ankle joint dorsiflexion moment (i.e. anterior tibial, extensor hallucis longus, extensor digitorum longus and peroneus tertius muscles). The increased contractile activity from one or a combination of these muscles increases the ankle joint dorsiflexion moment, which, in turn, causes the CoP to move posteriorly on the plantar foot.

    Therefore, the increase in dorsiflexor contractile activity causes the CoP to move posteriorly which acts to counterbalance the tendency for the individual to lean further backward due to the CoM being posterior to the ankle joint. It is only in the special circumstance when the CoM is directly over the ankle joint axis that gravity is not causing any ankle joint moment and, as a result, no muscular contractile activity of the ankle joint plantarflexors and dorsiflexors is necessary to maintain balance (Hicks, pp. 167-171).


    Even though the body seems, at first glance, to be relatively still during periods of bipedal standing, the mechanics of this “quiet” activity is much more dynamic and complex. Careful measurement of the sway of the CoM of the body in the sagittal plane relative to the changes in the location of the CoP and muscular contractile activity shows that the individual will constantly alter the contractile activity of the ankle joint dorsiflexors and plantarflexors in order to alter the locations of the CoP to maintain balance (Winter, pp. 5-14).


    Experimental studies of relaxed bipedal stance show that if the CoM is positioned anterior to the ankle joint and the individual shifts their CoP anterior to the CoG by increasing the contractile activity of their ankle joint plantarflexors, that a posterior acceleration of the CoM of the body will occur (Fig. 3). Posterior acceleration of the CoM then causes a progressive increase in displacement of the CoM in a posterior direction. If the CoM is moving posteriorly too rapidly then the ankle joint dorsiflexors may be activated to shift the CoP posterior to the CoM which, in turn, will cause an anterior acceleration of the CoM of the body. Anterior acceleration of the CoM, in this example, then causes, at first, a progressive deceleration of the posterior movement of the CoM that is then followed by a progressive acceleration of the CoM in an anterior direction. In this fashion, the body maintains sagittal plane balance during relaxed bipedal standing by causing an oscillation of the CoP anteriorly and posteriorly in response to the movements of the CoM in relation to the feet (Winter, p 5-14).


    Another interesting aspect of the inverted pendulum model is that the horizontal acceleration of the CoM is proportional to the difference in distance between the CoP and CoG. In other words, the larger the distance between the CoP and CoG, the larger the acceleration of the CoM and the smaller the distance between the CoP and CoG, the smaller the acceleration of the CoM. Therefore, by mechanical necessity, the range of CoP movement must be greater than the range of CoG movement in order to allow enough distance between CoP and CoG to generate sufficient backward or forward acceleration of the CoM toward the center of the foot in order to maintain balance (Winter, p. 6).


    In summary, the body is able to maintain sagittal plane balance during relaxed bipedal standing by selectively increasing or decreasing the contractile activity of the ankle joint plantarflexors and dorsiflexors to adjust the location of the CoP in relation to the CoM so that the CoM never falls outside the base of support of the body. Understanding the basic interrelationships involved in the balance control of bipedal standing helps create a basis for developing a better knowledge of the complex neuro-mechanical interrelationships which exist in other, more dynamic, weightbearing activities.

    [Reprinted with permission from: Kirby KA.: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 133-136.]
     
  2. Kevin Kirby

    Kevin Kirby Well-Known Member

    I first wrote "Maintenance of Balance in Relaxed Bipedal Standing" as the August 2000 Precision Intricast Newsletter. In writing this newsletter, my thoughts had been strongly influenced by the papers by John Hicks and the book by David Winter on their approach to the concept of how the bipedal human maintains balance on two relatively small feet. I thought that this information was so mechanically fundamental that it would be important for podiatrists to understand this subject more completely.

    We all take balance on two feet for granted because, as bipedal organisms, we have a highly specialized neuromuscular system that is adapted just for that purpose. However, the physics of maintaining the center of mass of the body approximately a meter off the ground by the mechanical interactions of two feet that have a relatively small contact area with the ground is quite complicated.

    In order to better appreciate this fact, do you remember as children how difficult it was to get your toy dolls or army figures to stand balanced on two feet? It had to be positioned just perfectly or it would fall over. When you attempt to balance an object that has the same dimensions as the bipedal human on both feet, it becomes much more apparent as to how complex of a system we must have to maintain this upright posture. This little project becomes even more interesting when unipedal standing is attempted by the human or the toy doll.

    Understanding these interactions between center of pressure and center of mass during bipedal standing are very important to allow the podiatrist to better understand the more complex dynamic interactions of center of pressure and center of mass during walking, running, skating, skiing and other athletic activities.
     
  3. Kevin Kirby

    Kevin Kirby Well-Known Member

    I would be interested in any comments on the above article. Is this too complicated for podiatrists?? Or does anyone really care to read this long of an article on this forum???
     
  4. PF 3

    PF 3 Active Member

    Great Stuff Kevin. This stuff should be taught at Podiatry school. Maybe it is now?

    Have any studies been done comparing person to person? I'm guessing that as long as the pressure plate was accurate enough you could measure the minute changes in GRF as the body adjusted itself from swinging forwards and backwards. Tight rope walker versus an average Joe.

    One drill we used to work on for sprinting was high knees with almost straight posture. You would move forward very slowly. 30 m up the track you begin to lean forward which would result in acceleration, you couldn't help it.


    Cheers

    Tom
     
  5. Kevin Kirby

    Kevin Kirby Well-Known Member

    One study actually looked at center of pressure (CoP) movements in single leg stance with and without orthoses. Significant decreases in frontal plane CoP length and velocity with medially posted orthoses were thought to indicate enhanced postural control when subjects stood on the orthoses in single leg stance (Hertel J, Denegar CR, Buckley WE, Sharkey NA, Stokes WL: Effect of rearfoot orthotics on postural control in healthy subjects. J Sport Rehabil, 10:36-47, 2001). Numerous other studies have been done on CoP movement and balance. CoP is not static even during bipedal standing and is almost always maintained anterior to the ankle joint axis and lateral to the subtalar joint axis.

    Anyone want to venture a guess why if the CoP is anterior to the ankle joint axis that the CoP will nearly always also be lateral to the STJ axis???
     
  6. Ian Linane

    Ian Linane Well-Known Member

    Hi Kevin

    I find the article interesting and will certainly be reading it again.

    Two questions I have long asked people learning biomechanics are:

    1. How many people remember learning how to walk?

    Usually, No one.

    2. How many people have seen someone learn to walk

    All the hands go up

    Real answer (arguably) is that no one learns how to walk. They learn how to balance!

    I still feel that biomechanics can be simply put as being about Bend, Balance and Stability. When you introduce the subject to people in such a way it's amazing how the lights seem to switch on!

    Thanks

    Ian
     
  7. efuller

    efuller Well-Known Member

    Kevin,

    Have you done a newsletter on David Winter's balance during gait articles. They are an extension of what you have written in the above newsletter. Two quick interesting things from those articles. Frontal plane balance is determined mainly by placement of the swing leg, but is fine tuned by shifting the center of pressure under the foot with muscle activity. The other fascinating thing is his examination of control of the swing leg.

    Eric
     
  8. Kevin Kirby

    Kevin Kirby Well-Known Member

    I havent seen these articles by Winter, Eric. Maybe you can provide the references for me. Are they in either of his books? (which I do have).
     
  9. efuller

    efuller Well-Known Member

    Hi Kevin,

    Winter DA. Foot trajectory in human gait: a precise and multifactorial motor control task. Physical Therapy. 72(1):45-53; discussion 54-6, 1992 Jan.

    Winter, DA Sagittal Plane Balance and Posture in Human Walking. IEEE Engineering in Medicine and Biology Magazine. Sept. 1987

    MacKinnon CD. Winter DA. Control of whole body balance in the frontal plane during human walking. Journal of Biomechanics. 26(6):633?44, 1993 Jun.

    I think that the gist of these may be in his book.

    Eric
     
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