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Optimization in Foot and Lower Extremity Biomechanics

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Kevin Kirby, Sep 25, 2012.

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    Robin made a comment on another thread that I thought should be more thoroughly discussed:

    Optimization is such an important concept that I thought it deserved its own thread. In his book Optima for Animals, R. McNeill Alexander made the following statement (Alexander, R. McNeill: Optima for Animals. Princeton University Press, Princeton, New Jersey, 1996, p. 2):

    Optimization is an important concept in understanding subtalar joint (STJ) axis location and STJ rotational equilibrium since the central nervous system (CNS) is continually performing optimization calculations to place the STJ axis in optimal position to:

    1. Allow painfree ambulation.
    2. To use the least metabolic energy during weightbearing activites.

    Too much of a medial STJ axis spatial location during weightbearing activities may overload the medial column and require more peroneal muscle activation than is optimal. Too much of a lateral STJ axis spatial location may overload the lateral column, may require more posterior tibial muscle activation, or might increase the risk of an inversion ankle sprain than is optimal.

    Like the CNS of other members of the animal kingdom, the CNS of the bipedal human, by trial and error, has learned to continually optimize the rotations and translations of STJ axis spatial location to perform all weightbearing tasks in the most metabolically efficient manner which also produces the least chance of injury.

    Therefore, to answer the question: why does there have to be an optimal?

    Because optimization is the way that the CNS of the members of the animal kingdom works on a continual basis to allow the animal to survive and thrive throughout their lives in their own respective environments.
  2. and a very important point is N = 1, Optimal is an individual thing, which must take in multiple of factors such as environment, stress etc .

    But as in all things there will be a more Optimal ;) - which gives us the theory of evolution but that would be for another day.
  3. RobinP

    RobinP Well-Known Member

    I think it is worth pointing out the context of the statement

    This was with reference to something on the aforementioned thread that Dennis had said. He feels that there is an optimal position for casting in which the foot would function in a "normal" or "optimal" way

    As is clearly laid out above, the CNS constantly makes optimisations based on feedback loops that determine optimal joint alignment at an instantaneous point in time.

    This is quite different to a single, absolute optimal alignment, as suggested by Dennis, regardless of terrain, footwear and any number of variables

    I hope this clarifies my position on the subject

    welcome back Mike
  4. pod29

    pod29 Active Member

    Great topic Kevin and co.....

    Is this a typo Kevin? Or did I miss something in you your books? :D
    Robin, I like your summation regarding CNS regulation, hit the nail on the head!:drinks:

  5. Luke and Colleagues:

    To clarify my previous statement: the CNS may, by efferent activity to the lower extremity muscles, position the STJ axis to any spatial location that is possible for that foot. The CNS may activate the peroneal muscles that will pronate the STJ and thus make the STJ axis more medially deviated. The CNS may also activate the posterior tibial muscle that will supinate the STJ and thus make the STJ more laterally deviated.

    In addition, the CNS may "decide" to use increased peroneal muscle activity in a foot which has a laterally deviated STJ axis in order allow the forefoot to remain plantigrade on the ground and to prevent inversion ankle sprains. Also, the CNS may "decide" to use increased posterior tibial muscle activity in a foot which has a medially deviated STJ axis in order to allow more normal gait function and reduce the strain on the spring ligament and other plantar ligaments of the medial longitudinal arch.

    In other words, the CNS is continually analyzing afferent neural activity from its body and environment to optimize the efferent neural activity to the lower extremity muscles so that the STJ axis spatial location is optimized for metabolic efficiency, comfort and injury prevention.

    Hope this explains things better.:drinks
  6. Optimization does not just apply, obviously, to the central nervous (CNS) control of subtalar joint (STJ) axis position. It also applies to CNS control of such things as whether it is metabolically more efficient to walk or run at a given locomotion velocity. For example, at 3 miles per hour (mph), nearly all individuals will choose to walk rather than run, but if the locomotion speed is 8 mph, then nearly all individuals will choose to run rather than to walk. When speeds are being gradually increased, at about 4.9 mph (2.2 m/sec), most individuals will be likely be making the transition from walking to running since it is at this speed of locomotion where running becomes more metabolically efficient than walking (Alexander, R. McNeill: Optima for Animals. Princeton University Press, Princeton, New Jersey, 1996, p. 53). This is yet another example of how the CNS will tend to optimize efferent activity to the lower extremity muscles to achieve optimal metabolic efficiency for any given activity.
  7. Rob7

    Rob7 Welcome New Poster

    Hi all,

    Am I correct in thinking that what is being said here is that:

    The STJ axis effectively moves around either medially or laterally depending upon the need of the individual. This need is governed by the CNS which employs either the post tib or peroneal muscles in response to an input (e.g. strain on a ligament).

    If so then I guess that in any given individual the location of the CNS influenced STJ axis could change depending upon the input the CNS receives. In other words a particular activity could cause the axis to move one direction whereas another could cause it to move in the other.

    So with this in mind, how do we go about treating patients when we consider that an STJ axis could be found to be deviating in a particular direction during our assessment, but then, when the the patient returns to their activity this axis could effectively move due to the influence of the CNS? Isn't there a danger that the treatment might be insufficient or even inappropriate?

    I realise that a good history take will have a big influence here but don't we need to know how much the CNS influences the STJ axis in a particular activity in a particular patient? Is this possible?

  8. RobinP

    RobinP Well-Known Member

    Yes, effectively. The CNS is rectionary. It does not "know" the needs of the individual or the exercise. It responds in a feedback loop.

    The spacial location of the STJ axis is influenced by numerous factors, not least terrain, footwear, activity etc etc. As said before, the CNS is reactionary to the demands of terrain, footwear activity et
    If a patient comes to you with pain in a given structure, your job should be to identify the injured structure, identify what is causing the stress in the damaged structure and devise a strategy for reducing the tissue stress.

    So, to answer your above question, you assess the sub talar joint axis location during the activity that is causing a problem. obviously, this will be easier with someone who gets the majority of their pain when standing stationary in a stationary job. Your static weight bearing axis assessment will be more relevant than in a marathon runner where sub talar joint axis is a constant variable

    Again, I would say that CNS activity is largely reationary. It does what is required to maintain equilibrium for a given activity

    Hope this helps
  9. The subtalar joint (STJ) axis will rotate and translate medially or laterally relative to the plantar foot and ground depending on the motion of the STJ which, in turn, is governed by the summation of pronation and supination moments acting on the STJ at any point in time. These moments will be determined largely by active efferent central nervous system (CNS) activity. The CNS, in turn, uses afferent neural input from the environment and from within the body in order 1) to perform weightbearing activities with optimal metabolic efficiency, and 2) to avoid pain and injury during those weightbearing activities.

    One must always assume that the CNS has an over-riding influence on the gait function of the individual when gait examinations are being done in a clinical setting. In addition, one must also assume that our mechanical interventions for patients (e.g. custom foot orthoses) will produce 1) direct mechanical effects on the foot and lower extremity, and/or 2) neuromotor effects on the foot and lower extremity. It is up to the clinician to determine how best to modify the biomechanics of the foot and lower extremity with mechanical interventions, such as foot orthoses, in order to produce both positive direct mechanical and neuromotor effects from that mechanical intervention for the therapeutic benefit of their patients.

    Further reading: Direct Mechanical vs Neuromotor Effects of Foot Orthoses
  10. efuller

    efuller MVP

    First off the location of the STJ axis relative to the talus and the calcaneus is determined by the shape of the articular facets of the talocalcaneal joint. So, if the talus externally rotates then the axis will externally rotate, etc.

    There are many factors that the CNS will evaluate when deciding on which muscles to activate when moving the body from one location to another. It seems that you are implying that someone would choose their foot position to put the axis in the best mechanical position for a particular activity. It's probably much more important to get the foot flat on the floor, or to use the muscles to shift the center of pressure under the foot for balance, or to interally or externally rotate the leg so the the body can change direction.

  11. It's 6:00 AM here at the Sacramento International Airport and I'm sitting in the terminal with my lovely wife, waiting for our flight to Washington DC's Dulles Airport (and then on to Madrid for the Spanish National Congress). Since I have about an hour before I need to board the plane, here are some more thoughts on this subject.

    As Eric states, the central nervous system (CNS) does not focus on the spatial location of the subtalar joint (STJ) axis. Rather, the CNS focuses on the mechanical effects that different spatial locations of the STJ axis have on the foot and lower extremity while the CNS is supplying efferent activity to the lower extremity muscles during physical activity in order to make certain that physical activity is metabolically efficient, pain-free and injury-free.

    For example, in the bipedal human, the CNS prefers to keep the forefoot plantigrade both in standing and during the midstance phase of walking and running. The CNS presumably prefers this forefoot plantigrade position since this forefoot-to-ground position, where the forefoot is flat on the ground, gives the foot added stability by providing a more broad base of support and which also decreases the plantar pressures on any one structural component of the plantar forefoot.

    Therefore, when the forefoot is plantigrade on the ground, with the metatarsal heads all receiving fairly equal plantar pressures, the STJ axis spatial location in this forefoot plantigrade position serves as a "starting point" by which the CNS may choose to either keep the STJ axis in that position or rather supinate or pronate the STJ from that position to orient the STJ in a different position. The CNS may choose to rotate the STJ and the STJ axis into a different rotational position in order to relieve stress from a painful structure and/or make the weightbearing activity more metabolically efficient and/or to protect the individual from injury.

    For example, in the foot with a high arch, high degree of metatarsus adductus or high degree of varus rearfoot deformity, or everted forefoot deformity, the forefoot plantigrade position will be such that the STJ axis will likely be signficantly laterally deviated from its normal position. As a result, the CNS wil sense, probably through afferent neural activity from skin, joint and tendon receptors, that the forefoot plantigrade position is a relatively unstable position for the foot since this position generates sufficient external STJ supination moment to cause the STJ to supinate to its end range of supination motion. As a result, probably through years of learned response, the CNS will constantly send efferent neural messages to the peroneal muscles during standing, walking and other weightbearing activities to the peroneal muscles to generate sufficient internal STJ pronation moments to keep keep the forefoot plantigrade and reposition the STJ axis to a more normal position. Careful clinical observation of these feet while in relaxed bipedal stance position will nearly always show that the peroneal muscles/tendons are tonically active, since without this constant peroneal muscle activity, the individual would supinate their STJ to the maximally supinated position and invert onto their lateral aspect of their forefoot.

    As a result of this "correction for abnormal STJ axis location" by the CNS, the individual with such a foot will often suffer from lateral leg fatigue within the peroneal muscles with prolonged standing, will often develop peroneal tendinitis/tendinopathy and will also be much more likely to suffer from inversion ankle sprains during certain weightbearing activities. Therefore, by understanding how abnormal STJ axis spatial location affects the response of the CNS to the abnormal STJ moments created by this abnormal STJ axis location, the clinician will be better able to predict what mechanical therapeutic treatments will work best at resolving the symptoms of the patient who has abnormal STJ axis spatial location.

    Now, off to Dulles Airport......

    Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989.

    Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001.
  12. RobinP

    RobinP Well-Known Member


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