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Biomechanical properties of ligaments and tendons

Discussion in 'Biomechanics, Sports and Foot orthoses' started by admin, Nov 3, 2006.

  1. admin

    admin Administrator Staff Member

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    I am grateful to Kevin Kirby and Precision Intricast for permission to reproduce this October 1998 Newsletter (you can buy the 2 books of newsletters off Precision Intricast):

    Ligaments and tendons are very important structural components of the foot and lower extremity. Both ligaments and tendons may be listed under the classification of regular connective tissue in which connective tissue fibers are regularly oriented to one another. Adipose tissue is an example of irregular connective tissue where there is less regular alignment of the connective tissue fibers (Warwick, R. and P.L. Williams (eds.): Grays Anatomy; 35th British edition. W.B. Saunders Co., Philadelphia, 1973).

    Tendons and ligaments are made up of collections of collagen, elastin, proteoglycans, glycolipids, water and cells. Water and proteoglycans, which make up about 60-80% of the total wet weight of tendons and ligaments, provide lubrication and spacing which is important to providing proper gliding of the fibers of collagen on each other and at points of stress. Together, the molecular structure of tendons and ligaments are such that they allow proper fiber orientation and distance in an organized fashion to allow for optimum load distribution and loading response characteristics. In addition, the insertions of tendons and ligaments onto bone are specially adapted to allow proper distribution and dissipation of forces at this critical interface (Woo, S.L-Y., G.A. Livesay, T.J. Runco, and E.P. Young: "Structure and Function of Tendons and Ligaments", in Mow, V.C. and W.C. Hayes (eds.), Basic Orthopaedic Biomechanics, Second Edition, Lippincott-Raven Publishers, Philadelphia, 1997, pp. 210-211).


    Figure 1. Ligaments and tendons have a characteristic sinusoidal wave microscopic structure known as crimp. As tension is applied to a ligament or tendon, the crimp straightens out which helps to explain their mechanical behavior when subjected to tensile loading forces.

    When ligaments and tendons are viewed under the microscope, they have a characteristic sinusoidal wave pattern which is known as crimp (Fig. 1). This unique microscopic wavy pattern of the collagen fibers, along with the parallel arrangement of the collagen fibers, are thought to play an important role in how both tendons and ligaments respond to loading forces. Once tension is first applied to a tendon or ligament the "crimp pattern" is straightened. This structural characteristic of ligaments and tendons helps explain their nonlinear mechanical behavior. Examples of structures which exhibit linear mechanical behavior would be a rubber band or steel wire which stretches a certain length with each increase in tensile force applied to it. However, in ligaments and tendons, the incremental increase in length becomes decreasingly smaller as more tensile force is applied to the structure. These load-deformation or stress-strain behaviors of ligaments and tendons are measured by in vitro experiments in a similar fashion to the way that a steel cable or a beam of wood would be tested for structural strength in an engineering lab (Woo, S.L-Y., G.A. Livesay, et. al., p. 211).

    Under laboratory conditions of performing load-deformation testing on ligaments and tendons, there are a number of factors which affect the results of the tests. First of all, it has been found that the orientation of the specimen and the joint angle of the specimen affects how much tension can be applied across a ligament specimen before ligament failure occurs. For example, experiments on the human anterior cruciate ligament (ACL) show that the orientation of the knee joint and how uniformly the loading force was applied across the ACL are important in determining its behavior under tension. ACLs tested under conditions which allowed for a smooth transition of load from bone to ligament and also allowed for uniform loading of the all the ligament fibers demonstrated higher ultimate loads before failure, increased linear stiffness, and greater energy absorbed at failure (Woo, S. L-Y., J.M. Hollis, D.J. Adams, R.M. Lyon and S. Takai: Tensile properties of the human femur-anterior cruciate ligament-tibia complex: The effect of specimen age and orientation. Am. J. Sports Med., 19: 217-225, 1991).

    The temperature of the tested ligament or tendon specimen also has a significant influence on its mechanical properties. It has been shown experimentally that there is a negative temperature-elastic modulus relationship between 00 to 700C and an inverse relationship between joint stiffness and temperature. In other words, ligaments show increasing stiffness as the temperature decreases which correlates clinically with increasing joint stiffness as joints becomes colder. In addition to temperature, the strain rate, or rate of elongation of the specimen, has been shown to have some effect on the stiffness of the ligament or tendon. Ligaments and tendons which are loaded at a faster rate will exhibit more stiffness (i.e. will stretch less per unit of tensile load) than ligaments and tendons which are loaded at a slower rate (Woo, S.L-Y., G.A. Livesay, et. al., p. 218-221).

    Other physiological factors also affect the behavior of ligaments and tendons under load. Age has been found to have a very significant effect on the mechanical behavior of ligaments and tendons. In experiments on animals of young and adult ages, it has been found that collagen fibril size and tensile strength of ligaments and tendons increase from puberty to adulthood. This may explain the increased ligamentous laxity which is seen in the feet of children compared to adults. In addition, experiments on ACLs from human cadaveric knees from both young donors (ages 22-41) and older donors (ages 60-97) showed a significant increase in ligamentous stiffness in the older knees compared to the younger knees. It was also found that the young knees were able to withstand increased loading forces before ligamentous tearing, or failure, than the older knees (Woo, S.L-Y., G.A. Livesay, et. al., p. 222-223).

    Immobilization also has been shown to have profound effects on the mechanical behavior of ligaments and tendons. Gross inspection of immobilized ligaments and tendons shows that they have less of a glistening appearance and also appear more woody than normally exercised ligaments and tendons. The thickness and number of collagen fiber bundles may be decreased secondary to immobilization. In fact, it has been reported that 9 weeks of immobilization may decrease ligamentous failure strength by 69% and the energy absorbing capacity of the ligament by 82%. Increased joint stiffness is a common clinical finding after prolonged immobilization and has been confirmed experimentally in rabbits which required an increased amount of torque to extend their knees after nine weeks of immobilization. It is interesting to note that with immobilization, the ligaments of the joint became less stiff, but joint stiffness increased. This paradoxical result is thought to be due to decreased lubrication within the joint and the formation of adhesions and pannus after immobilization (Woo, S.L-Y., G.A. Livesay, et. al., p. 223-225).

    [Reprinted with permission from: Kirby KA.: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 109-110.]
  2. Colleagues:

    I thought that this newsletter may help up redirect the biomechanics discussions on Podiatry Arena toward more important academic topics. This newsletter is meant to stimulate some meaningful discussion on the biomechanical properties of the important collagenous tension-load-bearing elements of the human body: ligaments and tendons. Interestingly, little research on the general biomechanical properties of foot ligaments and tendons has been performed. However, we do know, from both human and animal research, how these vital load-bearing elements respond to load, and how their biomechanical characteristics are alterered by temperature, loading rate, age, immobilization, exercise, hormone levels and surgery.

    I feel very strongly that in order for us to advance our knowledge regarding foot biomechanics and the production and resolution of foot and lower extremity pathologies, we must all learn basic engineering concepts and use these concepts for the benefit of our patients. As David Smith so nicely said in a recent thread, "Applying engineering science to the mechanics of the body is IMO the only way that we can reliably research, analyse, record and repeat the results and outcomes we see or expect."

    Does that mean we all need to go back and become engineers to understand how the foot works? No. However, does that mean we should attempt to gain a better understanding of many of the concepts and methods that engineers use so that we can further increase our own understanding of the biomechanics and mechanical properties of the structural elements of the human locomotor apparatus? Yes!!!

    Maybe we should start the discussion talking a little about stress-strain curves, ligament and tendon tensile stiffness, elastic modulus, elastic limit, stress-relaxation, creep phenomenom, and effects of strain rate to get the ball rolling on some interesting discussions on this fascinating topic. Who wants to start??
  3. Heather J Bassett

    Heather J Bassett Well-Known Member

    start no!?! But sounds good and I look forward to learning from the expertise that will follow. Great idea again Kevin,
    Heather Bassett
  4. One more thing, the viscoelastic behavior of biological tissues and the importance of the linear or Hookean region of the stress-strain curve as it applies to biological tissues would also be good topics of discussion.

    By the way, speaking of "Hooke's Law".......,here's some more questions for all of you: who was Robert Hooke anyway?, when did he live?, what country was he from?, and what other famous scientist did he "do battle with" in the Royal Society regarding the nature of gravitation?, among other things?

    [Hooke was probably one of the greatest scientists of his age, but for some reason is also one of the least appreciated of the great scientists of his era!!]
  5. markjohconley

    markjohconley Well-Known Member

    his detractor and plagiarist?? "isaac of falling apple fame"
  6. I'm probably jumping ahead too much for some people (sorry), and I've kind of talked a little about this one before, but this is the thing that interests me:

    The viscoelastic nature of soft tissues means that when they are loaded more rapidly they become stiffer and are then able to store more energy. Several studies reveal that orthoses decelerate pronation. This should then lead to a decrease in tissue stiffness and a reduced capacity to store elastic energy. So, the question becomes: is a gradual deceleration of pronation more beneficial than a rapid halt?
  7. I don't know that energy conservation during walking has been shown with foot orthoses. However, it may be more likely that running with orthoses may make the foot a "stiffer spring" which may allow it to store and release elastic energy more efficiently. The question I have is whether this decrease in tissue stiffness is not then offset by the increase in arch stiffness that the deformation and recoil of the foot orthosis has on the foot? In other words, the tissues (e.g. plantar fascia, plantar ligaments) may be under less stiff but the orthosis stiffness probably more than makes up for it??

    Good question to chew on, but I think we are not even close to an answer on that one.
    Last edited: Nov 7, 2006
  8. Phil Wells

    Phil Wells Active Member

    Having completed the Tissue Stress Module at Staffordshire Universitys Clinical Podiatric Biomechanics MSc, I cannot agree more with the applying of Engineering Principles to the body.
    On this note, I would appreciate comments on the concept of taking into consideration the type of loading occuring in tendons etc. The idea being that if a tendon e.g. tib post, is loaded rapdily e.g during running, then the tendon will display different properties to a slower loaded tendon. The question then being does this result in different pathologies such as non-insertional tendinopathy or insertional tendinopathy. Also, if the collagen alignment is different at the insertion of a ligament or tendon, and an insertional pathology is identified, are the forces creating the pathology most likely acting along the main tendons optimum operation zone(?) but antagositically to the alignment of the fibres at the insertion?

    Any thoughts?

  9. javier

    javier Senior Member

    Hooke http://en.wikipedia.org/wiki/Robert_Hooke . I recommend the following lecture: Neal Stephenson's The Baroque Cycle. They are three books about those exciting days.

    Human body is not only governed by the laws of physic. If it is, it would be possible to predict treatment outcomes using only principia mathematica .

    Interesting approach. But, how can you measure it?

    I have given more question than answers, but I hope it fuels this discussion. For those (like me) who are not very used to these concepts I recommend the link http://www.orthobiomech.info/basic_mechanics.htm

  10. Not sure what you are asking here Javier, measure what? At this stage it's probably only achievable to do this kind of research using cadaver specimens in jigs like Erin Ward produced. Unless you live in Sweden. ;)
  11. To an extent this comes back to the "tuned track". How did they achieve the "tuning" Kevin?
  12. Simon:

    Here is a nice little story about Tom McMahon and Peter Green's tuned track built at Harvard nearly 30 years ago. http://www.pponline.co.uk/encyc/0818.htm

    Here is the orignal abstract from JoB:

    Here is another of McMahon's articles on his research on human and mammalian gait efficiency/mechanics that is also a good read: http://jeb.biologists.org/cgi/reprint/115/1/263.pdf

    Let me know if you can't access this pdf and I'll e-mail it to you. Hope this helps stimulate more interesting discussion on this important subject that seems to have escaped the view of podiatry for the past 20+ years.
  13. javier

    javier Senior Member

    It was a rhetorical question. As you have pointed out soft tissues stiffness can only be measured in vitro. Moreover, as Kevin Kirby wrote, tendons and ligaments mechanical performance can vary depending on different conditions; also there are great variations from one individual to other. Thus your question:

    falls (using 17th century terminology) among metaphysics. We can guess a response but we are unable to validate this response. Can we actually say that foot orthotics have any influence about soft tissues stiffness? What is it the optimum soft tissues stiffness? How can we know if a decrease or an increase of soft tissues stiffness is less or more beneficial? Would this stiffness decrease/increase be related to all foot conditions or to certain ones?

  14. efuller

    efuller MVP

    This question has to get down to specifics. Which structure(s) are being loaded rapidly. To take subtalar joint pronation you have to look at the structure that is stopping pronation. A rapid pronation that is stopped by the PT tendon alone will just load the tendon. However, if the PT tendon is less loaded the STJ can pronate far enough to load the floor of the sinus tarsi. Now I would bet there would be more damage to the floor of the sinus tarsi if it is loaded rapidly and with high forces. You do see a lot of sinus tarsi pain with posterior tibial dysfunction.


  15. efuller

    efuller MVP

    What other laws are there? In terms of motion, you have to explain each motion with forces and moments. Sometimes motion will be caused by moments from muscles as opposed to altered moments from ground reaction force. So, if you put a vaurs heel wedge under a foot and see pronation, you should look to muscle function changes to explain the motion.

    Now if you mean outcomes in terms of patients feelling better, that's a different story.


  16. David Smith

    David Smith Well-Known Member


    At the risk of going over old ground.

    I believe this is a problem of limitation of data collection but not the limitation of physics. We cannot yet gather in vivo data about all the unknowns that we require to write equations to solve the problem of interest. Assumptions must be made.
    In biomechanics it is quite usual to make assumptions about the properties of materials and their actions. This enables us to construct a model that can be used to analyse the action of interest. Finite element modeling for instance assumes certain properties of stiffness of individual tissues that make up a whole limb and in that way it is possible to make conclusions about the deformation and and deflection of the whole limb. This model can only give an indication of the action. It cannot be directly applied to an individual unless all the data used was collected directly from that individual.
    This problem also arises when considering statistics. Statistics give us a useful guide to the outcomes in terms of a certain population. The statistics cannot guarantee anything about an individual in that population. However this is logical basis to make reasonable assumptions about expected outcomes for that individual. Further experimentation with the individual will reveal if those assumptions were valid or not. In this way we build a system of model, analysis, evaluation, conclusion and re evaluation, that daily or gradually adds to our database of knowledge. Eventually we can more frequently make reliable assumptions about an individual based on our more reliable data about populations. Even if those populations have to be quite small.

    Therfore the use of physics applied to biomechanics will gradually increase our knowledge database and the ability to reliably predict the outcomes of certain interventions of interest. Then, reliably communicate that knowlege thru out the profession.

    Does this make sense?

    All the best Dave Smith
  17. Javier:

    I agree more with Eric and David. All the processes within the human body can be explained by the principles of physics and biochemistry. The problem is that our models are not nearly sophisticated enough and our knowledge is not yet deep enough regarding the human neuromuscular system to allow accurate treatment predictions to be made, even though progress is being made in this regard. However, since medical ethics panels in nearly all countries routinely prohibit invasive research on human subjects (which is probably a good thing for those subjects, but a bad thing for medical knowledge), we will always be in the dark regarding many processes within the human body that are otherwise unobtainable by non-invasive methods.
    Last edited: Nov 9, 2006
  18. David Smith

    David Smith Well-Known Member

    Hello all

    In an effort to steer the thread back to the original discussion

    Do you agree with the following

    Primary function of Muscle and tendon is to move the joint.
    Primary function of Ligament is to stabilise the joint.
    Muscle and tendon has a strong secondary function to stabilse the joint in reaction applied load.
    Ligament has a secondary function in that it can move (or tend to move) the joint as an elastic reaction to applied load.

    The viscoelastic nature of Tendon and ligament allows the tissue to shrink back to its shortest potential if it is not regularly loaded beyond this. This may mean that the joint will have reduced passive and prehaps active RoM.

    The thickness of tendon is directly related to its stiffness and strength (but not its youngs modulus). Tendon thickness will increase, over time, with increased load. Stronger tendons may reduce risk of injury Therefore optimal loading of soft tissue is a good thing.

    Any orthosis should be designed to allow or encourage normal loading and RoM of a joint while reducing the possibility of pathological loads or stress within the tissues. The bias of this principle may change according to joint / tissue pathology.

    Therefore the most efficatious orthosis is symbiotic to the joint (and joint distal to it) and interferes the least with its optimal action (and the optimal action of joints distal to it.)

    This is the basis of the principle 'ZOOS' I believe.

    Cheers Dave Smith
  19. Here is a slightly different way of describing muscle, tendon and ligament function:

    Muscle: Exerts active tensile forces to either accelerate, decelerate or stabilize joint motions (e.g. the posterior tibial muscle may either accelerate STJ supination, decelerate STJ pronation or stabilize the STJ against antagonistic pronation moments).

    Tendon: Passively tranmits tensile forces from muscle to either accelerate, decelerate or stabilize joint motions.

    Ligament: Exerts passive tensile forces across joints to stabilize, decelerate or accelerate joint motion.

    I do believe that Young's modulus (i.e. elastic modulus) of ligament and tendon also does change with exercise, immoblization and hormonal status.

    Otherwise, Dave, I liked your overview.

  20. javier

    javier Senior Member

    Yes, that is the point.

    Mathematical models have shown an exciting potential utility for treating certain cancers. But, I wonder about their limitations related to our field.

    Yes, our work is mostly based on assumptions. Thus, it is quite difficult to rejecte or validate them if we are unable to collect consistent data.

    It would be very interesting if someone decides to develop a mathematical model from the foot and try to validate it. I would spent a good time trying to decipher it!

  21. efuller

    efuller MVP

    Javier, have you seen Kevin's drawings of his model of the STJ. It's pretty easy to add the math to make these mathematical models. Nigg and Morlock published a paper on the complexity of models and found that simple ones could be used to make predictions. To validate Kevin's models you could take a series of patients with posterior tibial dysfunction and assess their STJ axis position. (There is a cause and effect issue there :( ) Then you could look at the effectiveness in patient satisfaction with orthoses with 1) medial heel skive 2) no heel skive 3) lateral heel skive. In terms of patient/ subject protection the subjects should be told to immediately remove a device that increased pain. The model predicts that the medial heel skive would work better than the lateral.

    Eric Fuller
  22. In addition, Craig Payne (maybe Craig can comment here while his family is still not enlarged by a factor of 2 :eek: ) has some unpublished research using his supination resitance testing machine where the force required to supinate the foot was extremely high (to the point of breaking the machine) in a few patients who had posterior tibial dysfunction. Further evidence of this association between STJ axis location and force required to supinate the foot was demonstrated by another paper by Craig and coworkers (Payne C, Munteaunu S, Miller K: Position of the subtalar joint axis and resistance of the rearfoot to supination. JAPMA, 93(2):131-135, 2003). As Eric noted, these experimental results would be predicted using the relatively simplistic models demonstrating the mechanical effects of STJ axis location that I introduced in 1989 (Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989)..............17 years ago! :eek:
    Last edited: Nov 9, 2006
  23. javier

    javier Senior Member

    Do you think it would be changes related to nerve conduction velocities easily asses through EMG?

    Can you give me the complete reference, please? Nigg have published more than 200 papers :eek: !http://www.kin.ucalgary.ca/hpl/content/Nigg_publications.pdf

    Thanks in advance,
    Last edited: Nov 10, 2006
  24. David Smith

    David Smith Well-Known Member


    As you say the youngs modulus of a tissue can also change. Significant changes are seen when collagen rich tissues stiffen due to the effects glycolysation from aging and diabetes. Achilles tendon and sprain injuries are seen more in middle aged men who return to sport.
    Do you think therefore that it would be appropriate as a general rule to prescribe a stiffer orthosis for these groups.

    Since the tendon and ligament is stiffer because of increased youngs modulus,

    (For those who don't know - youngs modulus is an nominal index of material elasticity and can also be called modulus of elasticity (E) and is a ratio of the stress / strain or load / extension. The overall stiffness of a material/structure (a femur for instance) is a product of its Youngs Mod (E) and its cross sectional area or second moment of inertia.)

    The increase in stress will be proportionate to its increase in strain so for a certain length of stretch there will be a higher stress within the tissue.
    Therefore it may be useful to reduce the potential to stretch IE reduce RoM.

    What would you think?

    Cheers Dave
  25. Just to clarify, probably better to say that modulus of elasticity is load/deformation rather than load/extension since E may represent material properties that are in either compression and/or tension (i.e. shortening and lengthening of materials).

    In general, I will tend to use a more compliant orthosis material with "more stiff feet" and a more stiff orthosis material with "more compliant feet", of course, also depending on the anatomical location of their symptoms, nature of stresses that are causing the symptoms (i.e. tension, compression, torsion), their other biomechanical parameters including gait findings, shoes and activities.

    I think that there is a huge potential for this type of mechanical analysis in gaining a further understanding about a disease process such as posterior tibial dysfunction (PTD). In PTD, it is likely that the propensity for this disease to affect post-menopausal women is related to a reduction in elastic modulus in all the ligaments of the body due to hormonal changes. However, it is only those ligaments of the body that are under the greatest loads, in other words, the ligaments of the plantar foot and especially the spring ligament complex, that are significantly affected. These ligaments are mechanically affected by becoming lengthened, or having partial or complete tears (i.e. are farther along on their stress-strain curves past elastic region of curve). Something that I have been chewing on for the past 10+ years.....

    By the way, finite element analysis (FEA) should allow us to better predict how geometry of the foot skeleton would change under load with different elastic moduli of the plantar ligaments being plugged into the FEA computer model. Very exciting research potential here!
    Last edited: Nov 10, 2006
  26. David Smith

    David Smith Well-Known Member



    I know this is the norm, but why? Is it true to say that in compliant feet the muscle are more at risk and in stiff feet the ligaments are more at risk of injury?

    Is it neccesary to consider why the foot is compliant or stiff?

    Is it due to muscles or ligaments or joint geometry or joint pathology or a combination of these.

    Dave Smith
  27. I don't think I can say "this is the norm", but this is what I have arrived from empirical observation. More compliant feet do tend to suffer from different ailments than stiff feet but I don't think we can assume one type of foot has more ligamentous vs muscular injury since both ligaments and muscles are under load and we simply don't know where the injury will occur in each foot. As you mentioned, Dave, foot skeleton geometry, muscle strength, and ligamentous tensile stiffness are all factors that will affect the internal loading forces acting on the structural components of the foot during weightbearing activities.

    At this stage in our knowledge, however, there are far too many unknowns in foot mechanics and central nervous system control of locomotion patterns to predict injury. However, the good thing for our patients is that once they have the injury, we can use the tissue stress model of treatment to generally get them back again into their activities with a minimum of invasive procedures necessary.

    We have made significant progress in this regard since, 25 years ago when I was being taught podiatric biomechanics as a student at CCPM, we were told that all we needed to know how to make patients better was hip range of motion, malleolar torsion, STJ range of motion, STJ neutral, forefoot to rearfoot relationship, ankle joint dorsiflexion, first ray range of motion, NCSP, RCSP, take a good negative cast and balance the orthosis to heel vertical!! I guess we have made significant progress!!
  28. efuller

    efuller MVP

    Morlock, M; Nigg BM Theoretical considerations and practical results on the influence of the representation of the foot for the estimation of internal forces with models. Cl. Biom 6:3-13, 1991

    I'm not quite sure how to answer your question about EMG and nerve conduction velocities. EMG or the relative activity of a muscle, in gait is controlled by the central nervous system. Changes in external moments will necessitate responses from the central nervous system. Not all patients woud have the same response under the model. For example peroneal muscle EMG would be expected to be different for different positions of the STJ axis. Without intervention a person with a medially deviated STJ axis would be expected to have a lower EMG profile than someone with a laterally deviated STJ axis. If you added a varus heel wedge, there might not be any change in the peroneal EMG profile with a medially deviated axis foot, but there might be a large increase in EMG activity with a varus heel wedge placed under a foot with a laterally deviated STJ axis.


  29. musmed

    musmed Active Member

    Dear All

    This paper was probably read by our Olympic Committee (Sydney 200) in the vain hope that there would be a world record produced.

    Of course this did not happen. There is no nandranalone in Mondo 9 track. The track was the firmest that Mondo make.

    The intersting thing was that a few months prior to opening of the Olympics a major credit card filmed about 10 athletes running 60 metre bursts on the corner. They completed about 10 runs for the cameras.

    They all had to take at least a week of for treatment due to foot pain and especially tibial and tibialis anterior pain.

    It is now covered and not used........

    Paul Conneely

  30. Paul:

    The Harvard track has a history of reducing injury in the athletes that train on it due to its unique "suspended platform" construction. No outdoor synthetic track even comes close to using this construction, not even the track used in the Sydney Olympics.

    Even though I have not run on the Harvard track, I imagine it is very similar to the times I have run on city sidewalks where a hole in the sidewalk was covered with a thick piece of plywood. In this situation, when my foot hit the plywood during my running stride, I felt as if the plywood rebounded with just the right deflection/timing to actually reduce the muscular effort, lessen my impact force and actually feel as it was helping to push me forward. Synthetic outdoor track's current method of construction does not permit them to deflect with the same magnitude and rate of rebound that a suspended platform track can. However, there are some high quality treadmills now that do offer this type of rebound which I, and many of my patients, have found to be helpful at reducing frequency of injury.

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