Kevin and collegues,

> 
>  Kevin wrote:
>  
>  << >>Factors which affect the plantarflexion moment across the first ray 
> axis 
>   at  the time of heel off:  (A)  tension in the medial band of the central 
>   component of the plantar aponeurosis , (B)  tension in the peroneus 
longus 
>   tendon, (C) tension in the plantar intrinsic muscles which insert onto 
the 
>   medial and lateral sesamoids, (D) height of the medial longitudinal arch, 
>   (E)  spatial location of the subtalar joint (STJ) axis in relation to the 
>   plantar  oot, (F) tensile stiffness of the plantar ligaments of the 
medial 
>   longitudinal arch of the foot and (G) possibly more factors.>>
>  
>  Eric replied:
>   
>   <<The above is Kevin's list. The only alteration that I would make to 
change 
>   (F) to tensile load, rather than stiffness in terms of what things can 
alter 
>   the plantar flexion moment on the first ray. >>
>  
>  Kevin replies:
>  
>  The reason I chose the property of the tensile stiffness of the plantar 
>  ligaments rather than the tensile load of the plantar ligaments is because 
>  stiffness is defined as stress divided by strain (stiffness = 
stress/strain) 
>  and load is the absolute tensile force on the ligaments.  Since there is 
>  obvious interindividual variances in the deformation of joints to given 
>  external loading forces (i.e. a wide range of interindividual ligamentous 
>  laxity) among the feet of the human population, then there is likely to 
exist 
>  a relatively large range of ligamentous stiffness from one individual to 
>  another.
>  

Eric replies:  I agree with you if we are trying to describe the position of 
the first ray. However, if we are trying to describe moment acting on the 
first ray then I still think that load is the value that should be used if we 
are talking about moments acting on the first ray.  

A ligament with low stiffness can support the same load as a ligament with 
high stiffness, the only difference is the amount of deformation (stretch) 
between the two ligaments.  An example, take a small object on a hook and 
attach it to a chain and a rubber band and then lift it.  When either the 
chain or the rubber band is the sole support then the tension in either of 
them will equal the weight of the object.  The rubber band will stretch more, 
but the tension is the same.  Moment is force (tension) times distance.
 
Regards,

Eric Fuller


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