Looking at a foot in a clinical setting and from that seeing how the foot will function during dynamic tasks, has been a bedrock of foot biomechanics practice. How “static foot structure” reflects “dynamic foot function” continues to be an important topic.
When I say “static assessment” I mean the assessments used to evaluate foot characteristics under conditions that are not real world tasks. So, looking at the foot during standing, manipulating a non-weight bearing foot to look at ranges of joint motion, or asking someone to rise up onto their toes. What these have in common is that we use them to infer how that foot behaves under more real world conditions, such as walking, running, landing and so on. These real world tasks are what I mean by “dynamic” foot function.
But should the “static foot” ever reflect the “dynamic foot”? There is a good reason why it should not and to explain why we need to go back to my earlier definition of the foot (see here).
“…the foot is a complex matrix of at least 11 biological tissues [cut]… that responds to external loads through the symbiotic relationship between the motor control system and tissue properties. [cut]…at no time is the foot ever a purely mechanical structure, other laws are always at play.”
What this means is that the foot is a very adaptable structure. It responds to the external loads applied to it and when these vary so too does its response. When external loads are applied in static tests, such as standing, you get one response, when you apply different external loads during walking, running, landing etc., you get a different response. It follows that even two “static” assessments might not correlate well, because the forces in a weight bearing and a non weight bearing assessment are entirely different, even if both are “static”.
Let’s go one step further. The principle underlying the use of static assessments to infer dynamic foot function is that structure reflects function, and of course this is true. However, I believe it is wrong to say that we are assessing foot structure when we undertake a static assessment. The structures are the bones, ligaments, muscles and so on, and we are not assessing these. What we are assessing is the functional response of the foot (and therefore the interplay between all the structures) to the loads applied. When we look at the same foot under dynamic conditions we are simply looking at the same thing, the functional response of the foot to the loads applied, it is just that the loads are different and so the outcome is different too.
A key point of difference between static assessment and dynamic function is the contribution of the muscles. We recently published results of a correlation analysis between static foot posture (measured as the foot posture index) and structural measures of various foot and leg muscles, tendons and the plantar fascia (here). Remarkably, the plantar fascia thickness explained 52% of variation in foot posture. How can the plantar fascia be so influential and explain more than half of the variation in static foot posture?
The answer is probably that in standing the muscles of the leg and foot are contributing very little to foot posture, because there are ample passive tissues, such as the fascia, to take the relatively low loads involved. Hence, measures of muscle thickness or cross sectional area do not explain very much of the variation in static foot posture, but a measure of passive soft tissue does (remember too that muscles will morph their structures to the needs of more demanding functional tasks like walking, running etc., less so standing). It follows that in our static assessments we are not likely to be reflecting the actions of muscles, and yet in dynamic function muscles are critical to the delivery of the functional task.
This is not to say that measures of the “static foot” bear no relation to the dynamic behaviour of the foot. There are constraints on what the foot can do that impose themselves in both static and dynamic situations, such as articular surface geometry and tensions in passive tissues such as ligaments. But, the point is to see the two situations for what they are: two responses of the same foot to two different sets of forces being applied. It is not, therefore, an assessment of foot structure from which you can infer foot function. It is two assessments of the functional response of the foot to the loads applied. Moving forward, therefore, rather than ask whether static function relates to dynamic function, might it be more appropriate to ask: how do we apply forces to the foot in clinic that allows us to reveal the likely response of the same foot under dynamic situations?