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Frontal Plane Moments Do Not Accurately Reflect Ankle Dynamics During Running
Kristian M. O’Connor and Joseph Hamill
Quote:
The ankle joint has typically been treated as a universal joint with moments
calculated about orthogonal axes and the frontal plane moment generally used
to represent the net muscle action about the subtalar joint. However, this joint
acts about an oblique axis. The purpose of this study was to examine the differences
between joint moments calculated about the orthogonal frontal plane
axis and an estimated subtalar joint axis. Three-dimensional data were collected
on 10 participants running at 3.6 m/s. Joint moments, power, and work
were calculated about the orthogonal frontal plane axis of the foot and about
an oblique axis representing the subtalar joint. Selected parameters were compared
with a paired t-test (a = 0.05). The results indicated that the joint moments
calculated about the two axes were characteristically different. A moment
calculated about an orthogonal frontal plane axis of the foot resulted in a joint
moment that was invertor in nature during the first half of stance, but evertor
during the second half of stance. The subtalar joint axis moment, however,
was invertor during most of the stance. These two patterns may result in qualitatively
different interpretations of the muscular contributions at the ankle
during the stance phase of running.
I am going to take a risk and attach the full PDF verson of the paper to this message - I will delete it soon...
Thanks for the paper, Craig. Sure would have been nice if O'Connor and Hamill had looked into the podiatric literature for our published papers on the significance of STJ axis location on the kinetics of the foot.
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
I think this points to another of Mert Root's brilliant observations. You just can't do research on all feet, because they do not all behave the same. I'm not sure I like the Root classification system as a way to separate feet out for research. Position of the axis is a good way to separate feet for the effects of intervention.
Determination of subtalar joint axis location by restriction of talocrural joint motion. Gait Posture. 2006 Jan 30; [Epub ahead of print]
Lewis GS, Kirby KA, Piazza SJ.
Quote:
The location of the subtalar joint axis is an important determinant of the mechanical function of the foot. The moments of muscle forces and of the ground reaction force about the subtalar joint are dependent upon the location of this joint axis. There is substantial variation in subtalar axis location across subjects, but current methods for determining its location are often invasive or involve expensive imaging protocols. A novel technique for location of the subtalar axis is presented in which the talocrural joint is passively immobilized so that motion of the tibia relative to the calcaneus can be used to estimate the subtalar axis. This paper presents results of cadaver testing in which accuracy of the technique was assessed by comparing helical axes computed from calcaneus-tibia bone motions to axes computed from calcaneus-talus bone motions. Only small motions at the talocrural joint were observed, and good estimates of the subtalar axis (errors less than 15 degrees and 2mm) were achieved in four of six specimens
Determination of subtalar joint axis location by restriction of talocrural joint motion. Gait Posture. 2006 Jan 30; [Epub ahead of print]
Lewis GS, Kirby KA, Piazza SJ.
Here's a photo from our experimental setup for this project that we did a few summers ago at the Penn State Biomechanics Lab. Cool stuff!
Next planned project is to put live subjects into an MRI to track the talus during foot motion with a specially constructed device that maintains constant ankle joint moments throughout the range of motion of the STJ. Hopefully, in a few years we will be able to track the 3D location of the STJ axis in live subjects during gait on a force plate to come up with a relatively accurate assessments of STJ moments during gait. Then we will really start to understand the kinetics of the foot!!
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
Here's another photo of the cadaver setup in our experiment at Penn State Biomechanics Lab. The foot has three sets of retroreflective markers attached screwed/bolted to tibia, calcaneus and talus along with three motion analysis cameras to track 3D motions of calcaneus and talus relative to the tibia. This allowed us to determine STJ axis spatial location and to determine whether STJ axis location could be determined by calcaneal to tibial motion instead of by the standard calcaneal to talar motion.
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
STJ axis variation makes it into mainstream biomechanics literature
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