A comparison of foot kinematics in people with normal- and flat-arched feet using the Oxford Foot Model.
Levinger P, Murley GS, Barton CJ, Cotchett MP, McSweeney SR, Menz HB. Gait Posture. 2010 Aug 7. [Epub ahead of print]
I would guess that the forefoot plantarflexion in the flatfeet either occurred from heel contact to forefoot contact or more likely during propulsion.
Activation of the windlass (i.e. hallux dorsiflexion in a foot with an intact plantar fascia) in a flatfoot will often cause more forefoot plantarflexion than in a normal arched foot during propulsion.
That would be a reasonable explanation, so if the windlass was working in both normal and pronated feet and the pronated foot had a greater potential for forefoot plantarfelxion (as it was more dorsiflexed to start with as its flat)....then if the windlass is working fine, then how does that gel with the conclusion in the abstract:
If the windlass is working fine, then are these feet at risk for anything?
Without having read the full article, I'm not sure exactly what stage of gait this finding is occurring. However, assuming it is late midstance and into propulsion where you would expect to see forefoot plantarflexion it does make sense that a flat foot type would display more motion in a plantarflexion direction due to the relatively dorsiflexed position of the forefoot to begin with.
Craig, it is difficult to make any comments regarding injury etiology from this study as it was conducted on a healthy cohort. As you say, maybe these people aren't sufferring injury because this "windlass type" mechanism is in tact. It would be interesting to look at a case control study to identify any potential differences between plantar fasciitis sufferers and controls, as a starting point.
Another idea that springs to mind is what happens after prolonged activity (sport or work) in the same group. Previous studies have shown that flat arched feet display higher levels of muscle activity. You would expect that with higher levels of muscle activity the feet would be susseptable to fatigue earlier that normal arched feet. Does the forefoot still plantar flex after the intrinsic muscles have fatigued? If not, this may be where potential for injury arises?
Sounds like the authors are guessing that the more pronated feet will be more likely injured.
There are some studies that show that injury is associated with more pronated foot type, while some studies show no correlation.
From a strict biomechanical modelling approach, flatter medial arch contour will signficantly increase the tensile force within the plantar fascia versus higher medial arch contour.
In addition, since flatter medial arches almost always have abnormally medially deviated subtalar joint axes, then more pronated feet will therefore also generally experience greater external subtalar joint pronation moments than will feet with normal medial arch contour.
Greater subtalar joint pronation moments will also generally cause higher tensile loading forces on the posterior tibial muscle and increased interosseous compression forces within the sinus tarsi, increasing the risk for posterior tibial tendinitis/dysfunction and sinus tarsi syndrome.
Maybe the authors' guess isn't such a bad one after all?
I like Kevin's first idea.
Do they publish the point in time in gait at which maximal plantar flexion of the forefoot occurs.
If the "zero point" is in loaded stance (where the x-rays were taken?) then in gait there might be more plantar flexion of the forefoot during swing phase and at after heel contact and before forefoot loading.
That would require a different explanation than if the plantar flexion occured after toe off, which would be a different explanation than if it occured between heel off and toe off.
As in, if you jump from 10,000 feet a parachute has more to do than if you jump 10 feet. It is working fine but the 10,000 feet jump carries more risk of the parachute failing to open because the 10 feet the parachute is not actually required to open?