Studio Podologico Bascherini

06/11/2019

How Do the Plantar Fascia and Plantar Plate Cause Normal Digital Purchase Force?

The plantar fascia and plantar plate form one continuous soft-tissue structure from the medial calcaneal tubercle, proximally, and to the base of the proximal phalanx of the lesser digits, distally. With loading of the plantar forefoot by ground reaction force (GRF), the forefoot will dorsiflex on the rearfoot which will cause a flattening and elongation of the longitudinal arch of the foot. In turn, the plantar fascia and plantar plate will come under tension forces due to this longitudinal arch elongation to resist further arch flattening and helping to stabilize the longitudinal arch from flattening further.

The resultant increase in plantar fascia and plantar plate tension due to forefoot loading from GRF will also cause a metatarsophalangeal joint (MPJ) plantarflexion moment (i.e. a tendency to plantarflex the MPJ). As a result, the lesser digit proximal phalanx will plantarflex at the MPJ until the GRF under the digit is increased sufficiently to counterbalance the MPJ plantarflexion moment (see my illustration below). The result of this MPJ plantarflexion moment, therefore, is what is known as digital purchase force.

Rotational equilibrium within the sagittal plane at the MPJ will only occur once the internal MPJ plantarflexion moments from the plantar fascia and plantar plate is exactly counterbalanced by the external MPJ dorsiflexion moment from GRF acting on the plantar digit (assuming no flexor tendon tension forces). In this way, the passive plantar fascia and plantar plate force which automatically develop within the human foot with forefoot loading during the latter half of the stance phase of gait will also automatically cause a digital plantarflexion moment and a digital purchase force which tends to stabilize the digit within the sagittal plane during weightbearing activities.

References:

Kirby KA: Understanding the biomechanics of plantar plate injuries. Podiatry Today, 30(4):30-39, 2017.

Kirby KA: Longitudinal arch load-sharing system of the foot. Revista Española de Podología, 28(2), 2017.

Kirby KA: New concepts in longitudinal arch biomechanics. Podiatry Today, 31(6):20-27, 2018.

03/11/2019

31/10/2019

28/10/2019

21/10/2019

04/10/2019

Nike and Their "Breaking2" Pace Car with Huge, Oversized Digital Clock Virtually Eliminated Headwind on Eliud Kipchoge...Yet Another Example of Nike Cheating at Athletics?

This excellent article by Alex Hutchinson in Runner's World discusses how Nike virtually eliminated all headwind on Eliud Kipchoge during their May 2017 Breaking2 event by using a Tesla pace car with a huge, oversized digital clock on top of the Tesla. By eliminating this speed-reducing headwind, Nike made certain that Kipchoge could run up to 4.5 minutes faster in the marathon and increase their chance that they could claim that their shoe was the cause of the speed increase by Kipchoge in the Breaking2 event.

Is this yet another example, like in the recent Alberto Salazar drug-doping case where the CEO of Nike knew of the drug-doping by Salazar, of Nike cheating in athletic events all to increase their market share?

https://www.runnersworld.com/races-places/a20855370/did-the-tesla-pace-car-aid-eliud-kipchoges-2-00-25-marathon/

14/09/2019

The Maximally Pronated Subtalar Joint Position - Why the Concept of Rotational Equilibrium Helps Explain How the Subtalar Joint Behaves When Maximally Pronated

In the latter part of the 1980s, early on in my exploration of the STJ axis and its kinetics, one of the most difficult issues I had regarding the forces and moments acting across the subtalar joint (STJ) was the interesting mechanics of the STJ at its maximally pronated position. The STJ is said to be "maximally pronated" when the lateral process of the talus slides down the posterior facet of the calcaneus until it hits (i.e. abuts against) the floor of the sinus tarsi of the calcaneus (see my illustration below).

When a more normal foot is resting on the ground in relaxed bipedal stance, the STJ will be close to neutral position with the lateral process of the talus not touching the floor of the sinus tarsi (see illustration below). However, when a STJ pronation moment is applied to the foot, the STJ will pronate away from neutral position, and will continue to pronate until the maximally pronated position of the STJ is reached (i.e. when the lateral process of the talus abuts against the floor of the sinus tarsi of the calcaneus).

Now, in this maximally STJ position, where is just a small amount of compression force between the lateral talar process and the floor of the sinus tarsi of the calcaneus, a relatively small amount of external STJ supination moment will supinate the foot. Think about the Supination Resistance Test as one example of an external STJ supination moment. I first described the Supination Resistance Test in 1992 (Kirby KA, Green DR: Evaluation and Nonoperative Management of Pes Valgus, pp. 295-327, in DeValentine, S.(ed), Foot and Ankle Disorders in Children. Churchill-Livingstone, New York, 1992).

However, in this foot with the maximally pronated STJ, any external STJ pronation moment, or internal STJ pronation moment, added to the foot will not pronate the foot further, but will keep the STJ in the maximally pronated position. There, then, is the problem: STJ supination moment will cause STJ supination motion while STJ pronation moment will cause no STJ pronation motion, if the foot is already maximally pronated.

The solution to this problem of trying to understand "where does the force go" when further STJ pronation moments are added to a foot already in the maximally pronated position is to use the high school physics concept of "rotational equilibrium" to understand the STJ maximally pronated position more fully.

Once the STJ is already maximally pronated, adding further STJ pronation moment should cause STJ pronation motion, but it doesn't. Why? Because further STJ pronation moment in a foot with a maximally pronated STJ will not cause STJ pronation motion, but will cause the lateral process of the talus to press progressively harder onto the floor of the sinus tarsi of the calcaneus (see illustration below). Too much STJ pronation moment, such as that seen in feet with a severely medially deviated STJ axis, will lead to very large magnitudes of compression forces between the lateral process of the talus and the floor of the sinus tarsi of the calcaneus which, over time, may lead to sinus tarsiitis or sinus tarsi syndrome.

By learning more about the concept of rotational equilibrium, the STJ maximally pronated position can be more fully appreciated. This improved biomechanical understanding will then allow the podiatrist and foot-health clinician to better understand why some feet are much harder to supinate with an orthosis then other feet, in addition to other clinical observations.

22/07/2019

Good summary in latest Current Opinion in Rheumatology Purpose of review: The biomechanical aspects of gait and the impact of alignment have been...