Studio Podologico Bascherini

13/07/2022

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...

19/07/2019

Plantar Intrinsic Fatigue Syndrome

In addition to Plantar Ligament Stress Syndrome, which I described as a new diagnosis a few weeks ago, I also described, over a year ago, another new plantar arch condition that has, to my knowledge, not been discussed or named within the medical literature. This condition is Plantar Intrinsic Fatigue Syndrome (PIFS). I have seen this condition hundreds of times in my clinic over my last 34 years of private podiatric practice.

Patients describe the symptoms of PIFS as a vague, aching sensation deep within the plantar aspect of the MLA of their feet. Many times, the patient cannot isolate the aching sensation within the MLA of their feet to one pinpoint location but rather describe the symptoms of PIFS as simply being “an aching sensation within the arches of my feet”. Specific palpation of the plantar fascia and the medial plantar ligaments do not produce any tenderness in patients suffering only from PIFS. Therefore, the diagnosis of PIFS must come from taking a careful patient history combined with a detailed physical examination of the structures of the plantar MLA of the foot (see illustration below).

PIFS is most commonly seen in patients with a lower-than-normal MLA height during standing. There is rarely any trauma associated with the vague, aching plantar arch symptoms seen in PIFS. Most patients state that if they are on their feet for longer periods of time, the symptoms of PIFS are worse. Women and men over the age of 50 with flatter-than-normal MLA height seem to be affected more by PIFS than any other group. However, younger adults and children with pes planus deformity may also suffer from the symptoms of PIFS. In other words, PIFS seems to be caused by decreased longitudinal arch height, increased duration of weightbearing activity and increased age.

The plantar intrinsic muscles are activated by the central nervous system (CNS) when the CNS determines that increased longitudinal arch stiffness is necessary to optimize weightbearing function of the foot. When the CNS activates the plantar intrinsics, the resultant increase in plantar intrinsic contractile activity helps to prevent MLA flattening and MLA lengthening by stiffening the MLA. The plantar intrinsic muscles work together, along with the other tension load-bearing elements of the plantar longitudinal arch (i.e. the plantar fascia, posterior tibial, flexor hallucis longus, flexor digitorum longus and peroneus longus muscles and plantar ligaments), as a synergistic system to help prevent MLA lengthening and flattening.

Together, these four sets of tension load-bearing structures share the responsibility of supporting the longitudinal arch and make up what I have previously described as the Longitudinal Arch Load-Sharing System (LALSS) of the foot (Kirby KA: Longitudinal arch load-sharing system of the foot. Revista Española de Podología, 28(2), 2017).

As previously reviewed in the September, October and November 2016 Precision Intricast Newsletters, exciting new research has occurred over the past seven years regarding the activity and function of the plantar intrinsic muscles. Luke Kelly, PhD, and coworkers, using fine-wire electromyography in live subjects, has performed valuable research during this time which has helped us better understand the function of the plantar intrinsic muscles. In addition, their research provides evidence as to why some individuals may experience fatigue and pain within their plantar intrinsic muscles, such as those individuals complaining of the symptoms of PIFS during weightbearing activities.

In the first study by Kelly and colleagues, 10 male subjects had electrodes implanted into their abductor hallux (AH), flexor digitorum brevis (FDB), and quadratus plantae (QP) muscles to measure the electromyographic (EMG) activity of these muscles during standing activities. The subjects demonstrated more plantar intrinsic muscle activity during single leg standing than during double leg standing and more plantar intrinsic activity during medial-lateral sway motions than during anterior-posterior sway motions (Kelly LA, Kuitunen S, Racinais S, Cresswell AG: Recruitment of the plantar intrinsic foot muscles with increasing postural demand. Clin Biomech, 27:46-51, 2012).

In their second experiment, the distal femur was loaded with weights in seated subjects to increase the vertical loading force on the plantar aspect of the foot. MLA height decreased with increased load and EMG activation of plantar intrinsic muscles increased also with increasing loads. In the second part of their experiment, the AH, FDB and QP muscles were electrically stimulated during loading which showed that the plantar intrinsic muscles could significantly reduce MLA length and increase MLA height while the foot was placed under increased loads (Kelly LA, Cresswell AG, Racinais S, Whiteley R, Lichtwark G: Intrinsic foot muscles have the capacity to control deformation of the longitudinal arch. J. R. Soc. Interface 11: 20131188, http://dx.doi.org/10.1098/rsif.2013.1188, 2014).

In their third study, plantar intrinsic EMG activity was measured during both walking and slow and fast running activities. The EMG activity of the plantar intrinsic muscles increased the most during faster running and increased the least during walking. In addition, the increase in EMG activity of the plantar intrinsics corresponded to increases in ground reaction force acting on the plantar foot during both walking and running activities (Kelly LA, Cresswell AG, Lichtwark GA: Active regulation of longitudinal arch compression and recoil during walking and running (2014). J.R. Soc Interface. 6;12(102):20141076).

Together, these excellent experiments from Dr. Luke Kelly and coworkers provide evidence that the CNS increases the contractile activity of the plantar intrinsic muscles with increased weightbearing loads on the plantar foot. These studies indicate that the plantar intrinsic muscles, together with the other elements of the LALSS, are under increasing tension loads during increased weightbearing activities. Increased activation of these muscles, whether during prolonged standing, walking or other weightbearing activities, may cause fatigue of the plantar intrinsics in some individuals and the vague, aching plantar arch pain described by patients with PIFS. It is likely that lower-than-normal MLA height feet may experience the symptoms of PIFS more due to the increased plantar tension forces required to maintain MLA height in these feet. More research on plantar intrinsic muscle activity and the effects of MLA height will be required in the future to more fully determine the etiology of PIFS.

[Reprinted with permission from Kirby KA: "Plantar Intrinsic Fatigue Syndrome", February 2018 Precision Intricast Newsletter. In Kirby KA: Foot and Lower Extremity Biomechanics V: Precision Intricast Newsletters, 2014-2018. Precision Intricast, Inc., Payson, AZ, 2018, pp. 83-84.]

05/07/2019

Crowell HP, Davis IS. Gait retraining to reduce lower extremity loading in runners Clin Biomech (Bristol, Avon). 2010 Sep 30. [Epub ahead of print]...

04/07/2019

Biomechanics of ankle instability. Part 1: Reaction time to simulated ankle sprain. Mitchell A, Dyson R, Hale T, Abraham C. Med Sci Sports Exerc....

04/07/2019

Plantar Ligament Stress Syndrome

A new pedal pathology, Plantar Ligament Stress Syndrome (PLSS), is a clinical condition that I first described over a year ago in my January 2018 Precision Intricast newsletter. To my knowledge, PLSS has never been described previously within the medical literature.

Over the past 34 years of private practice, I have found that patients with PLSS complain of a deep aching sensation within the plantar aspect of the medial longitudinal arch (MLA) of their feet. The pain is worse during weightbearing activities and becomes better with rest. The patient describes the pain as being in the plantar aspect of the first cuneiform-first metatarsal joint and also possibly at the plantar navicular-first cuneiform joint. Tenderness to deep palpation is also found at these anatomical locations. The tenderness in PLSS is specifically not located within the plantar intrinsic muscles or within the central component of the plantar aponeurosis (i.e. plantar fascia).

PLSS is most commonly caused by repetitive overuse of the plantar ligaments of the MLA, but may also be caused by acute trauma. In other words, most patients with PLSS report a gradual increase in pain within the plantar ligaments of the MLA without any history of acute trauma. Patients with PLSS also always have a lower-than-normal MLA height during standing. I have never seen PLSS in individuals with a normal arch or a pes cavus deformity. Women and men over 50 years of age with a moderate pes planus foot structure seem to be the most common subgroup of patients affected by PLSS.

The plantar ligaments affected by PLSS are important passive structures which help maintain the height and integrity of the longitudinal arch. The longitudinal arch of the foot consists of four layers of tension load-bearing structures which share the responsibility of supporting it during weightbearing activities called the Longitudinal Arch Load-Sharing System (LALSS) of the foot. The four layers of tension load-bearing structures of the LALSS include the 1) the plantar fascia, 2) the plantar intrinsic muscles, 3) the deep flexor muscles (i.e. posterior tibial, flexor digitorum longus, and flexor hallucis longus) and peroneal muscles, and 4) the plantar ligaments. Therefore, the tension load-bearing structures of the LALSS work synergistically, both actively and passively, along with the bones and joints of the longitudinal arch, to maintain integrity of the longitudinal arch and to prevent the longitudinal arch from flattening and lengthening excessively during weightbearing activities (Kirby KA: Longitudinal arch load-sharing system of the foot. Revista Española de Podología, 28(2), 2017).

The most likely biomechanical reason that the plantar ligaments become symptomatic in cases of PLSS is due to the fact that the plantar ligaments of the MLA of the foot, like the other components of the LALSS, are subjected to great magnitudes of tension loads during weightbearing activities. The combination of vertical loading forces from the tibia acting on the talus, Achilles tendon tension force and ground reaction force acting on the plantar forefoot cause a large longitudinal arch-flattening moment, especially during late midstance, which greatly increases the tension loads on the plantar ligaments (see illustration below).

In trusses or in tied-arch structures, structures which are in many ways mechanically analogous to the longitudinal arch of the foot, compression load-bearing elements form the arch shape, and tension load-bearing elements help to prevent collapse of the arch structure. With lower-arched structures, the tension forces within the tension load-bearing structures of the arch will be greater than in higher-arched structures (Ambrose JE: Design of Building Trusses. J Wiley, New York, 1994, pp. 50-55). Because of this mechanical fact, the tension load-bearing elements of the LALSS will tend to be subjected to greater tension forces with pes planus deformities, in order to maintain arch height, than with pes cavus deformities.

There are a number of biomechanical reasons why the plantar ligaments of the MLA may be subjected to excessive, chronic tension forces that may result in PLSS. If the MLA is very flat, the plantar ligaments will be subjected to chronically high tension forces which may cause PLSS. Patients with weak plantar intrinsic muscles or patients that have had a spontaneous rupture or surgical sectioning of the plantar fascia may also develop PLSS. Also, any pathology which weakens the extrinsic muscles of the plantar longitudinal arch, such as posterior tibial tendon dysfunction (PTTD), can also increase the strain within the plantar ligaments, due to the increased subtalar joint pronation moments that always accompanies PTTD.

Treatment of feet with PLLS requires the use of custom foot orthoses which have anti-pronation features designed to mechanically reduce the collapse of the MLA and increase the subtalar joint supination moments acting on the foot. Orthosis modifications that should be used to treat patients with PLSS include medial heel skives, deeper heel cups, inverted heel balancing positions, minimal medial arch fill and orthosis shells that resist excessive deformation during weightbearing activities. These specially designed anti-pronation foot orthoses should be worn in shoes that have a heel height differential (i.e. heel drop) of at least 10 mm, and the patient should avoid barefoot walking or walking in flat heel shoes (i.e. 0 mm heel drop). In addition, icing therapy for 20 minutes, twice a day, and Low-Dye strapping can be very helpful in patients suffering from PLSS. Finally, one-time cortisone injections into the plantar ligaments, from a medial approach, may be required to best reduce symptoms in more resistant cases.

PLSS is yet another pathology caused by mechanical dysfunction of the foot. The podiatrist and foot-health clinician should be aware of the clinical presentation, biomechanical etiology, and treatment of PLSS in patients with plantar MLA symptoms. Understanding the biomechanical concepts of the LALSS of the foot is critical to developing an appreciation of the etiology and effective conservative treatment of PLSS which represents another important step toward becoming an expert in custom foot orthosis therapy.

[Reprinted with permission from Kirby KA: "Plantar Ligament Stress Syndrome", January 2018 Precision Intricast Newsletter. In Kirby KA: Foot and Lower Extremity Biomechanics V: Precision Intricast Newsletters, 2014-2018. Precision Intricast, Inc., Payson, AZ, 2018, pp. 81-82.]

01/07/2019