February 2021

BEHOLD the Human Arch! Biomechanics of Longitudinal Arch Load-Sharing System of the Foot

The human foot is an engineering marvel, consisting of 26 bones, 33 joints, and more than 100 muscles, tendons, and ligaments. But it is the unique and elegant load-sharing system of the longitudinal arch that makes human locomotion possible. This author explains how.

By Kevin A. Kirby, DPM

For more than a century, the importance of the longitudinal arch of the foot and its function for the bipedal human has been debated within the medical community. 125 years ago, an orthopedic surgeon, Royal Whitman, described how flattening of the longitudinal arch of the foot, “pes planus”, could create the condition known as “weak foot”.1,2 In 1853, Little first described “pes cavus”, the foot with a high longitudinal arch.3,4 Differences in longitudinal arch height are associated with a variety of foot and/or lower extremity pathologies. Pathologies associated with pes planus deformities may include pain, fatigue, joint degeneration and/or associated deformities such as hallux valgus, hammer toes, and metatarsalgia.5 Pathologies associated with pes cavus deformities include foot and ankle instability, abnormal plantar weightbearing patterns, and restricted foot mobility.6

Unfortunately, even with more than a century of discussion and research on the biomechanical significance of the longitudinal arch within the medical community, there has been relatively little discussion regarding how all the structural components of the longitudinal arch work together as a unit to perform their remarkable functions for the weightbearing individual. In order to emphasize the importance of the mechanical interplay among the various structural elements of the longitudinal arch during weightbearing activities, the Longitudinal Arch Load-Sharing System (LALSS) was introduced in 2012 to describe a new concept in longitudinal arch biomechanics (Figure 1).7,8

“The plantar fascia, like all fascial and ligamentous structures, is a passive structure, not being dependent on the central nervous system (CNS) to increase its tension forces.”

Figure 1. The longitudinal arch of the foot can be modeled as having an osseous framework, represented here to include the tibia, rearfoot and forefoot segments, along with the 4 plantarly-located tension load-bearing layers of the LALSS: the plantar fascia, plantar intrinsic muscles, plantar extrinsic muscles, and plantar ligaments. When GRF increases on the plantar foot and Achilles tendon tension and tibial loading force increase, the longitudinal arch will tend to flatten. All 4 tension load-bearing elements of the LALSS work together to stabilize the longitudinal arch and prevent its flattening and elongation during weightbearing activities. The plantar fascia and plantar ligaments are passive structures (i.e. not controlled by the CNS), which come under tension with dorsiflexion of the forefoot on the rearfoot and which provide a baseline stiffness to the longitudinal arch. The plantar intrinsic and plantar extrinsic muscles are active structures (i.e. controlled by the CNS) which may increase the longitudinal arch stiffness over what the tension forces within the plantar fascia and plantar ligaments create passively. If one element of the LALSS fails, such as in a rupture of the plantar fascia, the tension loading forces in all the remaining elements of the LALSS must increase in order to maintain longitudinal arch stiffness and longitudinal arch function. In this fashion, the longitudinal arch of the foot is a true load-sharing system, allowing the human foot to have an elegant and durable longitudinal arch stiffening mechanism that optimizes the biomechanics of the foot and lower extremity during a wide variety of weightbearing activities over the lifetime of the individual. CNS: central nervous system; GRF: ground reaction force; LALSS: Longitudinal Arch Load-Sharing System.

Load-Sharing Concepts

Load-sharing is a common concept in both mechanical and electrical systems where multiple components of the system are designed to operate together which, in turn, ensures reliability of the system as a whole. Examples of load-sharing systems include the use of multiple supporting cables in suspension bridges, multiple engines in aircraft, multiple electric generators in power-generation systems, multiple processors in computers, and multiple servers in distributed computer systems. Instead of just one component performing all the work for the system, multiple components work together as a unit within the load-sharing system. In doing so, if one component fails, the system as a whole will not fail; rather, the working loads on all the remaining components of the system will be increased in response to the failure of one of its components.9,10

A load-sharing system which is quite common and mechanically analogous to the longitudinal arch of the foot is the suspension system within the rear axle of many trucks. Truck rear-suspension systems generally include 2 elements: leaf-springs and shock absorbers. Both the leaf springs and the shock absorbers function to help absorb and dampen any vertical oscillations due to changes in loads between the rear axle and truck bed so that suspension will not “bottom-out” or bounce excessively when driving with heavy loads or on uneven roads. If the shock absorbers fail in the rear-suspension, the leaf springs will have increased load, and vice versa. However, if the leaf springs and shock absorbers both remain operational, these two elements of the rear-suspension will each have decreased loads, allowing optimum function of the truck rear-suspension.

Compression Load-Bearing Framework of the Longitudinal Arch

During the variety of weightbearing activities an individual performs on a daily basis, the longitudinal arch of the foot is subjected to significant external loads from ground reaction force (GRF). Peak GRF loads range from 1.1 to 1.5 times body weight (BW) during walking, 2.5 to 3.0 times BW during running, and can exceed 4.0 times BW during jumping activities.11,12 Much like the leaf spring in a truck rear-suspension that flattens and elongates with increased vertical load and then returns to its original shape as the vertical load is reduced, as the plantar foot loads from GRF are increased, the longitudinal arch will flatten and lengthen. Then, as the GRF on the plantar foot is reduced, the longitudinal arch will return to its original, unloaded arch height.  These cycles of longitudinal arch loading and unloading occur thousands of times a day during an individual’s daily weightbearing activities.

The structural framework of the LALSS is made up of the osseous components of the rearfoot and forefoot. The osseous framework of the LALSS has the important mechanical function of resisting compression loads, resisting bending, and resisting torsional loads that occur when GRF acts on the individual’s plantar feet.13 In combination with the plantarly-located tension load-bearing elements of the longitudinal arch, the osseous structural framework of the LALSS ensures stability of the longitudinal arch under a wide range of weightbearing loads and loading patterns for the individual.

Tension Load-Bearing Elements of LALSS

While the bones of the rearfoot and forefoot serve as the structural framework of the longitudinal arch by resisting compression, bending, and torsion loads, it is the plantar tension load-bearing elements of the LALSS which allow the longitudinal arch of the foot to possess both the flexibility and rigidity to accomplish the weightbearing needs of the active individual. The four layers of tension load-bearing elements of the LALSS consist of (from superficial to deep) the plantar fascia, plantar intrinsic muscles, plantar extrinsic muscles. and plantar ligaments.  These plantarly-located tension load-bearing structures work synergistically with each other within the LALSS to fine-tune its stiffness, thereby regulating the flattening and elongation of the longitudinal arch which, in turn, optimizes the weightbearing function of the foot.

Figure 2. As ground reaction force increases on the plantar forefoot from early midstance to late midstance during walking and running, an increase in Achilles tendon tension force results which tends to flatten and elongate the longitudinal arch. As a result of this longitudinal arch flattening and elongation, both the plantar fascia and plantar ligaments develop increased passive tension forces which automatically increase the stiffness of the longitudinal arch during walking and running gait. This Longitudinal Arch Auto-Stiffening Mechanism is dependent on the unique anatomical arrangement of the osseous structures and joints of the ankle and longitudinal arch combined with the geometry of the Achilles tendon, plantar fascia, and plantar ligaments relative to these joints. Thus, the Longitudinal Arch Auto-Stiffening Mechanism of the foot results not only in less longitudinal arch flattening motion during late midstance but also likely results in increased mechanical and metabolic efficiency during human bipedal gait.

The most superficial layer of the tension load-bearing elements of the LALSS is the plantar fascia, otherwise known as the central component of the plantar aponeurosis (Figure 2). The plantar fascia originates from the plantar aspect of the medial calcaneal tubercle and spreads distally to form five separate slips that each insert into the bases of the proximal phalanges of all five digits.14 The plantar fascia, like all fascial and ligamentous structures, is a passive structure, not being dependent on the central nervous system (CNS) to increase its tension forces.

The tension forces experienced by the plantar fascia have been estimated to be 0.96 times body weight in simulated walking experiments using cadaver legs and feet.15 In addition, transection of the plantar fascia has been shown experimentally to increase longitudinal arch flattening and elongation16,17 and to reduce the stiffness of the longitudinal arch to plantar loading forces.18

Immediately deep to the plantar fascia is the next layer of tension load-bearing elements of the LALSS, the plantar intrinsic muscles. The plantar intrinsic muscles span the longitudinal arch from the plantar rearfoot to the plantar forefoot and are active, being controlled by phasic efferent activity from the CNS. Recent research has confirmed the concepts that the plantar intrinsic muscles help stiffen the longitudinal arch, can raise the longitudinal arch, are more active during unipedal standing than in bipedal standing, and are also more active in running than during walking.19,20,21

Deep to the plantar intrinsic muscles are the next layer of tension load-bearing elements of the LALSS, the extrinsic muscles and tendons of the plantar foot (Figure 3). The extrinsic plantar foot muscles include the deep flexors, the posterior tibial (PT), flexor digitorum longus (FDL), flexor hallucis longus (FHL) muscles, and the peroneus longus (PL) muscle. The extrinsic muscles of the plantar foot, like their plantar intrinsic muscle counterparts, are actively controlled by the CNS which regulates longitudinal arch stiffness by actively controlling the distribution, magnitudes and temporal patterns of efferent neural activity to these muscles. In this fashion, the CNS of the individual may increase or decrease the stiffness of either or both the medial and lateral longitudinal arches to optimize the weightbearing function of the individual.6

The deepest layer of the tension load-bearing elements of the LALSS are the plantar ligaments (Figure 2). The tension forces acting on the plantar ligaments are passively increased when the forefoot dorsiflexes on the rearfoot and are passively decreased with plantarflexion of the forefoot on the rearfoot. Both passive tension load-bearing elements of the LALSS – the plantar fascia and plantar ligaments – work synergistically to give the longitudinal arch a baseline stiffness under weightbearing conditions that, even without CNS-controlled muscle activity, help prevent longitudinal arch flattening and elongation,

Plantar fascia and plantar ligament cadaver research published in 2003 from Crary et al found that plantar fasciotomy increased the average strain in the spring ligament by 52% and increased the average strain in the long plantar ligament by 94%.22 In other words, plantar fasciotomy increased longitudinal arch flattening in these cadaver experiments which then caused increased strain on the plantar ligaments. These experimental findings support the biomechanical concept that both the plantar fascia and plantar ligaments, acting as passive tension load-bearing elements within the LALSS, work synergistically to increase longitudinal arch stiffness and prevent excessive longitudinal arch flattening and elongation during weightbearing activities.

Figure 3. The posterior tibial, flexor digitorum longus, flexor hallucis longus, and peroneus longus muscles are all actively controlled by the CNS and form the layer of the LALSS between the plantar intrinsic muscles and plantar ligaments. These plantar extrinsic muscles, when activated by the CNS, cause an increase in longitudinal arch stiffness which helps to resist longitudinal arch flattening and elongation during weightbearing activities. Together the plantar intrinsic and plantar extrinsic muscles of the LALSS work to increase the longitudinal arch stiffness above the baseline stiffness provided by the passively-acting plantar fascia and plantar ligaments which allows the CNS to fine-tune the stiffness of the longitudinal arch of the foot depending on the nature and intensity of the chosen weightbearing activity. CNS: central nervous system; LALSS: Longitudinal Arch Load-Sharing System.

Functional Synergy of LALSS Elements

The main functions of the LALSS are to allow the longitudinal arch to be compliant enough to allow normal longitudinal arch deformation during the first half of stance phase and to be stiff enough during the second half of stance of walking to allow effective push-off force from the powerful gastrocnemius and soleus muscles through the plantar forefoot during propulsion. To accomplish these important functions, the LALSS uses the baseline stiffness from the passive tension forces within the plantar fascia and plantar ligaments along with the increased longitudinal arch stiffness which arises from the active, CNS-controlled, plantar intrinsic and plantar extrinsic muscles. In other words, the 4 layers of the LALSS work synergistically, both passively and actively, to form a load-sharing system that can continuously modulate the stiffness of the longitudinal arch, thereby optimizing the biomechanical function of the longitudinal arch during weightbearing activities.3

One of the most important biomechanical benefits of the 4 layers of tension load-bearing elements that comprise the LALSS is that each of these elements perform similar functions for the longitudinal arch. In this fashion, if one tension load-bearing element of the LALSS fails (eg, plantar fascia rupture or plantar ligament rupture), the other tension load-bearing structures of the LALSS will still be able to produce the necessary tension loading forces on the plantar rearfoot and forefoot so that the longitudinal arch may still function, indicating a true load-sharing system. However, if one of the elements of the LALSS does fail, the remaining tension load-bearing elements will be subjected to higher magnitudes of tension forces to allow the longitudinal arch to maintain enough strength and stiffness to allow proper weightbearing function to occur.

Passive Control and Active Control of LALSS

Since the LALSS tension load-bearing elements combine the plantar fascia and plantar ligaments to offer a baseline of longitudinal arch stiffness along with the CNS-controlled plantar intrinsic and extrinsic muscles to increase the longitudinal arch stiffness over and above this baseline passive stiffness, the human foot has a remarkable ability to adjust the stiffness of its longitudinal arch depending on the type and intensity of weightbearing activity performed.  The active CNS-controlled plantar muscles have the additional ability of being able to independently modify either medial or lateral longitudinal arch stiffness so that weight transfer and balance may be optimized during any given weightbearing activity.

In addition, the plantar fascia and plantar ligaments have the ability to perform their longitudinal arch stiffening functions automatically during both walking and running gait, without CNS control, due to the increase in tension force that occurs within these passive elements as GRF passes from the plantar rearfoot to plantar forefoot. The mechanism, previously described as the Longitudinal Arch Auto-Stiffening Mechanism,23 allows the plantar fascia and plantar ligaments to stiffen the longitudinal arch during propulsive activities without any additional metabolic demand on the muscles of the foot or lower extremity, likely significantly improving the metabolic efficiency of human bipedal gait (Figure 2).

Conclusion

Over six centuries ago, Leonardo Da Vinci observed that the human foot is “a masterpiece of engineering and a work of art”.24 One of these engineering marvels of the human foot is the longitudinal arch and its unique and elegant load-sharing system, the LALSS.  By combining metabolic energy-saving passive elements with its plantar fascia and plantar ligaments that provide a baseline of longitudinal arch stiffness, together with active muscle elements which allow rapid changes in longitudinal arch stiffness through CNS control, the LALSS provides the bipedal human with a remarkable structural system within its feet to optimize weightbearing function. All clinicians dealing with foot and lower extremity injury should strive toward fully appreciating these synergistic mechanisms within the structural elements of the longitudinal arch so that a better understanding of the biomechanics and mechanically-related pathologies of the foot and lower extremity may be achieved.

Kevin A. Kirby, DPM, is Adjunct Associate Professor in the Department of Applied Biomechanics at the California School of Podiatric Medicine at Samuel Merritt College Oakland, California, and in private practice in Sacramento.

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