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
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.
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.
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).
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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- Kirby KA: Foot and Lower Extremity Biomechanics IV: Precision Intricast Newsletters, 2009-2013. Payson, AZ: Precision Intricast, Inc.; 2014:31-34.
- Kirby KA: Longitudinal arch load-sharing system of the foot. Revista Española de Podología. 2017;28(2):e18-e26.
- Ye Z, Revie M, Walls L. A load sharing system reliability model with managed component degradation. IEEE Transactions on Reliability. 20154;63(3):721-730.
- Taghipour S, Kassaei ML. Periodic inspection optimization of a k-out-of-n load-sharing system. IEEE Transactions on Reliability. 2015;64(3):1116-1127.
- Keller TS, Weisberger AM, Ray JL, Hasan SS, Shiavi RG, Spengler DM. Relationship between vertical ground reaction force and speed during walking, slow jogging, and running. Clin Biomech. 1996:11(5):253-259.
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- Kelikian AS, Sarrafian SK. Sarrafian’s Anatomy of the Foot and Ankle: Descriptive, Topographic, Functional, 3rd ed. Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins;2011:144-154.
- Erdimir A, Hamel AJ, Fauth AR, Piazza SJ, Sharkey NA: Dynamic loading of the plantar aponeurosis in walking. J Bone Joint Surg Am. 2004;86A:546-552.
- Sharkey NA, Ferris L, Donahue SW. Biomechanical consequences of plantar fascial release or rupture during gait: part I – disruptions in longitudinal arch conformation. Foot Ankle Intl. 1998;19(12):812-820.
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- Kelly LA et al: Recruitment of the plantar intrinsic foot muscles with increasing postural demand. Clin Biomech, 27:46-51, 2012.
- 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. 2014:11(93):20131188.
- Kelly LA, Lichtwark G, Cresswell AG. Active regulation of longitudinal arch compression and recoil during walking and running. J. R. Soc. Interface. 2015;12(102): 20141076.
- Crary JL, Hollis M, Manoli A. The effect of plantar fascia release on strain in spring and long plantar ligaments. Foot Ankle Int. 2003;24(3):245-50.
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