Design Challenge:
Encourage forefoot/midfoot strike pattern change while distributing load to aid in injury mitigation. A novel structure “SPEEDWEB” serves as a responsive cushioning element located at the forefoot (anterior to transverse arch) with high energy return as well as longevity. Introducing this into the footbed at the forefoot, coupled with traditional foam at the rearfoot, provides incentive for runners to strike at the forefoot. The biomimetic “SPEEDWEB” structure distributes load more efficiently than traditional foams due to its hierarchical structure, which is further parametrized based on optimal strike patterns. This works to create “the ultimate ride” to encourage runners to alter their strike pattern from rearfoot to forefoot striking, while minimizing overload of impact stress.
Background:
Strike pattern in recreational and competitive running has endured controversy among athletes, biomechanists, and footwear companies alike. Effects on running economy, joint contact forces, and injury prevention have been explored through clinical studies, revealing tradeoffs between improving efficiency and adaptation limitations. Concerns over abrupt changes in strike pattern without proper adaptation and strengthening raise concern for those considering a switch from rearfoot strike to forefoot strike, despite substantial evidence on the benefits of forefoot-striking on performance and economy. A dramatic or unforgiving shift in running mechanics heavily strains untrained muscles, possibly leading to overuse injuries.
Biomechanical Findings:
Evidence suggests a forefoot-strike pattern improves economy through enhanced storage of elastic energy in the foot and ankle as a result of longer and earlier plantarflexion activation during the stance phase of gait.
A cross-disciplinary study conducted by the Claremont Colleges assessed the muscle activity and kinematics of forefoot and rearfoot strike runners, gathering evidence on timing of muscle activation and joint loading patterns. The study revealed that forefoot strikers activated the plantarflexor muscles 11% earlier and 10% longer than rearfoot strikers, and landed with more vertical shin angles at beginning of stance. These findings indicate a shift of joint loading from the patellofemoral joint to the ankle, increasing load on the gastrocnemius and Achilles tendon and decreasing load on the patella and quadriceps. The study inferred this to be desirable as the Achilles is better equipped to store and translate elastic energy through the foot and ankle, thus improving economy.
These findings are significant not only from the standpoint of running economy but from the perspective of injury etiology and prevention. Patellofemoral knee pain, often dubbed “runner’s knee” is one of the leading injuries in recreational and dedicated runners. Several studies on prevalence and epidemiological trends of patellofemoral pain cite the syndrome as having an incidence rate of 22 – 25% annually. A study conducted by The International Journal of Sports Physical Therapy compiled data from 10 studies previously conducted, finding patellofemoral pain to be the most prevalent pathology at sports medicine clinics, accounting for ⅕ to ⅓ of all recorded pathologies.
These findings showcase a pervasiveness of a syndrome closely correlated to rearfoot strike patterns, despite advances in cushioning systems and running shoe design. This suggests that some improvement can be made with regard to strike pattern adaptation with the intention of reducing patellofemoral pain syndrome through a shift to forefoot- and midfoot-strike pattern. Encouraging a strike pattern change through sensory queuing can be achieved through materials selection, specifically with midsole and footbed cushioning. Differentiation of materials in the desired “strikezone” provides a form of feedback for the runner to begin to make the shift to a forefoot or midfoot strike.
With this intention comes the challenge of properly protecting the Achilles tendon and gastrocnemius during the adaptation phase to reduce the risk of overload injury. A possible solution to this challenge of proper adaptation and mitigation of injury is impact diffusion. The “strikezone” cushioning system, constructed using the biomimetic principles of the spiderweb (described below) can successfully diffuse the force of a forefoot strike into the larger surface area of the strikezone itself to aid in the strike pattern adaptation process of the runner, freeing them of stress overload.
Structural Inspiration:
As mentioned above, the mitigation of stress concentration can be achieved through a latticework structure derived from the construction of spiders’ webs. This biomimetic inspiration is grounded in the research and analysis of web structures as meticulous engineering achievements of nature.
The 2-dimensional orb web is the structure many of us envision when thinking of spider webs. Though the properties of spider silk itself are quite impressive, the architecture of orb webs (as well as 3-dimensional funnel and cob webs) is an exceptional feat of structural engineering. The structure has evolved over millions of years to optimize the spider’s strict criteria. The ability of the web to withstand impact (generated by prey, wind, etc.) with minimum damage and at the lowest manufacturing cost showcases the opportunity for a biomimetic solution.
Looking deeper, the web possesses several characteristics and structural elements that can be replicated in a cushioning unit to yield the desired results. The hierarchy of threads used in the construction of a web, and the different properties of these threads creates a structure that is not only extremely efficient, but has a very high damage tolerance. In other words, when threads on the web are damaged and break, force distribution remains unchanged throughout the web, ensuring its structural integrity. The remarkable force distribution of the web structure provides a design opportunity for a lightweight latticework system that properly distributes force throughout the cushioning unit. This can be applied to our challenge of encouraging a change to forefoot striking without increasing risk of injury.
Closer examination of the web structure reveals how this can be achieved. In a study conducted at the University of Madrid, scientists noted an overlooked aspect of the web: the secondary frame. This feature, highlighted and analysed by the study, found that it allowed the web to successfully dissipate impact force through “load paths” in the web from a variety of impact points mapped out on the web itself. They further experimented with the length of the secondary frame radials to better understand the optimal radial length for maximum load distribution within the structure.
These findings are significant in designing a cushioning system that is not only lightweight and structurally sound, but offers an opportunity to explore load distribution in a forefoot cushioning unit that is forgiving enough to encourage training adaptations to forefoot strike patterns.
Prior Art:
Lattice systems are not necessarily novel in midsole design, but have come to the fore in recent years. Lattice systems provide structure similar to that of a foam midsole, but often with reduced weight and an eye-catching design. Functionality of this system depends on the architecture of the lattice and mechanical properties of the materials used in the system.
Perhaps the most notable existing lattice midsole system is Adidas’s Futurecraft 4D. Developed in collaboration with Carbon 3D, the liquid resin framework offers variable densities throughout the system. The patent for this technology was filed in 2018 under US20180271211A1. Key characteristics noted in the patent include the following:
“The interconnected unit cells are connected at nodes having a valence number defined by the number of struts connected at that node. The valence number of the nodes may vary to provide customized characteristics to zones or portions of the midsole. The plurality of interconnected unit cells may be organized in a warped cubic lattice structure. The warped cubic lattice structure and the size/shape of interconnected unit cells may vary to provide customized characteristics to zones or portions of the midsole.”
Nike, however, patented a lattice structure in 2002 under US6763611B1 that describes a “sole including a three-dimensional, polymer lattice structure formed of a plurality of connectors that extend between a plurality of masses” and “at least a portion of said connectors having a length extending in a direction that corresponds with a longitudinal length of said footwear, and at least another portion of said connectors having a length extending in a direction that corresponds with a lateral width of said footwear.”
Under Armour boasts three patents related to lattice structures in the midsole. The first, filed in 2016, US20160219976A1,describes a:
“Midsole that includes a platform extending along a perimeter portion of the midsole and a lattice structure integrally formed with the platform, the lattice structure including a network of laths. The platform is molded to the user’s foot then integrated with the lattice below it to provide the cushion system.”
The second patent, US10010134B2, a renewal of a previous version, references that the midsole includes a lattice network of laths and nodes with a recess formed within the structure. A resilient foam insert is positioned in the recess of the lattice structure. The third patent US20160324260A1 describes a sole member connected to the upper, the sole member including a plurality of tubular structures, the plurality of tubular structures at least partially filled with a loose granular material.
Its important to note that these existing patents are characterized by a lattice structure rather than a pre-tensioned web system. They offer insight on manufacturing processes and open-structure midsole designs but support a different mechanism than the proposed SPEEDWEB design. More akin to the SPEEDWEB proposal is three-dimensional weaving systems such as those developed and patented by Three-D Composites Research Corp. These systems use multi-axial groups of rods oriented to develop 3D woven structures that can withstand impact forces from many directions. Related patents include US5076330A (expired in 2009) and US5348056A, which describes :
“The present inventors proposed in their previous patent application, Japanese Laid-Open Patent Application H2-259148, a technology of weaving three-dimensional 4- or 5-axis woven fabrics with various fiber orientation angles in a facilitated manner by the use of a rotor-carrier type three-dimensional fabric weaving machine (hereinafter referred to simply as a “3D weaving machine” for brevity).”
Looking forward:
The proposed solution serves as an intermediary for runners seeking to transition from rearfoot-striking to forefoot-striking in a comfortable and safe fashion. It is important to understand the body’s ability to adapt requires time and training to influence these outcomes. Looking forward, individuals and institutions alike may strive to contradict the notion that forefoot striking is no more desirable than rearfoot striking. In understanding the evolution of bipedal locomotion and foot architecture, science arrives at the conclusion that humans were meant to run barefoot and with a forefoot strike pattern, but modern footwear has impeded our intrinsic foot strength abilities. Key evidence of this conclusion can be found in a valuable study from the Journal of Sport and Health Science. The study examined ground reaction forces in shod rearfoot strikers, shod forefoot strikers, and barefoot/minimalist forefoot strikers. Both the shod rearfoot- and forefoot-strikers bore similar ground reaction forces, while the minimalist forefoot strikers showed a markedly lower GRF than the other groups. The study suggests that any type of cushioning influences running mechanics, but acknowledges the dangers of abrupt transition from rearfoot to forefoot strike patterns. It is this grey zone that we can hope to tackle, with the goal of reducing anterior knee pain, regaining intrinsic foot strength, and improving running economy.
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