Vanderbilt University

Sr. Design Project

Group 7: Athletic Shoe Development

Date Due: 04/25/06

Group Members:

Michael Cannamela

Caroline de Monasterio

Robin Giannattasio

Brandon Jackson

Phillip O'Bannon

Dan Thomas
Abstract

50% of runners every year will acquire significant injuries such as Plantar Fasciitis, Iliotibial Band Syndrome, Achilles Tendonitis, and Patellofemoral Pain Syndrome which are a direct result of overuse and hyperpronation. Running shoes help to prevent injury and make the running gait more efficient, making a running shoe the most important piece of equipment a runner possesses. Shoes must be able to absorb shock, control motion, be flexible, and be durable. However, because of the complexity of biomechanics and the range of pronation between different individuals, it is difficult to find the perfect shoe to find all foot types. Our group proposed to create an adjustable running shoe that would be able to correct pronation and thereby reduce running injuries. By using a rigid plate and spring model developed in Matlab, we were able to implement a design that incorporated a suspension system as well as a tilt of the upper plate of the shoe to induce pronation control. Furthermore, we tested our design against the top of the line model shoe on the market today as well as a normal running shoe to see how it compared to various competitors for pronation control and Ground Reaction Force (GRF). We were able to correct rearfoot pronation of our test subject, however, there was a higher GRF during the toe-off as a result of the firm springless sole at the toe.

Introduction

Running is the primary exercise for 40 to 50 million people in the United States each year; however, significant injuries will occur in 50% of runners. Injuries vary from the foot and ankle, all the way up to the lower back. The main risk factors in running related injuries are overuse and hyperpronation. Biomechanical studies in running evaluate movement alternating between pronation and supination throughout the gait cycle. Pronation is defined by the simultaneous movement of calcaneal eversion, abduction of the forefoot, and dorsiflexion of the foot such that the sole of the foot moves away from the medial line. Supination is defined by simultaneous calcaneal inversion, adduction of the forefoot, and plantar flexion of the foot occurring together in a movement that causes the soles of the foot to turn inward toward the medial line. A runner will cycle between supination and pronation depending on what point of the cycle they are in; for normal gait, at midstance there is a transition from the dorsiflex position to a plantar flex position. From midstance to push off, the subtalar joint, the keystone to normal foot function and the site of pronation, will supinate. This helps to lock the foot into a rigid lever that the midtarsal joint can use for efficient propulsion. From the loading response right before the heel strike to midstance, the subtalar joint pronates causing the metatarsal joint to unlock and allow shock absorption.

Even slight deviation from the normal gait cycle can cause problems for runners along the biomechanical chain. When a person has an excessive angle of pronation, misalignment in the biomechanical chain will occur. Hyperpronation causes the "down and in" phenomenon to the normal line of action; the hip becomes internally rotated and the knees and knee caps bend inward and become flexed. This internal rotation causes injuries from the foot all the way up to the lower back.

The Posterior Tibialias runs from the back of the leg and helps to maintain the arch of the foot. The major action of the Posterior Tibialias is to invert the foot in the open kinetic chain and evert the foot when there is a loading response. The posterior tibialias will contract eccentrically such that the maximum stretch is in the pronated position. The Proneous Longus sits on the lateral side of the fibula, wraps under the foot, and attaches to the first metatarsal so that it pulls in to eccentrically control eversion. During overpronation, the Proneous Longus continually attempts to stabilize the foot and the Posterior Tibialias becomes overstretched so that there is a chronic abusing of both tendons leading to inflammation and tendonitis.

The Plantar Fascia, which runs along the bottom of the foot from the calcaneus to the metatarsal, normally lengthens and shortens depending on the angle of pronation. Since the subtalar joint stays in an unlocked relaxed position during pronation, hyperpronation will prevent the subtalar joint from entering supination so that it will not be rigid enough for propulsion. The Plantar Fascia responds by staying rigid in order to compensate for the lack of supination causing it to became abused and inflamed leading to Plantar Fasciitis.

In the pronated position, the normal amount of dorsiflexion causing a functional shortening of the calf muscles and a strain on the subtalar joint. This shortening results in a smaller range of motion for the ankle when walking and running which will displace the Achilles tendon into an incorrect position. The chronic micotrauma over time caused by the abuse of the tendon causes micro tears and the insidious onset of Achilles tendonitis. Furthermore, insufficient dorseiflexion prevents the tibia from traversing over the talus and causes the knee to overextend to correct for the ankle position. This pain from over extension is one cause of Patellofemoral Pain Syndrome.

Since the knee is flexed due to the internal rotation of the tibia and the femur, the quadriceps muscles become lengthened in response. The Vastus Medialis Oblique, the part of the quadriceps that is responsible for stabilizing the patella, becomes weak and can longer cause proper tracking of the patella. This will strain the patellar tendon resulting in its inflammation as well as the inflammation of the bursa, the sack of fluid between the tendon and the bone. Intrapatellar Bursitis and Patellar Tendonitis are another two contributing factors of Patellofemoral Pain Syndrome.

The abdominals are also lengthened and weakened by pronation since the pelvis becomes internally rotated and misaligned. The abdominals become too weak to posteriorly tilt the pelvis into the correct position; therefore, the hamstrings attempt to dynamically stabilize the pelvis by shortening their length. During the heel strike of the gait cycle, the hamstring needs to fully extend but this is prevented by their role as preferential dynamic stabilizer of the pelvis. Therefore, the perpetual quick stretch of the hamstring during the gait cycle will cause chronic micro tears in the muscle leading to a hamstring strain.

The ileotibial band is a ligament over the trochanter of the hip that attaches to the fascialatae. The anterior tilt of the pelvis during pronation causes the ITB complex to shorten which will squeeze down on the bursa between the ITB and the trochanter. Since the bursa is meant to help reduce friction by promoting gliding of the ligament over the bone, the constant pounding on the trochanter causes the bursa to become aggravated in an injury known as Iliotibial Band Friction Syndrome.

Finally, since overpronation causes misalignment all the way up the biomechanical chain, the internally rotated anterior tilt of the pelvis will increase lordosis of the spine causing an increase in the amount of shearing forces on the weight bearing lumbar vertebrae. This is especially stressful for the L5 vertebrae since its already precarious position juts out even further. Since the position of the pelvis is changed by the angle of pronation, the gaps between the sacrum and inominate bones through which the nerves and ligaments run can increase or decrease size. Sciatica can be caused by the pinching of sciatic nerve when it is run through a smaller than normal gap.

The numbers of running injuries that are caused by a deviation of the normal angle of pronation are vast and it is imperative for athletes to ensure that they are running correctly to prevent overuse injuries. A shoe can be used as an orthotic to support and effect weak joints and muscles in order to enhance performance by correcting the angle of pronation. By effectively changing the hyperpronation to a normal degree of pronation, the injuries that are caused by the misalignment of the biomechanical chain and overuse will automatically decrease because the source of the injury has been eliminated. However, the pronation angle fluctuates from person to person as well as other variables including the type of terrain and the current gait. Therefore, a shoe would ideally adjust to whatever the current angle of pronation was. The Adidas 1 is currently the only shoe on the market that dynamically adjusts to correct for pronation. Furthermore, the Adidas 1 is a very expensive option, $250, and it is not found in stores so it must be specially ordered from the company. Additionally, the ideal running shoe is lightweight so that it will not impede with running performance. A normal running shoe weighs 12 ounces, whereas the Adidas 1 weighs 15.3 ounces. Taking into account the drawbacks of the Adidas 1 and the amount of people affected by the various running injuries, there is limited competition and a large target market for such a device.

Methods

In order to achieve the goal of correcting overpronation and thus reduce injuries the shoe design had to accommodate the runner’s individual bio-mechanics and running preferences/styles. Upon research into the anatomy of a running shoe the decision was made to focus on the midsole region which is essentially the shoes suspension system. A running shoe is composed of four components: the upper, midsole, the last and the outsole. The upper is the part of the shoe that wraps around the foot and is usually constructed from a combination of lightweight nylon mesh and synthetic materials. Some parts of the upper contribute to midfoot support, motion control or provide torsional stability; however, its main purpose is comfort and friction prevention. The midsole is considered the most important part of the running shoe since its main purpose is to provide cushioning, stability and motion control. In most shoes, the midsole incorporates materials of various densities positioned in strategic places to achieve the desired effect. Commonly used materials are EVA (ethyl-vinyl acetate), polyurethane, air, gel and firm plastic. The last refers to shape and construction. The outsole is what comes into contact with ground and is usually made of carbon rubber, blown rubber or a combination of both. The outsole resists wear and provides traction. It may also have transverse flex grooves or longitudinal split grooves to enhance shoe flexibility while potentially sacrificing a degree of stability.

Before developing a prototype, a QFD was developed in order to determine the most important features of the design to ensure that it would meet all of our qualifications. Our primary goal was to create a shoe that was able to decrease the amount of injuries sustained because of running by adjusting the runner's angle of pronation and fixing the suspension of the midsole making stability control and adjustability key elements on the product planning matrix. Since many runners acquire high mileage throughout the year, running shoes can wear out the shock absorption of the midsole contributing to further running problems. Therefore, it is important that the shoe be durable so that it will continue to fix the problem instead of aggravating it. Finally, the main competition on the market for our prototype is the Adidas 1, which are too heavy and too expensive. By making our prototype cost effective and lightweight, it provides an edge to the current competition in the market.

The preliminary idea for an adjustable shoe suspension involved a series of inflatable air bladders. These bladders would be attached to a micro-pump located in the heel of the shoe so that upon heel strike air would be driven into the bladders positioned on either side of the shoe. Based on the resistance of each bladder (such as how open the exhaust valves were) one side would be stiffer than the other. In addition this design would provide viscous damping effects when the bladder was impacted by the runner’s foot. This approach never advanced past initial concept and some rough sketches because it was quickly discovered that the idea of integrating an inflatable air bladder into a shoe was extremely patented. Examples of these patents are #4263728 and #5179792.

The concept of combining piezoelectrics into the shoes midsole was another line of thought which was short-lived. The idea involved using PVDF (polyvinelydine fluoride) a piezoelectric polymer to recover some of the power lost in the process of running to drive an electrorheological fluid also located in the midsole. PVDF was chosen over any other available piezoelectrics because of its flexibility, high voltage output, good impact resistance and the fact that its small stiffness adds minimum local stiffening to the host structure. Dr. Goldfarb posed the idea of using this to interact with an electrorheological fluid which would stiffen into a semi-solid when subjected to the electric field. While the underlying theory was promising the idea proved to be too difficult to implement. There simply was not enough research or testing compiled to make it feasible. Furthermore, durability proved to be an important factor in our shoe prototype and the complexity of this particular design was likely to make the shoe's lifespan a short one.

For the final prototype design, we chose two rigid plates separated by springs. Stable spring placement was decided by using the two plate rigid shoe model developed in Matlab, although several methods of modeling were performed. The pressure map for the foot in Figure 2 shows areas of darker green where there is areas of high pressure and yellow where there is low pressure. Since the springs are used as a suspension, it is important for them to be placed at areas of higher pressure in order to maximize their efficacy. This placement is shown in Figure 2 by the red stars. It was essential that the prototype followed the parameters entered into the model so that we could see if our prototype had the desired results. Rigid plates were chosen to provide a stable platform for the springs to rest on and to ensure accurate force calculations. For the upper shoe, we used a Shimano bike shoe because it had a rigid carbon fiber sole to add to the stability of the upper plate. To provide flexibility in the forefoot of the shoe, a cut was made on each bike shoe at the ball of the shoe across the width. In order to provide traction so the runner would not slip during testing, the sole from a pair of Nike Prestos was attached to the bottom of the metal plate.