Midsole

The midsole is the focus of a shoe's cushioning system. An argument could be made that no part of the shoe has benefited from science more and affects performance more than the midsole. In early athletic shoes, especially in running shoes, midsoles didn't really exist. Early basketball shoes had some rubber cushioning inside them, but nothing along the lines of modern midsoles. Not until an athlete with an interest in shoe design started thinking about running shoes and how they could be improved to reduce injuries and allow for more training did the invention of the midsole as we know it exist. The original idea for the midsole, itself a study in how an inventive mind works, utilized a shower sandal glued between a nylon upper and outsole of a simple running shoe already on the market. This invention was given to running friends to test and provide feedback. Out of this simple innovation, for which a number of patents have been issued, grew the plethora of midsole "innovations" we see today in every type of shoe from walking to basketball. The innovations range from improved foam midsoles to complicated air and fluid systems.

Stability - As the importance of cushioning systems became the central focus of midsole research and design, the midsole's contribution to overall foot and ankle stability was somewhat ignored. The reason for this is interesting in that it was driven by the boom in the running movement, by people's in-store perception of what a good cushioning system "felt" like, and by surveys in running magazines as to the "best" cushioning offered in shoes. What cushioning is can be a complex question, and will be outlined later in this section. But the basic conflict set up by trying to find a cushioning system that was soft enough to satisfy a person's perception of a soft shoe when trying a shoe on in a store and, in turn would pass the tests set up by sport research labs under contract to running magazines to measure cushioning in a shoe often resulted in a shoe that was so soft that it provided minimal stability and contributed to over pronation or supination and therefore other injuries. The solution to this was to pay attention to cushioning as well as modify parts of the midsole to enhance stability. Support structures like heel collars that are extensions of the midsole that, in many cases are integrated with the upper, seem to answer the need to balance the cushioning as well as the stability needs of a particular sport and athlete.

Flexibility - Like stability, flexibility is directly related to the cushioning system used. As full length midsoles evolved flexibility tended to be compromised. As researchers started to define the nature of cushioning needs by sport and type of athlete, they in turn started to pay attention to the need to provide flexibility to the regions of the foot where flexible joints are found. Most midsole designs now incorporate some type of flex line in the midsole located near where the foot flexes naturally. This flex line is closely aligned and integrated with the outsole's flex lines.

Cushioning - As mentioned earlier, this is the main focus of the midsole system. The need to deal with the impact forces that an athlete encounters during sport, over seven times the body weight in sports like basketball, is of paramount importance when designing a midsole unit. How the midsole deals with these forces is dealt with under the section of this guide dealing with impact testing later, but the basic idea is to help spread out the force of impact so that it is not delivered full force straight to the foot and legs of the athlete. The cushioning needs of an athlete are closely tied to an individual's biomechanics as well as to the specific sport the athlete is participating in. For example, a person's biomechanics (as defined by the bones, muscles, ligaments and tendons) might be such that they are a heel striker rather than a forefoot striker when running. In general, a heel striker exhibits a key, potentially injurious, impact force at the beginning of a stride cycle that is not seen in the impact record of a forefoot striker. What this means is that the design for a midsole system for a heel striker must pay particular attention to cushioning in the heel area. In turn, the cushioning system also plays a key role in the overall energy consumption of an athlete during an event. The natural cushioning system of the body is the musculoskeletal system--the bones, muscles, ligaments and tendons, of the foot and leg. The fat pads in the foot also play a major role in protecting the foot from sudden and repeated impact. Operating this system requires the use of the body's stored energy as it is dependent on muscles contracting to help deal with the impact going through the system. This works fine under normal conditions, like relaxed walking over soft surfaces. When an athlete asks more of this system by increasing the frequency and force of impacts through training and participation in athletic events, more energy is needed to help maintain performance. A shoe's cushioning system can help by improving the efficiency of, or reducing, the energy needs of the athlete. A good midsole design is one that provides some type of energy return to the body when compressed. This energy return, or spring gives lift to the foot and leg that normally would require muscular contraction, and therefore cost the athlete in terms of energy.

Durability - In midsoles, traditional cushioning systems based on rubber or foam materials tend to break down, or compact, over time. As they break down they lose their ability to cushion and provide energy return. The way these materials break down is related to how they are constructed. Basically, the way to increase the cushioning characteristic in a rubber or foam based midsole is to increase the amount of air blown into the material. This air produces bubbles. The larger the bubbles, the thinner the walls surrounding them. The thinner the walls the shorter amount of time needed to break them down. Durability in midsoles is desirable since losing the cushioning properties, as happens as traditional midsole materials compact, can lead to injury. Many athletic shoes address this need by introducing some type of hybrid midsoles that integrate pressurized air, or liquid systems that do not lose their cushioning characteristics over time.