What is SSI?

Shoe Stability Index (SSI) is an indicator of shoe stability, as an index. SSI is derived through mathematically combining the quantified measurements of midfoot stability (TCI) and hindfoot stability (VCI), indicating the shoe’s overall ability to control the motion of the foot.

What is TCI?

Torsion Control Index (TCI) is a measurement of midfoot shoe stability, in inch-pound units (in.-lbs.). TCI is measured through actively twisting a shoe around the longitudinal axis of the shoe, from the heel to the area of the toes joints, simulating rotational forces of the foot, and measuring the shoe’s resistance to this motion.

The higher the TCI, the firmer the midfoot and more torsional stability. The lower the TCI, the softer the midfoot and less torsional stability.

What is VCI?

Vertical Compression Index (VCI) is a measurement of hindfoot shoe stability, in millimeters (mm). VCI is measured through compression of the heel portion of the shoe under a fixed amount of pressure, thereby measuring how the structure of the shoe controls rearfoot motion.

The higher the VCI, the softer the midsole and less hindfoot stability. The lower the VCI, the firmer the midsole and more hindfoot stability.

What is Loaded Heel to Toe Drop?

Loaded Heel to Toe Drop is the measurement in millimeters of the height of the heel relative to the fore foot when compressed to a fixed poundage. This can be defined as vertical support.

What is RI?

Rebound Index (RI) is an indicator of energy return of shoe to the foot, in millimeters (mm). RI is measured through compression of the heel portion of the shoe under a fixed amount of pressure, and then determining how much force the shoe exerts on the foot.

OPIS at Work

It is widely recognized that proper footwear reduces injuries and enhances athletic performance by improving biomechanics. The purpose of OPIS is to help determine the best footwear for an individual, given his or her physical stature, physical activity, foot pattern and injury history.

The OPIS Laboratory

The data found on this site comes from measurements made in the OPIS laboratory. All shoes tested for each gender are the same size, and are tested under identical temperature-controlled conditions, following the same protocols. The testing protocol followed in the OPIS lab produces quantified, objective data about each shoe’s support systems. This is done with machinery that simulates the forces that a human foot naturally puts on a shoe. The data obtained via this testing is then correlated with clinical observations and case studies in order to optimize the likelihood of matching the best shoe to the customer.

More specifically, OPIS testing quantifies a shoe’s support systems, specifically how well a shoe can control the rotational forces of the foot, seen in pronation and supination, as well as compression, seen in foot strike. Control of rotational force is measured in terms of the Torsion Control Index (TCI), which indicates stability through the ‘midfoot’ portion of a shoe. Control and cushioning of the foot at the striking phase of the gait cycle is indicated by the Vertical Compression Index (VCI), and indicates the ‘hindfoot’ stability of the shoe.

TCI = Torsional Support

Excessive motion of the foot, if not properly controlled, can contribute to or cause a variety of injuries, such as knee pain, shin splints, ilio-tibial band syndrome, as well as others. Using OPIS technology, we measure and quantify the ability of a shoe to control rotational motion of the foot by twisting the shoe around its long axis (from the heel to the first metatarsal) through a fixed degree of motion. The resistance to this twisting is the Torsion Control Index (TCI), and is measured in inch-pounds (a torque unit). A higher TCI indicates greater torsional control (and midfoot stability) in a shoe, and thus a greater effectiveness in controlling undesirable foot motion.

VCI = Vertical Support

Another important quality of a shoe is its ability to provide proper vertical support. Proper vertical support reduces impact forces during the weight-bearing phase of running and walking, and controls deviation in heel movement, thereby helping to control general foot motion such as pronation or supination. Using OPIS technology, we measure vertical support by mechanically compressing a shoe’s midsole to a fixed PSI (pounds per square inch). The amount of compression at this PSI is the Vertical Compression Index (VCI), which is measured in millimeters, and indicates the shoe’s vertical support. A higher VCI indicates that the shoe has a softer midsole and greater cushion, while a lower VCI indicates that the shoe has a firmer midsole and greater control of heel motion. In general, people of lighter stature, and those who do not require a high level of shoe support, perform well in shoes with softer midsoles (high VCI), while those of heavier stature, and those who need more shoe support, perform better in shoes with a firm midsole (low VCI).

SSI = Shoe Stability Index

Studies have shown the importance of both torsional and vertical support in athletic footwear, and how proper support can both improve performance and reduce the likelihood of injury by minimizing excessive foot motion. As both torsional and vertical support are important, OPIS has worked with statisticians to develop a method by which TCI and VCI can be combined to form a single measurement of overall shoe support. The Shoe Stability Index (SSI) is calculated using a formula that weights each of these two variables equally, combining them into one index, so that the overall level of shoe support is described by a simple, easily compared number.

OPIS In Practice

Data obtained in the OPIS lab is correlated with clinical observations from the sports medicine clinic affiliated with OPIS. Over 6,000 pairs of shoes per year are fit using the OPIS system, which combines the quantified measurements of the shoes with the application of established principles of suitability. Variables taken into consideration in determining proper support levels and shoe selection include gait, gender, stature, activity and orthopedic maladies. Shoes identified as having the proper type of structure to meet an individual’s needs, via OPIS and established principles, can thereby be identified as likely being good matches for the customer.

Case studies are also effective tools for establishing and substantiating shoe parameters. Through our clinical affiliation, we are able to document the treatment history of individuals with injuries related to running and walking activities. Through the use of OPIS, these individuals are able to bring in their current athletic footwear for evaluation of vertical and torsional support levels. If one’s current shoes are not suitably meeting his or her needs, we recommend shoes with the appropriate support levels needed to relieve the pain of relevant injuries. Follow-up on these cases continues to illustrate that effective use of OPIS translates to proper footwear fitting and increased customer satisfaction.

OPIS strives to develop parameters for athletic footwear so that consumers, retailers and manufacturers can benefit from a system of standards that will lead to the selection of appropriate footwear for any individual, and furthermore provide the optimal footwear for enhancing performance and reducing injuries.

October 2023 test data is now live!