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.


What is OPIS? How does it work?

The Optimal Performance Index System (OPIS) was developed out of a need for quantifying the support systems in athletic shoes and to provide objective data in regard to how each shoe performs. OPIS objectively tests and quantifies shoe support, thus allowing for direct comparison between various shoe models, and eliminating the need to rely solely on subjective opinions and catalog descriptions when determining proper athletic footwear.

All shoes are tested with patented OPIS machinery in a laboratory setting, under the same protocols and in the same testing environment. New shoe models are tested and added to our database on a regular basis, with major tests and site updates occurring in January and July of each year.

How does OPIS quantify support levels in shoes?

OPIS machinery is specifically designed to mimic the rotational and compressive forces of the human foot. As each shoe is loaded for testing, specific forces are applied, and a series of measurements are taken to determine the torsional support of the shoe (TCI), the vertical support of the shoe (VCI), and the energy return (RI). The variables indicating shoe support levels, TCI and VCI, can be used independently to compare shoe models, as well as jointly to provide a measurement of overall shoe support, in the Shoe Stability Index (see below). For more information on the testing process, please visit OPIS at Work.

What is TCI? How does it relate to shoe support?

The Torsion Control Index (TCI) is a measurement of shoe stability through the mid-foot portion of the shoe and is indicated in inch-pound units (in.-lbs.). TCI is measured through actively twisting a shoe along the long axis of the shoe, simulating rotational forces of the foot, and then recording the shoe’s resistance to this motion. After applying standard hindfoot and forefoot pressure, each shoe is twisted through the portion of the shoe where foot rotation naturally occurs, and through a set number of degrees. The amount of torque required to complete this motion is registered as the TCI. A higher TCI, as generally seen in shoes of higher stability, indicates a greater ability of the shoe to control the rotational forces of the foot, as seen in pronation and supination.

What is VCI? How does it relate to shoe support and cushion?

The Vertical Compression Index (VCI) is measurement of shoe stability through the hindfoot portion of the shoe and is measured 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 calcaneal (heel) deviation and rearfoot motion. Once compressed, the distance that the shoe has been depressed is recorded as the VCI. A higher VCI generally indicates that a shoe is soft in the heel area, corresponding to a greater cushioning effect, but less control of heel motion. A lower VCI indicates greater firmness and hindfoot stability in a shoe, and subsequently increased control of rearfoot motion.

What is the Shoe Stability Index? How is it calculated?

The Shoe Stability Index (SSI) is a measurement of overall shoe stability, and allows the consumer to identify how different shoe models compare to one another in regard to their functionality.  Running shoes in particular show a vast difference in the degrees of support that they offer, and the SSI is a quantified measurement of the support level of any given shoe.  The SSI is calculated using a formula that weighs the TCI and VCI (indicators of midfoot and rearfoot shoe stability) equally, converts them into a single index, and normalizes them to (approximately) a 100 pt scale. Shoes with a higher SSI have relatively more stability, while shoes with a lower SSI have less stability.

Why does the term “stability” mean? Does everyone need stability in their shoes?

Stability is a term used in the athletic shoe industry to indicate a higher level of support in a given shoe model.  There are as many features and materials used by various shoe companies to enhance stability as there are terms used by shoe manufacturers to describe the various levels of support and stability.  The purpose of OPIS is to not only quantify the support levels of shoes in simple terms (levels of stability), but also to simplify the language used in comparing running shoes to one another.  The level of stability one needs in their athletic footwear generally depends on his/her foot pattern, activities being pursued, and orthopedic history; physical stature can certainly play a role as well.  While for most people it is rare to have problems with too much stability in their footwear, many people are comfortable with lighter, softer shoes which generally are found in lower levels of stability.

How does OPIS determine which shoes are in each level of stability?

Shoes are classified into the four levels of stability based on their Shoe Stability Index (SSI), the quantified amount of support that the shoe offers.  Once calculated, the SSI values have been normalized to (approximately) a 100 point scale: shoes with a SSI value above 70 are Level 1 shoes, 60-69.9 are Level 2, 50-59.9 are Level 3, and those with a SSI under 50 are Level 4. While each level of stability includes a range of values, OPIS gives an accurate indication of how shoes in each stability level will perform.  By maintaining these standards, OPIS also allows for the comparison of shoe models from one test period to another, or from one version of a shoe model to another!

What is the Rebound Index?

The Rebound Index (RI) is the third measurement taken in the OPIS lab and indicates energy return from a shoe.  The midsole of the shoe is placed under a standard amount of pressure, and a measurement is taken from how the shoe responds to that pressure.  As the RI is not indicative of shoe stability, it is not used in calculation of the Shoe Stability Index (SSI).  However, there is generally a significant correlation found between the Vertical Compression Index (VCI) and Rebound Index (RI).  That is, shoes with a greater VCI (less firm, more cushioned) tend to have a higher RI as well.  Shoes with a higher RI may be suitable for runners who do not need much support in their footwear and who are concerned with better athletic performance.

How are the “Clinical Recommendations” determined?

Clinical recommendations can be found on the drop down info for any shoe listed on the OPIS website.  These recommendations are a result of combining the laboratory data from OPIS testing with clinical trials and information from our retail affiliate.  Over the last ten years, we have compiled data through the fitting of 60,000 pairs of shoes; approximately 6,000 pairs on an annual basis.  The correlations we have found between how shoes test in the OPIS lab, and how they perform on one’s feet, are the basis for the recommendations made for each shoe.  As with anything that combines science and craft, there are certainly some grey areas and unknowns in shoe fitting, but OPIS helps simplify and define the shoe selection process, and steers you toward the proper running shoe for your individual needs.

How are shoes selected for the injuries in the “Injury Profiler?”

The shoes found in the Injury Profiler portion of this site are models in Stability Levels 1 and 2, with some modifications made depending on the VCI and firmness of the shoe.  Our clinical trials and biomechanical research are in concurrence with much of the available research as to how proper shoe support can limit motion of the foot as well as tibial torsion forces.  Shoes with greater stability alleviate various orthopedic injuries/conditions as they perform better in controlling the undesirable foot motion that often strains various soft tissues and changes the alignment of the joints of the leg. The shoes listed for each injury/condition are ones that will generally alleviate pain associated with those maladies, but, as always, a shoe needs to fit properly to maximize its functionality and ability to help you!

What makes OPIS different from other sites that rate shoes?

OPIS uses a proven, patented system that allows for quantification of an athletic shoe’s support in a way that has never been done before.  The advantage of OPIS is it provides objective, technical information from laboratory test results, and correlates this information with findings from clinical and retail settings.  A variety of sites and publications make shoe recommendations based on the opinions of a few runners, or based on the specifications or features listed by the manufacturer, but which have not had appropriate end-product testing.  OPIS allows for the determination of a shoe’s overall support, as well as who shoes are suitable for, without relying on the marketing or branding that is often apparent with many shoe models.  Furthermore, OPIS does not receive any donations of shoes or funds, and remains a completely independent testing facility.  For more information, please see OPIS & You.

How can I tell what level of shoe stability I need?

The proper level of shoe stability is essential to keeping your athletic activities pain-free and improving your performance.  While we have included information on this site regarding how to determine your foot pattern, and how to use that information, and any current injury you may suffer from, to properly select footwear, there are other avenues available to you, if necessary.  While OPIS provides simple guidelines to accurately select the proper footwear, many people have questions related to selecting shoes based on physical stature or other factors.  The inexperienced buyer may benefit from consulting a footwear professional, but at OPIS, we will certainly continue to address any footwear fitting concerns or questions that you have. Contact us

Why is my shoe model not rated on this site? Why are only running shoes rated?

OPIS is constantly expanding its testing to include a greater number of shoe models.  While the depth of the running shoe market and the number of available shoe models continues to expand, we will continue to increase the number of shoe models undergoing OPIS testing.

At the current time, we are only testing running shoes, but in the future also plan to include test data for walking shoes, court shoes, and other sport-specific shoes.

October 2023 test data is now live!