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 Fore Flex?

Fore Flex is the measurement in millimeters of the fore foot when compressed to a fixed poundage. The higher the Fore Flex, the softer the sole, allowing more bend at the fore foot. The lower the Fore Flex, the firmer the sole, allowing less bend at the fore foot.

Personal Profiler

Shoe selected by gender, stature and foot pattern.

Shoe Query

Shoe selection through commonly asked questions.

Injury Profiler

Shoe selection for assisting in injury recovery and prevention.

Shoe Rankings

Shoe selection by overall shoe stability.

The Value of OPIS

The utilization of Optimal Performance Index System (OPIS)–an independent laboratory testing facility and database founded by physical therapist Craig Norton–recommends proper athletic shoes, basic exercises, orthotics, and products through a evidence-based platform designed to help resolve those with common subacute lower extremity issues. Our approach centers on data-driven footwear selection, stability indexing, and insole translation analysis, so every recommendation is grounded in testing rather than trends.

The Science Behind Our Recommendations

Our insights aren’t based on guesswork or subjective reviews. Every recommendation is built on a rigorous foundation of:

Structural Parameters: Analyzing exact shoe metrics (like heel-to-toe drop, stability indices, and torsion control).

Mathematical Modeling: Evaluating the differences between a variety of shoe parameters.

Clinical Observations: Real-world testing and insights from active patient care.

Injury Mechanism Knowledge: A deep understanding of how and why lower extremity injuries occur and how to mitigate them.

Why We Transitioned to a Subscription Model

To keep providing rigorous, data-driven insights that go beyond aesthetic footwear trends, OPIS has transitioned to a subscription platform. This model allows us to sustain our clinical research, mathematical modeling, and ongoing structural parameter testing—ensuring our community of practitioners and athletes continues to receive uncompromising, evidence-based guidance for lower extremity recovery.