The Foot Posture Index (FPI) is a clinically oriented, semi-quantitative tool that grades static standing foot posture along a continuum from supinated through neutral to pronated. It is designed to translate routine visual and palpatory observations into a single numerical score that can support diagnosis, risk stratification, and monitoring of treatment outcomes in both research and clinical practice
Concept and Development
The FPI was developed in the mid‑2000s (Redmond et al.) to address limitations of traditional static measures such as isolated rearfoot angles and arch indices, which often captured only one plane of motion or a single segment of the foot. The developers systematically reviewed over 140 papers and distilled 36 clinical measures down to a smaller set of items that together could represent foot posture across all three anatomical planes.
From this process emerged the currently used six‑item version, commonly called the FPI‑6, which balances practicality with sufficient biomechanical coverage. The intention was to create a method that clinicians could apply quickly in a busy clinic without specialized equipment, yet that would still show acceptable reliability and construct validity in research settings.
Structure and Scoring
The FPI‑6 evaluates six specific criteria of foot posture in relaxed bipedal stance, each scored on an ordinal scale from −2 to +2. Features judged to be approximately neutral receive a score of 0, pronated characteristics are given positive scores, and supinated characteristics negative scores, with larger magnitudes indicating more extreme postures.
Although the exact wording of the items varies slightly between teaching resources, the six criteria typically assess: talar head palpation, curves above and below the lateral malleolus, frontal plane position of the calcaneus, prominence of the talonavicular joint, medial longitudinal arch height/shape, and forefoot abduction/adduction in relation to the rearfoot. The six scores are summed to produce a total FPI value ranging from −12 (highly supinated) to +12 (highly pronated), with values around zero reflecting an overall neutral posture and intermediate ranges interpreted as mildly pronated or supinated.
Measurement Procedure
FPI is performed in relaxed standing, with the patient in double‑limb support and a comfortable, self‑selected stance width and foot angle. This position was chosen because it approximates the posture around which the foot operates during normal gait, while being easier and more reproducible than dynamic assessments.
The examiner typically stands behind and slightly to the side of the patient to visualize the rearfoot and midfoot, moving around the patient as needed to inspect each criterion. No goniometer is required; the scoring relies on standardized visual categories supported by illustrative reference images in training materials, which helps to improve inter‑rater agreement. Because the method is observational, training and calibration are recommended, particularly for research use or when multiple clinicians will collect data.
Reliability, Validity, and Normative Data
Multiple studies have reported that the FPI‑6 has acceptable inter‑rater and intra‑rater reliability when examiners follow standardized instructions. The inclusion of multiple segments and all three planes of motion has been shown to correlate more strongly with 3‑D kinematic measures of foot posture compared to single‑angle static methods, supporting its construct validity.
Normative data have been published for both adult and paediatric populations, indicating that a “normal” foot is often slightly pronated rather than perfectly neutral. In a large paediatric dataset, researchers established age‑related reference values and examined the influence of BMI, helping clinicians distinguish between physiologic flatness and potentially pathological pronation in children. In adults, anthropometric factors such as foot size, height, and BMI explain only a small proportion of the variance in FPI scores, suggesting that foot posture reflects a complex interplay of morphology and function beyond simple body dimensions.
Clinical and Research Applications
Clinically, the FPI is used to classify foot type for a range of purposes, including identifying pronated or supinated postures that may contribute to overuse injuries, informing orthotic prescription, and monitoring the effects of interventions such as footwear modification or exercise therapy. Because it captures a global picture of foot posture, it is well suited to patient classification in studies that explore relationships between foot type and pathology.
Numerous investigations have used FPI scores to examine associations between foot posture and conditions up the kinetic chain, such as medial compartment knee osteoarthritis. For example, pronated FPI scores have been positively associated with medial tibiofemoral osteoarthritis, whereas cavus postures appear relatively protective, supporting the rationale for including foot assessment when managing knee OA. The index is also commonly used in paediatric research to track developmental changes and to evaluate whether specific foot postures are linked with pain or functional limitation in children.
Strengths and Limitations
The major strengths of the Foot Posture Index are its simplicity, low cost, and multidimensional nature. It can be implemented quickly in almost any clinic, requires minimal equipment, and yields a single interpretable score that can be recorded longitudinally or used to stratify participants in trials. Its ability to incorporate multiple foot segments and planes offers a more holistic representation of static posture than traditional single‑measure approaches.
However, FPI is a static, weight‑bearing assessment and does not replace instrumented gait analysis or dynamic pressure measurement when those are available. Recent work has highlighted that static FPI scores do not always correlate strongly with dynamic parameters such as plantar pressure distribution or kinematic patterns during barefoot running or walking, reminding clinicians that posture does not fully predict function. In addition, as an ordinal, observer‑rated scale, the FPI is susceptible to rater bias and requires adequate training and periodic recalibration, particularly in research environments where small differences in scoring may be important
Despite these limitations, the FPI‑6 remains a widely used, pragmatic tool that bridges the gap between purely qualitative visual inspection and more complex quantitative biomechanical analyses. When interpreted within a broader clinical and functional assessment, it provides a structured way to document foot posture, contribute to differential diagnosis, and support evidence‑based decisions about orthoses, footwear, and exercise interventions in both adult and paediatric populations.