The objective of this study is to translate, cross-culturally adapt, and validate the Spanish version of the Foot Posture Index (FPI-6). This clinical tool is widely used to evaluate standing foot posture through six visual criteria. The study aims to provide Spanish-speaking clinicians and researchers with a reliable and valid instrument to assess foot posture in the Spanish population, ensuring that the psychometric properties of the original scale are preserved

This study follows a prospective, observational design and will be carried out in two phases:

Phase 1: Translation and Cross-Cultural Adaptation. Following international standards (forward-backward translation method), the original English FPI-6 will be translated into Spanish by two independent translators. A synthesis version will be created, followed by a back-translation into English to ensure conceptual equivalence. A pilot test will be conducted to ensure the clarity of the items.

Phase 2: Validation and Psychometric Evaluation. The Spanish version of the FPI-6 will be administered to a sample of participants. To evaluate reliability, intra-rater and inter-rater reliability will be calculated using the Intraclass Correlation Coefficient (ICC). Internal consistency will be assessed using Cronbach's alpha. Construct validity will be analyzed by comparing FPI-6 scores with other validated clinical measures of foot posture

Inclusion Criteria:

• Participants aged between 6 and 89 years
• Ability to understand and sign the informed consent form. In the case of minors, written informed consent from parents or legal guardians and child's assent (where applicable) are required
• Ability to maintain a relaxed standing position unassisted for the duration of the clinical assessment

Exclusion Criteria:

• History of major surgery or significant trauma in the lower limbs or feet.
• Presence of open wounds or active skin infections on the feet at the time of the examination.
• Neurological or musculoskeletal conditions that severely affect balance or standing posture.
• Cognitive impairment that prevents the participant from following simple clinical instructions

The study population consists of individuals from the general population in Spain, covering a wide age range that includes children (from 6 years old), adults, and the elderly (up to 89 years). Participants are recruited through convenience sampling from university clinics and local community centers to ensure a diverse representation of foot types (neutral, pronated, and supinated) across different stages of life