Comparing Foot-Type-Matched and Neutral Insoles for Foot Discomfort in Factory Workers

BACKGROUND Prolonged standing at work has been associated with discomfort in the feet and lower limbs. Prefabricated insoles are often supplied to industrial workers, but it is uncertain whether selecting the insole model according to foot type adds benefit compared with a neutral insole worn in the same footwear.

PROJECT STRUCTURE This trial was the second phase of a two-phase project conducted at the BSH Electrodomésticos España plant in Montañana (Zaragoza, Spain). In phase 1 (cross-sectional assessment, May to November 2024), foot posture (Foot Posture Index, FPI-6), three-dimensional foot morphology (DOMESCAN 3D scanner), plantar pressures and anthropometric variables were recorded. During the phase-1 assessment, workers signed a written informed consent form for the intervention phase.

FOOT-TYPE CLASSIFICATION Workers were grouped into three foot types (pronated, neutral and supinated) by k-means clustering (k = 3) of the standardized FPI-6 total score and a truncated arch height index (instep height divided by truncated foot length, heel to first metatarsal head) obtained from the 3D scan, using the whole phase-1 sample. The cluster labels used in this trial were those recorded in December 2024, before allocation. The approved protocol did not specify the classification algorithm. Cluster membership is not equivalent to the clinical FPI-6 categories: some workers in the pronated or supinated clusters had FPI-6 total scores in the neutral range. Workers in the neutral cluster were not eligible.

RANDOMIZATION AND ALLOCATION Eligible workers were randomized 1:1 to the foot-type-matched or the neutral insole before insole delivery, using blocks stratified by foot-type cluster, sex and body mass index. The allocation list was recorded in March 2025. The company occupational health service delivered the insoles according to the allocation list. Participants were not told which insole type they had received; the staff who delivered the insoles were aware of the allocation. Analyst blinding was not documented, and the success of blinding was not assessed.

INTERVENTION Both groups wore the same safety shoe model (Galena, Paredes Seguridad, Spain). The experimental group received a prefabricated TSTEP insole (Paredes Seguridad, Spain) matched to the foot-type cluster: the pronator model for workers in the pronated cluster and the supinator model for workers in the supinated cluster. The control group received the neutral TSTEP insole, of the same material and characteristics as the foot-type-specific models. Insole delivery dates, wearing time, adherence and tolerability were not recorded in the analysis dataset.

OUTCOME ASSESSMENT Musculoskeletal discomfort during the previous working week was self-reported with a Spanish-language version of the Cornell Musculoskeletal Discomfort Questionnaire (CMDQ), completed online through a link sent by e-mail by the company occupational health service. According to the protocol, the baseline questionnaire was to be completed at insole delivery and the follow-up questionnaire three months later. The outcome questionnaires were completed between 22 October 2025 and 3 February 2026; in participants who completed two questionnaires, the median interval between them was 57 days (range 44 to 103 days). Because insole delivery dates were not recorded, the timing of the questionnaires relative to insole delivery cannot be verified for each participant, and a three-month wearing period cannot be assumed.

CHANGES FROM THE APPROVED PROTOCOL The approved protocol (version 3, 20 November 2024) specified: (1) CMDQ musculoskeletal discomfort as the primary variable, without defining a body region; (2) within-participant comparisons before and after insole delivery (paired t test or Wilcoxon signed-rank test); (3) stratified block randomization by foot type, sex, age and body weight, generated with randomizer.org by an independent investigator not involved in recruitment; (4) follow-up three months after insole delivery; and (5) a sample size of 6 participants per group (GRANMO), based on the CMDQ discomfort index.

As conducted and reported: (1) randomization was stratified by foot-type cluster, sex and body mass index; the use of randomizer.org and the independence of the person who generated the allocation sequence are not documented; (2) no statistical analysis plan other than the approved protocol is documented before data collection; in the trial report, written after data collection, the primary outcome was the bilateral foot CMDQ score (sum of the right-foot and left-foot regional scores), which was not prespecified in the approved protocol, and the primary analysis was a between-group analysis of covariance adjusted for baseline score, sex, body mass index and foot-type cluster in participants with two questionnaires, with a sensitivity analysis including all participants with at least one questionnaire; within-participant comparisons were reported descriptively; (3) the observed interval between questionnaires was shorter than the planned three months (see above); (4) the number of participants randomized was determined by the eligible workers available, not by the protocol sample size calculation; and (5) analyses were performed in Python 3 rather than IBM SPSS.

REGISTRATION This trial was not registered before enrolment. This record was created retrospectively, after completion of data collection, to make the trial and its deviations from the approved protocol publicly available.

STUDY START DATE The study start date in this record (22 October 2025) is the date of the first trial questionnaire, which also recorded electronic agreement to participate in the trial. Written informed consent for the intervention phase had been signed during the phase-1 assessment (2024), and the allocation list was recorded in March 2025.

FUNDING The safety shoes and insoles were purchased by BSH Electrodomésticos España S.A. Publication costs are funded by the Cátedra Empresa Sana.

NOTES ON THE APPROVED PROTOCOL An English translation of the approved protocol (version 3) is provided in the study documents. Its first page lists the investigators' clarifications of clerical errors in the approved text: the funding years (2014-2015 should read 2024-2025), the meaning of the Annex I declaration, the summary statistic for non-normal variables (median rather than mean) and the source of the standard deviation used in the sample size calculation. The protocol assigned pseudonymization and data analysis to different team members; in the trial, both were performed by the same investigator (A. J. Almenar-Arasanz).

ETHICS The protocol was approved by the Research Ethics Committee of Aragón (CEICA, reference PI24/176; favourable opinion 17 April 2024; amendment approved 4 December 2024, protocol version 3). The doctoral thesis project also received a favourable report from the Ethics Committee of Universidad San Jorge (report 53/2/23-24, 18 March 2024).