Reliability of Custom-Made Platform for Trunk Flexion Isometric Strength Assessment

The custom-made platform is a standing device designed to measure maximal voluntary isometric trunk flexion force. During testing, participants stand on the platform and lean forward to a standardised angle of 17.5 degrees of trunk flexion. A trunk belt connected to a handheld dynamometer fixed to the rear platform records the force generated when the participant pushes forward against the belt during a maximal isometric contraction. Force is recorded in Newtons using Biopac Student Lab Pro software through the MP36 data acquisition unit.

A cross-sectional test-retest reliability design will be employed. Twenty-nine healthy male participants aged 18-25 years will be recruited from Riphah International University using non-probability convenience sampling. Each participant will attend two identical testing sessions spaced one week apart, conducted at the same time of the day to control for diurnal variation in muscle strength. During each session, participants will perform three maximal voluntary isometric trunk flexion contractions of five seconds duration, with sixty seconds of passive rest between trials. The average force across the three trials will be recorded in Newtons and used as the primary outcome variable for analysis.

Reliability will be assessed using the Intraclass Correlation Coefficients with two-way random effects model for absolute agreement, single measures - ICC (2,1) - reported alongside the 95% confidence interval. Values below 0.50 will be interpreted as poor reliability, 0.50 to 0.75 as moderate, 0.75 to 0.90 as good, and above 0.90 excellent. Absolute reliability will be quantified using the Standard Error of Measurement and the Minimal Detectable Change at 95% confidence level. Bland-Altman analysis will be used to assess systematic bias and limits of agreement between sessions. A paired t-test or Wilcoxon signed-rank test, depending on normality assessed by the Shapiro-Wilk test, will determine whether a statistically significant learning effect exists between Session 1 and Session 2. All statistical analyses will be conducted using SPSS Version 30 at a significance level of 0.05.

If the platform demonstrates good to excellent reliability, it will be validated as a feasible and affordable alternative to isokinetic dynamometry for trunk flexion isometric strength assessment, with direct implications for physiotherapy practice and sports science research in Pakistan.