Association but Limited Agreement Between the My Jump Lab App and the NordBord in Assessing Eccentric Hamstring Function in Soccer Players

Monitoring eccentric hamstring strength is critical for reducing injury risk in soccer players, yet laboratory-based technologies such as isokinetic dynamometry remain costly and impractical for field use. The purpose of this study was to examine the association and exploratory predictive relationship between variables derived from a smartphone application (My Jump Lab) and eccentric hamstring strength outputs obtained with an instrumented field device (NordBord, Vald Performance, Australia), while also quantifying their absolute agreement during the Nordic hamstring exercise (NHE). Thirty-one professional soccer players from a second-division United Arab Emirates team performed the NHE on the NordBord, while a simultaneous two-dimensional (2D) kinematic analysis was conducted using the My Jump Lab app (version 5.0 for iOS; My Jump Lab, Madrid, Spain). Pearson correlations, linear regression models, and Bland–Altman analyses were used to distinguish linear association/prediction from agreement/interchangeability. Results revealed a very large association between My Jump Lab-derived torque estimates and NordBord peak torque (r = 0.77, p < 0.001), with moderate associations for breakpoint angle (r = 0.42–0.43). A combined regression model using My Jump Lab torque and breakpoint angle explained 69.2% of the variance in NordBord torque (SEE = 15.30 N·m), although this predictive result should be interpreted as exploratory because the variables are task-specific and partly share anthropometric and mechanical determinants. Bland–Altman analysis revealed poor agreement, with a large systematic difference and proportional bias, indicating that My Jump Lab overestimated torque values at higher strength levels (mean bias = +511.9 N·m). Therefore, torque values derived from the app should be interpreted as relative indicators rather than absolute equivalents to instrumented measurements. From a practical perspective, My Jump Lab may offer a low-cost option for broad screening or relative group profiling when instrumented devices are unavailable, but it should not be used as a substitute for instrumented devices or for individual longitudinal monitoring based on absolute torque values.