# Agreement-Based Validation of ISOMETRO for Upper-Limb Isometric Tension Measurements

## TL;DR

ISOMETRO accurately measures upper-limb isometric strength in a lab setting, matching a reference force plate with high precision.

## Contribution

The study validates ISOMETRO's accuracy for vertical upper-limb isometric force measurements using rigorous statistical methods.

## Key findings

- ISOMETRO showed near-perfect agreement with a force plate for vertical upper-limb isometric force measurements.
- Bland–Altman analysis revealed negligible bias and narrow agreement limits between ISOMETRO and the reference device.
- Linear mixed-effects models confirmed strong predictive accuracy of ISOMETRO measurements (β = 0.999).

## Abstract

What are the main findings?
ISOMETRO demonstrated excellent agreement with an independent force-plate reference for upper-limb isometric tensile force under a standardized vertical laboratory configuration.Bland–Altman analysis, concordance metrics (CCC/ICC), and mixed-effects modeling showed near-unity agreement, negligible systematic bias, narrow limits of agreement, and confirmed internal measurement-chain consistency.

ISOMETRO demonstrated excellent agreement with an independent force-plate reference for upper-limb isometric tensile force under a standardized vertical laboratory configuration.

Bland–Altman analysis, concordance metrics (CCC/ICC), and mixed-effects modeling showed near-unity agreement, negligible systematic bias, narrow limits of agreement, and confirmed internal measurement-chain consistency.

What are the implications of the main findings?
Under controlled vertical alignment, ISOMETRO provides peak-force measurements that closely match an independent criterion reference system.The guided-rail architecture supports standardized laboratory-grade upper-limb tensile testing, although further studies are required to establish reliability and validate additional force directions and real-world applications.

Under controlled vertical alignment, ISOMETRO provides peak-force measurements that closely match an independent criterion reference system.

The guided-rail architecture supports standardized laboratory-grade upper-limb tensile testing, although further studies are required to establish reliability and validate additional force directions and real-world applications.

Muscular fitness is a key component of health and athletic performance, and isometric strength is a widely used indicator. This study reports an agreement-based validation of the Isometric Strength Measurement Device (ISOMETRO) for upper-limb isometric tension measurements under controlled laboratory conditions. Twenty-one healthy young amateur rock climbers (11 men and 10 women) performed four upper-limb tensile tests (shoulder adduction at 90°, shoulder adduction at 60°, shoulder extension at 90°, and elbow extension at 90°). Agreement with an independent criterion device was evaluated using a force plate, while a series-connected load cell was used as an internal consistency check of the measurement chain. Linear mixed-effects models showed that ISOMETRO strongly predicted force plate values (β = 0.999, SE = 0.002, p < 0.001), with a marginal R2 > 0.99. Bland–Altman analysis indicated negligible bias (−0.08 N) and narrow limits of agreement (−4.97 to 4.81 N), and concordance was excellent (CCC ≥ 0.996). The series-connected load cell comparison also showed near-unity agreement (β = 0.998, SE = 0.003, p < 0.001), supporting internal measurement chain integrity. These findings support excellent agreement between ISOMETRO and force plate measurements for upper-limb tensile isometric testing along the vertical axis in young amateur rock climbers under controlled laboratory conditions. However, given the specific sample characteristics and the strictly vertical laboratory configuration, these results should not be generalized to other populations, joint angles, force directions, or non-laboratory environments without further validation. Further studies are needed to confirm performance in more diverse contexts and to establish reliability for repeated-measurement applications.