Accurate measurement of internal/external rotation joint angle is critical in assessing the shoulder function, especially in the clinical practice as it plays a key role in evaluating activities of daily living and monitoring the rehabilitation progress. This study analyzed the effectiveness of using a marker cluster placed over the proximal epiphysis of the ulna to measure humeral axial rotation with respect to the thorax, comparing it with the traditional method that uses a cluster placed on the upper arm. Data were collected simultaneously using the proposed indirect approach and a conventional marker cluster to analyze three internal/external rotations performed in the Ski-Pose, frontal, and sagittal plane. Linear regressions for time series comparison reported a coefficient of determination R² > 0.9919 in all tasks. The linear coefficients (a₁) were as follows:​ Ski-Pose (a₁ = 0.64 ± 0.10), frontal plane (a₁ = 0.74 ± 0.05), and sagittal plane (a₁ = 0.73 ± 0.04). Three additional planar tasks were recorded for concurrent validity and RMSE was reported for the main joint angle, obtaining a maximum of 3.87° for the pure flexion/extension task and 1.94° for the abduction/adduction task. A forearm pronation/supination task without axial rotation yielded a maximum error standard deviation of 2.64°. Proximal ulna tracking showed a statistically higher maximum range of motion than humeral tracking in pure axial rotation tasks. This indirect tracking approach is a promising alternative to the traditional cluster technique due to its reduced sensitivity to soft tissue artifacts.