The purpose of this study was to establish the intra-rater and inter-rater reliability of isometric shoulder strength assessment using a hand-held dynamometer (HHD) in functional joint positions in student circus artists with symptomatic atraumatic shoulder instability. METHODS: Over two testing sessions, two experienced physiotherapists assessed the shoulder strength of 24 student circus artists with clinically diagnosed atraumatic shoulder instability. Both the symptomatic and asymptomatic shoulder was assessed using a HHD in 10 functional positions. Intra-class correlation coefficients (ICCs) were calculated to determine the reliability of strength measurements. RESULTS: All examined positions showed moderate-high intra-rater and inter-rater reliability. External rotation at 0° and internal rotation in horizontal flexion at 45° revealed the most reliable results, and the shrug position the least reliable. Inter-rater and intra-rater reliability was high and demonstrated similar results in symptomatic and asymptomatic shoulders by both raters. DISCUSSION: This study demonstrated clinical applicability in reliably measuring functional strength in symptomatic atraumatic instability or asymptomatic shoulders when assessed by experienced therapists using an HHD.