Results: Seventeen subjects (5 male and 12 female) participated; each having two intact thumbs. To detect a reduction of radial subluxation of 2.0 mm (SD 1.25), which is in agreement with Cheema et al. (2012) with a two-tailed 5% significance level and a power of 90%, a minimum sample size of 12 subjects was needed. Average age was 25.9 (21-59). Thirteen right handed, one left handed, and 3 ambidextrous subjects were included. Average maximal voluntary contraction of the FDI was 27N. Inter-rater reliability of subluxation scores across the three surgeons was high (ICC > .74). Twenty-eight thumbs demonstrated subluxation when stressed with reduction after activation of the FDI. Three thumbs were not subluxed at rest and did not sublux with stress or reduce with firing of the FDI, consistent with stiff CMC joints. Two thumbs were subluxed at rest but did not further sublux with stress. In the 27 thumbs that demonstrated increased subluxation with stress, subluxation while stressed averaged 0.6 cm (0.4-0.9) or 49% (29-75) of articular width. Across all 32 thumbs, FDI activation appeared to significantly reduce subluxation of a radially-stressed CMC by an average of 0.41 cm (t = 10.8, p <.001, d = 1.9) and the ratio of subluxation to articular width improved from .42 to .09 (t = 10, 3, p <.001, d = 1.9) after FDI activation (Table 1). When the CMC joint was stressed and FDI activated, maximum FDI strength explained 39% of the variability in subluxation (F(l,30) = 18.34, p <.001). Conclusion: Activation of the FDI appears to radiographically reduce subluxation of the thumb CMC joint. Strengthening the FDI may assist in preventing and managing thumb arthritis sympto-mology. Although the strength of the FDI explained much of the variability in the reducibility of a subluxed joint, factors such as joint laxity, stiffness, and topography are also likely factors which predict reducibility. Further exploration of the FDI's effect on reducibility; its carry-over effects on reduction; and, if warranted, the 'just-right' dosage of FDI training in clinical populations is needed. Purpose: Early in vitro biomechanical studies describe the superficial head of the First Dosal Interosseus (FDI) to stabilize the thumb Carpometacarpal (CMC) joint (Brand & Hollister, 1993). Boutan (2000) later report the force coupling effect of the FDI and the opponens pollicis on stabilizing the thumb CMC joint in an in vivo electromyographic study. Based on these principles, a dynamic thumb stabilization program, which includes training of the FDI, was developed to treat those with thumb CMC osteoarthritis and first metacarpal (MC) radial instability. When combined with comprehensive hand therapy, new evidence supports that this approach is effective in reducing pain and disability (O'Brien & Giveans, 2013). To date, however, the role of the FDI has not yet been validated by radiographic evidence. The purpose of the present study was to determine if activation of the first dorsal interosseous (FDI) reduces radial subluxation of the first metacarpal on the trapezium as measured by fluoroscopy. Methods: Seventeen healthy adults (12 female, 5 male; 32 hands), 18 years or older participated. Exclusion criteria included positive grind test, pregnancy, and major conditions of Iigamentous laxity. A certified hand therapist (CHT) performed a grind test on all subjects. Two hands were excluded due to a positive grind. Fluoroscopy was used to obtain true AP radiographs of the CMC joint: 1) at rest, 2) while stressed without activation of the FDI and 3) while stressed with maximal resistance of the FDI. A CHT maximally resisted 2nd MC abduction while quantifying maximal FDI strength via use of the Rotterdam Intrinsic Hand Myometer (RIHM) (Figure 1). Radial subluxation of the base of the first metacarpal and metacarpal width was measured by 3 blinded surgeons as described by Wolfe (2010) (Figure 2). The ratio of radial subluxation to the articular width was calculated. SPSS ver. 22 was used to run descriptive and inferential statistics (within subjects T-Tests, and Cohen's d).