The structural and mechanical properties of tendons influence postural balance and physical function. Changes in patellar tendon (PT) stiffness have been shown to occur in aging, with training and following stretching exercises, and there are differences of PT stiffness between genders (Reeves et al. 2003, Onambélé et al. 2007). However, PT stiffness has not been assessed in inflammatory arthropathies. This study therefore investigated the PT properties in stable rheumatoid arthritis (RA) and ankylosing spondylitis (AS) patients. We compared 18 RA patients (13 women, 59.0 ± 2.8 yrs, mean ± SEM) with 18 age- and sex-matched healthy controls (58.2 ± 3.2 yrs), and 7 male AS patients (54.9 ± 2.4 yrs) with 7 healthy men (56.0 ± 5.4 yrs) with paired t-tests. Force production was measured during ramped isometric voluntary knee extension contractions (MVCs) on an isokinetic dynamometer, with concurrent electromyographic recording of vastus lateralis and of biceps femoris activity to account for antagonist co-contraction torque. Resting PT length and cross-sectional area (CSA), and force-related PT elongation were assessed by ultrasonography to determine PT stiffness and Young’s modulus (YM = PT stiffness x PT length / PT CSA) (Reeves et al. 2003). Lower body physical function was assessed using one-leg standing balance, 8-foot up and go, 50-foot walk and 30-second sit to stand tests. The study had local NHS ethics committee approval. PT stiffness was significantly lower in RA patients compared to healthy controls by 26.6% (p = 0.04). There was no difference in PT CSA. YM tended to be lower in RA by 19.6%, and force production by 10.1%, but these did not reach statistical significance (p = 0.26 and 0.13, respectively). Physical function was reduced by 16-25% in the RA group. In the comparison of AS patients with healthy controls PT stiffness tended to be lower by 21.1%, albeit not significantly (p = 0.16). However, PT CSA was significantly larger in AS by 25.9% (p = 0.01), leading to a significant reduction in YM in AS by 38.3% (p = 0.02). Force production and physical function were similar in AS patients and their healthy controls. In conclusion, although the PT size and force are not reduced in RA, there is an impairment of PT stiffness which could possibly contribute to the reduced physical function observed in RA patients. In AS, however, a thickening of the PT was observed and, with a tendency of reduced PT stiffness, a significantly reduced YM was found, although this was not accompanied by lower body functional impairment. Thus, stable RA and AS patients display differing alterations of tendon characteristics and lower body function, perhaps due to their dissimilar pathogeneses.