Objective physical function of the lower body was assessed by the 30-second chair sit-to-stand, the 8-foot-up-and-go [20], the 50-foot-walk, and single-leg balance tests [21]. The Modified Health Assessment Questionnaire (MHAQ) [22] and the 36 questions of the Short-Form Health Survey (SF-36) [23] provided information on subjective physical function and health-related quality of life (QoL), respectively. These questionnaires and physical function tests have been used in RA and AS populations before [24�C28].2.3. Setup for Quadriceps Muscle and Patella Tendon MeasurementsParticipants sat upright on an isokinetic dynamometer (CSMI Medical Solutions, Stoughton, MA, USA) with their right leg strapped to the dynamometer arm above the ankle. Additional straps were secured to prevent extraneous movement at the hips and shoulders. The knee joint angle was fixed at 90�� from full leg extension and the hip angle at 90�� [29]. PT stiffness was then determined using the method of Onambele-Pearson and Pearson [2]. After a set protocol of warm-up contractions, participants performed three ramped maximal voluntary isometric knee extension contractions (MVC), building up to maximum force with increasing effort over 4-5 seconds. During these contractions, participants crossed their arms over their chest to avoid the addition of arm muscle force to the quadriceps force measurements. Verbal encouragement was given. The US 7.5MHz linear probe (MyLab50, Esaote, Firenze, Italy) was positioned sagittally over the PT and three video clips were recorded of PT excursion from the proximal and the distal attachments of the tendon to the bone, respectively (Figure 1). An external marker was fixed on the skin to detect accidental movement of the probe against the skin; when this occurred, recordings were repeated. The recordings were aligned by synchronization of force and US data. Real-time display of muscle torque on a computer screen provided feedback to the participants, and at least 1 minute rest between each MVC helped to minimise fatigue. US images were analysed using digitizing software (ImageJ, NIH, Bethesda, MD, USA), with the assessor blinded to the participant’s disease status.Figure 1Illustration of patellar (P) tendon (PT) elongation. PT elongation shown from a skin marker (vertical line) to the tendon insertion at the patella (star) using US at rest (a) and during contractions ((b) and (c)). 2.4. Calculation of Quadriceps Muscle ForceThe MVC with the highest torque was used for analysis. Calculation of quadriceps muscle force accounted for torque, PT moment arm length [2], and antagonist cocontraction (which was estimated from electromyographic (EMG) activity [2, 30]).