OBJECTIVE: To quantify resting lumbar erector myofascial stiffness in younger patients with ankylosing spondylitis (AS) and age-comparable healthy control subjects using a handheld mechanical impulse-based myotonometric device. DESIGN: A case-control study of 24 patients with AS and 24 age-comparable healthy control subjects. SETTING: University physical therapy department. PARTICIPANTS: Patients with AS (men: n=19; women: n=5; total: N=24) and healthy volunteers (men: n=19; women: n=5; total: N=24) without low back pain (age range, 18-46y). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURE: Lumbar myofascial stiffness. RESULTS: At the initial measurements, median stiffness (Nm) of the averaged right- and left-sided values was greater (P=.021) in 24 patients with AS than 24 control subjects (268.9 vs 238.9, respectively). Repeated measurements after a 10-minute prone resting period were also greater (P=.007) in patients with AS than control subjects (281.0 vs 241.4, respectively). The 48 averaged right- and left-sided values from baseline and 10-minute measurements were compared in each subject group. The patients with AS more frequently (P=.012) had stiffness values >250 Nm (35 [72.9%] vs 22 [45.8%] in control subjects). CONCLUSIONS: Lumbar myofascial stiffness was greater in 24 patients with AS than in the control subjects. A hypothesized biomechanical concept of increased resting lumbar myofascial stiffness in AS may be supported by this preliminary controlled study.