Erector spinae SEMG activity during forward flexion and re-extension in ankylosing spondylitis patients

Objective: To compare erector spinae muscle (ESM) activity as measured by surface electromyography (SEMG) in lumbar flexion from the upright position in men with ankylosing spondylitis (AS) and healthy males, and to study associations between pain, lumbar mobility and ESM activity. Methods: Surface EMG was undertaken at the L1-2 and L4-5 levels in 11 men with AS taking part in a rehabilitation course at the Rheumatism Foundation Hospital, and in 10 pain-free male controls, while the subjects were bending forward. Results: During full flexion ESM SEMG activity was significantly greater in patients with AS than in the controls. Relaxation was evident during flexion in all of the controls but in only some patients with AS. Lumbar mobility correlated negatively with ESM activities. No relationship between pain and ESM activity was evident. Some AS patients reported pain while ESM activity was being measured. Conclusion: Decreased lumbar mobility rather than pain explains ESM activity during full flexion in patients with AS.