Flexion-relaxation and clinical features associated with chronic low back pain: A comparison of different methods of quantifying flexion-relaxation.

OBJECTIVES: The purpose of this study was to simultaneously assess 5 surface electromyography (SEMG) ratios commonly used to quantify the flexion relaxation phenomenon in chronic low back pain patients relative to clinical and musculoskeletal abnormalities.
METHODS: Seventy-six persons with low back pain (LBP) were assessed through SEMG in standing, flexion, maximum voluntary flexion, and extension. Additionally, participants were assessed for clinical status (pain intensity, perceived disability, and pain-related fear) and musculoskeletal abnormalities (supine straight leg raise, Faber test, and lumbar restrictions).
RESULTS: Flexion-relaxation assessed as the ratio of maximum SEMG during flexion to average SEMG during maximum voluntary flexion, and maximum SEMG during extension to average SEMG during maximum voluntary flexion, demonstrated that highest associations with clinical and musculoskeletal status. Ratio of maximum SEMG during flexion to average SEMG during maximum voluntary flexion was significantly associated with 2 measures of perceived disability (both rs=-0.31, P<0.01), pain-related fear (r=-0.45, P<0.001), as well as range of motion during flexion (r=0.51, P<0.001), and elicitation of pain during straight leg raise (r=0.30, P<0.05). Ratio of maximum SEMG during extension to average SEMG during maximum voluntary flexion was also significantly associated with these same measures, at a slightly higher magnitude, in addition to a measure of clinical pain (r=-0.29, P<0.05). DISCUSSIONS: These data suggest that measures of flexion-relaxation that contrast SEMG during flexion or extension to MVF are more highly associated with clinical status compared with measures that contrast SEMG during flexion and extension, SEMG whereas standing to MVF, or SEMG during MVF alone.