The role of an active muscular subsystem in prone instability test during rest and leg raise conditions

BACKGROUND: Clinicians commonly used prone instability test (PIT) by assessing the posterior-to-anterior (PA) displacement to identify lumbar instability. Most studies focusing on passive subsystem found greater mobility in lower lumbar (L4-L5) than upper lumbar (L1-L3) spine. However, there is still a lack of evidence to demonstrate the role of active subsystem. Additionally, it is unclear whether sex affects PA displacements. AIM: To determine differences in displacement among five lumbar segments, between two testing positions (rest and leg raise), and between male and female during PIT in individuals with chronic non-specific low back pain (CNLBP). DESIGN: A cross-sectional study design. SETTING: Spine biomechanics laboratory. POPULATION: Individuals with CNLBP. METHODS: An electromagnetic tracking system was used to measure PA displacement with sensors attached at T12, S2 and a hand-held dynamometer. Participants were asked to perform PIT, while a 100N force was applied to each lumbar segment during resting and leg raise positions. RESULTS: Significantly less PA displacement (P<0.05) was seen in lower compared to upper lumbar spine and in leg raise compared to rest at L1 to L4. No significant interaction of sex with different lumbar levels and conditions (P>0.05) during PIT was found. CONCLUSIONS: Although previous studies have reported that the lower lumbar spine had greater mobility, the lower amount of displacement during the rest position suggests the role of an active subsystem contributing to lumbar stability regardless of sex. CLINICAL REHABILTATION IMPACT: A reduction in displacement during the leg raise position across L1 to L4 suggesting an interaction of stabilizing subsystems of the spine to provide lumbar stability.