INTRODUCTION
Maintenance of balance when sitting on an unstable surface requires coordinated out-of-phase motion of the lumbar spine and provides a sensitive method to study the quality of trunk postural control in individuals with low back pain (LBP). This study investigated whether quality of hip and spine coordination to maintain balance in individuals with acute LBP predicts recovery at six-months.
METHODS
Participants in the early acute phase (<2 weeks) of an LBP episode (n=94) underwent baseline assessment of balance while sitting on an unstable surface. Seat, hip and spine (lower-lumbar, lumbar, upper-lumbar, thoracic) angular motion and force plate data were recorded. The coordination between the seat and hip or spine segments was quantified using cross-spectral (frequency-domain) analyses to describe the relation between two signals (e.g., body segments) with respect to their relative timing (phase angle; in-phase [0°] = both segments move together, out-of-phase [180°] = one segment moves opposite to the other segment) and the consistency of this timing (coherence; 1 = consistent relation, 0 = inconsistent relation). Center of pressure (CoP) and upper thorax motion were calculated to assess overall balance performance. Hip and spine coordination with the seat were compared between those who did not recover from baseline (increased/unchanged pain/disability), partially-recovered (reduced pain and/or disability) and recovered (no pain and disability) at six-months.
RESULTS
In the sagittal plane, coherence between the seat and lower-lumbar spine was lower at baseline in those who went on to be unrecovered (coherence=0.25) and partially-recovered (coherence=0.34) from their pain/disability after six-months than recovered participants (coherence=0.45). In conjunction with low coherence, unrecovered participants moved the lumbar spine in the same direction (more towards in-phase; 57°) as the seat which does not counteract the postural disturbance, while recovered participants used out-of-phase (112°; opposite direction to the seat) movement. In the frontal plane, coherence between the seat and lumbar spine was lower in unrecovered (coherence=0.62) than recovered participants (coherence=0.78), and again this was less out-of-phase (144°) than recovered participants (173°). Coherence between the seat and hip was higher in partially-recovered than recovered participants in both planes, which implies that the hip might compensate for poor lumbar spine coherence in partially-recovered participants. Although the three LBP groups were equally successful in overall balance performance (no differences between groups in CoP and upper thorax motion), partially-recovered and unrecovered participants used a less than optimal strategy that might have consequences for long term health of the spine.
DISCUSSION
These longitudinal data revealed that individuals who demonstrated a compromised contribution of the lumbar spine to the balance maintenance when sitting on an unstable surface during an acute LBP episode are less likely to recover in the long term. These results highlight a potential role for trunk neuromechanics in clinical outcomes.