Local dynamic stability of trunk movements during the repetitive lifting of loads.

The local dynamic stability of trunk movements was assessed during repetitive lifting using nonlinear Lyapunov analyses. The goal was to assess how varying the load-in-hands affects the neuromuscular control of lumbar spinal stability. Thirty healthy participants (15M, 15F) performed repetitive lifting at 10 cycles per minute for three minutes under two load conditions: zero load and 10% of each participant's maximum back strength. Short- and long-term maximum finite-time Lyapunov exponents (λ(max-s) and λ(max-l)), describing responses to infinitesimally small perturbations, were calculated from the measured trunk kinematics to estimate the local dynamic stability of the system. Kinematic variability was also assessed using mean standard deviations (MeanSD) across cycles. The results of a mixed-design repeated-measures ANOVA showed that increasing the load lifted significantly reduced λ(max-s) (μ(0%-LOAD)=0.379, μ(10%-LOAD)=0.335, p<.001), but not λ(max-l) (μ(0%-LOAD)=0.46E-03, μ(10%-LOAD)=2.41E-03, p=.055) or MeanSD (μ(0%-LOAD)=2.57, μ(10%-LOAD)=2.89, p=.164). There were no between-subject effects of sex, or significant interactions (α<.05). The present findings indicated improved dynamic spinal stability when lifting the heavier load; meaning that as muscular and moment demands increased, so too did participants' abilities to respond to local perturbations. These results support the notion of greater spinal instability during movement with low loads due to decreased muscular demand and trunk stiffness, and should aid in understanding how lifting various loads contributes to occupational low back pain.