Does the personal lift-assist device affect the local dynamic stability of the spine during lifting?

The personal lift-assist device (PLAD) is an on-body ergonomic aid that reduces low back physical demands through the restorative moment of an external spring element, which possesses a mechanical advantage over the erector spinae. Although the PLAD has proven effective at reducing low back muscular demand, spinal moments, and localized muscular fatigue during laboratory and industrial tasks, the effects of the device on the neuromuscular control of spinal stability during lifting have yet to be assessed. Thirty healthy subjects (15M, 15F) performed repetitive lifting for three minutes, at a rate of 10 lifts per minute, with and without the PLAD. Maximum finite-time Lyapunov exponents, representing short-term (λmax-s) and long-term (λmax-l) divergence were calculated from the measured trunk kinematics to estimate the local dynamic stability of the lumbar spine. Using a mixed-design repeated-measures ANOVA, it was determined that wearing the PLAD did not significantly change λmax-s (μNP=0.335, μP=0.321, p=0.225), but did significantly reduce λmax-l (μNP=0.0024, μP=−0.0011, p=0.014, η2=0.197). There were no between-subject effects of sex, or significant interactions (p>0.720). The present results indicated that λmax-s was not statistically different between the device conditions, but that the PLAD significantly reduced λmax-l to a negative (stable) value. This shows that subjects’ neuromuscular systems were able to respond to local perturbations more effectively when wearing the device, reflecting a more stable control of spinal movements. These findings are important when recommending the PLAD for long-term industrial or clinical use.