Low-intensity functional electrical stimulation can increase multidirectional trunk stiffness in able-bodied individuals during sitting

The inability to voluntarily control the trunk musculature is a major problem following spinal cord injury as it can compromise functional independence and produce unwanted secondary complications. Recent developments suggest that neuroprostheses utilizing functional electrical stimulation (FES) may be able to facilitate or restore trunk control during sitting, standing, and other tasks involving postural control. In spite of these efforts, no study to date has used low-intensity FES to increase multidirectional trunk stiffness and damping in an attempt to bolster stability while minimizing muscle fatigue. Therefore, we set out to investigate how multidirectional trunk stiffness changes in response to low-intensity FES of a few selected trunk muscles. Fifteen healthy participants sitting naturally were randomly perturbed in eight horizontal directions. Trunk stiffness and damping during natural and FES-supported sitting conditions were quantified using force and trunk kinematics in combination with two models of a mass-spring-damper system. Our results indicate that low-intensity FES can increase trunk stiffness in healthy individuals, and this specifically for directions associated with the stimulated muscles. In contrast, trunk damping was not found to be altered during FES. The presented results suggest that low-intensity FES is a simple and effective method for increasing trunk stiffness on demand.