Alterations in trunk bending stiffness following changes in stability and equilibrium demands of a load holding task

Abstract

The contribution of the trunk neuromuscular system (TNS) to spine stability has been shown in earlier studies by characterizing changes in antagonistic activity of trunk muscles following alterations in stability demands of a task. Whether and/or how much such changes in the response of TNS to alteration in stability demand of the task alter spinal stiffness remains unclear. To address this research gap, a repeated measure study was conducted on twenty gender-balanced asymptomatic individuals to evaluate changes in trunk bending stiffness throughout the lumbar spine's range of flexion following alterations in both stability and equilibrium demands of a load holding task. Trunk bending stiffness was determined using trunk stiffness tests in upright posture on a rigid metal frame under different equilibrium and stability demands on the lower back. Increasing the stability demand by increasing the height of lifted load ∼30 cm only increased trunk bending stiffness (∼39%) over the lower range of lumbar flexion and under the low equilibrium demand condition. Similarly, increasing the equilibrium demand of the task by increasing the weight of lifted load by 3.5 kg only increased trunk bending stiffness (55%) over the low range of lumbar flexion and under the low stability demand condition. Our results suggest a non-linear relationship between changes in stability and equilibrium demands of a task and the contribution of TNS to trunk bending stiffness. Specifically, alterations in TNS response to changes in stability and equilibrium demand of a given task will increase stiffness of the trunk only if the background stiffness is low.

Trunk Muscle Activation Patterns Differ Between Those With Low and High Back Extensor Strength During a Controlled Dynamic Task

“…This study expands on this model to hypothesize that individuals with lower active spinal system function (WEAK) might experience greater adaptations to ensure spinal stability that is beyond a margin of safety, whereas those with higher active system function (STRONG) have fewer adjustments. This hypothesis is consistent with recent work showing that changes in trunk muscle activation was greatest when the spine has lower stiffness presenting a higher stability demand (Shojaei et al, 2018). While the results of this study are not definitive, they do support and expand upon the motor adaptation to pain theory.…”

In vivo through-range passive stiffness of the lumbar spine: a meta-analysis of measurements and methods

“…Here the angle of the platform which moved the legs was used to represent the ROM, and the moment was derived from the force applied to a pressure sensor located behind the torso, detecting changes in the amount of force with which the torso involuntarily attempted to extend as a result of the legs being raised. This clear difference in methodology yielded consistently higher stiffness values being reported by Shojaei et al [15] across the first three quartiles, most significantly the first quartile. The mean reported by Shojaei et al [15] for the first quartile was over double that of the next nearest value [10] and nearly five times that of the lowest value [17].…”

“…Included within this study were thirteen articles, ten of which investigated flexion/extension [2,4,5,7,9,10,15,17,18,20], four that studied lateral bending [6,9,10,20] • Assessed flexion in standing • Participants stood in a rigid metal hinged frame with their torso fixed in an upright position using a harness connected to a rigid rod • The participants legs and pelvis were constrained to the bottom half of the frame • The frame was adjusted such that the frame's hinge was aligned with the participant's S1 spinal level…”

“…In the studies investigating flexion in the quantitative synthesis, Shojaei et al's study [15] was the only one to perform their assessment in standing, whereas the others all performed their stiffness assessments in sidelying. This may therefore further explain the relatively large reduction in mean stiffness when Shojaei et al's [15] results were removed from the comparison. This is in keeping with in vitro studies of the spine where stiffness characteristics increase in the spine with greater compressive loading [21].…”

“…Within the ten articles (Table 2) that studied passive flexion/ extension stiffness, eight conducted their measurements in side-lying [2,4,5,7,9,10,17,20] and two performed theirs in standing [15,18]. Six of these [4,7,10,15,17,20] presented their values in a format suitable for comparison with the others in this context. The synthesised weighted mean stiffnesses and confidence intervals for the quartiles can be seen in Table 5 and graphically represented in Fig.…”