Feeling stiffness in the back: a protective perceptual inference in chronic back pain

Abstract

Does feeling back stiffness actually reflect having a stiff back? This research interrogates the long-held question of what informs our subjective experiences of bodily state. We propose a new hypothesis: feelings of back stiffness are a protective perceptual construct, rather than reflecting biomechanical properties of the back. This has far-reaching implications for treatment of pain/stiffness but also for our understanding of bodily feelings. Over three experiments, we challenge the prevailing view by showing that feeling stiff does not relate to objective spinal measures of stiffness and objective back stiffness does not differ between those who report feeling stiff and those who do not. Rather, those who report feeling stiff exhibit self-protective responses: they significantly overestimate force applied to their spine, yet are better at detecting changes in this force than those who do not report feeling stiff. This perceptual error can be manipulated: providing auditory input in synchrony to forces applied to the spine modulates prediction accuracy in both groups, without altering actual stiffness, demonstrating that feeling stiff is a multisensory perceptual inference consistent with protection. Together, this presents a compelling argument against the prevailing view that feeling stiff is an isomorphic marker of the biomechanical characteristics of the back.

Figure 1

Study 1a results. (a) Picture of the indentor and participant set-up (b). Scatterplots depicting the correlation between feelings of stiffness and objective measures of stiffness in people with chronic back pain and stiffness (c). Mean and standard error for objective spinal stiffness measures at 60 N in healthy control participants and participants with chronic back pain and stiffness. Abbreviations: AvgStiff = Average stiffness; TermStiff = Terminal stiffness.

Figure 2

Study 1b results comparing healthy controls and people with chronic LBP and stiffness (a). Mean and standard error for force estimation error at 60 N for the first and last indentation (differed significantly between groups for both the first and last indentation at 60 N) (b). Mean and standard error for force discrimination threshold (differed significantly between groups). Abbreviations: LBP = low back pain; HC = healthy controls; *p < 0.05; **p < 0.001.

Figure 3

Study 2 results (a). Mean and standard error for force estimation error during the control, no-sound, and creaky sound conditions, compared between groups. A significant indent number x condition interaction occurred: significant differences were found between force estimation errors for Indent 1 between the creaky sound and control sound conditions and between the control sound and no-sound conditions; estimation errors did not differ between conditions for the other indentations, nor did they differ between groups. (b) Mean and standard error for force estimation error during the creaky sound and creaky decrease sound conditions, compared between groups. A significant indent number x condition interaction occurred: significant differences were found between force estimation errors for Indent 3 between the creaky sound and the creaky decrease conditions in posthoc analyses; estimation errors did not differ between conditions for the other indentations, nor did they differ between groups. Abbreviations: LBP = low back pain participants; HC = healthy controls; *p < 0.05.