Feeling stiffness in the back may be all in the head?

Bodily feelings are commonly thought to reflect the biological state of our body tissues. A new research from the University of South Australia now stated that  feelings of back stiffness are a protective perceptual construct, rather than reflecting biomechanical properties of the back. This idea has implications for treatment of pain/stiffness but also for our understanding of bodily feelings.  The research investigated the long-held question of what informs our subjective experiences of bodily state was published in the journal: Scientific Reports.

In an experiment, they recruited 15 people with chronic low back pain (LBP) who also reported having chronic feelings of back stiffness, and selected 15 age- and gender-matched healthy controls. The researchers compared the subjective stiffness rating with an objective stiffness measure using displacement over force. In another experiment, the participants were advised that they would receive indentation forces between 50 newtons and 70 newtons to their back; and were asked to estimate, as accurately as possible, the magnitude of force delivered

All experiments showed 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.

The authors also found that 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.

Note: This research should not be interpreted to mean that stiffness in the back does not exist. Rather, that the “feeling of stiffness” might be more related to the nervous system’s desire to protect a region of the body that it believes to be fragile or vulnerable.