A feeling of stiffness is an important predictor of disability, but the assumption that feeling stiff reflects actually being stiff has rarely been investigated. Our results provide compelling evidence that a feeling of back stiffness relates poorly to biomechanical measures of back stiffness. Consistent with our hypothesis, we show that a protective response exists in people who report feeling back pain and stiffness: they over-estimate applied force and are better than healthy controls at detecting any change in this force. Last, congruent auditory information applied with pressure to the spine modulates perception of the back, supporting the idea that a feeling of stiffness is a multisensory perceptual inference that serves bodily protection.

This research makes several important contributions to the existing literature. First, it provides empirical evidence that our conscious perception of stiffness is not derived solely from joint relevant sensory information: perceived and actual stiffness do not relate in our sample. In rheumatoid and osteoarthritis, the duration of feeling stiff does not relate to the degree of joint changes24,25,26, and our work suggests that the relationship is tenuous for stiffness intensity, in real-time, as well. Further, the lack of a difference in objective spinal stiffness between those with and without reported stiffness and LBP, is consistent with the idea that our bodily feelings reflect multimodal and evaluative processes, serving as behavioural drivers rather than markers of a biological or biomechanical state27. That is, people with back pain may feel stiff but it seems that this is not because they are objectively stiff; instead stiffness may be an effective perceptual mechanism to drive down movement and thus avoid provocation of nociception or injury. This also raises an important discussion of what people are describing when they refer to experiencing feelings of stiffness. While often described as a perceived resistance to movement, there is also suggestion that reports of feelings of stiffness may be a learned concept for what is actually a feeling of a lack of movement velocity28. It is interesting to consider that for some people, feelings of stiffness may also reflect more complex constructs such as fear of movement. Regardless, our results clearly show that the bodily experience of stiffness differs from the biomechanical tissue state.

Our results are in contrast to previous studies that found differences in objective spinal stiffness measures between those with and without acute/subacute LBP29 or neuropathic LBP (duration unspecified)30. In chronic LBP, treatment altered objective spinal stiffness measures and this alteration related to self-reported measures of disability31 – that is, actual back stiffness is changeable and may be important to some body-relevant perceptions. However, none of the previous studies evaluated whether the participants specifically had feelings of back stiffness, whereas we specifically recruited these participants. This makes direct comparison to our work difficult.

Our work also contributes to the knowledge of perceptual dysfunction in chronic conditions. There is consistent evidence of perceptual abnormalities in people with LBP: alterations in perceived shape of the back;32 reduced tactile acuity at the back;32,33,34 impaired motor imagery of the back;35 and impaired trunk voluntary motor control36. Our work suggests that this dysfunction extends to the perception of force applied to the back. Changes in the perception of touch have been linked to changes of receptive fields and response profiles of primary sensory cortex (S1) neurons37 – so-called cortical reorganisation37, 38 – raising the possibility that alterations in force perception might also represent cortical changes related to the back. That people with back pain are less accurate than healthy controls on a task requiring intact cortical proprioceptive representation supports this possibility35.

It is intriguing that people with LBP and stiffness could detect a smaller difference in force than healthy controls. It seems counter to findings of poor tactile acuity in people with LBP32. Superior force acuity yet inaccurate estimation of force magnitude, while seemingly incongruent, are together consistent with high attentional demands of pain39. Expecting pain can impair attentional disengagement40, 41 which may uniquely facilitate sustained attention to the back in the LBP group where indentations occurred over the painful area. Indeed, indentation evoked pain in those with LBP and stiffness but not in healthy controls. Expectations of pain and changes in attentional focus (e.g., spatial attention) are relevant because acuity in somatosensory (i.e., tactile) change detection has been shown to be improved by both42, 43 as has enhanced perception of threat44, which in other non-nociceptive paradigms increases protective reflexes45 – a finding that is largely consistent with protective force over-estimation seen here. Although the specific role of attention and expectation cannot be elucidated in our paradigm, that enhanced change detection extends to force magnitude in people with back pain is sensible and consistent with the ecological value associated with bodily protection. Given that this increased acuity in detecting differences in force was not accompanied by group differences in objective spinal stiffness suggests that it is more likely to reflect a top-down mechanism (e.g., attention-mediated), rather than a bottom-up one (e.g., superior mechanical detection).

While it is possible that features of the force discrimination task, such an indentation duration and response timing, played a role in the outcome seen, the likelihood of this is low. Regarding indentation duration, the indentor advanced at a constant velocity, meaning that differences in force targets (50 N vs 100 N) took slightly different time durations to reach. However, given our small range in force targets (50N-70N) this difference was less than a maximum of 1.5 seconds, and post-test questioning revealed that all participants based their force estimation judgements solely on the feel of the force on their back. None reported using time (indentation duration) to make these judgements. Second, regarding response timing, there was a 30 second interval between receiving the first and second indentation for the forced choice paradigm (response: same or different). This timing was essential given viscoelastic properties of the spine. Given that both groups received training to become comfortable with this task suggests that the impact of response timing is likely minimal.

Our findings of a heightened protective response in people that report back pain and stiffness raise the possibility that the mechanisms that subserve pain also contribute to the perception of stiffness. Such a possibility would be predicted by several relevant theoretical frameworks, for example: associative learning46 – pain and stiffness often go together; adaptation to pain model47 – pain-related muscle activation changes alter trunk stiffness48, 49 to prevent ongoing irritation of sensitive tissues; and the more recent link between motor effort and proprioception50 – painful movements are predicted to be more effortful [Tabor 2016, unpublished data], which in turn might make them feel stiffer. Together, these suggest that pain and stiffness likely compel similar behavioural responses that limit movement and thus re-injury. Such issues could be disentangled with further studies based on the current experimental paradigm. Indeed, further research is warranted to determine if this protective perceptual inference, e.g., a feeling of stiffness, results in protective behaviour such as movement avoidance in people with back pain and stiffness. That there was no difference between groups in force perceptual error over time suggests that perceptual differences are unlikely to be due to differences between groups in cognitive function (e.g., working memory capacity51).

This work provides crucial information to suggest how conscious perceptions are created, relevant to multisensory integration in chronic painful conditions. Our results suggest that auditory information is integrated with temporally congruent pressure information to create a perceptual experience and that this effect is not merely due to sound alone, but rather reflects the nature of this auditory information. Indeed, we found that the type and context of congruent sound had specific and varying effects on the modulation of perceptual error in force magnitude estimation. It is well-established that multisensory neurons in the superior colliculus receive information from auditory, visual and somatosensory inputs52. Further, substantial communication exists between the primary auditory cortex and the primary somatosensory cortex53, and auditory-somatosensory interactions exist in the early stages of cortical processing54. Last, integration of body-relevant information also occurs in the premotor55 and posterior parietal cortex56, 57 – the latter being an area that provides cognitive representations of our body and the space surrounding it.

Our discoveries support previous work that has shown that congruent auditory and tactile information can induce changes in the perceived properties of the hand: the so-called Marble Hand illusion58. Our work extends this finding by showing that the ability to integrate multimodal information and alter perception can also occur in a chronic pain state. Previous work has shown that touch and visual information can still be integrated in chronic pain: when a rubber hand illusion is applied people experience disownership of their painful limb in a similar manner to that observed in healthy controls59; the visual illusion of touch (using mirrored reflection of the good leg) improves tactile perception in the leg with chronic tactile deficits60. Our results extend these findings by confirming that this integration extends to audition and touch/pressure.

Our findings may have relevant implications for treatment. Improved outcomes in people with tendon pain are seen when exercises are paired with auditory cues61. Given the positive relationship between force estimation error and feelings of stiffness (the more stiff you feel – the greater the error), this raises the possibility that we may directly target feelings of stiffness with such auditory cues. Indeed, that we saw perceptual effects of sound in people with LBP and stiffness suggests that processes of multisensory integration are likely intact and thus may be a new and relevant treatment target. Finally, our work suggests that subjective and objective measures of stiffness are not proxies for one another, but likely capture different aspects of a similar domain. This raises the possibility that either self-perception or the physiological, objective aspects of back stiffness can be normal or abnormal. It follows then that exploring how various combinations of these states play out in intervention may reveal new mechanisms underlying back pain and/or approaches for its effective treatment.