Spinal Stiffness

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. Bodily feelings constitute a fundamental aspect of self-awareness and provide critical homeostatic functions – e.g., feeling cold makes one seek warmth 1 ; feeling pain makes one seek protection 2 ; feeling parched makes one drink 3. We assume that these bodily feelings reflect the biological state of our body tissues – a 'read-out' , so to speak, of somatosensory and visceral input – particularly when the feeling is located somewhere in the body, as it is for pain or stiffness. There is growing evidence for pain however, that it is highly modulated by a wide range of cognitive and contextual variables 4, 5. For example, visually manipulating the perceived size of one's hand alters the pain experienced in experimental contexts 6 and during movement of a chronically painful limb 7 , and illuminating a blue or red light in synchrony with delivering a noxious cold stimulus can transform the feeling evoked from uncomfortably cold to painfully hot 5. That somatic input triggered by physiological responses is important in cognitive processes – so-called 'embodied cognition' – has recently been countered with the idea that bodily awareness (e.g., our sense of bodily ownership) might directly modulate physiological regulation of body tissue in an anatomically specific way 8, 9. Indeed, we now know that the sense of bodily ownership, the felt location, and the anthropometric characteristics of our body parts, are tightly linked to their physiological regulation in a bi-directional manner 7, 10. Extensive human data from both healthy and diseased populations have led to the proposal of the cortical body matrix theory 11 , the predictions of which are yielding new developments in our understanding and treatment of some pathological pain conditions 12. One feeling that is almost universally experienced, which impacts on our daily lives 13, 14 , but which remains relatively ignored, is feeling stiff. Feeling joint stiffness is unanimously attributed to actually having stiff joints. Take for example having a 'stiff back'. We assume that this feeling reflects altered force-response profiles of the tissues in our back (objective biomechanical stiffness). But does it? A feeling of joint stiffness has been shown to Published: xx xx xxxx OPEN

Background: Low back pain is often associated with increased spinal stiffness which thought to arise from increased muscle activity. Unfortunately, the association between paraspinal muscle activity and paraspinal stiffness, as well as the spatial distribution of this relation, is unknown. The purpose of this
investigation was to employ new technological developments to determine the relation between spinal muscle contraction and spinal stiffness over a large region of the lumbar spine.
Methods: Thirty-two male subjects performed graded isometric prone right hip extension at four different exertion levels (0%, 10%, 25% and 50% of the maximum voluntary contraction) to induce asymmetric back muscle activity. The corresponding stiffness and muscle activity over bilateral paraspinal lumbar
regions was measured by indentation loading and topography surface electromyography, respectively. Paraspinal stiffness and muscle activity were then plotted and their correlation was determined.

Findings: Data from this study demonstrated the existence of an asymmetrical gradient in muscle activation and paraspinal stiffness in the lumbar spine during isometric prone right hip extension. The magnitude and scale of the gradient increased with the contraction force. A positive correlation between
paraspinal stiffness and paraspinal muscle activity existed irrespective of the hip extension effort (Pearson correlation coefficient, range 0.566–0.782 (P < 0.001)).

Interpretation: Our results demonstrate the creation of an asymmetrical gradient of muscle activity and paraspinal stiffness during right hip extension. Future studies will determine if alterations in this gradient may possess diagnostic or prognostic value for patients with low back pain.

Approximately 60-80% of people experience low back pain at least once in their lifetime. Given the poor correlation between radiological findings and low back pain, physicians and physiotherapists usually palpate patients with low back pain to assess the severity of back pain and to make differential diagnosis. However, this clinical assessment has been criticized for its subjectivity and low reliability. This book includes comprehensive reviews on different spinal stiffness assessments and paraspinal surface electromyography, as well as two empirical studies using state-of-the-art technology to investigate the relation between spinal stiffness and back muscle activity. The results of these studies revealed that there was a directional specific relation between the two in healthy individuals. The results help develop a new method that may improve the accuracy of back assessment in clinical practice. This book proivdes up-to-date information on spinal research for both researchers and clinicians.