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

Instrumented spinal stiffness measurements have shown high test-retest reliability. However, factors that may affect reliability have yet to be investigated. The objective of this study was to compare the: 1) within- and between-day reliability of a mechanical indentation device (MID) in measuring spinal stiffness, 2) measurement precision of averaging multiple measurements, and 3) reliability of stiffness measurements between individuals with and without LBP.
The spinal stiffness of 26 volunteers with and without LBP was measured 3 times by MID in each of two visits 1 to 4 days apart. Two stiffness measures were calculated from the resulting force-displacement data: global stiffness and terminal stiffness. Intraclass correlation coefficients were used to estimate reliability. Measurement precision was measured by minimal detectable changes, bias and 95% limits of agreement.
Using the mean of three spinal stiffness measurements, the measurement precision was improved by 33.7% over a single measurement. Averaging three measurements, the within- and between-day reliability point estimates of both global and terminal stiffness were 0.99 and 0.98, respectively. The reliability estimates of spinal stiffness measurement using MID were not significantly altered by the participants’ LBP status across all circumstances (95% confidence intervals overlapped). 
With our experimental protocol, averaging three spinal stiffness measurements using MID produces reliable stiffness measurements regardless of individuals’ LBP status.

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.

Alkaptonuria is a rare cause of backache. A 55 years old gentleman, a retired army personal had been having pain and stiffness in the spine for the last 15 years, accompanied by bluish black discolouration of face and hands. Kyphosis and reduced movement of spine were present. Other joints were spared. Osler spots were visible on the sclera. X-rays revealed calcification of intervertebral discs. Asymptomatic renal stone was identified. There was no cardiac involvement. Symptomatic care was provided.

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.