Pascal Madeleine

Sensitization in patients with painful knee osteoarthritis

&NA; Pain is the dominant symptom in osteoarthritis (OA) and sensitization may contribute to the pain severity. This study investigated the role of sensitization in patients with painful knee OA by measuring (1) pressure pain thresholds (PPTs); (2) spreading sensitization; (3) temporal summation to repeated pressure pain stimulation; (4) pain responses after intramuscular hypertonic saline; and (5) pressure pain modulation by heterotopic descending noxious inhibitory control (DNIC). Forty‐eight patients with different degrees of knee OA and twenty‐four age‐ and sex‐matched control subjects participated. The patients were separated into strong/severe (VAS ≥ 6) and mild/moderate pain (VAS < 6) groups. PPTs were measured from the peripatellar region, tibialis anterior (TA) and extensor carpi radialis longus muscles before, during and after DNIC. Temporal summation to pressure was measured at the most painful site in the peripatellar region and over TA. Patients with severely painful OA pain have significantly lower PPT than controls. For all locations (knee, leg, and arm) significantly negative correlations between VAS and PPT were found (more pain, more sensitization). OA patients showed a significant facilitation of temporal summation from both the knee and TA and had significantly less DNIC as compared with controls. No correlations were found between standard radiological findings and clinical/experimental pain parameters. However, patients with lesions in the lateral tibiofemoral knee compartment had higher pain ratings compared with those with intercondylar and medial lesions. This study highlights the importance of central sensitization as an important manifestation in knee OA.

Shoulder muscle co-ordination during chronic and acute experimental neck-shoulder pain. An occupational pain study

Abstract Little is known about the mechanisms leading to chronic neck-shoulder musculoskeletal disorders (MSD). The aim of the present study was to investigate and compare motor function during controlled, low load, repetitive work together with chronic or acute experimental neck-shoulder pain. The clinical study was performed on workers with (n = 12) and without (n = 6) chronic neck-shoulder pain. In the experimental study, experimental muscle pain was induced in healthy subjects by intra-muscular injection of hypertonic saline into the trapezius muscle (n = 10). The assessed parameters related to motor performance were: work task event duration, cutting forces, surface electromyogram (EMG) activity in four shoulder muscles, displacement of the centre of pressure, and arm and trunk 3D movements. For controlled cutting force levels, chronic and acute experimental pain provoked a series of changes: a decreased working rhythm and a protective reorganisation of muscle synergy (experimental study), higher EMG frequency contents which may indicate altered motor unit recruitment, and greater postural activity and a tendency towards increased arm and trunk movements. These pain-related changes can play a role in the development of MSD. The present clinical and experimental study demonstrated similar interactions between motor co-ordination and neck-shoulder pain in occupational settings. We therefore suggest that this experimental model can be used to study mechanisms related to MSD. Information on such modulatory processes may help in the design of new strategies aimed at reducing the development of MSD.

Sex differences in temporal characteristics of descending inhibitory control: an evaluation using repeated bilateral experimental induction of muscle pain.

&NA; Little is known about sex differences in the temporal pattern of descending inhibitory mechanisms, such as descending noxious inhibitory control (DNIC). Sex differences in temporal characteristics of DNIC were investigated by measuring pressure pain thresholds (PPTs) over time in the trapezius muscles (local pain areas) and the posterolateral neck muscles (referred pain areas) following repeated bilateral injection of hypertonic versus isotonic saline into both trapezius muscles. Ten females and 11 males received two consecutive bilateral injections, with 15 min interval, of either 5.8% hypertonic saline (0.5 ml in each side for each bilateral injection) or isotonic saline as a control in a randomized manner. Following hypertonic saline injection, the maximal pain intensities of the first and second bilateral injections were significantly higher in females than in males. The PPTs in the trapezius muscles were significantly lower in females than in males. Significantly higher PPTs (hypoalgesia) in men than in women were shown 15 min after the first bilateral injection, and 7.5 and 15 min after the second bilateral injection in the referred pain areas. Importantly, the second bilateral injection failed to further increase the PPTs for both sexes. These results showed that there were sex differences in temporal characteristics of descending inhibition with long‐lasting hypoalgesia in men than in women. Repeated noxious muscular stimuli may inhibit further build‐up of DNIC, which may reflect a mechanism of plasticity of the descending inhibitory systems following recurrent nociceptive barrage for both sexes.

Subjective, physiological and biomechanical responses to prolonged manual work performed standing on hard and soft surfaces

Abstract The aim of this laboratory study was to examine the subjective, physiological and biomechanical responses to prolonged light repetitive manual work during standing on soft (polyurethane standard mat) and hard (aluminum casting) surfaces. The subjects stood on the hard (10 subjects) and on the soft surfaces (11 subjects) for 2 h. Intensity of unpleasantness, shank circumference, electromyograph (EMG) activities from the right soleus and tibialis anterior muscles, mean amplitude and total angular displacement around the left and right ankle in the saggital plane, centre of pressure (CoP) displacement in the frontal and saggital planes, calf surface temperature, and pain intensity in experimentally induced muscle pain were recorded. Maximal voluntary contraction and fatigue tests were performed before and after the 2 h experiment. Standing on a soft surface caused a lower intensity of unpleasantness. During standing on a hard surface compared to a soft one the results showed an enhanced swelling of the shank, an increased EMG activity (right soleus muscle) of the lower leg, a greater amplitude and total angular displacement, and a larger CoP displacement in the frontal plane. Indications of more pronounced muscle fatigue while standing on the hard surface were also noticed. After 105 min, experimental muscle pain was elicited by injecting hypertonic saline. The intensity of the induced pain was lower when standing on the soft surface. Amplitude, angular distance and CoP displacement showed a tendency to be greater after injection of the hypertonic saline. It was found that the experimentally induced pain influenced postural activity, underlining central interactions between proprioceptors and nociceptors. The results highlighted a higher feeling of comfort when standing on the soft surface. In addition, postural activity was lower when standing on the soft surface, but the activity was sufficient to prevent swelling of the lower legs.

Topographical mapping and mechanical pain sensitivity of myofascial trigger points in the infraspinatus muscle

Objectives: To screen for the presence of latent and active myofascial trigger points (MTrPs) in patients with unilateral shoulder and arm pain and perform topographical mapping of mechanical pain sensitivity bilaterally in the infraspinatus muscles.

On functional motor adaptations: from the quantification of motor strategies to the prevention of musculoskeletal disorders in the neck-shoulder region.

Background:  Occupations characterized by a static low load and by repetitive actions show a high prevalence of work‐related musculoskeletal disorders (WMSD) in the neck–shoulder region. Moreover, muscle fatigue and discomfort are reported to play a relevant initiating role in WMSD. Aims: To investigate relationships between altered sensory information, i.e. localized muscle fatigue, discomfort and pain and their associations to changes in motor control patterns.

Experimental muscle pain changes the spatial distribution of upper trapezius muscle activity during sustained contraction.

OBJECTIVE To investigate the effect of local excitation of nociceptive muscle afferents on the spatial distribution of muscle activity. METHODS Surface electromyographic (EMG) signals were recorded from the upper trapezius muscle of 10 healthy volunteers with a 5 x 13 electrode grid during 90-s isometric contractions before, during, 15 and 30 min after intramuscular injection of hypertonic (painful) or isotonic (non-painful) saline. From the multi-channel EMG recordings, two-dimensional maps of root mean square and mean power frequency were obtained. The centre of gravity of the root mean square map was used to quantify global changes in the spatial distribution of muscle activity. RESULTS During sustained contractions, average root mean square increased, average mean frequency decreased and the centre of gravity moved cranially. During experimental muscle pain, compared to before injection, the average root mean square decreased and there was a caudal shift of the centre of gravity. Fifteen minutes after the painful injection the centre of gravity returned to its original position. CONCLUSIONS Short-term dynamic reorganization of the spatial distribution of muscle activity occurred in response to nociceptive afferent input. SIGNIFICANCE The study furnishes an extension of the pain adaptation model indicating heterogeneous inhibition of muscle activity.

Contribution of the local and referred pain from active myofascial trigger points in fibromyalgia syndrome

ABSTRACT The generalized hypersensitivity associated with fibromyalgia syndrome (FMS) may in part be driven by peripheral nociceptive sources. The aim of the study was to investigate whether local and referred pain from active myofascial trigger points (MTrPs) contributes to fibromyalgia pain. FMS patients and healthy controls (n = 22 each, age‐ and gender‐matched) were recruited. The surface area over the upper trapezius muscle on each side was divided into 13 sub‐areas (points) of 1 cm in diameter for each point. Pressure pain threshold (PPT) and the local and referred pain pattern induced by manual palpation at 13 points bilaterally in the upper trapezius were recorded. Results showed that PPT levels at all measured points were significantly lower in FMS than controls. Multiple active MTrPs (7.4 ± 2.2) were identified bilaterally in the muscle in FMS patients, but no active MTrPs were found in controls. The mid‐fiber region of the muscle had the lowest PPT level with the largest number of active MTrPs in FMS and with the largest number of latent MTrPs in controls. The local and referred pain pattern induced from active MTrPs bilaterally in the upper trapezius muscle were similar to the ongoing pain pattern in the neck and shoulder region in FMS. In conclusion, active MTrPs bilaterally in the upper trapezius muscle contribute to the neck and shoulder pain in FMS. Active MTrPs may serve as one of the sources of noxious input leading to the sensitization of spinal and supraspinal pain pathways in FMS.

Changes in the amount and structure of motor variability during a deboning process are associated with work experience and neck-shoulder discomfort

In this field study, the size and structure of kinematics variability were assessed in relation to experience and discomfort during a deboning task. Eighteen workers divided in groups with low/high experience and with/without neck-shoulder discomfort participated. Standard deviation and coefficient of variation (amount of variability), as well as approximate entropy and sample entropy (complexity) and, correlation dimension (dimensionality) were computed for head-shoulder, shoulder-hip and elbow-hip displacement in the vertical direction. A longer work experience was associated with shorter work cycle duration and decreased amount of variability while complexity increased for the head-shoulder displacement, P<0.05. Shorter work cycle, lower amount of variability and, lower dimensionality for the head-shoulder displacement were found in relation to discomfort, P<0.05. While the amount of variability, complexity and dimensionality increased for the elbow-hip displacement, P<0.05. These findings suggest a functional role of experience via learning effects and discomfort through compensatory mechanisms on the size and structure of motor variability.

The size of cycle-to-cycle variability in biomechanical exposure among butchers performing a standardised cutting task.

The effects of employment duration and pain development on motor variability were investigated during repetitive work. Electromyographic (EMG) and kinematics data from two previous studies were re-analysed. Newly employed butchers were followed prospectively in relation to employment duration and pain development. Healthy butchers with long-term experience were compared with novices. The variability of the cycle time, EMG ratio and arm and trunk movement was expressed as cycle-to-cycle standard deviations. During the first 6 months of employment, cycle time variability decreased, while posture and movement variability increased (p < 0.05). In presence of pain, the variability of the initial arm position decreased while it increased for the trunk (p < 0.05). Experienced butchers showed a larger variability than novices for work cycle and several kinematic variables, but a smaller EMG ratio variability (p < 0.05). These findings indicate that the variability of motor patterns in repetitive work changes with experience and pain. A change towards a more variable motor strategy may protect workers from work-related musculoskeletal disorders.