The following important results and conclusions of the present pilot study will be discussed below:
Methodological considerations
Tissue velocity imaging measures velocities in the axial direction, i.e., in the direction towards and away from the probe. This is a drawback as the fascicles of the upper trapezius are distributed in a more horizontal direction. However, the direction of fascicles according to the ultrasound images is relatively homogeneous. Based on the incompressibility of muscles (i.e., if a muscle is compressed in one direction it has to expand in the other directions) an indirect measurement is still possible. It is also possible to confirm the contraction by visual inspection of the ultrasound loops where the diameter of the muscle belly becomes thicker/thinner as the shoulder elevation enters either of the concentric or eccentric phases of the movement. During elevation, the muscle expands in a vertical direction. Therefore, we used relative rather than absolute measurements.
Another methodological aspect concerns the fact that the probe is handheld. Of course reproducibility can be questioned. However, reproducibility according to cardiac applications that are similar to the situation of registering motions from a skeletal muscle has been shown to be reliable [
41,
42]. To optimize the location of the probe, the SENIAM standard point was used for centering the probe and the contour of the probe was drawn. Yet a moment of uncertainty occurs when the angle of the probe is placed on the trapezius muscle, but as far as possible the applicator is aware of this fact and was as careful as possible to achieve the same projection image.
As this is a pilot study, further studies are needed in order to confirm the results indicated in this study. To improve the study design, full EMG should be provided. Interpretations can then be with respect to the relationship between neuromotor activation and muscle tissue response. This matter will be presented in a forthcoming study. Pain intensity ratings should also be made after the first two elevations to evaluate the intermediate pain ratings. Also a larger study including more patients would be of great value.
The number of performed repetitions could have been chosen differently. There is a variation in pain intensity between 6–8 (VAS) in the patient group. It is in accordance with a reasonable variation as the response to a certain activity is not easy to foresee and the estimation according to the VAS scale is not a direct correlation to biological activity. The VAS scale is used as a reference instrument to standardize a cut off for the purpose of providing a moderate pain rather than a mild pain.
It could be argued that some subjects used less force after pain provocation and thus lower degree of activation and this can of course be a relevant objection. In this study, the task to be performed was not a heavy one. On the contrary, a 3-cm shoulder elevation carrying a weight of just 1 kg was performed. The shoulder elevation was short and standardized in terms of height of elevation and tempo of the movement. The interesting finding from a qualitative point of view is that less of the muscle seems to be contracted and this part is in the middle/deeper part surrounded by more passive trapezius muscle tissue segments after pain provocation compared to before pain provocation
In future studies it will be an advantage if the applicator of the US registration is blinded with respect to group belonging.
Multivariate statistics have been used in this pilot study. The strength of this procedure is that during the analysis subjects are not clustered a priori, e.g., in categories as patients and controls. Instead, subjects are seen as uncategorized and are graphically projected based on the results from the variable analysis. Furthermore, the variables have been scaled and mean-centred so each variable has the same impact, i.e., the same possibility to affect the model. Yet another benefit is that clusters of subjects can be related to a variable profile rather than just a single variable: the distribution of variables provides patterns of variables that in turn describe the internal variable structure in relation to the identified cluster of subjects.
Before pain provocation/exercise
This study focuses on the intra-muscular activity in the upper part of the trapezius muscle. Strain rate is a variable that represents the rate at which small (8 mm) muscle tissue segments deform and hence contribute as an important variable to describe muscle activity and intra-segmental coordination pattern. Because the rate of deformation is calculated by the tissue velocity algorithm, the presence of force is at least to some extent part of the measurement. Hence, strain rate can also be thought of in terms of contractility. Strain, on the other hand, is a regional tissue deformation parameter that presents muscle tissue activity as an additive presentation of a regional tissue progress as a function of the progress of a movement. In other words, there are no rapid fluctuations between positive and negative strains as a consequence of muscle activation as is the case in the strain rate parameter, but instead the gradual distribution of tissue contraction beyond a certain degree of deformation is compared to rest. The benefit of the strain parameter is that a certain degree of deformation can be chosen and hence variations in the coordination pattern can be visualised and compared as a function of a certain degree of deformation. As a consequence, activation patterns according to fine-tuned tissue activations can also be studied.
It is interesting that strain rate and strain did not differ significantly before pain provocation, neither according to the univariate (Table
1) or multivariate analyses (Figures
4a and
4b). This could be seen as indicating a relative homogeneous reference movement. One objection to this finding could be that the TM group is weaker than the controls [
43]. In the present study, the strength was not measured. Clearly, the importance of this factor cannot be determined. Hence, future studies ought to consider aspects of biomechanical output, such as strength and fatigue, when validating the tissue velocity method within this application.
After pain provocation/exercise
The univariate analysis revealed significant differences according to mean strain rate and RMS strain rate in the healthy controls indicating a higher strain rate after exercise than before. A combination of a warming-up effect together with a dynamic muscle tissue could be one explanation of the increase of the deformation rate after several repetitions. The multivariate analyses (Figures
4a,b,
5 and
6) revealed that patients with chronic trapezius myalgia after pain provocation due to exercise on a group level showed decreased strain and unchanged strain rate whereas healthy controls had unchanged strain and increased strain rate.
In healthy subjects, from a muscle physiological point of view, these results taken together could be interpreted as a more dynamic tissue. Possible explanations for these findings might be warming-up effects due to increased blood flow (exercise hyperemia) and increased neuromuscular control. The pain in itself may also be factor to be considered. Studies have shown that the interplay and coordination according activities in prime and secondary movers may be affected by pain in the sense that a decrease in an agonist muscle activity may result in an increase in an antagonist muscle activity [
13‐
17,
19]. In future studies of healthy subjects, it is important to analyze whether other factors – such as sex, degree of fitness, strength, and fatigue – influence strain and strain rate both in prime movers and postural muscles
In agreement with the physiological interpretation above, the TM group has a less flexible or dynamic muscle tissue as a consequence of exercise and pain provocation. As briefly referred in the introduction, several studies indicate morphological, biochemical, and neuromuscular alterations in chronic trapezius myalgia when compared to healthy controls. The morphological alterations, such as increased prevalence of RR-fibres, are less likely as direct explanations to decrease strain. If such changes were important for strain and strain rate, then a lower strain would reasonably have been observed before pain provocation. This was not the case, however.
Based on studies using microdialysis of the trapezius, our group has reported that patients with chronic work-related trapezius myalgia have increased interstitial concentrations of potassium, serotonin, lactate, pyruvate, and glutamate at rest [
9,
10]. The interstitial concentrations of these substances also increase during brief exercise associated with a significant increase in pain intensity. One possible explanation could be that these concentrations are so high, even though exercise hyperaemia, that they directly affect the mechanical properties of the active muscle fibres.
A third explanation is related to altered neuromuscular control due to increased pain as a consequence of exercise. Le Pera et al. [
44] describe how motor control mechanisms are affected by muscle pain by way of reflex and central mechanisms. Experimental studies modulating the noxious system also demonstrate altered responses to acute pain due to injections of algogenic substances, mechanical pressure, ischemia, and electrical stimulation [
45,
46]. Injecting hypertonic saline into the upper trapezius muscle resulted in a decreased EMG activity both during static [
46,
47] and dynamic tasks [
48]. Acute pain can result in reorganization among trapezius muscle subdivisions during repetitive shoulder flexions [
49]. Changes in the pattern of coordination during parts of the contraction cycle have been shown during dynamic activities in patients with chronic pain [
13‐
16]. Although differences have been observed at a group level, it is obvious that not all patients have altered coordination patterns in the trapezius muscle [
50], which is interesting with respect to the fact that we found a certain blend between controls and TM. The studies are in reasonable agreement with the pain-adaptation model [
51,
52]. This model states that a decrease in agonist muscle activity and increase in antagonist muscle activity is a consequence of pain. Hence the decreased strain and unchanged strain rate in TM could be a consequence of such an altered neuromuscular control resulting in a less activated trapezius as a consequence of the increasing pain due to exercise. Another contributing factor in this respect could be a hyper-sensitised pain system (i.e., central sensitization and/or decreased descending control) as a consequence of plastic changes in the pain system due to nociceptive impulses of long duration in TM.