Muscle metabolism and acid-base status during exercise in forearm work-related myalgia measured with31P-MRS

Neuromuscular manifestations of work-related myalgia in women specific to extensor carpi radialis brevis

This study assessed neuromuscular function in the extensor carpi radialis brevis (ECRB) of female workers diagnosed with work-related myalgia (WRM, n = 14, age 45.2 ± 1.9 years) and the ECRB of healthy controls (CON, n = 10, age 34.6 ± 2.5 years). Groups were compared on voluntary and electrically evoked functional responses at rest (Pre), immediately following a 5 min repetitive task (Post-0) performed at 60% maximal voluntary contraction (MVC), and after 5 min of recovery (Post-5). Despite near complete motor unit activation (MUA) (CON 98% ± 1% vs. WRM 99% ± 1%), at Pre, WRM produced 26% less (P < 0.05) MVC force than CON. Following an MVC, twitch force was increased (P < 0.05) by 94% ± 13% and 54% ± 11% in CON and WRM, respectively (CON vs. WRM; P < 0.05). The peak force and the maximal rates of force development and decline of electrically evoked contractions (10-100 Hz) were generally depressed (P < 0.05) at Post-0 and Post-5 relative to Pre. The response pattern to increasing frequencies of stimulation was not different (P > 0.05) between groups and MUA was not impaired (CON 97% ± 1% vs. WRM 97% ± 1%; P > 0.05). In conclusion, the peripheral weakness observed in the ECRB in WRM at rest does not result in abnormal fatigue or recovery responses after performing a task controlled for relative demand (60% MVC).

Cellular properties of extensor carpi radialis brevis and trapezius muscles in healthy males and females

In this study, we sought to determine whether differences in cellular properties associated with energy homeostasis could explain the higher incidence of work-related myalgia in trapezius (TRAP) compared with extensor carpi radialis brevis (ECRB). Tissue samples were obtained from the ECRB (n = 19) and TRAP (n = 17) of healthy males and females (age 27.9 ± 2.2 and 28.1 ± 1.5 years, respectively; mean ± SE) and analyzed for properties involved in both ATP supply and utilization. The concentration of ATP and the maximal activities of creatine phosphokinase, phosphorylase, and phosphofructokinase were higher (P < 0.05) in ECRB than TRAP. Succinic dehydrogenase, citrate synthase, and cytochrome c oxidase were not different between muscles. The ECRB also displayed a higher concentration of Na(+)-K(+)-ATPase and greater sarcoplasmic reticulum Ca(2+) release and uptake. No differences existed between muscles for either monocarboxylate transporters or glucose transporters. It is concluded that the potentials for high-energy phosphate transfer, glycogenolysis, glycolysis, and excitation-contraction coupling are higher in ECRB than TRAP. Histochemical measurements indicated that the muscle differences are, in part, related to differing amounts of type II tissue. Depending on the task demands, the TRAP may experience a greater metabolic and excitation-contraction coupling strain than the ECRB given the differences observed.

Preliminary observations on high energy phosphates and metabolic pathway and transporter potentials in extensor carpi radialis brevis and trapezius muscles of women with work-related myalgia

This study compared both the extensor carpi radialis brevis (ECRB) and the trapezius (TRAP) muscles of women with work-related myalgia (WRM) with healthy controls (CON) to determine whether abnormalities existed in cellular energy status and the potentials of the various metabolic pathways and segments involved in energy production and substrate transport. For both the ECRB (CON, n = 6-9; WRM, n = 13) and the TRAP (CON, n = 6-7; WRM, n = 10), no differences (P > 0.05) were found for the concentrations (in millimoles per kilogram of dry mass) of ATP, PCr, lactate, and glycogen. Similarly, with one exception, the maximal activities (in moles per milligram of protein per hour) of mitochondrial enzymes representative of the citric acid cycle (CAC), the electron transport chain (ETC), and β-oxidation, as well as the cytosolic enzymes involved in high energy phosphate transfer, glycogenolysis, glycolysis, lactate oxidation, and glucose phosphorylation were not different (P > 0.05). The glucose transporters GLUT1 and GLUT4, and the monocarboxylate transporters MCT1 and MCT4, were also normal in WRM. It is concluded that, in general, abnormalities in the resting energy and substrate state, the potential of the different metabolic pathways and segments, as well as the glucose and monocarboxylate transporters do not appear to be involved in the cellular pathophysiology of WRM.

Excitation–contraction coupling properties in women with work-related myalgia: a preliminary study

We investigated the potential role of selected excitation–contraction coupling processes in females with work-related myalgia (WRM) by comparing WRM with healthy controls (CON) using tissue from extensor carpi radialis brevis (ECRB) and trapezius (TRAP) muscles. For the ECRB, age (mean ± SE) was 29.6 ± 3.5 years for CON (n = 9) and 39.2 ± 2.8 years for WRM (n = 13), while for the TRAP, the values were 26.0 ± 2.1 years for CON (n = 7) and 44.6 ± 2.9 years for WRM (n = 11). For the sarcoplasmic reticulum (SR) of the ECRB, WRM displayed concentrations (nmol·(mg protein)−1·min−1) that were lower (P < 0.05) for Total (202 ± 4.4 vs 178 ± 7.1), Basal (34 ± 1.6 vs 30.1 ± 1.3), and maximal Ca2+-ATPase activity (Vmax, 168 ± 4.9 vs 149 ± 6.3), and Ca2+-uptake (5.06 ± 0.31 vs 4.13 ± 0.29), but not SERCA1a and SERCA2a isoforms, by comparison with CON. When age was incorporated as a co-variant, Total, Basal, and Ca2+-uptake remained different from CON (P < 0.05), but not Vmax (P = 0.13). For TRAP, none of the ATPase properties differed between groups (P > 0.05) either before or following adjustment for age. No differences (P > 0.05) were observed between the groups for Ca2+-release in the SR for either TRAP or ECRB. Similarly, no deficiencies, regardless of muscle, were noted for either the Na+–K+-ATPase content or the α and β subunit isoform distribution in WRM. This preliminary study provides a basis for further research, with expanded numbers, investigating the hypothesis that abnormalities in SR Ca2+-regulation are involved in the cellular etiology of WRM.

Blood flow to the extensor carpi radialis brevis muscle following adrenaline infusion in patients with lateral epicondylitis

PURPOSE

Based on previous evidence of muscle fiber injury and decreased blood flow in the extensor carpi radialis brevis (ECRB) muscle in lateral epicondylitis (LE), we hypothesized that there would also be an abnormal (vasoconstrictive) vascular response to adrenaline in the ECRB muscle in LE.

METHODS

In a case-control study, we measured skeletal muscle blood flow in 8 patients with LE and in 8 healthy controls in response to a 30-minute intravenous infusion of adrenaline. We used local clearance of technetium-99m in the main portion of the ECRB muscle to calculate muscle blood flow.

RESULTS

In support of the hypothesis, the blood flow response to the adrenaline infusion was markedly different in the 2 study groups. Whereas the continuous decrease in technetium-99m clearance rate over time was interrupted by the adrenaline-induced vasodilatory effect in the control group, we detected no such effect in the patient group.

CONCLUSIONS

In the ECRB muscle in LE, there is a shift in the balance of vasodilatory and vasoconstrictory influences of adrenaline, leading to vasoconstriction during low-dose adrenaline infusion. The adverse adrenaline effect is similar to what was previously observed after minor muscle injury. Whether the vasoregulatory change, by causing relative muscle ischemia, represents the primary etiology in LE or results from muscle injury cannot be determined, but it is likely to contribute to the development and continuation of chronic muscle pain in LE. New ways of thinking about the condition may be required, and pharmacological treatment might be an option to improve the blood supply and avoid further damage to the affected ECRB muscle-tendon unit.

A novel system for transcutaneous application of carbon dioxide causing an "artificial Bohr effect" in the human body

Background

Carbon dioxide (CO₂) therapy refers to the transcutaneous administration of CO₂ for therapeutic purposes. This effect has been explained by an increase in the pressure of O₂ in tissues known as the Bohr effect. However, there have been no reports investigating the oxygen dissociation of haemoglobin (Hb) during transcutaneous application of CO₂in vivo. In this study, we investigate whether the Bohr effect is caused by transcutaneous application of CO2 in human living body.

Methods

We used a novel system for transcutaneous application of CO₂ using pure CO₂ gas, hydrogel, and a plastic adaptor. The validity of the CO₂ hydrogel was confirmed in vitro using a measuring device for transcutaneous CO₂ absorption using rat skin. Next, we measured the pH change in the human triceps surae muscle during transcutaneous application of CO₂ using phosphorus-31 magnetic resonance spectroscopy (³¹P-MRS) in vivo. In addition, oxy- and deoxy-Hb concentrations were measured with near-infrared spectroscopy in the human arm with occulted blood flow to investigate O2 dissociation from Hb caused by transcutaneous application of CO₂.

Results

The rat skin experiment showed that CO₂ hydrogel enhanced CO₂ gas permeation through the rat skin. The intracellular pH of the triceps surae muscle decreased significantly 10 min. after transcutaneous application of CO₂. The NIRS data show the oxy-Hb concentration decreased significantly 4 min. after CO₂ application, and deoxy-Hb concentration increased significantly 2 min. after CO₂ application in the CO₂-applied group compared to the control group. Oxy-Hb concentration significantly decreased while deoxy-Hb concentration significantly increased after transcutaneous CO₂ application.

Conclusions

Our novel transcutaneous CO₂ application facilitated an O₂ dissociation from Hb in the human body, thus providing evidence of the Bohr effect in vivo.

Work-related pain in extrinsic finger extensor musculature of instrumentalists is associated with intracellular pH compartmentation during exercise

Background

Although non-specific pain in the upper limb muscles of workers engaged in mild repetitive tasks is a common occupational health problem, much is unknown about the associated structural and biochemical changes. In this study, we compared the muscle energy metabolism of the extrinsic finger extensor musculature in instrumentalists suffering from work-related pain with that of healthy control instrumentalists using non-invasive phosphorus magnetic resonance spectroscopy (³¹P-MRS). We hypothesize that the affected muscles will show alterations related with an impaired energy metabolism.

Methodology/Principal Findings

We studied 19 volunteer instrumentalists (11 subjects with work-related pain affecting the extrinsic finger extensor musculature and 8 healthy controls). We used ³¹P-MRS to find deviations from the expected metabolic response to exercise in phosphocreatine (PCr), inorganic phosphate (Pi), Pi/PCr ratio and intracellular pH kinetics. We observed a reduced finger extensor exercise tolerance in instrumentalists with myalgia, an intracellular pH compartmentation in the form of neutral and acid compartments, as detected by Pi peak splitting in ³¹P-MRS spectra, predominantly in myalgic muscles, and a strong association of this pattern with the condition.

Conclusions/Significance

Work-related pain in the finger extrinsic extensor muscles is associated with intracellular pH compartmentation during exercise, non-invasively detectable by ³¹P-MRS and consistent with the simultaneous energy production by oxidative metabolism and glycolysis. We speculate that a deficit in energy production by oxidative pathways may exist in the affected muscles. Two possible explanations for this would be the partial and/or local reduction of blood supply and the reduction of the muscle oxidative capacity itself.