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

Abstract

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 ((31)P-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 (31)P-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 (31)P-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 (31)P-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.

Figure 1. Representative ³¹P spectra of the finger extensor muscles at rest and during exercise.

A, MRS pattern A (control); B, MRS pattern B (patient). Peak assignments: ATP, adenosine triphosphate; PCr, phosphocreatine; Pi, inorganic phosphate (N-Pi, neutral-pH peak; L-Pi, low-pH peak). The spectra are the sum of 32 scans (64 s) The displayed spectra times are the centre points of such time periods. Note the appearance of a second and acidic Pi peak (L-Pi) in pattern B spectra. Exercise in the chosen example of pattern A was performed for a longer time period without Pi splitting, but only the same time points as in pattern B example are shown for comparison purposes.

Figure 2. Representative time courses of metabolite raw amplitudes, intracellular pHs, and total Pi/PCr ratios during exercise.

A, MRS pattern A; B, MRS pattern B; PCr, phosphocreatine; Pi, inorganic phosphate (N-Pi, neutral-pH peak; L-Pi, low-pH peak). Data were obtained from 64 s spectra and the displayed symbol times are the centre point of the time periods.