Experimental muscle pain increases trapezius muscle activity during sustained isometric contractions of arm muscles

Abstract

Objective

In the present study, the influence of experimental muscle pain on muscle co-ordination and fatigue development during sustained isometric elbow flexion was investigated.

Methods

Conventional surface electromyography (EMG) was recorded from the biceps brachii, brachioradialis, deltoideus and trapezius muscle during isometric elbow flexion at 40% maximum force. Single motor unit (MU) conduction velocity in the biceps brachii was assessed using a high spatial resolution surface EMG technique. Measurements were performed on 15 healthy subjects before, during and after (1) injection of hypertonic (pain condition) and (2) isotonic saline (control) into the biceps brachii. The pain intensity was assessed on a 10 cm visual analogue scale.

Results

The experimental results showed in both experimental sessions a fatigue-related increase of the root mean square value of EMG (222±164% of the baseline), and a decrease of the median frequency (118±16% of the baseline) in all investigated muscles. A maximum pain level of in average 3.2 cm on the visual analogue scale was reached after injection of hypertonic saline during contraction. Differences between painful and control condition were seen in an increased trapezius activity (230±141%) during pain. The global EMG activity of the brachioradialis and biceps brachii was unaffected by experimental muscle pain in line with unaffected single MU conduction velocity in the biceps brachii. Differences in endurance time (mean 89.3 and 102.3 s, pain and control, respectively) were not significant.

Conclusions/Significance

The findings suggest that upper extremity pain could be a possible source for overloading the trapezius muscle and as such is an important factor in occupational settings.

Introduction

Muscle pain in the upper extremities is common for a number of occupations (e.g. Vereisted et al., 1993, Ohlsson et al., 1994). However, little is known about the effects of muscle pain on the motor control of the painful muscle and its influence on local muscle fatigue.

Experimental muscle pain decreased the firing rates of active motor units (MUs) during isometric constant force contractions (Sohn et al., 2000, Farina et al., 2004). Other studies have shown that muscle pain was associated with a decrease in surface electromyographic (EMG) activity during maximal contractions, with respect to the non-painful contraction (e.g. Graven-Nielsen et al., 1997, Wang et al., 2000a). Reduced firing rates of single MUs and decreased EMG activity at a constant force level during pain might be explained by changed co-ordination of synergistic and antagonistic muscles. The effect of muscle nociception on the motorneuron pool is not clear; both excitation and inhibition of motorneurons have been reported (e.g. Anastasijevic et al., 1987, Hayward et al., 1988, Kniffki et al., 1981, Mense and Skeppar, 1991). An inhibition of motorneurons to the agonist muscles and a simultaneous excitation of motorneurons to the antagonist muscles is hypothesised in the pain adaptation model (Lund et al., 1991). A consequential limitation of the range of movements might serve to prevent further damage to the painful muscle. Findings supporting the pain adaptation model were reported in various studies involving moderate-to-maximal force levels in dynamic conditions like gait (Arendt-Nielsen et al., 1996, Graven-Nielsen et al., 1997), free shoulder movement (Madelaine et al., 1999) and chewing (Svensson et al., 1995). On the other hand, no pain-induced changes in EMG parameters were found in several studies of isometric contractions of different muscles at submaximal force levels (Ashton-Miller et al., 1990, Birch et al., 2000, Graven-Nielsen et al., 1997) and the force levels could be maintained.

In case of sustained contractions during muscle pain, an initially reduced activity in the painful muscle as predicted by the pain adaptation model, could be expected leading to a reduction in the electrophysiological fatigue development in this muscle. However, a decrease in endurance during experimental muscle pain (Ciubotariu et al., 2003, Graven-Nielsen et al., 1997) also limits the maximal degree of local muscle fatigue in the painful muscle.

Changes in the activation levels of the painful muscle and its synergists may be hypothesised to enhance with fatigue, as to shift load from the painful to unaffected muscles, and thus to further unburden the affected muscle while holding the required force level. Along this line, experimental muscle pain in the biceps brachii might lead to an increased activity of the synergistic brachioradialis muscle during sustained isometric elbow flexion. Another conceivable way to unload a painful biceps brachii is a partly shift in force generation from the elbow to the shoulder joint. Performing an isometric elbow flexions task, an activation of the trapezius was found (Mamaghani et al., 2001). Thus, a decrease in biceps activity during muscle pain might be accompanied by an increased activity of shoulder muscles, including the upper trapezius.

In addition to global EMG analysis from surface EMG, single MUs can be non-invasively traced and their properties can be estimated by the combination of spatial sampling and spatial filtering approaches (Reucher et al., 1987a). In particular, muscle fibre conduction velocity (CV), an indirect indicator of changes in membrane properties, can be estimated from high spatial resolution surface EMG recordings at the single MU level (Schneider et al., 1989). A decrease in global (Merletti et al., 1990) or single MU CV (Farina et al., 2002) has been shown to be an indicator of local muscle fatigue. Sustained isometric contractions lead to a decrease in muscle fibre CV (Farina et al., 2002). At the same time, additional MUs are recruited to keep a constant force level during progressing fatigue, backing up already fatigued MUs (e.g. Thorn et al., 2002, Olsen et al., 2001, Jensen et al., 2000, Enoka et al., 1989).

The present study investigated changes in the muscular activation level and in single MU CV and firing during sustained isometric elbow flexion with and without experimentally induced muscle pain.