SIAMOC Best Methodology Paper Award 2007Assessment of force and fatigue in isometric contractions of the upper trapezius muscle by surface EMG signal and perceived exertion scale
Introduction
Work-related musculoskeletal disorders of the shoulders and neck have been reported for different occupations [1], [2]. Shoulder and neck conditions (e.g., rotator cuff tendonitis, myalgia, thoracic outlet syndrome, and radiculopathy [2]) can be caused by repetitive or sustained work, short work cycles, and localised muscular loadings.
Perceived exertion ratings (e.g., VAS, Likert, OMNI, and Borg) have been used to study the subjective feeling during an exercise. Different scales were compared in the literature [3]. The Borg scale CR10 was considered in this study. This is a subjective scale with values from 1 to 10 indicating the perceived level of effort. A good correlation between quantitative measure of physiological response (e.g., metabolic acidosis, ventilation, oxygen intake, heart rate, and respiration frequency) and perceived exertion has been shown [4]. The Borg scale was applied to both resistance [5] and endurance [6] exercises, and to both isometric [7] and dynamic [8] contractions. Thus, perceived exertion ratings can be considered an acceptable approach to work-related studies [7], [9] measuring the perceived response at different loads [10] or testing the effect of rest periods on recovering perceived efficiency [11].
An objective and non-invasive assessment of muscle activity can be provided by surface electromyography (EMG). The use of surface EMG techniques in ergonomic studies has been documented in different tasks, including repetitive work at a car assembly line [12], work at the visual display terminal [13], and other studies [1]. In occupational health, upper trapezius (UT) muscle is usually investigated by surface EMG, as it is a superficial muscle and its activity is influenced by neck or shoulder pain [14]. The structure of UT is complicated. Contradictory descriptions of the direction and disposition of its motor units (MUs) are available in the literature [15].
The relation between EMG and force strongly depends on MUs control by the central nervous system. This can change depending on muscle pain [16] or fatigue [17]. Due to the high inter-subject and inter-muscle variability, the estimation of the EMG–force relation requires a calibration model [18].
Muscle fatigue consists of myoelectric and mechanical phenomena, the former ones preceding the latter. Myoelectric manifestation of fatigue includes both “peripheral” and “central” adaptations of the muscles [19]. Interesting indications have been obtained from EMG studies concerning fibre type distribution of the muscle [20], prediction of endurance time (ET) [21], and pathological conditions [22].
To increase the reliability of the information extracted from surface EMG, high density, two-dimensional (2D) detection systems have recently been applied. Information pertaining to a large spatial area of a muscle was obtained, supporting the investigation of the spatial–temporal recruitment of the MUs [23], [24].
This paper focused on the application of subjective (based on Borg scale CR10) and objective (based on surface EMG signals) methods to investigate UT. Surface EMG signals were recorded by a 2D detection system. The relation between subjective/objective estimations and muscle force, expressed as percentage of maximal voluntary contraction (MVC), and muscle fatigue, expressed as a function of ET, was assessed. Differences related to gender were investigated. Furthermore, the analysis of topographical maps of RMS supported the analysis of the activation pattern of UT during the isometric task performed by the subjects.
Section snippets
Subjects
Fourteen healthy subjects, seven males and seven females (mean ± standard deviation; age: 25 ± 3 years; height: 172 ± 10 cm; weight: 63 ± 13 kg) participated in the measurements. The study was approved by the Local Ethics Committee of the Health Department of Piemonte Region, Italy, and a written informed consent was obtained from all participants. All subjects had right side dominance, assessed by the Edinburgh Handedness Inventory.
EMG and force measurement
Selective and isometric contractions (shoulder elevation) were performed
Representative signals and results
Portions of signals at 20%, 40%, 60%, and 80%MVC are shown in Fig. 2a. Channels chosen by visual analysis for the subsequent global analysis are indicated by arrows. Estimates of CV, MNF, RMS, FD, entropy, and Borg rate at different force levels are shown in Fig. 2b. Two examples of topographical maps obtained for 20%MVC (upper map) and 80%MVC (lower map) are shown in Fig. 2c.
Representative signals recorded during the endurance contraction are shown in Fig. 3a. Global parameters estimated from
Subjective and objective estimation of force
A high correlation (R = 0.99) was found between the perceived effort (Borg ratings) and the objective measure of exerted force (RMS values) at different force levels (Fig. 4d). This is in line with the good correlation between Borg scale and force levels documented in the literature for different tasks [27].
A statistically significant dependence was demonstrated between force level and EMG amplitude (measured by RMS). Averaging RMS values from a large number of channels was useful to reduce the
Conflict of interest
The authors declare that they have no conflict of interest, financial or otherwise, related to the submitted manuscript or the associated research.
Acknowledgements
The authors are grateful to Mr Luca Manetta who built the isometric brace and carried out part of the experiments and to Mr Roberto Bergamo who helped us with the anatomy of UT. This work was supported by the European project no. 016712 “Cybernetic Manufacturing Systems (CyberManS)” and the enterprise “Ergonomia Prevenzione Ambiente (EPA)”.
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