Effect of mechanical compression due to load carrying on shoulder muscle fatigue during sustained isometric arm abduction: an electromyographic study

Abstract

The use of surface electromyography (EMG) for studying the effect of mechanical compression of occupational origin on muscle fatigue has been the subject of poor attention in ergonomic research. This study examined the effect of backpack carrying on fatigue of two shoulder muscles during sustained low force static contraction: the middle deltoid (MD) muscle and the upper trapezius (UT) muscle on which the backpack strap exerted direct compressive force. EMG activities of MD and UT muscles, of the dominant and non-dominant sides, were studied on eight subjects during two tasks, a maximal and an exhausting submaximal bilateral isometric 90 degrees arm abduction, which were performed while carrying a backpack load of 0, 10, and 20 kg, respectively. EMG amplitude (root mean square, RMS) and spectral (mean power frequency, MPF) parameters were computed from the recorded signals. No significant differences between the dominant and non-dominant sides were found for none of the parameters whatever the load-carrying mass. Load-carrying masses which were tested did not influence significantly the maximal bilateral arm abduction performance contrary to the time to exhaustion during the submaximal task, which decreased significantly with increasing load-carrying mass. A significant increase in fatigability, defined by the slope of MPF decrease, was observed for both muscles when load-carrying mass increased; but only UT presented a significant increase in muscle fatigue level, defined by the MPF value with respect to its initial value, at the end of the exhausting submaximal task. Furthermore, the increase in muscle activation of UT, quantified by RMS, during the exhausting task was not significantly higher with increasing load-carrying mass. So, the increased signs of local fatigue of UT may be interpreted by a localised blood flow impairment resulting from the direct compressive force exerted by backpack on this muscle.

Effect of carrying school backpacks on cardio-respiratory changes in adolescent students

“…Most load carriage studies have focused on the metabolic, biomechanical and subjective perceptual changes associated with load weight, walking speed, gradient, terrain and other factors as well as medical hazards and performance limitations [3,13]. Many authors in the past have studied the effect of carrying load on different electromyography and pathological [14][15][16], biomechanical [17,18] and physiological [8,[19][20][21][22] parameters in adults. Most of these studies have focused on soldiers and hikers with the purposes of improving the techniques of load carriage.…”

Characteristics of power spectrum density function of EMG during muscle contraction below 30%MVC

“…Parameters used in this analysis have been, in addition to amplitude, those that describe mean frequency (MPF) and median frequency (MF) of the power spectrum density function (PSDF) and the number of crossings of the EMG signal through the zero level (Cook et al, 1998;De Luca, 1997;Hägg and Suurkü la, 1991;Masuda et al, 1999;Pincivero et al, 2006;Coorevits et al, 2005;Hummel et al, 2005;Iridastadi and Nussbaum, 2006). Parameters MF and MPF are well accepted as indicators of muscle fatigue at higher levels of muscle contraction (Crenshaw et al, 1997;Gayda et al, 2005;da Silva et al, 2005;Beck et al, 2004;Piscione and Gamet, 2006). However, there is some doubt as to their role in an analysis of muscle fatigue at low levels of muscle contraction (Hägg and Ojok, 1997).…”

The Influence of Fatigue on Muscle Temperature

“…This method makes it possible to estimate fatigue on the basis of the electrical signal registered from muscles during their activation [3,4,5,6,7,8,9,10,11,12]. The registration of an electrical signal with surface electrodes is commonly used to assess muscle load and fatigue during occupational tasks [13,14,15,16,17]. Numerous studies indicate the relationship between the amplitude of the EMG signal and the force developed by muscles [18,19,20,21].…”