Muscular activity of the shoulder and neck region during sustained and intermittent exercise

Sustained isometric contractions of the neck and shoulder muscles in laboratory conditions are often assumed to be representative of muscular load in occupational tasks. The present study aims to investigate whether differences can be found between these contractions and those in intermittent tasks. Ten subjects performed forward flexions of the right arm during three tests to the limit of endurance: (1) a sustained contraction; (2) an intermittent contraction with a cycle time of 10 s and a duty cycle of 0.7; and (3) the same intermittent contraction with the possibility of changing the shoulder angle during relaxation. Muscular activities of the upper trapezius, deltoid pars anterior and medialis and infraspinatus were registered using surface electromyography, and changes in the signal were quantified by root mean square (RMS) and mean power frequency (MPF) values. During the continuous test, highly significant changes in both parameters were found for the four muscles. Significant changes were found during both intermittent conditions, although to a lesser degree than in the continuous test. Significant differences between the regression coefficients of the sustained test and the two intermittent tests were found on all occasions: for RMS and MPF of the four muscles. Significant differences were also found regarding the intermittent tests. The test in which no variation in limb position was allowed during relaxation caused a more pronounced MPF decrease for the trapezius (P < or = 0.01), which was confirmed by the subjective scores of perceived exertion and a higher RMS increase (P < or = 0.01) for the middle part of the deltoid. The intermittent condition with changes in shoulder angle showed more MPF decrease for the deltoid, significant for the middle part (P < or = 0.05), not significant for the anterior part and a larger RMS increase for infraspinatus (P < or = 0.05). It can be concluded that the latter condition represents more the trapezius and deltoid activity as in occupational tasks.