G. David et al., EMG and strength correlates of selected shoulder muscles during rotations of the glenohumeral joint, CLIN BIOMEC, 15(2), 2000, pp. 95-102
Objective. To identify activation patterns of several muscles acting on the
shoulder joint during isokinetic internal and external rotation.
Design. Combined EMG and isokinetic strength analysis in healthy subjects,
Background. EMG studies of the shoulder region revealed intricate muscular
activation patterns during elevation of the arm but no parallel studies reg
arding pure rotations of the joint could be located,
Methods. Fifteen (n = 30 shoulders) young, asymptomatic male subjects parti
cipated in the study. Strength production during isokinetic concentric and
eccentric internal and external rotations at 60 and 180 degrees/s was corre
lated with the EMG activity of the rotator cuff, biceps, deltoid and pector
alis major. Analysis of the smoothed EMG related to the timing of onset of
the signal and to the normalized activity at the angle of the peak moment.
Determination of the association between the EMG and the moment was based o
n strength ratios.
Results. Findings indicated that for both types of rotations, the rotator c
uff and biceps were active 0.092 +/- 0.038-0.215 +/- 0.045 s prior to the i
nitiation of the actual movement and 0.112-0.034 s prior to onset of deltoi
d and pectoralis major activity. These differences were significant in all
of the eight conditions (P < 0.05). In terms of the strength ratios, strong
association was found between electrical activity and moment production in
the subscapularis and infraspinatus (r(2) = 0.95 and 0.72, respectively) a
t the low and high angular velocities.
Conclusions. Prior to actual rotation of the shoulder joint, normal recruit
ment of the rotator cuff and biceps is characterized by a non-specific pres
etting phase which is mainly directed at enhancing the joint 'stiffness' an
d hence its stability. Once movement is in progress, the EMG patterns of th
ese muscles become movement specific and are correlated with the resultant
moment.