Does Supraspinatus Initiate Shoulder Abduction?

Journal of Electromyography and Kinesiology 23 (2013) 425–429 Contents lists available at SciVerse ScienceDirect Journal of Electromyography and Kinesiology journal homepage: www.elsevier.com/locate/jelekin Does supraspinatus initiate shoulder abduction? ⇑ Darren Reed a, , Ian Cathers a, Mark Halaki b, Karen Ginn a a Discipline of Biomedical Science, Sydney Medical School, The University of Sydney, Australia b Discipline of Exercise and Sport Science, Faculty of Health Sciences, The University of Sydney, Australia article info abstract Article history: Purpose: It is commonly stated that supraspinatus initiates abduction; however, there is no direct evi- Received 21 August 2012 dence to support this claim. Therefore, the aims of the present study were to determine whether supra- Received in revised form 22 October 2012 spinatus initiates shoulder abduction by activating prior to movement and significantly earlier than other Accepted 12 November 2012 shoulder muscles and to determine if load or plane of movement influenced the recruitment timing of supraspinatus. Methods: Electromyographic recordings were taken from seven shoulder muscles of fourteen volunteers Keywords: during shoulder abduction in the coronal and scapular planes and a plane 30 anterior to the scapular Shoulder ° Rotator cuff plane, at 25%, 50% and 75% of maximum load. Initial activation timing of a muscle was determined as Muscle activation the time at which the average activation (over a 25 ms moving window) was greater than three standard Timing deviations above baseline measures. Electromyography Results: All muscles tested were activated prior to movement onset. Subscapularis was activated significantly later than supraspinatus, infraspinatus, deltoid and upper trapezius, while supraspinatus, infraspinatus, upper trapezius, lower trapezius, serratus anterior and deltoid all had similar initial activation times. The effects of load or plane of movement were not significant. Conclusions: Supraspinatus is recruited prior to movement of the humerus into abduction but not earlier than many other shoulder muscles, including infraspinatus, deltoid and axioscapular muscles. The com- mon statement that supraspinatus initiates abduction is therefore, misleading. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction quired to indicate that supraspinatus is activated prior to the activation of other shoulder muscles and prior to the beginning of the Despite evidence from both ‘in vivo’ (Howell et al., 1986; abduction movement. Only one electromyographic (EMG) study McCully et al., 2007; Van Linge and Mulder, 1963) and ‘in vitro’ has compared the onset of muscle activity in supraspinatus with (Thompson et al., 1996) studies dating back to 1963, that paraly- other shoulder muscles during shoulder abduction in asymptom- sing supraspinatus does not impede the ability to abduct the shoul- atic participants (Wickham et al., 2010). These authors concluded der, it is still universally reported in anatomy textbooks that that supraspinatus initiates abduction, reporting that deltoid, mid- supraspinatus initiates shoulder abduction (Moore and Dalley, dle trapezius as well as supraspinatus were activated prior to the 2006; Palastanga et al., 2008; Standring, 2008). Numerous studies commencement of the abduction movement but found no signifi- confirm that supraspinatus is active during the full range of shoul- cant difference in onset of activity between supraspinatus and se- der abduction (Basmajian and DeLuca, 1985; Inman et al., 1996; ven other shoulder muscles examined: middle, anterior and Kronberg et al., 1990) however, there is no direct evidence to sup- posterior deltoid, middle and upper trapezius, serratus anterior port the claim that supraspinatus initiates abduction. and rhomboid major (Wickham et al., 2010). Wickham et al. For a muscle to act as an initiator of a movement it must acti- (2010) determined the initiation of muscle activation by the sub- vate prior to other muscles moving that joint and prior to the jective method of visual inspection of the data signals to identify movement occurring in order to generate enough tension to pro- the first detectable point where the EMG signal rose above the duce the movement, or in the case of the shoulder joint, to dynam- baseline. An objective and more reliable method of determining ically stabilise the joint. Therefore, to investigate the claim that muscle activation onset time is by the use of an algorithm supraspinatus initiates shoulder abduction, direct evidence is re- (Di Fabio, 1987; Hodges and Bui, 1996), which has been utilised by other researchers investigating onset of muscle activity in the back (Hodges and Richardson, 1996), around the knee (Bennell ⇑ Corresponding author. Address: Discipline of Biomedical Science, Sydney et al., 2010) and at the shoulder (Barden et al., 2005; Brindle Medical School, Cumberland Campus C42, The University of Sydney, Lidcombe, NSW 1825, Australia. Tel.: +61 2 9351 9144; fax: +61 2 9351 9715. et al., 2007; Santos et al., 2007). It remains to be determined if sig- E-mail address: [email protected] (D. Reed). nificant differences would be revealed in the timing of supraspina- 1050-6411/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jelekin.2012.11.008 426 D. Reed et al. / Journal of Electromyography and Kinesiology 23 (2013) 425–429 tus activation – compared to the activation of other shoulder muscles and the commencement of movement into shoulder abduction – if this more objective criterion for determining the onset of muscle activity was applied. The aim of the current experiment, therefore, was to determine, using an objective measure of the initial muscle activation timing, if supraspinatus initiates shoulder abduction based on whether it is activated prior to the activation of other shoulder muscles and prior to movement of the upper limb during abduction. Secondary aims were to determine if load or plane of movement affected supraspinatus initial activation timing. 2. Methods 2.1. Participants The dominant shoulder (13 right and one left) of fourteen asymptomatic volunteers (five female and nine male), aged between 18 and 49 years (mean 22.5 years) was tested in this study. All volunteers were involved in recreational sporting activities, had no history of shoulder pain in the previous 2 years and had never sought treatment for shoulder pain. In addition, they were required to demonstrate normal range of shoulder movement (0–180°), with normal scapulohumeral rhythm, assessed visually by an experienced physiotherapist, and be pain-free during maximal iso- metric shoulder rotation tests. Informed consent was obtained and the study was approved by the University of Sydney Human Re- search Ethics Committee. 2.2. Instrumentation Activity was recorded from seven muscle sites around the dominant shoulder using a combination of surface and fine wire intramuscular electrodes (Fig. 1). Paired silver/silver chloride surface Fig. 1. Experimental setup. electrodes (Red Dot, 2258, 3 M) were placed at a distance 2 cm tween 10 Hz and 1 kHz) before transferring to a personal computer apart over upper trapezius and middle deltoid. Studies indicate with a 16 bit analogue to digital converter (1401, Cambridge Elec- that the use of surface electrodes to record activity from these tronics Design) at a sampling rate of 2564 Hz using Spike2 soft- large, superficial muscles is valid i.e. unlikely to be affected by ware (version 4.00, Cambridge Electronics Design). crosstalk from surrounding or underlying muscles (Giroux and Lamontagne, 1990; Oberg et al., 1992). Bipolar intramuscular electrodes were used to record activity in deep muscles inaccessible to 2.3. Experimental protocol surface electrode recordings (supraspinatus, subscapularis), in serratus anterior which may shift position in relation to the skin dur- Abduction testing was performed in the coronal, scapular (30° ing shoulder abduction, and in the thin lower trapezius where anterior to the coronal plane of the body), and the scapular +30° surface electrodes may pick up underlying muscle activity. In addi- (30° anterior to the scapular plane of the body) planes using visual tion, an indwelling electrode was used to record activity from cues to guide the plane of the movement. Participants stood up- infraspinatus where the use of surface electrodes has been shown right with feet shoulder width apart and the opposite hand resting to result in significant crosstalk from surrounding muscles in some on the adjacent hip to limit compensatory trunk movements shoulder movements (Johnson et al., 2011). The intramuscular (Fig. 1). Prior to testing, they were trained to abduct the dominant electrodes were prepared in accordance with Basmajian and DeLu- arm from the anatomical position, with the thumb pointing out- ca (1985) and placement followed the recommendations of Geirin- ward in the line of the movement and leading the arm through full ger (1994) for all but the subscapularis which was inserted range of movement over a 3 s period. The maximum abduction according to Kadaba et al. (1992). A digital ultrasonic diagnostic load able to be lifted in one repetition using normal scapulohu- imaging system (Mindray, DP-9900) was used to accurately place meral rhythm was determined as maximum load. the electrodes into lower trapezius. All indwelling electrode place- During testing, the participants performed full range abduction, ments were confirmed by visual inspection of EMG signals

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