Three-dimensional scapulothoracic motion during active and passive arm elevation

Abstract

Background. Scapulothoracic muscle activity is believed to be important for normal scapulothoracic motion. In particular, the trapezius and serratus anterior muscles are believed to play an important role in the production and control of scapulothoracic motion. The aim of this study was to determine the effects of different levels of muscle activity (active versus passive arm elevation) on three-dimensional scapulothoracic motion.

Methods. Twenty subjects without a history of shoulder pathology participated in this study. Three-dimensional scapulothoracic motion was determined from electromagnetic sensors attached to the scapula, thorax and humerus during active and passive arm elevation. Muscle activity was recorded from surface electrodes over the upper and lower trapezius, serratus anterior, anterior and posterior deltoid, and infraspinatus muscles. Differences in scapulothoracic motion were calculated between active and passive arm elevation conditions.

Findings. Scapular motion was observed during the trials of passive arm elevation; however, there was more upward rotation of the scapula, external rotation of the scapula, clavicular retraction, and clavicular elevation under the condition of active arm elevation. This was most pronounced for scapular upward rotation through the mid-range (90–120°) of arm elevation.

Interpretation. The upper and lower trapezius and serratus anterior muscles have an important role in producing upward rotation of the scapula especially throughout the mid-range of arm elevation. Additionally, it appears that capsuloligamentous and passive muscle tension contribute to scapulothoracic motion during arm elevation. Assessment of the upper and lower trapezius and serratus anterior muscles and upward rotation of the scapula should be part of any shoulder examination.

Introduction

Motion of the scapula on the thorax is essential for normal function of the upper extremity (Kibler and McMullen, 2003). Scapulothoracic motion has been studied using two-dimensional (2-D) (Doody et al., 1970, Poppen and Walker, 1976) and more recently three-dimensional (3-D) (Karduna et al., 2001, Ludewig and Cook, 2000, McClure et al., 2001) measurement techniques. The orientation of the scapula relative to the thorax and the position of the scapula on the thorax are used to describe 3-D scapulothoracic motion (Karduna et al., 2001). Scapular rotations used to describe the orientation of the scapula relative to the thorax include upward and downward rotation, external and internal rotation, and posterior and anterior tilting (Karduna et al., 2001). Rotations of the clavicle are used to describe the position of the scapula on the thorax and include protraction and retraction and elevation and depression (Karduna et al., 2001). As the arm is elevated the scapula progressively upwardly rotates, externally rotates, posteriorly tilts (McClure et al., 2001, Ludewig et al., 1996), and the clavicle retracts and elevates (McClure et al., 2001, Ludewig et al., 2004). This pattern has been demonstrated in asymptomatic individuals under static (Ludewig et al., 1996, Lukasiewicz et al., 1999) and dynamic conditions (Karduna et al., 2001, Ludewig et al., 2004).

Amongst the 14 muscles that surround and attach to the scapula, the upper and lower portions of the trapezius and the serratus anterior muscles are believed to be important for scapulothoracic motion (Inman et al., 1944, Bagg and Forrest, 1988, Ludewig et al., 1996). These muscles are typically described as producing upward rotation and retraction of the scapula (Bagg and Forrest, 1988, Inman et al., 1944, Ludewig et al., 1996). Additionally, the upper and lower trapezius and serratus anterior muscles have been proposed to play a role in producing external rotation and posterior tilt of the scapula (Ludewig et al., 1996, Kent, 1971, Perry, 1978).

One way to determine how muscle activity influences scapulothoracic motion is to compare scapular motion during active and passive arm elevation. Qingyun and Gongyi (1998) used X-ray fluoroscopy to investigate active and passive shoulder motion in eighteen healthy subjects. From the X-rays, an angle between the glenoid surface and pivot of the humerus (GHA) was measured. Their description of the GHA angle was not clear and this makes the interpretation of their results difficult. However, their findings indicate that motion at the scapulothoracic and glenohumeral joints was different during active arm elevation compared with passive arm elevation. McQuade and Smidt (1998) used 3-D measures of scapular and humeral motion to study scapulohumeral rhythm (scapular upward rotation relative to humeral elevation) in 25 healthy subjects under three different conditions of arm elevation; (1) active elevation, (2) active elevation against resistance, and (3) passive elevation. Total arm elevation was divided into five different phases and the scapulohumeral rhythm for each condition in each phase was determined. For the first three phases of motion there was less upward rotation of the scapula during the passive condition compared to the active condition. In the final two phases of motion there was more upward rotation of the scapula for the passive condition compared to the active condition. Price et al. (2000) used 3-D measurement techniques to compare scapular motion during active and passive arm elevation. Bilateral measurements were obtained on 10 healthy subjects at 10° intervals from 10° to 50° of humeral elevation in the coronal plane. The authors noted that elevation beyond 50° was not performed in order to avoid humeral impingement against the coracoacromial arch and decrease the chance of the scapula being pulled along the thorax by the humerus. No differences in 3-D scapular motion were found between active and passive arm elevation trials.

Collectively the results from these studies (McQuade and Smidt, 1998, Price et al., 2000, Qingyun and Gongyi, 1998) indicate that scapulothoracic motion is influenced by whether the arm is actively or passively elevated. However, these studies are limited by the use of 2-D measurement techniques (Qingyun and Gongyi, 1998), assessment of motion through a limited range (Price et al., 2000), reporting on only one scapular rotation despite the use of 3-D measurement techniques (McQuade and Smidt, 1998), and lack of muscle activity quantification (McQuade and Smidt, 1998, Qingyun and Gongyi, 1998, Price et al., 2000) which makes it is difficult for the reader to get a sense of how relaxed the muscles were during passive arm elevation trials. Additional studies that address these issues are needed to further the understanding of how scapulothoracic muscle activity influences scapulothoracic motion. This information will provide a basis for clarifying the role that scapulothoracic muscles have in the production and control of scapulothoracic motion. Additionally it will provide a basis for further understanding of the contribution of muscle dysfunction to shoulder pathologies. Therefore, the purpose of this study was to determine the effects of different levels of muscle activity (active versus passive arm elevation) on 3-D scapulothoracic motion.