The EMG and load relationship is commonly measured with multiple submaximal isometric contractions. This method is both time consuming and may introduce fatigue. The purpose of this study was to determine if the electromyography (EMG) amplitude from the middle deltoid was reliable during isometric ramp contractions (IRCs) at different angles of elevation and rates of force application. Surface EMG was measured at 3 shoulder elevation angles during IRCs at 4 submaximal levels of maximum voluntary contraction (MVC). Data were reliable in all conditions except during the rate relative to the subjects’ MVC at 90° for 30% and 40% MVC. The main effect for angle on EMG amplitude was found to be significant, p < .01. EMG at 90° was greater than at 60° (p < .017) and at 30° (p < .017). The main effect of force level on EMG amplitude was significant, p < .01 and follow-up contrast demonstrated a significant (p < .001) linear increase of EMG amplitude with force level. We conclude that EMG amplitude from IRCs are reliable across all shoulder elevation angles and up to 40% MVC. IRCs are a feasible method for recording EMG at the deltoid.
David Phillips and Andrew Karduna
Yin-Liang Lin and Andrew Karduna
While synchronous movement of the glenohumeral and scapulothoracic joints has been emphasized in previous kinematics studies, most investigations of shoulder joint position sense have treated the shoulder complex as a single joint. The purposes of this study were to investigate the joint position sense errors of the humerothoracic, glenohumeral, and scapulothoracic joints at different elevation angles and to examine whether the errors of the glenohumeral and scapulothoracic joints contribute to the errors of the humerothoracic joint. Fifty-one subjects with healthy shoulders were recruited. Active joint position sense of the humerothoracic, glenohumeral, and scapulothoracic joints was measured at 50°, 70°, and 90° of humerothoracic elevation in the scapular plane. The results showed that while scapulothoracic joint position sense errors were not affected by target angles, there was an angle effect on humerothoracic and glenohumeral errors, with errors decreasing as the target angles approached 90° of elevation. The results of a multiple regression analysis revealed that glenohumeral errors explained most of the variance of the humerothoracic errors and that scapulothoracic errors had a weaker predictive relationship with humerothoracic errors. Therefore, it may be necessary to test scapular joint position sense separately in addition to the assessment of the overall shoulder joint position sense.
Yin-Liang Lin and Andrew R. Karduna
The measurement of humeral kinematics with a sensor on the humerus is susceptible to large errors due to skin motion artifacts. An alternative approach is to use data from a forearm sensor, combined with data from either a scapular or thoracic sensor. We used three tasks to assess the errors of these approaches: humeral elevation, elbow flexion and humeral internal rotation. Compared with the humeral method, the forearm methods (using either a scapular or thoracic sensor) demonstrated significantly smaller root mean square errors in humeral elevation and humeral internal rotation tasks. Although the errors of the forearm methods were significantly larger than those of the humeral method during elbow flexion, the errors of the forearm methods still were below 3°. Therefore, these forearm methods may be able to accurately measure humeral motion. In addition, since no difference was found between the forearm methods using the scapular or thoracic sensor, it may be possible to accurately assess both shoulder and elbow kinematics with only two sensors: one on the forearm and one on the scapula.
Katya Trousset, David Phillips and Andrew Karduna
Proprioception is assessed more often through joint position sense and kinesthesia than force sense. The purpose of this study is to investigate force sense at the shoulder. A total of 12 subjects were recruited. An ipsilateral force reproduction protocol at the shoulder at 50°, 70°, and 90° and 120%, 140%, and 160% baseline torque. Dependent variables were constant error (CE) and root mean square error. An effect was found for load on absolute (p = .001) and normalized CE (p < .001). CE decreased with increased load. An effect for angle was found for absolute root mean square error (p = .002), more accurate at 50° (p = .01), but no effect when normalized (p = .19). With increased loads, subjects undershot the target and CE approached zero. Because of the differing behavior in CE and root mean square error, and absolute and normalized data, force sense studies should examine error from these perspectives.
Jun G. San Juan, Peter Kosek and Andrew R. Karduna
Subacromial impingement syndrome is the most common shoulder disorder. Abnormal superior translation of the humeral head is believed to be a major cause of this pathology. The first purpose of the study was to examine the effects of suprascapular nerve block on superior translation of the humeral head and scapular upward rotation during dynamic shoulder elevation. The secondary purpose was to assess muscle activation patterns during these motions. Twenty healthy subjects participated in the study. Using fluoroscopy and electromyography, humeral head translation and muscle activation were measured before and after a suprascapular nerve block. The humeral head was superiorly located at 60 degrees of humeral elevation, and the scapula was more upwardly rotated from 30 to 90 degrees of humeral elevation after the block. The differences were observed during midrange of motion. In addition, the deltoid muscle group demonstrated increased muscle activation after the nerve block. The study’s results showed a compensatory increase in humeral head translation, scapular upward rotation, and deltoid muscle activation due to the nerve block. These outcomes suggest that increasing muscular strength and endurance of the supraspinatus and infraspinatus muscles could prevent any increased superior humeral head translation. This may be beneficial in reducing shoulder impingement or rotator cuff tears over time.
Lucas Ettinger, Jason Weiss, Matthew Shapiro and Andrew Karduna
In this study, we aimed to determine if electromyography (EMG) normalization to maximal voluntary isometric contractions (MVIC) was influenced by subacromial pain in patients with subacromial impingement syndrome. Patients performed MVICs in unique testing positions for each shoulder muscle tested before and after subacromial injection of local anesthetic. In addition to collection of MVIC data, EMG data during an arm elevation task were recorded before and after injection. From a visual analog pain scale, patients had a 64% decrease in pain following the injection. Significant increases in MVICs were noted in 4 of the 7 shoulder muscles tested: anterior, middle and posterior deltoid, and lower trapezius. No significant differences were noticed for the upper trapezius, latissimus dorsi, or serratus anterior. MVIC condition (pre and post injection) had a significant influence on EMG normalization for the anterior deltoid and lower trapezius muscle. Results indicate that subacromial pain can influence shoulder muscle activity, especially for the deltoid muscles and lower trapezius. In addition, normalization to MVIC in the presence of pain can have unpredictable results. Caution should be taken when normalizing EMG data to MVIC in the presence of pain.
Harpa Helgadottir, Eythor Kristjansson, Sarah Mottram, Andrew Karduna and Halldor Jonsson Jr.
Clinical theory suggests that altered alignment of the shoulder girdle has the potential to create or sustain symptomatic mechanical dysfunction in the cervical and thoracic spine. The alignment of the shoulder girdle is described by two clavicle rotations, i.e, elevation and retraction, and by three scapular rotations, i.e., upward rotation, internal rotation, and anterior tilt. Elevation and retraction have until now been assessed only in patients with neck pain. The aim of the study was to determine whether there is a pattern of altered alignment of the shoulder girdle and the cervical and thoracic spine in patients with neck pain. A three-dimensional device measured clavicle and scapular orientation, and cervical and thoracic alignment in patients with insidious onset neck pain (IONP) and whiplash-associated disorder (WAD). An asymptomatic control group was selected for baseline measurements. The symptomatic groups revealed a significantly reduced clavicle retraction and scapular upward rotation as well as decreased cranial angle. A difference was found between the symptomatic groups on the left side, whereas the WAD group revealed an increased scapular anterior tilt and the IONP group a decreased clavicle elevation. These changes may be an important mechanism for maintenance and recurrence or exacerbation of symptoms in patients with neck pain.