Shoulder joint and muscle exposure in violin musicians: a three dimensional kinematic and electromyographic exposure variation analysis.

Abstract

Subacromial impingement syndrome is a common disorder in the right shoulders of orchestral violinists. Studies performed to date on this population have been limited in terms of kinematic methods used, resulting in inability to relate recorded motions to clinically relevant motions. They have also used temporal EMG analysis, which can be misleading in terms of exposure. Previous kinematic research on non-musician populations, and using non-occupational tasks, have indicated that subacromial impingement is associated with reduced posterior tilting and upward rotation of the scapula, as well as increased upper trapezius and decreased serratus anterior activation.This study compared 20 violinists (3 males and 17 females) with right sided SIS to 30 normal controls (11 males and 19 females) in the performance of 30-second, randomized performances of slow and fast standardized musical repertoire (each score being played 4 times). Surface EMG of upper trapezius and serratus anterior were sampled at 1,000 Hz using pre-amplified electrodes and signals were further amplified and RMS processed at 100 ms to improve signal to noise ratio. Signals were normalized to resting EMG and relative voluntary electrical activity (RVE). Three-dimensional kinematic data were captured in a standardized fashion by mathematically embedding local coordinate systems within the trunk, scapula and humerus, and rotations of these segments about the embedded axes were sampled at 100 Hz. EMG and 3-D kinematic data were then analyzed using Exposure Variation Analysis (EVA) methods, which expresses 3 amplitudes of the EMG or kinematic signal in terms of time spent at each amplitude level. EVA arrays were expressed in 3X3 graphs and were analyzed in terms of speed effects (fast and slow), and injury effects (injured and uninjured) using Mixed-Effects Multinomial Logistic Regression statistical methods. The reliability of the EVA method was evaluated by calculating intraclass correlation coefficients (ICC) and standard error of the measurement (SEM) statistics for all EVA cells and for all dependent variables. The EVA methods used in this study were found to have moderate to high levels of reliability (moderate to high ICC and low SEM). The EVA method was able to discern differences in terms of speed and injury in both injured and uninjured participants. Musicians in both groups were observed to play in positions of increased glenohumeral internal rotation compared to non-musician subjects participating in other research studies at similar humeral elevation angles. Injured musicians were noted to play in positions characterized by increased posterior scapular tilting for longer durations of time, increased scapular upward rotation for longer durations of time, and increased scapular internal rotation at slow speeds. Injured musicians were also noted to adopt positions of increased scapular posterior tilting, increased scapular upward rotation for longer durations of time, and increased scapular internal rotation at fast speeds. When compared to their uninjured counterparts, injured musicians were also noted to perform with reduced amplitude but more static (longer duration) glenohumeral flexion, as well as with slightly increased glenohumeral external rotation compared to uninjured musicians. Injured musicians were noted to perform with increased short duration, low amplitude upper trapezius activity at slow speeds, while they played with reduced long duration, high amplitude recruitment at fast speeds. Lastly, injured musicians were noted to demonstrate increased amplitude of recruitment of serratus anterior at both slow and fast speeds. The EVA method of data reduction employed in this study was instrumental in identifying these differences where more traditional methods (also attempted in this study) failed to identify group differences with respect to injury. This study has therefore identified a reliable kinematic and EMG data reduction technique that can be used to assess the kinematics of shoulder motion, as well as the upper trapezius and serratus anterior muscle activation in violin musicians. The findings of this study suggest that violinists develop SIS because of the positions they adopt in playing the violin, and that the injured musicians may have developed compensatory strategies to avoid discomfort.