## 3 Muscle Activity and Disorders
### Muscle Electrodiagnostic Methods
#### 3.2.5 EMG Applications in Performers  
(a) Co-Activation of Muscles  
Co-activation (or co-contraction) refers to the simultaneous activity of antagonist muscles. The consequence of co-activation is that the same joint torque can be produced in various different ways, depending on the activation level of each of the acting antagonist muscles [76]. From the purely mechanical aspect, such co-activation is unwanted because, despite the fact that the torque obtained is the same, it can cause a greater resultant joint force. Co-activation is, however, physiologically advantageous because it enhances the controllability of posture and the motion of the joint. In playing upper string instruments, co-activation takes place when the left forearm, the wrist and the finger flexor, as well as extensor muscles, are used to control the fingering movements in the hand. Similarly, in the right forearm both the flexors and extensors are used to control the bow of the instrument [1; 4; 35; 56]. Co-contraction is also found in drum players at both extremely slow and very high tempi [73]. However, reduced co-contraction is achieved as drum players gain expertise, whereath they tend to display a muscle activation pattern that involves the reciprocal firing of the antagonist flexor/extensor muscles, which improves performance. (b) Muscle Imbalance and Asymmetry  
To reveal muscle imbalance, the quantitative EMG results of the bilateral trapezius, deltoid, biceps, and triceps muscles are compared between players with performance-related pain in the neck and shoulder region and players without pain [35; 67]. The EMG activity, as measured by the rectified signal of the pain group of violin players, is reported to be significantly higher in the trapezius (highest), deltoid and biceps muscles on the painful side, indicating that the group suffering from pain employed significantly more force in these muscles when playing, as compared to the pain-free group [47; 67]. (c) Risk Assessment (e.g., Increased Deltoid Activity)  
Specifically, an increased deltoid muscle activity bears risks due to the following: the deltoid, as the prime abduction mover of arm, is in fact an antagonist muscle of the adduction pectoralis major and latissimus dorsi muscles. Additionally, with raising the arm the deltoid also raises the humeral head. To counterweight the subsequent compression of the humeral head against the undersurface of the acromion and reduce the risk of injury of the supraspinatus tendon, simultaneous contractions of the infraspinatus, trees minor and subscapularis rotator cuff muscles occur [87]. An increased deltoid muscle activity will usually evoke a decrease in subacromial space and an increased compression of the humeral head, thus augmenting the hazard of injuring the supraspinatus tendon. (d) Subacromial Impingement Syndrome (SAIS)  
Subacromial impingement syndrome (SAIS), the commonest disorder of the shoulder, is a painful disorder due to loss of subacromial space [88]. It can be divided into two categories: structural and functional [89]. The first category is characterized by the physical loss of contact in the subacromial space due to inflammation (e.g., of the supraspinatus tendon), or bony growth. In functional impingement, the relative loss of subacromial contact is secondary to the altered scapulohumeral mechanics resulting from glenohumeral instability and muscle imbalance. In players of the violin and viola instruments, a mixture of these two categories takes place [67; 90]. (e) Assessment of Shoulder Rest and Backrest Support  
EMG of the upper arm muscles of the violin players can also be used to assess the efficacy of a shoulder rest attached to the violin. The EMG signals (rectified EMG) of the left trapezius and right sternocleidomastoid muscles reveal a significant reduction in muscle activity when the shoulder rest is used [62; 91]. EMG reduction in these muscles (thus their forces) is associated with their increased lever-arm. EMG is also an indicator of the efficacy of a backrest support.