## 3 Muscle Activity and Disorders
### Muscle Electrodiagnostic Methods
#### 3.2.5 EMG Applications in Performers
With the presence of a backrest support, the muscular activity of both left and right erector spinae muscles in both violin and cello players is found to be reduced (about 20% lower) [68]. (f) Comparison Between Upper Strings and Cello Playing  
To characterize the asymmetric nature of upper string and cello playing, the spatial distribution of surface EMG of the trapezius and lumbar muscles can be studied. For both upper string and cello groups, the following is found: higher asymmetry of muscle activation in fast compared to slow playing, and a twofold increase in the trapezius muscle activity index (MAI) from the most medial to the most lateral string. In the presence of backrest support, a reduction in the MAI of both left and right erector spinae muscles by about 20% is noticed [68]. Opposed asymmetry differences in the middle and lower trapezius MAI in the upper string and cello groups are also noted between bowing down (from tail to tip) and bowing up (from tip to tail). In upper strings, sliding the bow up requires higher muscle activity (about 50% higher than sliding it down) [56; 59]. The reason is that there is a static loading of the left shoulder to support the instrument, and a dynamic, repetitive loading of the right shoulder to facilitate the bowing, the latter being eased by gravity during the bowing down movement [57; 68]. Conversely, in cello players, sliding the bow from the tail toward the tip requires higher activity (about 50% higher than sliding it from the tip toward the tail). Here, there is a smaller static load, and the left shoulder has a less supporting role compared to the upper string instruments. In the bowing side, the effect of gravity is less important because the player keeps the bow nearly horizontal throughout playing. Bowing from tip to tail, however, requires dynamic abduction of the right arm, obtained by the elevation of the elbow and depression of the scapula [12; 68]. (g) Piriformis Muscle Syndrome  
A potential syndrome of great discomfort in cells is piriformis syndrome, associated with the piriformis muscle, which can particularly become tight and cause pain through pinching of the sciatic nerve [43]. Although a monitoring of the surface EMG of the piriformis is possible [92], it might be preferable to make use of fine-wire electrodes, due to the deep location of the muscle [93]. (h) Muscle Fatigue  
Through the processing of the EMG signal, information about muscle fatigue can be obtained both in the time domain (e.g., amplitude of the normalized the rectified signal) and/or in the frequency domain (e.g., median frequency) [79; 80]. In musicians, EMG analysis has been found to be efficient for detecting muscle fatigue with PRMD players compared to those without PRMD [94]. (i) Efficacy of \"Taping\" for Muscle Movement Restriction  
As above-described, altering the resting position of the scapula due to muscle imbalance can stimulate pain [60]. This pain can be alleviated by taping the scapulae of violinists into a position that prevents excessive elevation and protraction whilst playing. EMG measurements can be used to evaluate the efficacy of taping treatment by recording the activity from the upper trapezii, the scapula retractors and the right sternocleidomastoid muscles. (j) Biofeedback  
The recording of surface EMG is also useful for biofeedback in the treatment of pain problems, and to train players to reduce excessive force, such as that produced in the left hand and wrist of violinists from gripping the neck of the violin too tightly [24]. EMG feedback has also been shown to reduce muscle tension while playing, by reducing excessive left arm extensor muscle activity in violinists [95]. (k) Assessment of Low Back Pain (LBP)  
Using EMG measurements, it has been demonstrated that the presence of low back pain (LBP) can cause a modification of the activation between synergistic muscles of the lumbar back, suggesting that subjects with LBP experience higher fatigability of the erector spinae muscles at the thoracic part than at the lumbar part [96; 97; 98; 99].