The purpose was to investigate muscle potential during the vibrato motion for successful, healthy violin/viola performers. Electromyography was used to analyze parameters of muscle potentials during performance of a standardized exercise. These parameters were (a) evidence of potentials, (b) patterns of potentials, and (c) timing relationships (24 muscles). This study also sought to replicate and expand performance data from previous studies. Procedures from three pilot studies were used to standardize collection of EMG data. Synchronized video recordings were used to determine vibrato speed and conduct motion analysis. EMG data processing prior to analysis included power spectrum analysis and rectification, low-pass filtering, and smoothing data. Motion analysis findings (£D) were 1.09 for the elbow joints and 3.25 for the wrist joints. which was an indication of range of motion, suggested much greater activity in muscles controlling wrist movement than those moving the elbow. The degree of muscle potential and control were generally related to distance from the vibrating hand. Forearm muscle groups (8) demonstrated the greatest evidence of potential (76.5%) and were 18.1% non-periodic. Muscles of the upper arm (7) were off 59.4% and 57.0% non-periodic. Upper arm muscles had greater individual differences. Muscles of the chest and back (9) were collectively inactive (89.1%) and non-periodic (73.3%). With timing relationships, the forearm muscles demonstrated consistent firing patterns. Inconsistent firing patterns were evident in the upper arm, and to a greater degree in the chest and back muscles. Based on evaluations of performer motion and muscle potentials, it was strongly implied that there are (a) distinct roles for various muscles during vibrato (control vs. stabilization/support), (b) significant differences in potential between variables of rest, playing position, and performing, (c) significant differences in potential between some fingers, and (d) no significant differences between violinists and violists. The vibrato motion appeared to be controlled primarily by forearm muscles.