Muscle activation

Muscle activation and activation time of the LD, deltoid, teres major, serratus anterior and trapezius (upper, middle and lower) were measured with wireless EMG (Delsys Trigno Wireless, Boston, MA, USA). Location of the sensors can be viewed in Table I and patient setup in Figure 1.

Sensor placement electromyography.

Patient setup.

EMG was used to measure the activity of the muscles during active maximum shoulder elevation movements for the LDT and ACS: forward elevation, scapular abduction and abduction in coronal plane.

To scale EMG max for all muscles in the LDT and ACS, Maximal isometric voluntary contractions (MIVC) were performed in 6 different movements in a standardized order as follows: forward flexion at 45°, flexion in the scapular plane at 45°, internal and external rotation at 90°of shoulder abduction, retroflexion, amd horizontal adduction at 90° of shoulder forward flexion. Each resisted task was performed 3 times and patients had a 1-minute rest period, the largest value was used for further MIVC analysis. The researcher held and resisted the arm at the level of the wrist while the patient was asked to elevate or rotate the arm as forcefully as possible for that specific movement. Muscle activity during the MIVC was measured in millivolts (mV).

A linear envelope was achieved by correcting the Raw EMG data for offset before rectification and low-pass filtering (2Hz recursive Butterworth).

The maximal EMG value measured during the MIVCs for each muscle was used to scale the EMG signal to the maximal performance and this maximal value was set as 100% EMG max.

For further analysis, the highest EMG value during each elevation movement was selected and reported as a percentage of the EMG max of that muscle.

The timing of recruitment was reported by observing the start of the kinematic elevation curve and measuring the time to recruitment of each shoulder muscle.