2.4. Electromyographic Analysis

The EMG signals were recorded from the superficial quadriceps muscles (RF, VMO and VL) using a four-channel EMG unit system (MP 100, BIOPAC Systems, Goleta, CA, USA). The sampling rate was 1000 Hz, with a signal amplification of gain ×1000 and a standard mode rejection ratio of 110 dB. The frequency band-pass filter was set between 10 Hz and 500 Hz [18]. The notch filter was set at 50 Hz. Disposable bipolar Ag-AgCl disc surface electrodes (1 cm in diameter) were affixed over the chosen muscle groups, with the inter-electrode distance being approximately 20 mm [14]. The surface EMG electrodes were aligned with the muscle fibers of each muscle individually. The three muscles were palpated to ensure that the electrodes were placed over the bulk of the muscle belly. The assessor confirmed the EMG signals by testing the muscles and evaluating the signals for crosstalk. The RF electrode was placed halfway between the anterior superior iliac spine (ASIS) and the superior part of the patella. The VMO electrode was placed 80% of the way along a line between the ASIS and the joint space in front of the anterior border of the medial collateral ligament. The VL electrode was placed two-thirds of the way along a line from the ASIS to the lateral side of the patella [20]. In addition, a ground electrode was placed over the ipsilateral tibial tubercle of the medial malleolus. The electrodes were connected to an EMG data-collection system, and the signals were collected using customized software (Acqknowledge 3.4 for MP Systems, Biopac Systems Inc., Goleta, CA, USA). The root mean square (RMS) of the EMG signals was used for the analysis. These values were normalized as a percentage of the maximum voluntary isometric contraction (MVIC), which represented the activity of the RF, VMO and VL during each exercise.

The MVIC of activity of each muscle was determined in a sitting position with 90 degrees of hip flexion and 60 degrees of knee flexion. Prior to the MVC test, the participants warmed up their legs by cycling using a stationary bicycle without a load for 5 min. During the maximum knee extension measurement, the participants placed their arms across their chest and kept their hips and trunk on the chair. Verbal encouragement was provided throughout the test to motivate the participants to expend their maximum effort against a resistance above the ankle joint. The participants exerted their maximum isometric force as explosively as possible for ~2 to 3 s in order to retain a force plateau. The participants performed three trials with a 30 s rest period between trials. The average of the three MVIC trials was used for the subsequent calculations [21].

EMG activities were analyzed in portions of a 3 s isometric eccentric holding for the SQ exercise and a 3 s isometric concentric holding for the KE and SLR exercises. A finding provides evidence that isometric muscle contractions loaded in either concentric or eccentric manners elicit similar EMG amplitudes, and are therefore comparable in research settings [22]. The level of difference in EMG activity of the quadriceps femoris was statistically insignificant during eccentric contraction, isometric and concentric contraction [23].