2.5.1. Muscle Activity in the Lower Extremities

Muscle activity of the lower extremities was also measured by using EMG Noraxon Mini DTS (Noraxon, Inc., Scottsdale, AZ, USA). The common mode rejection ratio (CMRR) was 100 dB or more, and the input impedance was 100 Mohm or more. EMG signals were collected at a sampling rate of 1500 Hz with the notch filter set at 60 Hz. The EMG signal was obtained from 10 to 500 Hz using MyoResearch-XPME 1.07 (Noraxon, Inc., Scottsdale, AZ, USA) software. The first order high-pass and fourth order low-pass were filtered using Butterworth filters. The collected data were converted into root mean square values for analysis. To analyze the isometric contraction phase—which occurred during the 5 s-long pause following the execution of motion—only the data from the middle 3 s were used, with the exclusion of the initial and final second. To reduce the skin impedance of subjects, alcohol prep pads were applied on the point of attachment of the electrodes before evaluation. The point of attachment of the electrodes for the vastus medialis muscle was 4 cm above the medial surface of the patella, 3 cm on the medial side, 55° from the long axis of the patella, and the vastus lateralis muscle was 10–15 cm above the top surface of the patella, with a position of 15° from the long axis of the patella at a height of 6–8 cm. The flat-legged root was attached two-thirds of the way along the straight line between the knee bone and the lower front part [17]. The measurement posture measured three full squats and three half squats in the same posture as the squat exercise.