Previous studies have generally investigated patellar tendon stiffness, cross-sectional area of patellar tendon, patellar tendon length, patellar force and patellafemoral forces during decline squat. However, there is just a few study about electromyographic (EMG) activity during the eccentric squating, which is about rectus femoris (RF) recruitment. In the present study the investigators primarly aimed to quantify the activation of RF, vastus lateralis (VL), and vastus medialis obliqus (VMO) during eccentric squat on different decline angle (0, 5°, 10°, 15°, 20°). An additional purpose of our study was to compare the changes in muscle activation at various arc of knee flexion motion (degrees of 0-30, 30-60, 60-90, 0-60, 30-90, 0-90).The investigators hypothesised that the recruitment of RF, VL, and VMO during eccentric squat would differ according to decline angle and arc of motion.

The squat procedure in each condition will be repeated 10 times on a treadmill. The treadmill will be used to adjust the degree of decline in a standard manner. There will be a metronome to keep the speed of exercise in the same rhythm. All squats will be performed with a weighted barbell. The weight of it will be set as in ratio 30% of the participant's body weight

Inclusion Criteria:

• had exercised regularly (>3 days in a week) for at least six months.
• older than 18 years old.

Exclusion Criteria:

• had patellofemoral pain, knee pain otherwise unrelated to the knee extensor mechanism and other injuries that would restrict performance of the tasks (e.g. ankle pathology).

Asymptomathic older than 18 years, voluntary subjects who are students in a high school of physical education and sports.