Amir Ali Jafarnezhadgero - Department of Physical Education and Sport Sciences, University of Mohaghegh Ardabili, Ardabil, Iran
Mohammad Mahdi Bahramisharif - Department of Sport Biomechanics, Faculty of Humanities, Islamic Azad University, Hamedan Branch, Hamedan, Iran
Mahdi Majlesi - Department of Sport Biomechanics, Faculty of Humanities, Islamic Azad University, Hamedan Branch, Hamedan, Iran

Background: The determinant role of different movements to strengthen the quadriceps on rate of knee joint co-contraction, hamstring to quadriceps muscle activity ratio and vastus medialis to vastus lateralis muscle activity ratio provides useful information for therapists, coaches and athletes about the role of each movement in the rehabilitation of patients with anterior cruciate ligament injury, osteoarthritis and patellofemoral pain syndrome. The aim of this study was to compare the rate of the generalised and directed co-contraction of knee joint muscles during free weights squat, smith machine squat, smith machine squat with one leg and the dead lift movements. Methods: ۱۴ healthy power lifters (age: ۲۶±۷ years) were participated in this study.A portable EMG system with six pairs of bipolar surface electrodes was used to record the activity of the gastrocnemious medialis, long head of biceps femoris, semitendinosus, vastus lateralis, rectus femoris, and vastus medialis muscles at a sampling frequency of ۱۲۰۰ HZ. Participants had enough experience to perform free weights squat, smith machine squat, smith machine squat with one leg and the dead lift movements. Participants carried out each movement ۵ times at an intensity equal to ۵۰% of one-repetition-maximum level. Repeated-measure ANOVA test was used for statistical analysis. Results: Rate of medial co-contraction in dead lift movement was higher than that in smith machine squat with one leg (P= ۰.۰۴۲), and free weights squat (P= ۰.۰۴۴), respectively. Ratio of hamstring muscles activity to quadriceps during the implementation of dead lift was higher than that in free weights squat (P=۰.۰۲۲). The generalised co-contraction value at the knee joint was the lowest in dead lift movement and highest in smith machine squat with one leg. Conclusion: In order to strengthen quadriceps muscles in people suffering from anterior cruciate injury, dead lift movement is more effective than free weights squat. For athletes exercising to strengthen muscular groups, smith machine squat with one leg is more effective compared with the other three movements.

electromyography, Weight lifting, Knee joint