Background and Aim: Biomechanical conditions of limbs can affect on motor unit recruitment (MUR) markedly. Surface Electromyography (SEMG) is an approach for evaluation of muscle activities and onset time during different functions. The purpose of this study is to use SEMG for detection of central motor control in different functions of knee extensor muscles.

Materials and Methods: Ten healthy subjects contributed in this study. They were done five repetitions of knee flexion/extension in open and close chain separately during recording of SEMG simultaneously. Integrated EMG (iEMG) of three middle repeated cycles were detected according to chain (open- close) and kind of contraction (concentric- eccentric).then the Average of three slope of iEMG compared to each other. 

Results: There were not significant differences in MUR of three extensor muscles for concentric and eccentric contractions in open chain but vastus medialis and lateralis muscles showed significant changes in close chain (P<0.05). Result showed that there were significant differences of MUR for two kind of muscle contractions between open and close chains (P<0.05).  

Conclusion: Recruitment of motor units has not shown difference in open chain between concentric and eccentric contractions. This process indicates representative similarity of control pattern in two kind of contraction in open chain. Concentric contraction at close chain cycle has more MUR in comparison with eccentric contraction. It is equal to more interference of contractible compartment in concentric to eccentric contraction. This happens due to the nature of two kind of contraction particularly in close chain cycle. Comparison between two chains indicated that more motor units contribute in close chain (concentric and eccentric). It means that motor unit recruitment increased due to effect of increase of load in close chain.