introduction
During human walking, agonist and antagonist muscles of the lower extremities act simultaneously to provide support, dynamic balance and propulsion
(Doorenbosch et al., 1994; Van Ingen Schenau et al., 1992) : The simultaneous contraction of agonist and antagonist muscles, might be a mechanism to improve movement efficiency
(Falconer and Winter, 1985) : The simultaneous contraction of agonist and antagonist muscles and increase joint stabilization
(Falconer and Winter, 1985; Peterson and Martin, 2010): Excessive or prolonged co-contraction could also increase metabolic cost
(Den Otteret al., 2006): The estimation of muscle co-contraction is a useful clinical tool to better understand how pathology can affect the muscle strategies during gait
(Olney, 1985; Solomonow et al., 1986): method used to examine the simultaneous actions of agonist and antagonist muscles can vary from advanced mathematical models
(Rudolph et al., 2000; Crenna et al., 1992; Unnithan et al., 1996): method used to examine the simultaneous actions of agonist and antagonist muscles can vary from simple electromyography (EMG) measurements
(Kellis,1998) : Due to the complexity in estimating the muscle moment, normalized EMG measurement were generally used to represent the muscle co-contraction