EMG Activity and Voluntary Activation during Isometric and Concentric Knee Extensions

It is well documented that the ability of human muscles to develop tension is dependent on the angular velocity. In concentric actions, the decline of maximal voluntary torque, consequential to a velocity increase, has been extensively studied. The underlying mechanisms may be attributed to either or both chemical and mechanical muscle properties and/or motor unit activation by the central nervous system. However, during maximal voluntary contractions, neural drive remains controversial. While a neural drive inhibition has been suggested at slow concentric angular velocities (e.g. Pinniger et al., 1991) and verified using EMG activity (Westing et al., 1991), the twitch interpolation technique has proven useful to show similar activation pattern whatever the concentric angular velocity (Gandevia et al., 1998). Therefore, this study was designed to re-examine and clarify the neural drive during isometric and concentric contractions using both EMG activity and activation level (twitch interpolation technique).