EMG recordings are frequently used to obtain a better understanding in the coordination of movements. However, EMG activity is made up by the weighted summation of activity of many motor units with different contractile properties. Recent studies have revealed that different motor units contribute to muscle force in different motor tasks. The flexible recruitment of motor units with various contractile properties allows a flexible tuning of muscle properties, but also complicates the interpretation of EMG activity.
Stan Gielen is with the Department of Medical Physics and Biophysics, University of Nijmegen, Geert Grooteplein 21, 6525 EZ Nijmegen, The Netherlands.