Muscle Activation Patterns During Level Walking and Stair Ambulation

Dynamic electromyographic (EMG) data recorded using surface electrodes is commonly used to measure muscular activity during the performance of motor tasks and investigate normal and pathological motor strategies (Frigo & Crenna, 2010; Ferdjallah, 1998). In clinical gait analysis, the determination of the timing of muscle activation (“on-off”) is of paramount importance (C.J. De Luca, 1997; Shiavi, 1985). The evaluation of the “on-off” pattern of one or more muscles, particularly when examined together with kinematics (joint angles) and kinetics (joint moments and powers), provides an insight into the performance of muscles and their role in accomplishing a motor task (Gage, 1992; Benedetti et al., 1999; Davis, 1997). The relevance of considering the muscle activation timing is supported by several publications demonstrating its usefulness in orthopaedics (Andriacchi et al., 1982; Winter, 1984), treatment of cerebral palsy (Rose et al., 1999; Lyons et al., 1983; P.A. De Luca et al., 1987), and a number of other clinical applications (C.J. De Luca, 1997). More specifically, the timing of activation of muscles (“on-off” pattern) is the only reliable information that can be obtained from surface EMG signals recorded during dynamic contractions. In fact, the amplitude of the EMG signal is not only influenced by the actual electrical activation level of the muscle, but also by the electrical characteristics of the tissue between the active muscle fibers and the surface electrodes. Furthermore, since the relationship between electrical activity level and muscle force is non-linear, caution has to be taken in the interpretation of the EMG amplitude as an indicator of the muscle force output when data are recorded in dynamic conditions (C.J. De Luca, 1997). Unfortunately, this approach does not allow one to investigate completely the advanced control features of the central nervous systems, since muscle activation is simplified considering a muscle either “on” or “off”. However, in a great number of the clinical queries, the muscle activation timing represents the information of interest for the clinician, provided that reliable data are obtained for the activation intervals. Methods proposed in literature to determine the onset and offset of muscle activation intervals generally rely on an arbitrary choice of the threshold value that discriminates