Surface electromyographic amplitude normalization methods: A review

The electromyogram is the summation of the motor unit action potentials occurring during contraction measured at a given electrode location. The voltage potential of the surface electromyographic signal detected by electrodes strongly depends on several factors, varying between individuals and also over time within an individual. Thus, the amplitude of the EMG signal itself is not useful in group comparisons, or to follow events over a long period of time. The fact that the recorded electromyographic amplitude is never absolute is mainly because impedance varies between the active muscle fibers and electrodes and its value is unknown. The EMG signal is highly variable and is dependent upon many factors. Thus, the amplitude of the temporally processed electromyography can only be used to assess short-term changes in the activity of a single muscle from the same individual when the electrode setup has not been altered. To allow comparison of activity between different muscles, across time, and between individuals, the EMG signal should be normalized, i.e. expressed in relation to a reference value obtained during standardized and reproducible conditions. Notwithstanding the importance of electromyographic amplitude normalization, studies on functional activities, such as gait, do not seem to show a uniform methodology. Taking this into account, the main purpose of this chapter is to review and discuss different normalization procedures to relate the most appropriate method for specific situations, based on how the normalization method might influence data interpretation. In addition, this review supports the development of proper normalization procedures for biomechanical studies of functional activities like human gait.