Spatial filtering has become a common way to improve the resolution of surface electromyographic signals (SEMG) when used in connection with electrode arrays. The goal of this study is to observe the behavior of S-EMG amplitude and spectral descriptors when signals are submitted to a longitudinal quadruple differentiating spatial filter. Signals were acquired at 20% and 60% of the maximum voluntary contraction using a linear array of eight surface electrodes in order to understand the impact of the filtering technique in the S-EMG variables during fatiguing and non-fatiguing contractions. The final results show that the filtering procedure yields better selectivity, suggesting that single motor units can be better observed if spatial filters and measurement configurations with smaller pick-up areas are used. During fatiguing contractions, however, further analysis is needed.