Muscle synergies have been proposed as building blocks that simplify the construction of movements, and as a method to study motor behavior. However, the pre-processing of the EMG signals and the factorization algorithm may impact on their extraction and meaning. This preliminary work aimed at investigating the influence of the selection of the muscles on muscle synergies analysis. In particular, muscle synergies have been extracted from datasets including different upper limb muscles, recorded during the execution of planar reaching and grasping movements. Results suggested that the amount of degrees of freedom activated by the muscles considered in the analysis determines the number of muscle synergies, while muscle redundancy does not affect muscle synergies extraction. In conclusion, the number of muscle synergies is influenced not only by the task constraints and the performance of the movement, but also by the choice of the muscles. Therefore, both aspects have to be carefully considered to design muscle synergies analysis and to compare results from different studies.