## 1 Introduction
EMG signals provide information about the structure and activation sequence of muscles that form a specific movement pattern. The acquired data are used in sports for learning and improving sports techniques and in the physiotherapy of patients with injuries. The advantage of EMG is the possibility to assess the degree and sequence of muscle activation before the execution of a motor task in the anticipation phase of a given movement. This issue should be considered in two regards. First, a precise motor program should activate only those muscles that determine the quality and speed of movement execution in a fully rational and economical manner in accordance with the objective of the motor task. Secondly, in every motor activity the key role is played by mechanisms of postural balance, usually subconsciously preceding a particular motor task. Through the use of EMG, it is then possible to determine which muscle groups, with how much bioelectrical tension and at what time, maintain balance and stabilize posture before performing a specific sport technique. The activities of the following muscles were examined in the present study: deltoideus middle head (DEL), triceps brachii (TRI), biceps brachii (BC), extensor carpi radialis longus (ECR), flexor carpi radialis (FCR), right latissimus dorsi (LDRT), left latissimus dorsi (LDLT), right external abdominal oblique muscle (EAORT), and left external abdominal oblique muscle (EAOLT). The above approaches are considered to be the classic methods of EMG signal analysis. In this study, however, a novel approach, i.e., frequency spectrum analysis of the EMGsignal, was proposed as an important complement. The so-called wavelet transform allows for the analysis of non-stationary signals including detailed information about the time-frequency changes in the EMG curve. This phenomenon is used in medicine, for example, to assess disorders of the neuromuscular and cardiovascular systems. In the present study two groups of antagonist muscles (EDA and LD) were selected and data for both right-side and left-side muscles were analyzed. By monitoring the EMG signal amplitude of the examined muscles, a coherence analysis between muscles was performed for four possible variants of muscle synchronization. This made it possible to analyze anteroposterior and lateral movements. The results of this study show that in Paralympic wheelchair fencers, muscle activation occurs at low frequency levels, regardless of disability category. Moreover, significantly higher coherence indices were observed in category B fencers at different frequency levels. This may suggest that fencers with more severe neurological deficits (category B) require greater stabilization of trunk muscles in order to maintain stable posture and skillfully perform difficult technical tasks. The recording of muscle bioelectrical activity makes it possible to demonstrate significant differences in athletes' performance, even in repetitive motor activities. These differences usually concern the value of the EMG signal, duration of muscle activation, and the simultaneous activity of muscle pairs, i.e., muscle co-activation, occurring when muscles from agonist and antagonist groups simultaneously exhibit co-contraction within a given joint [11,12]. Co-activation involves two groups of muscles contracting at the same time, thereby expressing the percent of movement in which both muscles are simultaneously active. Most commonly, co-activation can be measured from pre-recorded muscle contractions (activity) using surface EMG as the co-activation index calculated as the antagonistic muscle activity divided by the agonistic muscle activity. A decrease in this ratio means that there is more agonist activity or less antagonist activity resisting the movement [13]. The use of the co-activation mechanism is essential in assessing the stability of a given joint and it promotes motor control of particular motor activities [14,15].