## 2 Materials and Methods  
The Declaration of Helsinki was used to carry out this work, to safeguud ethical principles for clinical research involving human beings, thus all procedures were approved by the local Ethics Committee (14_CEIPC2/2019). Before starting data collection, all volunteers were fully informed about this study and agreed by signing an informed consent form. To maintain anonymity, all data is confidential, and volunteers are identified by code numbers. To participate in this study, no volunteer could be under 18 years old, and they had to be a practitioner of the modality for at least 6 years and train at least 3 h a week. Athletes who had an injury in the last 6 months, who were using medication that could influence the musculoskeletal response, or who felt any muscle pain or discomfort were not allowed to participate in this study. Initially, the group was formed by eight athletes, four females and four males. However, before data collection, two female athletes were injured, and the group was reduced to six. The average and standard deviation of the volunteers characteristics were as follows--age: 21.8 \(\pm\) 4.9 years, height: 175.2 \(\pm\) 13.3 cm, weight: 66.5 \(\pm\) 12.2 kg, wingspan: 171.8 \(\pm\) 14.3 cm, and years of practice: 10.7 \(\pm\) 5.8. All volunteers were right handed. Figure 1 shows a flow diagram of the experimental procedure. The 6 athletes each performed 10 backhand smashes per racket, and used two rackets of different stiffness, resulting in a total of 120 smashes. Surface electromyography was used to analyze the muscular activity during each backhand (BH) smash. Normally, this movement is used when the player is not in position to play a forehand shot, and it is used to earn time by forcing the other player to the back of the court. The two rackets used by all volunteers were Yonex Duora 33 and Yonex Duora 88. The frame and string were the same for both rackets, namely Yonex BG3, with a diameter of 0.74 mm and 125 N tension. The Yonex Duora 33 (Figure 2) is a low-range racket that is usually used by less experienced players and is more flexible than the Yonex Duora 88 (Figure 3), which is a mid-range racket. The principal difference between these two rackets is the racket flex. Figure 1: Flow diagram of the experimental procedure. According to [PERSON] et al., this racket type has more stiffness, and is therefore more normally used by athletes of an advanced level. We used the SnowPeak C1101 shuttlecocks for this study, which have been approved by the Badminton World Federation for international tournaments [29]. The feathers of the C1101 are Class A synthetic, and the shuttle head is made of natural cork with a weight of approximately 5 g.  
Each athlete performed at 10 valid BH smashes with each racket, according to the protocol presented in Table 1.  
For the acquisition of data, sEMG BioSignalPlus equipment was used (Figure 4), which has a 1000 Hz frequency. AMBU BlueSensor N electrodes (AMBU, Ballerup, Denmark) were used, which are a compound of Ag-AgCl, pre-gelled, disk shaped, 10 mm in diameter, disposable, and adherent to the skin. BioPLUX telemetric equipment (PLUX, Lisbon, Portugal) with Bluetooth connectivity was used to record and amplify the EMG signals with the following characteristics [30]: gain (ratio between input and output signal): 1000; range: \(\pm\)1.5 mV; bandwidth: 25-500 Hz; input impedance: \(>\)100 GOhm. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Athlete** & **Yonex Duora 33** & **Yonex Duora 88** \\ \hline Muscle warm-up & & \\ \hline \multicolumn{3}{c}{Executed 5 BH smashes} \\ \hline Rest 30 s & & \\ \hline \multicolumn{3}{c}{Executed 5 BH smashes} \\ \hline Rest 30 s & & \\ \hline \multicolumn{3}{c}{Executed 5 BH smashes} \\ \hline Rest 30 s & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Protocol for the sEMG evaluation. Figure 3: Yonex Duora 88. Length: 675 \(\pm\) 3 m; width: 189 \(\pm\) 3 mm. Figure 2: Yonex Duora 33. Length: 675 \(\pm\) 3 m; width: 189 \(\pm\) 3 mm.