## 1 Introduction  
Surface electromyography (sEMG) is a non-invasive technique used to assess muscle activity in both humans and animals. Among animal species, horses are of particular interest due to the essential role that balanced, symmetrical, and efficient locomotion plays in racing, sports, and pleasure riding [1]. However, the comparability and reproducibility of equine sEMG research are limited due to the lack of standardized guidelines, such as those established for human sEMGstudies [2; 3]. These human guidelines cover aspects such as muscle fiber direction, surface electrode placement, and sEMG sensor characteristics, as well as signal processing and analysis. In equine medicine, efforts have been made to standardize horse skin preparation and habituation to sEMG measurements [2], surface electrode placement along specific muscles [1; 4], and signal processing from large muscles [5; 6]. To date, equine studies focusing exclusively on sEMG signal processing have examined signal filtering and normalization from the m. biceps femoris during center [5], as well as the effects of high-pass filter cut-off frequencies on signals recorded from the m. biceps femoris and m. triceps brachii during trot and canter [6]. However, these studies did not consider the smaller, distally located extensor muscles, which play a crucial role in the forelimb motion [7]. Among these muscles, four major extensors--the m. extensor carpi radialis, m. extensor digitorum communis, m. extensor digitorum lateralis, and m. extensor carpi ulnaris--are responsible for extending and stabilizing the carpi joints and/or extending the digit [7; 8; 9]. Of these extensor muscles, the m. extensor carpi radialis [4; 10; 11; 12], m. extensor digitorum communis [4; 10; 13; 14; 15], and m. extensor digitorum lateralis [10] have been investigated using sEMG, while the myoelectric activity of the m. extensor carpi ulnaris remains unexplored. To date, the myoelectric activity of the m. extensor carpi radialis was measured during walk and trot and filtered using low-pass filtering with a 10 Hz cut-off frequency to assess the effects of Kinesio Taping on forelimb motion [11]. During trotting, it was analyzed using bandpass filtering with a 40-450 Hz cut-off frequency to investigate the effects of clen-buterol on muscle myoelectric activity [12] and to develop recommendations for electrode placement to measure muscle myoelectric activity [4]. In research focused on standardizing electrode placement, the m. extensor digitorum communis was also investigated [4]. Among carpal extensors, three muscles--the m. extensor carpi radialis, m. extensor digitorum communis, and m. extensor digitorum lateralis [10], or the m. extensor digitorum communis alone [13; 14; 15]--were studied in the context of the nociceptive withdrawal reflex evoked by transcutaneous electrical stimulation. In these specific sEMG studies, signals were filtered using bandpass filtering with a 7-200 Hz cut-off frequency [10; 13; 14; 15]. However, the comparability of previous equine sEMG studies is limited due to the different filtering methods employed. Even when assessing the myoelectric activity of a group of muscles with similar functions, such as carpal extensors, the signal filtering methods used in published sEMG studies vary considerably. These included low-pass filtering with a 10 Hz cut-off frequency [11], band-pass filtering with a 40-450 Hz cut-off frequency [4; 12], and band-pass filtering with a 7-200 Hz cut-off frequency [10; 13; 14; 15]. It is important to note that filtering with different cut-off frequencies reduces noise and artifact contamination in various ways while also influencing the preservation of the myoelectric signal [5; 16]. High-pass filtering is used to remove low-frequency noise and movement artifact contamination, preserving signal components above the chosen cut-off frequency. Conversely, low-pass filtering is applied to eliminate high-frequency contamination, retaining signal components below the cut-off frequency [16].