## 4 Discussion
### Effect of the Studied Cut-Off Frequencies on Signal Features  
The presented results underscore the importance of selecting appropriate signal filtering methods to enhance the reliability and interpretability of sEMG data in equine biomechanics research, particularly for applications in training, rehabilitation, and diagnosis. The first study to highlight the significance of equine sEMG-filtering methods investigated the effect of cut-off frequency on signal quality by comparing raw signals with four high-pass filtered signals (using cut-off frequencies of 20 Hz, 40 Hz, 60 Hz, and 80 Hz) to assess data loss [5]. The authors demonstrated that a 40 Hz high-pass filter completely attenuated the low-frequency peak while optimally preserving the non-artifact portion of the sEMG signal [5]. High-pass filtering is used to remove low-frequency components that are especially susceptible to baseline drift and movement artifacts [1,16]. [PERSON] et al. [5] subsequently recommended bandpass filtering with a 40-450 Hz cut-off frequency for processing sEMG signals from large equine muscles during trotting and centering [5,6]. A similar study was currently conducted on four small equine muscles during walking and trotting, evaluating additional signal features, amplitude, MF, and SNR, beyond the RMS analyzed by [PERSON] et al. [5]. Both the study by [PERSON] et al. [5] and the present study demonstrated that the specific choices of filtering method matter, and that future research is needed for unified signal-processing protocols and filtering constraints to ensure comparability across future equine sEMG studies. Efforts to standardize signal filtration methods in equine sport and medicine have already led to promising developments in subsequent biomechanics research. The filtering method recommended by [PERSON] et al. [5]--bandpass filtering with a 40-450 Hz cut-off frequency--has since been adopted in various studies on equine training, including examination of movement and muscle-activation strategies in horses with different jumping techniques [19] and muscle activity of leading limbs during canceling [20]. It has also been applied in diagnostic studies examining muscle activity in horses with induced lameness [21,22], as well as research exploring the effects of \(\beta\)-hydroxy \(\beta\)-methylbutyrate [23] and clenbuterol [12] on equine muscle function. However, despite the foundational work of [PERSON] et al. [5,6], more recent studies have continued to employ different filtering approaches, such as bandpass filtering with a 30-500 Hz cut-off frequency filtering when analyzing the infraspinatus, deltoids, splenius, and brachiocephalic muscles [24] and bandpass filtering with a 5-420 Hz cut-off frequency when measuring activity of the superficial gluteus muscle [25]. These inconsistencies underscore the ongoing need to raise awareness about the influence of filtering choices on sEMG signal characteristics and to further advance standardization in this area. It is worth noting that the bandpass filtering with a 40-450 Hz cut-off frequency was found to be favorable for signals recorded from the m. extensor digitorum lateralis and m. extensor carpi ulnaris during walk and trot. However, for signals recorded from the m. extensor carpi radialis and m. extensor digitorum communis, this filtering method resulted in significantly higher signal loss, a lower residual signal, and notable variations in signal features. Interestingly, the electrodes for these latter two muscles were placed closer to the proximal section of the distance between anatomical landmarks, compared to the placements for the former two muscles. This shift in electrode placement was necessitated by the considerable difficulty in obtaining high-quality signals for all four muscles at the same anatomical level.