## 2 Materials and Methods  
### Signal Collection  
In this preliminary study, a sample of sEMG signals was collected from three warm-blood horses (the Polish half-bred; geldings; range age: 6-12 years; range body mass 480-560 kg; female). The horses were selected from the available pool of didactic horses at the Didactic Stable of the Horse Breeding Division at the Warsaw University of Life Sciences (WULS) based on their similarity in age, body weight, height at the withers, and body conformation, as well as being free from lameness and any history of neurological disorders. Horses were deemed sound by an experienced veterinarian (B.T.) and had no known history of neurological disorders. Horses were fed individually calculated ratios of hay and concentrate, with feed distributed across three meals per day. A mineral salt block and fresh water were available at all times. Horses were regularly exercised and housed in boxes with daily turnout. Procedures were approved by the II Local Ethical Committee on Animal Testing in Warsaw on behalf of the National Ethical Committees on Animal Testing (protocol code WAW2/089/2020 approved on 29 July 2020). On the day of signal collection, horses were clipped (0.8 mm hair length) and cleaned (ethanol 80%) on the proximal aspect of the right forelimb from the carpal joint area to the elbow joint area (Figure 1A). In this area of the forelimb, four extensor muscles were identified by palpation: the m. extensor carpi radialis, m. extensor digitorum communis, m. extensor digitorum lateralis, and m. extensor carpi ulnaris. For each muscle, specific anatomical landmarks were identified by palpation (performed by an experienced equine surgeon and certified specialist in equine diseases, B.T.) and confirmed ultrasonography (performed by an equine surgery resident who had completed training in equine ultrasound, E.S.). Ultrasound examination was performed using a SonoSite Edge II ultrasound machine (Fujifilm, Valhalla, NY, USA) with a 6-13 MHz linear probe (Fujifilm, Valhalla, NY, USA). For the m. extensor carpi radialis, the proximal landmark was the lacertus fibrosus, the distal landmark was the medial styloid process of the radius [4], and two electrodes were placed at 30-40% of the proximal section of the distance between landmarks. For the m. extensor digitorum communis, the proximal landmark was the lateral tuberosity of the radius, the distal landmark was the lateral styloid process of the radius, and two electrodes were placed at 30-40% of the proximal section of the distance between landmarks [4]. For the m. extensor digitorum lateralis, the proximal landmark was the lateral epicondyle of the humerus, the distal landmark was the ulnar carpal bone [7], and two electrodes were placed at 20-30% of the proximal section of the distance between landmarks. For the m.  
extensor carpi ulnaris, the proximal landmark was the lateral epicondyle of the humerus, the distal landmark was the accessory carpal bone [7], and two electrodes were placed at 20-30% of the distal section of the distance between landmarks. In each location, two surface electrodes (pre-gelled Ag/AgCl; 4 cm diameter electrode; 4 cm inter-electrode distance; 270 Bx, Noraxon, Scottsdale, AZ, USA) were placed next to each other, parallel to the direction of the muscle fibers (Figure 1B). Tiny drops of conductive gel (Indiba, Las Rozas, Spain) were applied between the electrodes and the skin. Two electrodes were connected to one wireless sEMG sensor (DTS, Noraxon, Scottsdale, AZ, USA), composing bipolar derivations (Figure 1C). The surface electrodes and sEMG sensors were fixed on the skin by a tiny layer of Rudaspray medical glue (Nobamed, Bielefeld, Germany). The sEMG sensors were additionally stabilized using Pino Tape Sport kinesiology tape (Pino, Ahrensburg, Germany). One piece of kinesiology tape was applied horizontally, perpendicular to the long axis of the limb and the direction of the studied muscle fibers, encircling the entire limb at the level of the sEMG sensors and adhering to the skin between the sensors.