## 2 Material and Methods
### Electromyography: Measurement of Muscle Activity  
The muscle activity was measured electromyographically. A portable Biomonitor ME 6000 16\({}^{\circ}\) (Mega Electronics Ltd., Cambridge, UK) was used for the measurement (for more detailed specifications, see Figure S1). The device can process up to 16 channels simultaneously, and all channels are designed to receive signals from paired flat electrodes.  
Special surface bipolar electrodes from Medico Lead-Lok were used to detect the signal. The electrodes were attached to the proband's well-cleaned skin on tested muscles, in parallel in the middle line of the muscular abdomen, and the ground electrode was placed according to the protocol of the Megawin program. Electrodes were not manipulated during the measurement. When gluing the electrodes, the proband was positioned in such a way that allowed the palpation of the examined muscle. The electrodes were attached directly onto the skin not to break the contact between the skin and the electrode, thus preventing signal interference by the movement of the electrodes [22]. Eight channels were used for the measurement: two for _m. anterior serratus (dexter, sinister)_, two for _m. rectus femoris (dexter, sinister)_, two for _m. externus obliquus (dexter, sinister)_, and two for _m. erector spinae (dexter, sinister)_. The data from the ME 6000 Biomonitor were processed using the MegaWin SW program.  
The raw EMG signal was cut to record only the targeted movement we explored. The signal was then converted to the absolute value. After the conversion to the absolute value, before the actual thresholding (detection of the beginnings and ends of light activity), the course (electromyographic signal envelope) was smoothed by passing through the low-pass filter in the form of an FIR filter (including a moving average filter). Thus, we obtained the envelope by filtering the absolute value of the low-pass electromyographic signal. To suppress the delay of the signal when passing through the filter, we used a combination of the forward and backward filtration (_filltrilt_ function in Matlab). A low-pass filter with a cutoff frequency of 5.5 Hz and a non-pass band with an attenuation of approximately 107 dB starting at 8 Hz was used to calculate the envelope. The low-pass filter was implemented with an FIR filter with a length of 1501 coefficients at a sampling frequency of 1000 Hz [23].  
All subjects were instructed not to consume alcohol, drugs, or other addictive substances 48 h before the experiment to avoid affecting their locomotor behavior. Before starting the measurement, each proband was informed about the exact course of the experiment and had enough time to try out the exercises without electrodes. Probands performed the exercises precisely according to the instructions. The exercise was explained and demonstrated to probands at the beginning of the experiment. Each proband then performed a trial exercise before conducting it on a gymnastic horse and a walking horse. The familiarization of the exercises ensured that all probands achieved adequate quality in all exercises during the experiment.  
After being instructed, each proband practiced the exercises to eliminate execution errors. This was followed by the attachment of the electrodes and the configuration of the ME 6000 16\({}^{\circ}\). [PERSON]'s strength muscle test [24] was then performed to determine maximum voluntary contractions (MVC), followed by measuring the muscle activity during all three exercises. Electrodes were not changed during the measurements.  
All probands were measured under the same conditions in the gym and the riding facilities of the Premium Stables in Prague. The gymnastic and living horse sessions were performed on the same day. Each exercise was measured three times, and there was aone-minute break between each measurement and a two-minute break between different exercises. The interval between the sessions was 15 min. Probands carried out exercises in a randomized order, first on the gymnastic horse, then on the living horse.