## 2 Materials and Methods
### Data Collection  
The personal scull boat of each rower was equipped with Powerline systems (Peach, Cambridge, UK). Such a system consisted of bi-axial force and position sensors at port and starboard gates, as well as force sensors at the left and right foot stretchers, allowing for the quantification of the applied effective forces and the oar angles and angular velocity. This system also included a GPS sensor and an inertial measurement unit (IMU). Based on the fusion of these data, the Powerline system computed the speed and acceleration of the boat. All Powerline data were exported at 50 Hz. The Powerline box was installed in front of the rowers and provided them with direct feedback on their stroke rate as they are used to in training. In addition, we collected neuromuscular parameters of the upper limb and trunk via surface electromyography (EMG) at 2000 Hz with Trigno Snap Lead sensors (Trigno(tm), Delsys, Natick, MA, USA). Prior to measurements, the skin was shaved, abraded, and cleaned with alcohol. An impedance below 5 k\(\Omega\) was set as a criterion for a good preparation. Afterwards, the electrodes (Ambu(r) BlueSensor SP, Ambu A/S, Ballerup, Denmark) were attached parallel to muscle fibers of 13 muscles, including left arm muscles (Figure 1): biceps brachii (BB), brachioradialis (BR), deltoideus anterior (DA), deltoideus posterior (DP), flexor carpi ulnaris (FCU), and triceps brachii longus (TBL); and left core muscles: rectus abdominis (RA), external obliquus (EO), internal obliquus (IO), lumbar erector spinae (ESL), thoracic erector spinae (EST), tensor fasciae latae (TFL), and gluteus maximus (GM). Electrode placements were in line with the SENIAM project's recommendations [30] and previous literature [6; 21; 31]. The right GM was also recorded as a backup measurement for the left GM. This decision was motivated by the fact that in preliminary studies, the clothes of the rowers, combined with the cyclical motion, proved to cause artefacts and patch detachment on the GM sensor. One last EMG channel was dedicated to the acquisition of data coming from a trigger adapter sensor (Trigno(tm), Delsys,Natick, MA, USA). This trigger was connected to the Powerline system and transmitted the signal from the starboard oar position to the Delsys system, thus allowing for the synchronization between the two systems. Finally, pelvis, spine, and left arm kinematics were measured via seven IMUs (iSen, STT Systems, San Sebastian, Spain) registering at 100 Hz the 3D spatial orientations [14]. The IMUs (Figure 1) were placed between the two posterior superior iliac spines, on the spine at L2 vertebra level, between the two scapulae at the T6 verteba level, and at the C7 verteba level. On the left arm, they were placed on the left third metacarpal bone for the hand, between the radius and ulna for the forearm, and on the lateral side of the arm. IMUs were placed only on the left arm to avoid interfering with the rowing motion. Indeed, in sculling, the hands overlap during the drive phase, with the left hand passing a few centimeters above the right one, possibly interfering with any sensor placed on the back of the right hand. One additional IMU was placed on the boat to measure the boat's acceleration and allow for synchronization with the Powerline system. Both the Trigno and STT systems were connected wirelessly to a receiver placed on a boat driven by the rowers' coach that closely followed the rowers throughout their training. Powerline data were downloaded from the logger after the end of the session.