## 2 Materials and Methods
### Experimental Design
Following a 10 min submaximal continuous warm-up run, all runners completed the circuit with one of the researchers guiding the route to guarantee they received the same instructions and full details. At the end of the session, participants were re-familiarized with the RPE and vertical stiffness tests and familiarized, for the first time, with the footwear comfort test described below. **Running time trials (3 rd Day). ** Seven days after previewing the circuit, the participants performed two running time trials on this circuit (CON and AFT) with a 30 min passive recovery in between. The order of the sets was randomized, as previously described. The circuit was located in Urrez (Burgos, Spain) at an altitude between 1.190 and 1.457 m above sea level, and it had an elevation gain of +305 m, which was representative of a typical mountain race (e.g., OCC Mont-Blanc) [26]. It consisted of three different segments: 1.68 km uphill, 1.56 km downhill, and 1.95 km of mixed terrain (a combination of uphill, flat, and downhill segments) (see Figure 2). The circuit was well-marked with trails and roads as in official competitions. The type of terrain represented the most common trails in mountain races (i.e., grass, loose stone, gravel, and mud) with low technical difficulty. Weather conditions on the testing day were also recorded (temperature: 11-15 \({}^{\circ}\)C; humidity: 85-92%; wind: 20-25 km/h WSW), and the test took place in autumn of the North hemisphere (November). The timeline of the measurements is shown in Figure 1 (the specific protocols are described below). The warm-up was performed 18 min before the first time trial and consisted of 10 min of continuous running and two 30 s progressive runs [36]. Following  
Figure 2: Profile of the mountain circuit. Distance: 5.19 km; elevation gain: 305 m; elevation loss: 305 m. Maximum altitude: 1.457 m; minimum altitude: 1.190 m. Abbreviations used: meter (m), kilometer (km). the warm-up, vertical stiffness was measured 4 min before (pre-) and 5 min after (post-) each time trial. Participants were weighed 2 min before and 1 min after each running set (Mobba Missil F2-150, Mobba Inc., Barcelona, Spain) to calculate the vertical stiffness. Blood lactate samples were taken 1 min before (pre-) and 3 min after (post-) each time trial. Heart rate and running performances were collected during each time trial by using an HR monitor and a running power meter attached to the right foot. The shoes' comfort and the RPE were recorded 8 and 10 min after each time trial, respectively. After the first RPE recording, participants had 10 min to rest and rehydrate with 250 mL of water and 250 mL of an isotonic drink, and to change the footwear model and adjust the running power meter for the next set. Ten minutes before starting the second time trial, participants completed a brief re-warm up consisting of four minutes of submaximal continuous running. **Heart rate and running measurements. ** The HR was recorded using a chest strap (Polar(r) H10, Polar Electro Oy, Finland) and an HR monitor (Polar(r) Vantage V2, Polar Electro Oy, Finland), previously used during familiarization sessions (1 st and 2 nd days). This same device was used to record the positioning coordinates via GPS, providing data on total time, segment times, and maximum pace [20; 37]. Running power, step frequency, step length, contact time, leg stiffness, and the vertical oscillation of the center of gravity were obtained from a validated running power meter (Stryd(r) Power meter, Stryd Inc., USA) [38]. Data from both devices were analyzed using a specific software (Polar Flow(r), Polar Electro Oy, Finland), except for contact time, leg stiffness, and vertical oscillation of the center of gravity, which were analyzed using Stryd PowerCenter (Stryd Inc., USA). **Vertical stiffness measurements. ** The vertical jumps were performed on a delimited grass area next to the start and finish lines.