## 2 Materials and Methods
### Experimental Design  
This study comprises a transverse analysis of the association between markers of muscle function/morphology and changes in CMJ jump height (\(\Delta\)CMJ) following a pre-activation protocol. As such, the participants visited the laboratory on three distinct occasions (1) to provide their informed consent and relevant anthropometric information, and familiarize with dynamometry, squatting exercise, and CMJ; (2) to determine their five-repetition maximum (5 RM) loads, further familiarize with CMJ, and determine dynamometry- and muscle morphology-related variables; and (3) to determine CMJ-related variables and responsiveness to PAPE.  
On their first visit to the laboratory, participants performed a light warm-up consisting of five minutes of pedaling with a 25 W load (Excalibur, Lode, The Netherlands), followed by 10 unweighted squats. Five minutes following the warm-up, they performed three five-second maximal isometric contractions with the knee extensors of their dominant leg in an isokinetic dynamometer (System 3, Biodex Systems, Shirley, NY, USA) with a one-minute rest interval between contractions. They were allowed to rest for five minutes and then performed three sets of five barbell back squats with no load other than the bar itself, which weighed 10 kg. Each squat was carefully supervised by an experienced examiner who provided feedback on proper squat technique (participants were instructed to maintain an upright posture with their chest lifted, scapulae adducted, and knees tracking in line with their toes). They were instructed to squat until their knees were flexed at 90\({}^{\circ}\) and then return to a standing position. After familiarization to the squat exercise, they performed three sets of five CMJs, also receiving immediate feedback. For CMJs, they were instructed to rapidly squat until approximately 90\({}^{\circ}\) and jump as high as possible immediately after. The 90\({}^{\circ}\) knee flexion angle used for squats and CMJs was controlled visually by experienced examiners.  
The second visit to the laboratory was conducted 48 h after the first visit. Prior to any procedures, thickness of the vastus lateralis muscle (VLMT) was determined. The same warm-up performed in the first visit was repeated, followed by the same procedures for familiarization to CMJ. After that, they performed two maximal voluntary isometric contractions with their dominant knee extensors in the dynamometer to determine isometric peak torque (IPT). Fifteen minutes later, individual 5 RM load in the barbell back squat was determined using a custom-built squat cage for safety purposes. The procedures for determination of the 5 RM load were followed according to what has been established in the literature and are described in detail elsewhere [15; 16].  
In the third and last visit to the laboratory, responsiveness to PAPE was determined. The participants repeated the same warm-up performed in the previous visits prior to the experimental procedures. CMJ height was assessed 10 min following the warm-up (control condition--CON). Ten minutes after CON CMJ assessment, participants performed a pre-activation protocol consisting of five squats with a 5 RM load. After four minutes, CMJ assessment was repeated (PAPE condition--PAPE). The timeline of the experimental procedures is illustrated in Figure 1.