# External Validation of Accelerometry-Based Mechanical Loading Prediction Equations
## 2 Materials and Methods
### Protocol  
Data collection was conducted at the Porto Laboratory of Biomechanics (LABIOMEP-UP). Ground reaction force data were collected using five force plates embedded in the floor: three FP4606 (Bertec, Columbus, OH, USA) and two FP6090 (Bertec, Columbus, OH, USA).  
During data collection, participants wore three activity monitors (GT9X Link: 100 Hz, \(\pm\)16 g range; ActiGraph, Pensacola, FL, USA): one placed at the right hip (along the anterior axillary line at the level of the iliac crest), one at the lower back (at the midpoint between the two posterior superior iliac spines), and one at the right ankle (immediately superior to the lateral malleolus). Accelerometer placements at the waist line (right hip and lower back) were selected for their proximity to the body's center of mass, providing a representative measure of overall movement. Right hip placement was chosen due to its widespread use in research involving physical activity measurement with accelerometers. The placement at the right ankle was selected due to its closeness to the ground contact point, allowing it to capture impact forces during the activities. Hip and lower back accelerometers were securely fastened with clips on an elastic belt, while the ankle accelerometer was held in place with adhesive tape and an elastic belt. The researchers positioned the accelerometers in order to ensure their correct positioning and orientation.  
The protocol included data collection during walking, running, and jumping activities. For walking and running, a 10 m path was marked on the laboratory floor, including the force plates in its center. Participants performed normal, slow, and fast walking, followed by slow and fast running, always with a self-selected speed. Three valid attempts were collected for each condition, with a 30 s rest between the walking trials and 1 min between the running trials. Attempts were considered valid if the participant's foot landed entirely on at least one force plate, without stepping on its borders. Participants were not informed about the force plate placement, nor of the necessity to completely step on them during each trial, to avoid altering their natural movement patterns during walking and running.  
Participants also completed a series of jumping trials, starting with two types of jumps using a 30 cm high box: (i) drop jumps (from the box to the floor) and (ii) box jumps (from the floor to the box). Then, they performed countermovement jumps with and without arm movements (hands on hips for the latter), horizontal forward jumps (from the floor to the platform and vice versa), side hops (multiple lateral jumps between two force plates, with either both legs together or landing with one foot on each platform), and continuous jumps (starting from a standing position with feet hip-width apart, at two heights: approximately 5 cm for 10-15 repetitions and approximately 15 cm for 5 repetitions). Three valid trials were recorded for drop, box, countermovement, and horizontal forward jumps, while multiple jumps were tested for side hops and continuous jumps. A 1 min rest interval was applied between the jumping trials.  
Activity monitors incorporated built-in primary and secondary triaxial accelerometers, with the primary having a dynamic range of \(\pm\)8 g and a proprietary filter and the secondary having a dynamic range of \(\pm\)16 g with unfiltered data. To avoid filtering bias and increase accuracy and replicability, only data from the secondary accelerometer were used. Data from force plates (Qualysis Track Manager, version QTM 2021.1; Qualysis AB, Gothenburg, Sweden) and accelerometers (ActiLife, version 6.13.3; Actigraph, Pensacola, USA) were processed with the manufacturer's software and exported as raw x, y, and z vector data. Ground reaction forces were reported in Newtons (N), and acceleration in units of gravitational acceleration (1 g = 9.807 m\(\cdot\)s\({}^{-2}\)).