## 2 Materials and Methods
### Data Analysis  
Motive software (NaturalPoint, Inc., Corvallis, OR, USA) was used to process the initial data. During 30 s of recording, a minimum of 45 stride cycles was registered approximately in each condition. Marker data were filtered with a fourth-order low-pass Butterworth filter with a cut-off frequency of 6 Hz [27]. Angular kinematics and spatiotemporal parameters were calculated using a custom routine performed with the MatLab R2013b program (Mathworks Inc, Natick, MA, USA). To detect the movements of hip and knee flexion-extension, shank oscillation, ankle dorsif flexion-plantar flexion, and rearfoot version-inversion, the angle convention shown in Figure 3 was used. Thereby, the absolute angle of the thigh with respect to the vertical angle and shank with respect to the horizontal and relative angles between the thigh and shank, shank and foot [28], and posterior leg and heel [23] for knee, ankle, and rearfoot respectively were calculated. Before recording, a standing calibration trial was collected, and the angular positions of body segments analyzed were considered as the anatomical position. Positive values represented the hip flexion, knee flexion, greater shank oscillation, ankle plantar flexion, and rearfoot inversion, while negative values described the hip extension, knee extension, lower shank oscillation, ankle dorsiflexion, and rearfoot even. Angular kinematics were calculated using an XYZ Cardan sequence of rotation, and root mean square error (RMSE) was calculated to determinate the 3D reconstruction accuracy obtaining a systematic error of 0.005, 0.012, and 0.037 mm for X (mediolateral), Y (interop posterior), and Z (vertical) axes, respectively. Figure 2: Protocol followed for fixing the retro-reflective markers. (**A**): fit shorts, (**B**): shave the area, (**C**): mark the anatomical point, (**D**): add adhesive spray, (**E**): wait for it to dry, (**F**): stick the marker, (**G**): reinforce with a non-woven adhesive material, (**H**): add adhesive spray again on the non-woven adhesive material. Angular kinematics was registered at the initial contact, maximum knee flexion, toe-off, and maximum oscillation. Running phases and gait cycles were normalized to 100 data points. The stance was divided into absorption phase--from the initial contact to maximum knee flexion in the midstance--and generation phase--from maximum knee flexion to toe-off [29] (Figure 4). Traditionally, initial contact has been identified using the vertical velocity of the heel but only is valid in rearfoot strike runners [30]. [PERSON] and [PERSON] [31] showed that the vertical velocity of the pelvis describes a similar behavior in all foot strike patterns, representing a better method to identify the initial contact. Thus, initial contact was identified as the frame of maximum downward velocity of the trochanter. Maximum knee flexion during the stance phase was detected as the knee flexion peak located between the two knee extension peaks produced in the initial contact and take-off. Toe-off was identified as the second knee extension peak [30], and maximum oscillation was defined as the knee flexion peak during the swing phase, which is located between the two knee extension  
Figure 4: Representation of maximum knee flexion (MKF), toe-off (TO), and maximum oscillation (MO) events using the knee angular kinematics once the initial contact (IC) instant is known through the vertical velocity of the trochanter. The black line represents the mean movement pattern, and the green lines represent every running stride. Figure 3: Conventions used for lower extremity angles. peaks produced in the toe-off and initial contact. Figure 5 represents the kinematic shape of every anatomical segment during the running test. Regarding spatiotemporal parameters, stride frequency (Hz) was calculated using (1) the time (s) between consecutive initial contact of the same foot and (2) stride length (m) dividing treadmill speed (3.89 m/s) by stride frequency.