Hip extension exercise performed under inertial loading conditions using the EPTE Inertial Concept (Ionclinics SL, L'Alcudia, Spain) during six weeks with two sessions per week. The EPTE Inertial Concept is an inertial device that combines a series of output pulleys and telescopic arms, capable of implementing 6 disks of different weights and radii.

Hip extension exercise performed under gravitational loading conditions using free-weight equipment during six weeks with two sessions per week.

Participans was advised to continue their regular levels of physical activity but not to perform any lower body resistance training

Hip extension exercise with inertial load

Two-dimensional images of the Biceps femoris long head (BFlh) architecture were captured using B-mode ultrasonography. All imaging was conducted with participants lying prone with their hips and knees in a neutral and fully extended position.Three ultrasound images were taken and stored for analysis through the ImageJ software (National Institutes of Health, USA).

The most visible fascicle in each image was used for measurement of fascicle pennation angle (i.e., angle between the fascicle and the intermediate aponeurosis) and estimation of fascicle length using a validated equation:

FL = sin (AA + 90°) × MT ÷ sin (180° - (AA + 180° - PA))

Knee flexion was assessed with subjects in prone position performing knee flexor maximal isometrics contractions with a knee angle of approximately 15 degrees. For hip extension, a resisted hip extension was requested with the subjects lying in the prone position with their legs straight. Hip adduction was evaluated in a supine position with the hips at 45º flexion, requesting an maximal isometrics contraction adduction with a resistance in the internal condyle.

CMJ and SJ, utilizing the MyJump 2 application on an iPad Pro (at 240 frames per second, high-definition video resolution). In the CMJ, participants initiated the jump from a standing position, performed a preparatory movement by bending their knees to 90° flexion, and then jumped upward as high as possible without pausing between movements. Similarly, for the SJ, participants began from a standardized position with knees flexed at 90°, and then jumped as high as possible with hands on their hips. Throughout both tests, participants were instructed to maintain their hands on their hips to minimize lateral and horizontal displacement, ensuring accurate vertical jump measurement.