Comparison of Architectural and Performance Adaptations of Hip Extension Exercise Under Gravitational or Inertial Loading Conditions

Primary outcomes

1. Fascicle length

   Time frame: Baseline, at weeks 2 and 3 of the intervention, at the end of the 6 weeks of training and 4 weeks after this (detraining period)

   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))

2. Muscle Strength Assessment

   Time frame: Baseline, at the end of the 6 weeks of training and 4 weeks after this (detraining period)

   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.

3. Jump performance

   Time frame: Baseline, at the end of the 6 weeks of training and 4 weeks after this (detraining period)

   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.

4. Power

   Time frame: Baseline, at the end of the 6 weeks of training and 4 weeks after this (detraining period)

   Power was calculated in both modalities (i.e., HE gravitational and inertial) as the product of velocity and force. The velocity was calculated using the MuscleLab 4020e linear encoder (Ergotest Technology AS, Porsgrunn, Norway). For gravitational loading (i.e. constant mass), the force was calculated as the product of mass and acceleration. For inertial loading, the force was calculated using a force gauge that was anchored along the rope, between a pulley and the proximal end of the second rope.

5. Soreness

   Time frame: From session 1 (the beginning of week 1) to session 12 (the end of week 6), at the beginning and end of each session.

   Participants were asked "How sore do you feel in your hamstrings?" and rated their soreness on a 10-point Likert Scale (1 = No Soreness, 3 = Minimal Soreness, 5 = Moderate Soreness, 8 = Very Sore, 10 = Extremely Sore)