Comparison of Architectural and Performance Adaptations of Hip Extension Exercise Under Gravitational or Inertial Loading Conditions: A Randomized Controlled Trial

Increasing the length of the biceps femoris long head (BFlh) fascicle is one of the objectives of hamstring injury prevention programs. This study aimed to investigate training-induced adaptations in BFlh architecture and performance measures after a six-week resistance training program with gravitational or inertial 45° hip extension (HE) exercise and a subsequent detraining period. Thirty-two resistance-trained males were assigned to a group doing a HE training program with gravitational loading (HE-G), HE by inertial loading (HE-I), or to a control group (CG). BFlh architecture, knee flexion and hip extension strength, jumping performance by means of the countermovement (CMJ) and squat jump (SJ) tests, and soreness before and after each training session were evaluated. HE-I participants modified the initial length of the BHlh fascicle (9.86 cm), increasing it by 1.64 cm at the end of the intervention and remaining 1.48 cm longer than baseline after the detraining period. Both HE modalities improved hip extensor strength (51.2 N for HE-G and 65.0 N) and only HE-I improved knee flexor strength (69.9 N). Soreness for HE-I exercise showed lower values than gravity-loaded performance, with at least one point difference across sessions, and more than two in some of the final sessions. This study provides evidence suggesting that HE performed with inertial loading is an effective exercise for lengthening the BFlh fascicle and promoting improvements in knee flexion strength and SJ performance, aspects not modifiable by the gravitational modality.