Stretch-mediated hypertrophy and strength increases and their impact on dynamic balance performance - a randomized controlled intervention study

Optimizing morphological and functional muscle parameters is of crucial importance in sports and rehabilitation. While resistance training is the most common intervention when improving muscle strength as well as general motor function such as balance, this option is not always applicable, calling for sufficient alternatives. One such opportunity is high-volume, high-intensity stretching. While previous research focused on lower leg muscles, this is the first study exploring effects on the quadriceps. Forty-nine recreationally active participants were allocated into a control group and an intervention group, which underwent twelve sessions of 15-min unilateral supervised hip flexor stretch for both legs over 4–5 weeks. Measurements included muscle thickness, isometric leg extension strength, and dynamic balance. Regional stretch-mediated ultrasound muscle thickness increased significantly in the rectus femoris (proximal: ηp²=0.24–0.41, p = 0.002-<0.001; distal: ηp²=0.14–0.40, p = 0.001-<0.001), but showed inconsistent effects in the vastus lateralis (ηp²=0.18, p = 0.008 only in the left leg). Isometric strength effects were significant in 70° (shortened position, right leg: ηp²=0.09, p = 0.04; left leg: ηp²=0.21, p = 0.001), but not at 110° (lengthened position) knee angle. Balance performance improved significantly leg-dependent in anterior (ηp²=0.13, p = 0.02, left leg), posterolateral (ηp²=0.14, p = 0.009, left leg) and posteromedial directions (ηp²=0.13, p = 0.02, right leg). This study provides first results on stretch-mediated muscle thickness and strength increases in the quadriceps with the potential for implementation in balance training, if participation in an active, supervised training program is not feasible. Future research should include additional balance outcomes, other muscle groups and clinical settings. German Clinical Trials Register ID: DRKS00036510 (03.02.2026). URL: https://drks.de/search/en/trial/DRKS00036510/details.