The Effects of Long Muscle Length Isometric versus Full Range of Motion Isotonic Training on Regional Quadriceps Femoris Hypertrophy in Resistance-Trained Individuals

This study explored the effects of isometric training at long muscle lengths (ISOM) vs. full range of motion (ROM) isotonic training (ISOT) on quadriceps femoris regional hypertrophy. Twenty-three healthy, resistance-trained men and women completed a 6-week, twice-per-week intervention. A within-subject study design was employed with limbs randomized to unilateral ISOM or ISOT knee extension. Muscle thickness was assessed pre- and post-intervention at proximal, middle, and distal sites of the anterior thigh and lateral thigh. Data was analyzed using Bayesian linear mixed-effects models. The between-condition estimate for summed anterior thigh muscle thickness was -0.20 cm (high-density credible intervals [HDI]: -0.54, 0.16), with 87% probability of direction (pd) and 75% of the posterior distribution exceeding the region of practical equivalence (ROPE). At the proximal site of the anterior thigh, between-condition estimates showed the greatest directional shift in favor of ISOM (contrast estimate: -0.11 cm [95% HDI: -0.24, 0.02]), with 82% of the posterior distribution exceeding the ROPE. Minimal to negligible changes in summed and regional lateral thigh muscle thickness were found for both conditions. Overall, ISOM and ISOT elicited similar quadriceps hypertrophy in resistance-trained individuals. Isometric training at long muscle lengths may elicit a superior hypertrophic effect in the proximal anterior thigh; however, uncertainty in the effect estimates precludes definitive conclusion in this regard and further investigation is warranted.