High- and Low-Load Resistance Training Produce Distinct Skeletal Muscle Growth but Similar Changes in Tendon Morphology.

Abstract

To compare the effects of high- versus low-load isotonic resistance training performed to task failure on skeletal muscle growth and distal biceps brachii tendon thickness, while incorporating a time-matched non-exercise control group. 201 non-resistance trained individuals [20.9 (3.0) years] were randomly assigned to one of four groups: High-Load (n=45), Low-Load (n=48), Mid-Point (n=54), or Control (n=54). High- and Low-Load completed 4 sets of unilateral elbow flexions to task failure 3x/week for 6-weeks (8-12RM and 20-30RM, respectively). Mid-Point completed pre- and post-testing, plus a 1RM test at 3-weeks, whereas Control performed pre- and post-testing only. Muscle thickness of the elbow flexors and distal biceps brachii tendon thickness (short- and long-head regions) were assessed via B-mode ultrasound before and after the intervention. Group differences in pre- to post-intervention changes were evaluated using Bayesian ANCOVA with informative hypotheses, supplemented by Bayesian regression models to estimate adjusted between-group differences. Low-Load produced greater increases in muscle thickness than High-Load (~0.30 cm versus ~0.20 cm), with both exceeding changes observed in Mid-Point and Control. In contrast, distal biceps brachii tendon thickness demonstrated small increases at the short-head across all training groups (High-Load, Low-Load, and Mid-Point; ~0.02-0.03 cm) relative to Control, whereas no detectable changes occurred at the long-head. When resistance exercise is performed to task failure, skeletal muscle and tendon tissues demonstrate distinct adaptation patterns; muscle growth may be augmented by low-load failure training, whereas tendon morphological adaptations may be driven more by localized strain exposure rather than the external load lifted per repetition.