Abstract
This study examined the effects of different velocity loss (VL) thresholds during resistance training (RT) on adaptations to concurrent training (CT), with particular focus on strength, endurance, neuromuscular, and hypertrophic outcomes. Forty-one moderately trained men were randomly assigned to one of four groups: CT with RT at 0% (VL0; n = 10), 15% (VL15; n = 10), or 40% (VL40; n = 11) VL, or endurance training (ET; n = 10) alone. Over 8 weeks, CT groups performed squat-based RT at 70%-85% of one-repetition maximum followed by ET (separated by 10 min), consisting of running at 90%-105% (from 18 to 8 min) of maximal aerobic speed (MAS) two times per week. Assessments included cross-sectional area of the vastus lateralis, maximal isometric squat, progressive loading squat, countermovement jump, sprinting, MAS, fatigue resistance, and electromyography (EMG) during squat tests. All CT groups significantly increased muscle mass, with VL40 achieving the greatest gains (group × time interaction, p < 0.05), while ET showed no changes. MAS improved in all groups (p < 0.001), with ET achieving the greatest gains; within CT, the lower the VL, the higher the effect size (group × time interaction, p = 0.04). VL15 and VL40 obtained greater 1RM gains than ET (group × time interaction, p = 0.009). VL15 and VL40 significantly improved strength-related variables. ET showed no strength gains and significantly reduced the rate of force development at 400 ms (p = 0.01). VL0 increased EMG amplitude across loads, while ET reduced it (group × time interactions, p < 0.05). CT improved strength and endurance performance. However, fatigue induced during RT may attenuate endurance adaptations. ET enhanced aerobic performance but impaired neuromuscular function and failed to improve strength.