Acute Effects of Complex-Contrast Exercise Performed With Two Different Intracontrast Rest Intervals on Muscle Activation, Heart Rate, and Countermovement Jump Performance: A Randomized Crossover Study

ABSTRACT: Thapa, RK, Babu, TS, Saini, P, and Bagchi, A. Acute effects of complex-contrast exercise performed with two different intracontrast rest intervals on muscle activation, heart rate, and countermovement jump performance: a randomized crossover study. J Strength Cond Res XX(X): 000-000, 2026-This study compared the acute effects of complex-contrast training (CT) exercise sets performed with 2 different intracontrast rest intervals (ICRI; i.e., the rest period between resistance and ballistic exercise within CT) on muscle activation, heart rate, and countermovement jump (CMJ) performance. Eleven male subjects (age: 20.9 ± 1.6 years, height: 172.4 ± 6.2 cm, body mass: 66.2 ± 7.6 kg, relative back squat strength: 1.3 ± 0.2 kg·kg-1) completed 2 experimental CT sessions (with 85% of 1 repetition maximum back squat and CMJ as a contrast pair of exercise) using either 30-second or 4-minute ICRI in a randomized, crossover manner. Neuromuscular function was assessed pre- and post-CT using maximal voluntary isometric knee extension. Muscle activation (using surface electromyography) of rectus femoris, vastus lateralis, and vastus medialis was recorded during squats and CMJs across 3 CT sets and during the knee extension test. Countermovement jump kinetics and kinematics were assessed using force platforms. No differences between ICRI conditions were observed in peak force (p = 0.319; ηp2 = 0.05) or muscle activation (p = 0.203-0.722; ηp2 = 0.12-0.67) during the knee extension test, nor during squats or CMJs (p = 0.060-0.773; ηp2 = 0.02-0.50). Similarly, most CMJ kinetic and kinematic variables did not differ between conditions (p = 0.287-0.992; ηp2 < 0.01-0.28). However, CMJ peak relative force at set 3 (p = 0.001, g = 1.26) and post-CT exercise heart rate (p = 0.044, g = 0.67) differed between ICRI conditions, with the 4-minute ICRI maintaining peak relative force and eliciting a lower heart rate. These findings suggest that shorter ICRI does not exacerbate acute neuromuscular fatigue or impair CMJ performance but may increase cardiovascular strain. Therefore, shorter ICRI may be used to increase training session density when time is limited, while acknowledging the associated physiological stress.