Acute Effect of the Bounce Technique on Joint Angles, Net Joint Moments, and Muscle Activity in the Free Weight Back Squat.

Achermann, BB, Kong, T, Drewek, A, Lee, G, Lorenzetti, SR, and Kim, H. Acute effect of the bounce technique on joint angles, net joint moments and muscle activity in the free weight back squat. J Strength Cond Res XX(X): 000-000, 2026-This study investigated the biomechanical effects of the bounce technique during the free-weight back squat on joint kinematics, net joint moments, and muscle activation patterns. The bounce technique is characterized by a rapid transition between eccentric and concentric phases and has been proposed to exploit elastic energy and enhance concentric performance. However, its biomechanical effects remain poorly understood. Ten resistance-trained subjects (7 men and 3 women) participated in this study. Subjects performed 4 trials of bounce and standard squats across 5 external load conditions (0, 25, 50, 75, and 100% of body mass). Three-dimensional motion capture, force plates, and electromyography data were used to estimate Joint Angles, Net Joint Moments, and Muscle Activity. Linear mixed effect models were used to evaluate the effect of the bounce condition across the 5 load conditions. Joint angles were greater in the bounce condition, with increased hip (+4.1°) and knee flexion (+5.8°), anterior pelvic tilt (+4.7°), and greater forward trunk lean (reduced lumbar extension, -6.4°). Net joint moments were higher across joints, particularly at the hip (+38%), knee (+50%), and ankle (+21.3%), with the largest increase in lumbar extension moment magnitude (182%). Muscle activation shifted toward the posterior chain in the concentric phase, with greater activity in the biceps femoris (+40%), gluteus maximus (+37%), and gluteus medius (+28), while quadriceps activation remained unchanged in the concentric phase. The bounce technique substantially alters lower-limb biomechanics in the squat, producing higher load on the lower back and inducing a shift toward a more hip-dominant movement strategy.