This study investigated the effects of knee braces with differing stiffness on in vivo knee kinematics and neuromuscular control during single-leg lateral landings. 14 healthy males performed landings under three conditions: no brace (Control), low-stiffness (Type-1), and high-stiffness (Type-2). Kinematics were quantified via dual fluoroscopic imaging, and sEMG recorded seven lower-limb muscles. Brace mechanics were assessed via three-point bending. Statistical analysis used repeated-measures ANOVA (α = 0.05). Kinematically, neither brace restricted knee flexion. Both significantly reduced varus angle (Type-1: 27–100% stance, p=0.043 ; Type-2: 60–100% stance, p=0.033 ), and Type-2 also lowered peak sagittal flexion acceleration (5.0 rad/s2, p=0.013 ). Neuromuscularly, Type-1 enhanced multiplanar control, advancing rectus femoris (154.7 ms vs. Type-2, p=0.005 ) and vastus lateralis (35.6 ms vs. Control, p=0.046 ) activation without increasing rotational instability. Conversely, Type-2 demonstrated a trade-off: despite earlier vastus medialis activation (43.6 ms vs. Control, p=0.011 ), it significantly delayed gluteus medius activation (23.9 ms vs. Type-1, p=0.037 ) and, critically, exacerbated compensatory internal-rotation acceleration (3.3 rad/s2 vs. Type-1, p=0.006 ) at peak flexion. The low-stiffness brace leveraged neuromuscular coordination for multiplanar stability, whereas the high-stiffness brace improved frontal-plane protection at the cost of rotational instability. These findings provide biomechanical evidence for the synergistic optimization of mechanical support and neuromuscular adaptation in knee brace design for populations with similar characteristics to the young male athletes studied herein.