The force produced by a muscle depends on both the neural drive it receives and several biomechanical factors. When multiple muscles act on a single joint the nature of the relationship between the neural drive and force-generating capacity of the synergistic muscles is largely unknown. This study aimed to determine the relationship between the ratio of neural drive and the ratio of muscle force-generating capacity between two synergist muscles (vastus lateralis[VL] and medialis[VM]) in humans. Twenty-one participants performed isometric knee extensions at 20% and 50% of maximal voluntary contractions (MVC). Myoelectric activity (surface electromyography [EMG]) provided an index of neural drive. Physiological cross-sectional area (PCSA) was estimated from measurements of muscle volume (magnetic resonance imaging) and muscle fascicle length (3D ultrasound imaging) to represent the muscles’ force-generating capacities. Neither PCSA nor neural drive was balanced between VL and VM. There was a large (r=0.68) and moderate (r=0.43) correlation between the ratio of VL/VM EMG amplitude and the ratio of VL/VM PCSA at 20% and 50% of MVC, respectively. This study provides evidence that neural drive is biased by muscle force-generating capacity, the greater the force generating capacity of VL compared to VM, the stronger bias of drive to the VL.