Muscular coactivation is fundamental in stabilizing and protecting the articulations during voluntary contractions and plays an important role in movement control. Numerous studies have shown the contribution of supraspinal and spinal mechanisms to the regulation of muscular coactivation but the implication of the primary motor cortex (M1) is still unclear. We studied the modulation of cortical oscillations and cortico-muscular interactions during isometric contractions in athletes having different levels of muscular coactivation as a consequence of their training orientation (strength (ST) vs. endurance (ED). We found that in ST, reduced muscular coactivation was associated with greater M1 activation, which could be explained by the control of a greater number of muscles, including antagonist muscles. Using a novel method to analyze cortico-muscular interactions, we show that M1 is directly involved in the control of antagonist muscles in all participants. However, the magnitude of cortico-muscular interactions with antagonist muscles was lower than in agonist muscles, which could be explained by a greater involvement of spinal mechanisms in the regulation of muscular coactivation. The estimation of agonist and antagonist muscle group moments opens the perspective to investigate the cerebral correlates of the modulation of muscular torque. Our results obtained through an approach combining biomechanics and neurosciences highlighted the involvement of M1 in the regulation of the muscular coactivation during isometric voluntary contractions.