During the last decades, rugby union focused on players' physical development as the main way of performance optimization. Consequently, a tremendous amount of research attention has been devoted to better understanding the main training parameters, neglecting however to study the effects of physical conditioning on group dynamics. In the present research, a quasi-experimental design in ecological conditions was used to analyze the effects of effort intensity on social interactions, especially during the rest periods of physical training. Semiprofessional rugby union players (N = 61; M age = 23.78, SD = 7.03 years) participated in this study. They were randomly assigned to four conditions consisting of three 5-min runs. With counterbalancing, the participant sample was divided into four groups. A control group ran at 50% Maximum Aerobic Speed (MAS) while the three experimental groups ran at three different effort intensities (80%, 90%, and 100% MAS) in a counterbalanced order (i.e., linear increase/linear decrease/and quadratic evolution of effort intensity). The results showed a positive relationship between effort intensity and the number of intragroup social interactions during rest periods, this result being accrued at the end of the training session. Interestingly, the results suggested that ending a conditioning session at 100% MAS after a linear progression of effort intensity might influence social identity mobility between different levels of self-abstraction, leading to more cooperation between group members. Overall, this research offers a psychosocial view of training effects that invites academics and coaches to consider effort intensity as a way to develop social bonding in rugby.  Lay summary: This study supports the perspective that "working hard" during conditioning influence social interactions between players. As a consequence, our results suggest that conditioning may be a powerful approach to build a feeling of "us-ness" that participates in optimizing group dynamics, and ultimately, performance in rugby. IMPLICATIONS FOR PRACTICE   In order to benefit from the positive influence of conditioning on social interactions within a team coach should:   * Prefer incremental conditioning ending at the highest intensity, to non-linear training    * Encourage emotional expression norms by appointing congratulating behaviors during rest periods and more particularly at the end of the training session    * Employ rest zone on the pitch so as to facilitate social bonding between players