Purpose <p>To investigate rectus abdominis (RA) activity during forced expiration under conditions where RA activity is assumed to be prominent, including trunk flexion and sustained expiration.</p> Methods <p>Fourteen males performed maximal voluntary forced expirations sustained from total lung capacity (TLC) to residual volume with and without slow trunk flexion. Maximal voluntary forced expiration from TLC with rapid trunk flexion was also performed as a single effort. As a reflexive forced expiration, sneezing was performed. Submaximal sustained expirations at 20, 30, and 50% of maximal expiratory mouth pressure were also conducted. During voluntary expirations, mouth pressure was measured using a mouth pressure meter. During all tasks, EMG activity of the RA and internal oblique/transversus abdominis (IO/TrA) were measured using surface electromyography.</p> Results <p>RA activity during maximal voluntary forced expiration did not change with slow trunk flexion but increased with rapid trunk flexion; however, neither condition increased expiratory mouth pressure. RA activity was significantly lower than IO/TrA activity during maximal voluntary forced expirations with and without trunk flexion, but did not differ significantly from IO/TrA activity during sneezing. During maximal voluntary sustained expiration, RA activity increased without trunk flexion but decreased with trunk flexion. RA activity increased during submaximal voluntary sustained expirations at all mouth pressure levels, but the magnitude of the increase was smaller than that of the OI/TrA.</p> Conclusion <p>The RA was less active and less responsive during voluntary forced expiration than the other abdominal muscles, whereas it appeared to be similarly active and responsive during reflexive forced expiration.</p>