The exact mechanism underlying the coupling between breathing and locomotion is still unknown; the visceral piston theory is a putative mechanism but has gone largely untested. The study assessed the visceral piston theory by examining the effect of perturbations (vocalization and fluid ingestion) on the coupling of breathing and locomotion. 10 experienced runners performed a speed running trial (6.5 to 8.0 mph in 0.5 mph increments), followed by a vocalization trial and a fluid ingestion trial at their preferred running speed while on a treadmill. Continuous recordings of respiratory flow and single leg, tibial acceleration was acquired (500 Hz). Rating of perceived exertion for breathing (RPEB), effort (RPEE), stitch (RPES) and heart rate (HR) were also recorded during the trials. The magnitude (SIFTMAG) and timing (SIFTT) of step induced flow transients were determined, along with the breath cycle duration (BCD) and respiratory locomotor entrainment (RLE) ratios. RPEB, and HR showed an increase in all three trials, RPEE showed an increase in the speed trial and the fluid ingestion trial but not the vocalization trial. SIFTMAG had a significant increase post perturbation (vocalization and fluid ingestion). SIFTT increased significantly post fluid ingestion but not after the vocalization perturbation. RLE ratios were increased post fluid ingestion but not post vocalization perturbation, however BCD was reduced after vocalization trials. The SIFTMAG was increased as result of perturbations corresponding to respiratory flow suppression which could lead to increased respiratory load. A shift in timing after fluid ingestion is consistent with an increased mass of a visceral spring mass system. Four of ten runners experienced a stitch after fluid ingestion. These findings strongly support the visceral piston theory of RLE, and this becomes a plausible mechanism for the induction of a stitch in the side.