Most of the modelling of body dynamics in sports and other activities assumes that every anatomical segment is solid and moves 'as a whole'. In the past a technique has been provided to estimate the relative motion of the visceral mass with respect to the trunk segment, a movement potentially affecting respiration and jump performance in general. Here we refine and expand that methodology by testing its ability to detect a smaller viscera excursion, within the trunk, expected in a respiratory-assisted jumping strategy, which stiffen the abdominal content and is expected to lower the degrees of freedom of the bouncing body. Six subjects where analysed, by using both inverse and direct dynamics, while repeatedly jumping vertically before and after a specific respiratory training period. The viscera excursion showed a consistent intra-individual time course, and decreased by about 30 ± 9% when the subjects had been familiarized with the trunk stiffening manoeuvre. We conclude that: 1) the present methodology proved capable to detect subtle visceral mass movement within the trunk during repetitive motor acts and, particularly 2) the new respiratory technique increased stiffness of the trunk and reduced visceral displacement.