Measurements of vertical displacement in running, a methodological comparison

The aim was (1) to evaluate measurements of vertical displacements (Vdisp) of a single point on sacrum as an estimate of the whole body centre of mass (CoM) Vdisp during treadmill running and (2) to compare three methods for measuring this single point. These methods were based on a position transducer (PT), accelerometers (AMs) and an optoelectronic motion capture system. Criterion method was Vdisp of the whole body CoM measured with the motion capture system. Thirteen subjects ran at 10, 12, 14, 16, 18, 20 and 22 km h−1 with synchronous recordings with the three methods. Four measurements of the (Vdisp) were derived: (1) Vdisp of CoM calculated from a segment model consisting of 13 segments tracked with 36 reflective markers, (2) Vdisp of the sacrum recorded with the PT, (3) Vdisp of the sacrum calculated from the AM, and (4) Vdisp of the sacrum calculated as the mid point of two reflective markers (sacrum marker, SM) attached at the level of the sacral bone. The systematic discrepancy between the measurements of sacrum Vdisp and CoM Vdisp varied between 0 and 1.5 mm and decreased with increasing running velocity and decreasing step duration. PT and SM measurements showed strong correlation, whereas the AM showed a variability increasing with velocity. The random discrepancy within each subject was 7 mm for all three methods. In conclusion single-point recordings of the sacrum Vdisp may be used to monitor changes in Vdisp of CoM during treadmill running.