Running pattern changes depending on the level of subjects' awareness of the measurements performed: a "sampling effect" in human locomotion experiments?

The aim of this study was to know whether subjects involved in a locomotion experiment modified their running pattern when made aware that data were being collected, and of the specific parameters studied. We used standardised actual and deceptive messages to induce increasing levels of awareness, and analysed subjects' running pattern on a treadmill dynamometer, using lower limb stiffness as the key mechanical parameter. The five levels of awareness were: (1) subjects thought no sampling was performed, (2) they knew a sampling was about to take place, without knowing the parameter studied, (3) they knew this sampling was in progress, (4) they knew that sampling of lower limb stiffness was about to take place, and (5) they knew this sampling was in progress. Subjects' running pattern significantly changed with the increasing level of information given, with a higher stiffness and step frequency, a reduced contact time and a lower change in leg length during contact. Post hoc tests showed that subjects changed their running pattern when knowing (i) that a sampling was performed and (ii) the mechanical parameter studied. These preliminary results suggest that experimental designs in locomotion protocols should take this potential "sampling" effect into account, whenever possible.