Effects of cadence on the acquisition and expression of podokinetic after-rotation

Podokinetic after-rotation (PKAR) occurs as blindfolded subjects inadvertently rotate when asked to step in-place following stepping in-place on a rotating surface. We examined the effects of using different cadences on PKAR to test the following hypothesis: the position signal indicating the amount of rotation between the trunk and the feet during each stance period of treadmill stimulation is used to determine the amount of rotation between the trunk and the feet that is expressed during each stance period of PKAR. Based on this hypothesis, we predicted that use of different cadences during treadmill stimulation would alter PKAR velocity because use of different cadences alters stance duration, thus changing the amount of limb rotation under the trunk during each stance phase of stimulation. We also predicted that use of different cadences during PKAR would alter PKAR velocity because the more steps that are taken in a given time the higher the velocity of PKAR given that the same rotation between trunk and feet occurs during each stance period. Use of different cadences during treadmill stimulation did not alter PKAR velocity, suggesting that PKAR velocity is not determined based upon a position signal regarding the relative rotation between the trunk and feet during stimulation. Use of different cadences during PKAR resulted in lower and higher velocities, respectively, than using a medium cadence. Based on these results, we now hypothesize that the PK system likely uses a velocity or acceleration signal present during stimulation to recalibrate the amount of relative rotation between the trunk and limbs that is expressed with each step during PKAR.