Watching quiet human stance to shake off its straitjacket

Abstract

The single-inverted pendulum (SIP) model is still the paradigm describing dynamics and control of quiet human stance in the sagittal plane. We used two methods to verify this paradigm. First, in an experimental approach we acquired kinematic data of both legs of ten subjects at high spatial resolution while quietly standing on two force platforms. We calculated all leg joint angles, the belonging joint torques using inverse dynamics and estimates of joint stiffnesses. Some linear correlations and regressions of both local (joint) and global (COM, COP: centre of mass respectively pressure) variables predicted by the SIP model were investigated. All three verification criteria applied to mean values extracted from experimental data revealed that the SIP is not a valid model for quiet human stance. As a second method, we used computer synthesis to demonstrate that a double-inverted pendulum (DIP) model enters a stable attractor when just the "hip" joint torque is regulated, whereas no torque is applied to the "ankle" joint. Here, angle and torque fluctuations are necessary because such a DIP strategy is of inevitable dynamic character. The two predicted eigenfrequencies of this regulated DIP model approximate the upper and lower limits of the main part of measured power spectra of quiet human stance. We suggest this dynamic necessity to be representative of the biological constraints under which a mechanically unstable inverted multi-segment chain must be stabilised.

What does head movement tell about the minimum number of mechanical degrees of freedom in quiet human stance?

“…Rather, multi-segment dynamics has moved into the limelight [6,14,15,20,24,34]. Nevertheless, the question remains open whether actual rather than mean anterior-posterior head accelerations are usually below threshold.…”

“…Most of the processing of kinematic data (in particular the translational and angular kinematics of the leg segments), of force platform data, of the whole body centre of mass (COM), of the centre of head, arms and trunk (HAT) mass, and of the net centre of pressure underneath the feet (COP) has been described in detail elsewhere [15]. Here, we give a short summary.…”

“…The first DIP attractor is achieved by just actuating the "hip" joint [15] whereas the "ankle" torque is constantly zero. Particularly, here, the "hip" torque M hip = −G · COM x runs simply in proportion to the difference in horizontal displacements COM x of the COM and the "ankle" positions, the latter being fixed in space.…”

“…Different from former analyses [14][15][16][17], here, the marker positions were digitised by means of "TraXXol" ("Biomotion Solutions", Tübingen, Germany) using a centroid algorithm plus a black-white threshold. Therewith, we achieved spatial resolutions of 0.002 mm for ankle, knee and hip positions and 0.01 mm for the head position, respectively.…”

“…In the meanwhile, the SIP was shown to not provide a valid mechanical model base for explaining the emergence of quiet human stance [15]. Rather, multi-segment dynamics has moved into the limelight [6,14,15,20,24,34].…”