title: All leg joints contribute to quiet human stance: a mechanical analysis.
abstract: According to the state of the art model (single inverted pendulum) the regulation of quiet human stance seems to be dominated by ankle joint actions. Recent findings substantiated both in-phase and anti-phase fluctuations of ankle and hip joint kinematics can be identified in quiet human stance. Thus, we explored in an experimental study to what extent all three leg joints actually contribute to the balancing problem of quiet human stance. We also aimed at distinguishing kinematic from torque contributions. Thereto, we directly measured ankle, knee, and hip joint kinematics with high spatial resolution and ground reaction forces. Then, we calculated the six respective joint torques and, additionally, the centre of mass kinematics. We searched for high cross-correlations between all these mechanical variables. Beyond confirming correlated anti-phase kinematics of ankle and hip, the main results are: (i) ankle and knee joint fluctuate tightly (torque) coupled and (ii) the bi-articular muscles of the leg are well suited to fulfil the requirements of fluctuations around static equilibrium. Additionally, we (iii) identified high-frequency oscillations of the shank between about 4 and 8 Hz and (iv) discriminated potentially passive and active joint torque contributions. These results demonstrate that all leg joints contribute actively and concertedly to quiet human stance, even in the undisturbed case. Moreover, they substantiate the single inverted pendulum paradigm to be an invalid model for quiet human stance.
authors.affiliation: Friedrich-Schiller-Universität, Institut für Sportwissenschaft, Lehrstuhl für Bewegungswissenschaft, Seidelstrasse 20, D-07749 Jena, Deutschland, Germany. s7gumi@uni-jena.de
authors.forename: Michael
authors.initials: M
authors.lastname: Günther
authors.forename: Sten
authors.initials: S
authors.lastname: Grimmer
authors.forename: Tobias
authors.initials: T
authors.lastname: Siebert
authors.forename: Reinhard
authors.initials: R
authors.lastname: Blickhan
mesh_terms: D000328:Adult; D000843:Ankle Joint / Q000502:physiology*; D003198:Computer Simulation; D005260:Female; D006621:Hip Joint / Q000502:physiology*; D006801:Humans; D007719:Knee Joint / Q000502:physiology*; D008297:Male; D008954:Models, Biological; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D019415:Torque