Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, PC72
Human standing: does the postural set pre-program a rigid knee?
I. Di Giulio1, C. N. Maganaris1, V. Baltzopoulos1, I. D. Loram1
1. Manchester Metropolitan University, Manchester, United Kingdom.
Human standing requires control of balance and configuration. Observation of the initiation of standing (previous abstract) predicts that most humans inwardly rotate the femur and extend the knee tending to lock the knee. This observation cannot determine whether a straight leg is prioritised in the control of standing and defended through passive or active mechanisms. External perturbations reveal the pre-set intention driving motor control to restore balance. Perturbation to activate ankle mechanisms (1) and also hip perturbations (2) have been used to study balance. The knee has been understudied and regarded as rigid taking advantage of the passive locking mechanism to maintain stance. Here we tested this assumption. We applied gentle, unpredictable mechanical perturbations to the knee. We ask: does the postural pre-set of normal standing defend a rigid leg? 10 healthy participants, aged 35±11 years (mean±s.d.), stood symmetrically on two force plates for 230sec. Gentle pulls of variable force (1-10N) and duration (0.2-2sec) were applied, unpredictably to either knee via Kevlar string connected to a servo motor. A 10-camera motion analysis system recorded 3D body kinematics. We report undisturbed standing sway of the centre of gravity (CoG) (Fig 1A), and displacement of sacral and knee markers relative to their undisturbed initial position. From the sacral displacement the predicted displacement of the perturbed knee assuming a rigid leg or inverted pendulum (IP) configuration (Fig 1B) was calculated. From the knee and sacral displacement the displacement of the perturbed knee beyond the rigid leg prediction was calculated (nonIP, Fig 1C). The ratio nonIP/IP (Fig 1D) was calculated. Differences between people were tested using one way ANOVA. All subjects could stand successfully, but normal standing sway differs between people (Fig. 1A, p<0.001) as does sacral marker displacement following knee perturbation (Fig 1B, p<0.001). Most subjects showed little or no knee displacement beyond inverted pendulum like prediction in response to perturbation (Fig 1C, D). Subjects 2 and 4 shown a large nonIP knee displacement and did not defend a rigid leg configuration (Fig 1 C, D, p<0.001), allowing the knee to flex when pulled. Different strategies to maintain standing and equilibrium are possible. Standing is possible with low leg stiffness (S2, S4), though for the majority, knee perturbations revealed a postural pre-set to defend a rigid leg. This confirms the prediction of the previous abstract that most humans adopt a pre-set postural configuration which is distinct from the one of movement and is associated with rigid legs.
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