Mike Sigman wrote:
.... and if you still think that he's doing that rooted standing in some sort of "gyro-dynamic" way, then I don't know what to tell you.
Paradoxical muscle contractions and the neural control of movement and balance
Richard C Fitzpatrick and Simon C Gandevia
Prince of Wales Medical Research Institute, Sydney, New South Wales, 2031, Australia
" Loram et al. (2005b) go on to show how the brain and the muscle solve this load problem. Since ‘static' equilibrium cannot be achieved by continuous muscle contraction, the system adopts a pattern of cyclic muscle activation, producing repeated ballistic, catch-and-throw movements of the body. The behaviour resembles keeping a balloon in the air by repeated hits. Over time, the balloon maintains a mean position, which might seem an equilibrium point, but at no time does it stay in static equilibrium; it is either being accelerated upward as it is hit or it is in free fall.
Standing is a dynamic activity. It has been believed that normal body sway comes from small perturbation forces, either internal to the body (respiration) or external (breezes); limited sensory acuity to detect body movement; receptor noise; motor output noise; or movement generated by the brain. What Loram et al. (2005b) show is entirely different. A major cause of human body sway arises from this cyclic pattern of catch-and-throw ballistic muscle activation."
Human postural sway results from frequent, ballistic bias impulses by soleus and gastrocnemius.
* Loram ID,
* Maganaris CN,
* Lakie M.
Applied Physiology Research Group, School of Sport and Exercise Sciences, University of Birmingham, Birmingham B15 2TT, UK. email@example.com
It has been widely assumed for nearly a century, that postural muscles operate in a spring-like manner and that muscle length signals joint angle (the mechano-reflex mechanism). Here we employ automated analysis of ultrasound images to resolve calf muscle (soleus and gastrocnemius) length changes as small as 10 mum in standing subjects. Previously, we have used balancing of a real inverted pendulum to make predictions about human standing. Here we test and confirm these predictions on 10 subjects standing quietly. We show that on average the calf muscles are actively adjusted 2.6 times per second and 2.8 times per unidirectional sway of the body centre of mass (CoM). These alternating, small (30-300 microm) movements provide impulsive, ballistic regulation of CoM movement. The timing and pattern of these adjustments are consistent with multisensory integration of all information regarding motion of the CoM, pattern recognition, prediction and planning using internal models and are not consistent with control solely by local reflexes. Because the system is unstable, errors in stabilization provide a perturbation which grows into a sway which has to be reacted to and corrected. Sagittal sway results from this impulsive control of calf muscle activity rather than internal sources (e.g. the heart, breathing). This process is quite unlike the mechano-reflex paradigm. We suggest that standing is a skilled, trial and error activity that improves with experience and is automated (possibly by the cerebellum). These results complement and extend our recent demonstration that paradoxical muscle movements are the norm in human standing.
PMID: 15661824 [PubMed - indexed for MEDLINE]