Dear All,

Happy New Year!

Chris Kirtley switched the topics of “centrifugal forces” to a more

important question on how the nervous system perceives and controls

movements. I reproduce here his question:

“As far as I know, we have no sensors for segment acceleration - only

(conceivably) joint angular acceleration, via spindles, joint afferents

and skin receptors. Would this variable be sufficient, I wonder, for the

CNS to compute the inverse dynamics?”

I would like to point out that the term “inverse dynamics hypothesis”

(in the most explicit form formulated by Hollerbach but the idea is as

old as Newton’s mechanics) implies that the nervous system pre-plans the

desired movement kinematics and then, based on some intrinsic

representation of equations of motion, computes and specifies the

electromyographic activity, muscle forces and torques, which are

necessary to actualise the movement plan.

Chris’s question implies that the nervous system does compute the

“inverse dynamics”. I suppose that the majority of those who work in the

field of biomechanics and maybe somewhat smaller % of physiologists

share this view. I would be pleased to know if this is an exaggeration

since I belong to those who, following the implicit arguments of Von

Holst (1969/1973) and very explicit arguments of Bernstein (1967), are

convinced that the nervous system cannot and does not need to compute

inverse dynamics to produce perfect movements.

The inverse computational strategy works well for robotics. I would

like to point out some simple physical and physiological principles that

bring us directly to the conclusion that the inverse dynamics control

strategy cannot be realised in biological systems.

1. Many human actions consist of movements from one stable posture

to another. A stable posture is associated not only with the equilibrium

position at which all forces are balanced but also with the ability of

the system to generate forces resisting deflections from this position.

2. According to a general rule of physics (e.g., Glansdorff &

Prigogine 1971), the spatial coordinates at which the equilibrium is

established in any physical system are determined not by output

variables (like EMG, forces, torques) but independently of them, by the

system’s parameters. For example, in a pendulum, the equilibrium

(vertical) position is determined not by variable forces but the

parameters of the pendulum, such as the length of the rope at which the

pendulum’s mass is suspended, the coordinates of the suspension point,

and the direction of gravity. Since these parameters are constant, the

equilibrium position of the pendulum remains the same even when the

system is put in motion. Thus, the ability of the nervous system to

change the equilibrium position implies that the nervous system has the

capacity not only to maintain but also to change appropriate parameters

or determinants of the equilibrium position and thus produce active

movements.

3. Suppose control levels computed and specified EMG signals and

forces according to the planned kinematics, as suggested in the

inverse-dynamic approach. If the system left the parameters that

determine the equilibrium position unchanged, the programmed forces

would drive the system from the existing (initial) equilibrium position.

The inverse dynamic approach does not account for the fact that, like in

a pendulum, the system will produce additional, resisting forces trying

to return the system to the initial position and thus destroy the

programmed action. Even if the inverse-dynamic specifications of the

computed forces were combined with a shift in the equilibrium position,

the emerging, additional forces arising due to the difference between

the initial position and the new equilibrium position would also

interfere with the computed forces. As a result, the programmed motion

would again be destroyed. The idea of EMG and force programming thus

conflicts with the natural physical tendency of the system to generate,

without programming, muscle activity and forces associated with

deflections from equilibrium. Briefly, the inverse dynamic approach

conflicts with the natural dynamics of biological systems.

4. Experimentally, one parameter (lambda) controlling the

equilibrium position of the system has been found by Asatryan and

Feldman (1965).

I am looking forward to seeing reactions to my comments. In my view,

the inverse-dynamic approach is just a revival of our old illness - the

mechanistic tradition of thinking inspired by remarkable successes in

robotics. The robotics view may, however, be misleading in educating,

especially, young scientists on how movements are controlled in living

systems.

Best wishes in the New Millenium!

--

Dr. Anatol Feldman

Professor

Neurological Science Research Center

Department of Physiology

University of Montreal and

Rehabilitation Institute of Montreal

6300 Darlington, Montreal, Quebec, Canada H3S 2J4

feldman@med.umontreal.ca

Tel (514) 340 2078 ext. 2192

Fax (514) 340 2154

Web Site: http://www.crosswinds.net/~afeldman/

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Happy New Year!

Chris Kirtley switched the topics of “centrifugal forces” to a more

important question on how the nervous system perceives and controls

movements. I reproduce here his question:

“As far as I know, we have no sensors for segment acceleration - only

(conceivably) joint angular acceleration, via spindles, joint afferents

and skin receptors. Would this variable be sufficient, I wonder, for the

CNS to compute the inverse dynamics?”

I would like to point out that the term “inverse dynamics hypothesis”

(in the most explicit form formulated by Hollerbach but the idea is as

old as Newton’s mechanics) implies that the nervous system pre-plans the

desired movement kinematics and then, based on some intrinsic

representation of equations of motion, computes and specifies the

electromyographic activity, muscle forces and torques, which are

necessary to actualise the movement plan.

Chris’s question implies that the nervous system does compute the

“inverse dynamics”. I suppose that the majority of those who work in the

field of biomechanics and maybe somewhat smaller % of physiologists

share this view. I would be pleased to know if this is an exaggeration

since I belong to those who, following the implicit arguments of Von

Holst (1969/1973) and very explicit arguments of Bernstein (1967), are

convinced that the nervous system cannot and does not need to compute

inverse dynamics to produce perfect movements.

The inverse computational strategy works well for robotics. I would

like to point out some simple physical and physiological principles that

bring us directly to the conclusion that the inverse dynamics control

strategy cannot be realised in biological systems.

1. Many human actions consist of movements from one stable posture

to another. A stable posture is associated not only with the equilibrium

position at which all forces are balanced but also with the ability of

the system to generate forces resisting deflections from this position.

2. According to a general rule of physics (e.g., Glansdorff &

Prigogine 1971), the spatial coordinates at which the equilibrium is

established in any physical system are determined not by output

variables (like EMG, forces, torques) but independently of them, by the

system’s parameters. For example, in a pendulum, the equilibrium

(vertical) position is determined not by variable forces but the

parameters of the pendulum, such as the length of the rope at which the

pendulum’s mass is suspended, the coordinates of the suspension point,

and the direction of gravity. Since these parameters are constant, the

equilibrium position of the pendulum remains the same even when the

system is put in motion. Thus, the ability of the nervous system to

change the equilibrium position implies that the nervous system has the

capacity not only to maintain but also to change appropriate parameters

or determinants of the equilibrium position and thus produce active

movements.

3. Suppose control levels computed and specified EMG signals and

forces according to the planned kinematics, as suggested in the

inverse-dynamic approach. If the system left the parameters that

determine the equilibrium position unchanged, the programmed forces

would drive the system from the existing (initial) equilibrium position.

The inverse dynamic approach does not account for the fact that, like in

a pendulum, the system will produce additional, resisting forces trying

to return the system to the initial position and thus destroy the

programmed action. Even if the inverse-dynamic specifications of the

computed forces were combined with a shift in the equilibrium position,

the emerging, additional forces arising due to the difference between

the initial position and the new equilibrium position would also

interfere with the computed forces. As a result, the programmed motion

would again be destroyed. The idea of EMG and force programming thus

conflicts with the natural physical tendency of the system to generate,

without programming, muscle activity and forces associated with

deflections from equilibrium. Briefly, the inverse dynamic approach

conflicts with the natural dynamics of biological systems.

4. Experimentally, one parameter (lambda) controlling the

equilibrium position of the system has been found by Asatryan and

Feldman (1965).

I am looking forward to seeing reactions to my comments. In my view,

the inverse-dynamic approach is just a revival of our old illness - the

mechanistic tradition of thinking inspired by remarkable successes in

robotics. The robotics view may, however, be misleading in educating,

especially, young scientists on how movements are controlled in living

systems.

Best wishes in the New Millenium!

--

Dr. Anatol Feldman

Professor

Neurological Science Research Center

Department of Physiology

University of Montreal and

Rehabilitation Institute of Montreal

6300 Darlington, Montreal, Quebec, Canada H3S 2J4

feldman@med.umontreal.ca

Tel (514) 340 2078 ext. 2192

Fax (514) 340 2154

Web Site: http://www.crosswinds.net/~afeldman/

---------------------------------------------------------------

To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl

For information and archives: http://isb.ri.ccf.org/biomch-l

---------------------------------------------------------------