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Spinal mechanisms may provide a combination of intermittent and continuous control of human posture: predictions from a biologically based neuromusculoskeletal model.

Elias LA, Watanabe RN, Kohn AF - PLoS Comput. Biol. (2014)

Bottom Line: Simulation results showed that the neuromechanical outputs generated by the NMS model resemble experimental data from subjects standing on a stable surface.These results suggest that the spinal cord anatomy and neurophysiology (e.g., motor unit types, synaptic connectivities, ordered recruitment), along with the modulation of afferent activity, may account for the mixture of intermittent and continuous control that has been a subject of debate in recent studies on postural control.Another finding was the occurrence of the so-called "paradoxical" behaviour of muscle fibre lengths as a function of postural sway.

View Article: PubMed Central - PubMed

Affiliation: Biomedical Engineering Laboratory, Escola Polit├ęcnica, University of Sao Paulo, Sao Paulo, Brazil.

ABSTRACT
Several models have been employed to study human postural control during upright quiet stance. Most have adopted an inverted pendulum approximation to the standing human and theoretical models to account for the neural feedback necessary to keep balance. The present study adds to the previous efforts in focusing more closely on modelling the physiological mechanisms of important elements associated with the control of human posture. This paper studies neuromuscular mechanisms behind upright stance control by means of a biologically based large-scale neuromusculoskeletal (NMS) model. It encompasses: i) conductance-based spinal neuron models (motor neurons and interneurons); ii) muscle proprioceptor models (spindle and Golgi tendon organ) providing sensory afferent feedback; iii) Hill-type muscle models of the leg plantar and dorsiflexors; and iv) an inverted pendulum model for the body biomechanics during upright stance. The motor neuron pools are driven by stochastic spike trains. Simulation results showed that the neuromechanical outputs generated by the NMS model resemble experimental data from subjects standing on a stable surface. Interesting findings were that: i) an intermittent pattern of muscle activation emerged from this posture control model for two of the leg muscles (Medial and Lateral Gastrocnemius); and ii) the Soleus muscle was mostly activated in a continuous manner. These results suggest that the spinal cord anatomy and neurophysiology (e.g., motor unit types, synaptic connectivities, ordered recruitment), along with the modulation of afferent activity, may account for the mixture of intermittent and continuous control that has been a subject of debate in recent studies on postural control. Another finding was the occurrence of the so-called "paradoxical" behaviour of muscle fibre lengths as a function of postural sway. The simulations confirmed previous conjectures that reciprocal inhibition is possibly contributing to this effect, but on the other hand showed that this effect may arise without any anticipatory neural control mechanism.

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Recruitment phase plots.(A) Phase plot showing the recruitment of Medial Gastrocnemius (MG) motor units (MUs) in angle-velocity bi-dimensional graph. The point (0,0) is a reference due to the subtraction of the mean value of each variable obtained in the simulations. Circle diameters are proportional to the number of recruited MUs (see Methods). It is noteworthy that MUs seem to be preferentially recruited in the first quadrant, i.e., when the pendulum is leaning forward as it comes from a backward position. (B) Phase plot showing the recruitment of MG MUs in angle-torque bi-dimensional graph. Again, the point (0,0) is a reference that was used because each simulation produced slightly different mean torques and angles. Note that MG MUs are mostly activated in the fourth quadrant, i.e., when the pendulum is leaning forward and the plantar-flexion torque produced by Triceps Surae (TS) is higher than the mean.
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pcbi-1003944-g006: Recruitment phase plots.(A) Phase plot showing the recruitment of Medial Gastrocnemius (MG) motor units (MUs) in angle-velocity bi-dimensional graph. The point (0,0) is a reference due to the subtraction of the mean value of each variable obtained in the simulations. Circle diameters are proportional to the number of recruited MUs (see Methods). It is noteworthy that MUs seem to be preferentially recruited in the first quadrant, i.e., when the pendulum is leaning forward as it comes from a backward position. (B) Phase plot showing the recruitment of MG MUs in angle-torque bi-dimensional graph. Again, the point (0,0) is a reference that was used because each simulation produced slightly different mean torques and angles. Note that MG MUs are mostly activated in the fourth quadrant, i.e., when the pendulum is leaning forward and the plantar-flexion torque produced by Triceps Surae (TS) is higher than the mean.

Mentions: The intermittent recruitment of MG MUs was evaluated on the basis of two phase plots that relate angular velocity and muscle torque with ankle angle data obtained from the postural control model (Figure 6). Figure 6A shows that most of the MG MUs (60%) were recruited when the inverted pendulum was leaning forward from its equilibrium position irrespective of its velocity (first and fourth quadrants of the angle-velocity phase plots). Nonetheless, a large number of MUs (28%) were recruited when the inverted pendulum was at a backward position but with a positive velocity (second quadrant), i.e., the pendulum was starting to return to a forward position. Similarly, most of the MG MUs (50%) were recruited when the pendulum was leaning forward and producing a higher plantar flexion torque (fourth quadrant in the Figure 6B), i.e., the pendulum was at a forward position and decelerating. Almost 35% of the MUs were recruited when the pendulum was at a backward position and with a lower (more positive than the mean value) plantar flexion torque. In general, the discharges in the first and third quadrants were mainly involved in the generation of a basal torque, while the discharges in the second and fourth quadrants represented the phasic corrective torque control produced by the MG muscle.


Spinal mechanisms may provide a combination of intermittent and continuous control of human posture: predictions from a biologically based neuromusculoskeletal model.

Elias LA, Watanabe RN, Kohn AF - PLoS Comput. Biol. (2014)

Recruitment phase plots.(A) Phase plot showing the recruitment of Medial Gastrocnemius (MG) motor units (MUs) in angle-velocity bi-dimensional graph. The point (0,0) is a reference due to the subtraction of the mean value of each variable obtained in the simulations. Circle diameters are proportional to the number of recruited MUs (see Methods). It is noteworthy that MUs seem to be preferentially recruited in the first quadrant, i.e., when the pendulum is leaning forward as it comes from a backward position. (B) Phase plot showing the recruitment of MG MUs in angle-torque bi-dimensional graph. Again, the point (0,0) is a reference that was used because each simulation produced slightly different mean torques and angles. Note that MG MUs are mostly activated in the fourth quadrant, i.e., when the pendulum is leaning forward and the plantar-flexion torque produced by Triceps Surae (TS) is higher than the mean.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4230754&req=5

pcbi-1003944-g006: Recruitment phase plots.(A) Phase plot showing the recruitment of Medial Gastrocnemius (MG) motor units (MUs) in angle-velocity bi-dimensional graph. The point (0,0) is a reference due to the subtraction of the mean value of each variable obtained in the simulations. Circle diameters are proportional to the number of recruited MUs (see Methods). It is noteworthy that MUs seem to be preferentially recruited in the first quadrant, i.e., when the pendulum is leaning forward as it comes from a backward position. (B) Phase plot showing the recruitment of MG MUs in angle-torque bi-dimensional graph. Again, the point (0,0) is a reference that was used because each simulation produced slightly different mean torques and angles. Note that MG MUs are mostly activated in the fourth quadrant, i.e., when the pendulum is leaning forward and the plantar-flexion torque produced by Triceps Surae (TS) is higher than the mean.
Mentions: The intermittent recruitment of MG MUs was evaluated on the basis of two phase plots that relate angular velocity and muscle torque with ankle angle data obtained from the postural control model (Figure 6). Figure 6A shows that most of the MG MUs (60%) were recruited when the inverted pendulum was leaning forward from its equilibrium position irrespective of its velocity (first and fourth quadrants of the angle-velocity phase plots). Nonetheless, a large number of MUs (28%) were recruited when the inverted pendulum was at a backward position but with a positive velocity (second quadrant), i.e., the pendulum was starting to return to a forward position. Similarly, most of the MG MUs (50%) were recruited when the pendulum was leaning forward and producing a higher plantar flexion torque (fourth quadrant in the Figure 6B), i.e., the pendulum was at a forward position and decelerating. Almost 35% of the MUs were recruited when the pendulum was at a backward position and with a lower (more positive than the mean value) plantar flexion torque. In general, the discharges in the first and third quadrants were mainly involved in the generation of a basal torque, while the discharges in the second and fourth quadrants represented the phasic corrective torque control produced by the MG muscle.

Bottom Line: Simulation results showed that the neuromechanical outputs generated by the NMS model resemble experimental data from subjects standing on a stable surface.These results suggest that the spinal cord anatomy and neurophysiology (e.g., motor unit types, synaptic connectivities, ordered recruitment), along with the modulation of afferent activity, may account for the mixture of intermittent and continuous control that has been a subject of debate in recent studies on postural control.Another finding was the occurrence of the so-called "paradoxical" behaviour of muscle fibre lengths as a function of postural sway.

View Article: PubMed Central - PubMed

Affiliation: Biomedical Engineering Laboratory, Escola Polit├ęcnica, University of Sao Paulo, Sao Paulo, Brazil.

ABSTRACT
Several models have been employed to study human postural control during upright quiet stance. Most have adopted an inverted pendulum approximation to the standing human and theoretical models to account for the neural feedback necessary to keep balance. The present study adds to the previous efforts in focusing more closely on modelling the physiological mechanisms of important elements associated with the control of human posture. This paper studies neuromuscular mechanisms behind upright stance control by means of a biologically based large-scale neuromusculoskeletal (NMS) model. It encompasses: i) conductance-based spinal neuron models (motor neurons and interneurons); ii) muscle proprioceptor models (spindle and Golgi tendon organ) providing sensory afferent feedback; iii) Hill-type muscle models of the leg plantar and dorsiflexors; and iv) an inverted pendulum model for the body biomechanics during upright stance. The motor neuron pools are driven by stochastic spike trains. Simulation results showed that the neuromechanical outputs generated by the NMS model resemble experimental data from subjects standing on a stable surface. Interesting findings were that: i) an intermittent pattern of muscle activation emerged from this posture control model for two of the leg muscles (Medial and Lateral Gastrocnemius); and ii) the Soleus muscle was mostly activated in a continuous manner. These results suggest that the spinal cord anatomy and neurophysiology (e.g., motor unit types, synaptic connectivities, ordered recruitment), along with the modulation of afferent activity, may account for the mixture of intermittent and continuous control that has been a subject of debate in recent studies on postural control. Another finding was the occurrence of the so-called "paradoxical" behaviour of muscle fibre lengths as a function of postural sway. The simulations confirmed previous conjectures that reciprocal inhibition is possibly contributing to this effect, but on the other hand showed that this effect may arise without any anticipatory neural control mechanism.

Show MeSH
Related in: MedlinePlus