Limits...
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 Politécnica, 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

Modulation of muscle fibre lengths during postural sway (typical simulation performed on Model 2).(A) Centre of mass (COM; gray curve) and centre of pressure (COP; black curve) displacements. (B-D) Muscle fibre lengths from Soleus (SO), Medial Gastrocnemius (MG), and Tibialis Anterior (TA) muscles. Lateral Gastrocnemius (LG) was not shown here because its behaviour is quite similar to that of the MG muscle. Dashed lines represent the 3-s duration windows used to perform the correlation analysis between COM and muscle fibre lengths (see Methods for details and Figure 8).
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4230754&req=5

pcbi-1003944-g007: Modulation of muscle fibre lengths during postural sway (typical simulation performed on Model 2).(A) Centre of mass (COM; gray curve) and centre of pressure (COP; black curve) displacements. (B-D) Muscle fibre lengths from Soleus (SO), Medial Gastrocnemius (MG), and Tibialis Anterior (TA) muscles. Lateral Gastrocnemius (LG) was not shown here because its behaviour is quite similar to that of the MG muscle. Dashed lines represent the 3-s duration windows used to perform the correlation analysis between COM and muscle fibre lengths (see Methods for details and Figure 8).

Mentions: In the present study, we tested the hypothesis that reciprocal inhibition may be responsible for the negative correlation between muscle fibre length and COM/COP displacement [29]–[31]. This was tested by performing a correlation analysis between COM displacements and muscle fibre lengths for the SO, MG, and TA. Typical signals representing these variables are shown in Figure 7. For this typical simulation (Model 2) one can notice that MG muscle fibre length was positively correlated with COM, while TA muscle fibre length and COM displacement were negatively correlated. The latter results are a typical “orthodox” behaviour that has been shown for some healthy subjects during upright quiet standing [29]. For the SO muscle, a more quantitative analysis was performed (see Figure 8). Correlation analysis between SO muscle fibre length and COM displacement was performed on 3-s windows (see dashed vertical lines in Figure 7) according to the method adopted in [29]. Correlation coefficients calculated from Model 1 and Model 2 were pooled into two groups: positively correlated () and negatively correlated (). For Model 1 most of the intervals (80%) showed a positive correlation between SO muscle fibre length and COM displacement (i.e., “orthodox” behaviour), whereas for Model 2 the number of negatively correlated intervals increased to 50%. -test revealed a statistically significant difference () between the responses of the two model structures. This suggests that, at least for the SO muscle, reciprocal inhibition might be involved in the genesis of the “paradoxical” behaviour of muscle fibre length. Nonetheless, a small percentage of negatively correlated intervals (20%) might also be generated in the absence of any reciprocal inhibition from antagonistic Ia afferents.


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)

Modulation of muscle fibre lengths during postural sway (typical simulation performed on Model 2).(A) Centre of mass (COM; gray curve) and centre of pressure (COP; black curve) displacements. (B-D) Muscle fibre lengths from Soleus (SO), Medial Gastrocnemius (MG), and Tibialis Anterior (TA) muscles. Lateral Gastrocnemius (LG) was not shown here because its behaviour is quite similar to that of the MG muscle. Dashed lines represent the 3-s duration windows used to perform the correlation analysis between COM and muscle fibre lengths (see Methods for details and Figure 8).
© Copyright Policy
Related In: Results  -  Collection

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

pcbi-1003944-g007: Modulation of muscle fibre lengths during postural sway (typical simulation performed on Model 2).(A) Centre of mass (COM; gray curve) and centre of pressure (COP; black curve) displacements. (B-D) Muscle fibre lengths from Soleus (SO), Medial Gastrocnemius (MG), and Tibialis Anterior (TA) muscles. Lateral Gastrocnemius (LG) was not shown here because its behaviour is quite similar to that of the MG muscle. Dashed lines represent the 3-s duration windows used to perform the correlation analysis between COM and muscle fibre lengths (see Methods for details and Figure 8).
Mentions: In the present study, we tested the hypothesis that reciprocal inhibition may be responsible for the negative correlation between muscle fibre length and COM/COP displacement [29]–[31]. This was tested by performing a correlation analysis between COM displacements and muscle fibre lengths for the SO, MG, and TA. Typical signals representing these variables are shown in Figure 7. For this typical simulation (Model 2) one can notice that MG muscle fibre length was positively correlated with COM, while TA muscle fibre length and COM displacement were negatively correlated. The latter results are a typical “orthodox” behaviour that has been shown for some healthy subjects during upright quiet standing [29]. For the SO muscle, a more quantitative analysis was performed (see Figure 8). Correlation analysis between SO muscle fibre length and COM displacement was performed on 3-s windows (see dashed vertical lines in Figure 7) according to the method adopted in [29]. Correlation coefficients calculated from Model 1 and Model 2 were pooled into two groups: positively correlated () and negatively correlated (). For Model 1 most of the intervals (80%) showed a positive correlation between SO muscle fibre length and COM displacement (i.e., “orthodox” behaviour), whereas for Model 2 the number of negatively correlated intervals increased to 50%. -test revealed a statistically significant difference () between the responses of the two model structures. This suggests that, at least for the SO muscle, reciprocal inhibition might be involved in the genesis of the “paradoxical” behaviour of muscle fibre length. Nonetheless, a small percentage of negatively correlated intervals (20%) might also be generated in the absence of any reciprocal inhibition from antagonistic Ia afferents.

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 Politécnica, 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