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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 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.

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Related in: MedlinePlus

Cross-correlation functions and centre of pressure (COP) power spectra for typical simulations carried out on Model 1 (graphs A, C, and E) and Model 2 (graphs B, D, and F).(A-B) Cross-correlation functions between centre of mass (COM) and COP. Note that for both models, cross-correlation peaks occurred at zero lag (dashed lines). (C-D) Cross-correlation functions between COP and muscle electromyograms (EMGs). Black, red, and blue curves represent cross-correlation functions for Soleus (SO), Medial Gastrocnemius (MG), and Lateral Gastrocnemius (LG), respectively. Irrespective of the model structure, there was a lag of about 300 ms between COP and EMG envelopes from the three muscles. (E-F) COP power spectra. Dashed line represents the 50% power frequency (). It is noteworthy that for Model 2 there was a broader bandwidth in comparison to Model 1.
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pcbi-1003944-g002: Cross-correlation functions and centre of pressure (COP) power spectra for typical simulations carried out on Model 1 (graphs A, C, and E) and Model 2 (graphs B, D, and F).(A-B) Cross-correlation functions between centre of mass (COM) and COP. Note that for both models, cross-correlation peaks occurred at zero lag (dashed lines). (C-D) Cross-correlation functions between COP and muscle electromyograms (EMGs). Black, red, and blue curves represent cross-correlation functions for Soleus (SO), Medial Gastrocnemius (MG), and Lateral Gastrocnemius (LG), respectively. Irrespective of the model structure, there was a lag of about 300 ms between COP and EMG envelopes from the three muscles. (E-F) COP power spectra. Dashed line represents the 50% power frequency (). It is noteworthy that for Model 2 there was a broader bandwidth in comparison to Model 1.

Mentions: A quantitative analysis was performed to validate the model with respect to the available data from the literature. Typical time-domain metrics were calculated from the COP time series and compared to data from normal subjects and vestibular loss patients standing on a force plate without visual information (see Table 1). Root mean square (RMS) and mean velocity (MV) of simulated COP were higher than the values observed experimentally in normal subjects, but compatible with data from vestibular loss patients. Another quantitative validation was based on a cross-correlation analysis performed between the COM and COP time series (Figure 2A-B), as well as between COP and EMG envelopes (Figure 2C-D). COM and COP were highly correlated () at lag zero. COP and EMG envelopes were positively correlated with the correlation peak occurring at a positive lag. Correlation coefficients () and cross-correlation peak lag values were compatible with experimental data from healthy subjects (see Table 1). In general, correlation coefficients were higher for Gastrocnemii in comparison to the SO, and muscles' activations (EMGs) were advanced by approximately 200–300 ms in relation to the postural sway (COP). The 50% power frequency () estimated from the COP power spectrum (see Figure 2E-F) resulted quite similar to the value from healthy subjects (see Table 1). COP power spectra of both model structures were limited to 1 Hz.


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)

Cross-correlation functions and centre of pressure (COP) power spectra for typical simulations carried out on Model 1 (graphs A, C, and E) and Model 2 (graphs B, D, and F).(A-B) Cross-correlation functions between centre of mass (COM) and COP. Note that for both models, cross-correlation peaks occurred at zero lag (dashed lines). (C-D) Cross-correlation functions between COP and muscle electromyograms (EMGs). Black, red, and blue curves represent cross-correlation functions for Soleus (SO), Medial Gastrocnemius (MG), and Lateral Gastrocnemius (LG), respectively. Irrespective of the model structure, there was a lag of about 300 ms between COP and EMG envelopes from the three muscles. (E-F) COP power spectra. Dashed line represents the 50% power frequency (). It is noteworthy that for Model 2 there was a broader bandwidth in comparison to Model 1.
© Copyright Policy
Related In: Results  -  Collection

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

pcbi-1003944-g002: Cross-correlation functions and centre of pressure (COP) power spectra for typical simulations carried out on Model 1 (graphs A, C, and E) and Model 2 (graphs B, D, and F).(A-B) Cross-correlation functions between centre of mass (COM) and COP. Note that for both models, cross-correlation peaks occurred at zero lag (dashed lines). (C-D) Cross-correlation functions between COP and muscle electromyograms (EMGs). Black, red, and blue curves represent cross-correlation functions for Soleus (SO), Medial Gastrocnemius (MG), and Lateral Gastrocnemius (LG), respectively. Irrespective of the model structure, there was a lag of about 300 ms between COP and EMG envelopes from the three muscles. (E-F) COP power spectra. Dashed line represents the 50% power frequency (). It is noteworthy that for Model 2 there was a broader bandwidth in comparison to Model 1.
Mentions: A quantitative analysis was performed to validate the model with respect to the available data from the literature. Typical time-domain metrics were calculated from the COP time series and compared to data from normal subjects and vestibular loss patients standing on a force plate without visual information (see Table 1). Root mean square (RMS) and mean velocity (MV) of simulated COP were higher than the values observed experimentally in normal subjects, but compatible with data from vestibular loss patients. Another quantitative validation was based on a cross-correlation analysis performed between the COM and COP time series (Figure 2A-B), as well as between COP and EMG envelopes (Figure 2C-D). COM and COP were highly correlated () at lag zero. COP and EMG envelopes were positively correlated with the correlation peak occurring at a positive lag. Correlation coefficients () and cross-correlation peak lag values were compatible with experimental data from healthy subjects (see Table 1). In general, correlation coefficients were higher for Gastrocnemii in comparison to the SO, and muscles' activations (EMGs) were advanced by approximately 200–300 ms in relation to the postural sway (COP). The 50% power frequency () estimated from the COP power spectrum (see Figure 2E-F) resulted quite similar to the value from healthy subjects (see Table 1). COP power spectra of both model structures were limited to 1 Hz.

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