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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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Intermittent recruitment of Medial Gastrocnemius (MG) motor units (MUs) and modulation of proprioceptive feedback (typical simulation performed on Model 2).(A) Centre of mass (COM; gray curve) and centre of pressure (COP; black curve) displacements. (B) Raster plots (black dots) of 40 MG MUs intermittently recruited during quiet standing. Red curve represents the global MG electromyogram (EMG) envelope. Note the ballistic-like (phasic) activation of this muscle during postural sway. (C) Raster plots for the population of Ia afferents from the MG muscle. Note the clear modulation in the recruitment of primary afferents. In addition, continuous curves show the instantaneous firing rate (estimated by a Gaussian kernel convolved with the spike trains) for three Ia afferents (4, 16, 28). (D-F) Raster plots for group II excitatory interneurons (INs), Ib inhibitory INs, and Ia inhibitory INs. Continuous curves in panels D and E represent the instantaneous firing rate for two type-specified INs (2 and 94 for group II INs; 24 and 94 for Ib INs). (G-I) Raster plots for type II afferents from MG muscle spindle, Ib afferents from MG muscle spindle, and Ia afferents from TA muscle spindle. Continuous curves in panels G and H represent the instantaneous firing rate of two type-specified afferent fibres (9 and 28 for type-II afferents; 22 and 39 for Ib afferents).
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pcbi-1003944-g004: Intermittent recruitment of Medial Gastrocnemius (MG) motor units (MUs) and modulation of proprioceptive feedback (typical simulation performed on Model 2).(A) Centre of mass (COM; gray curve) and centre of pressure (COP; black curve) displacements. (B) Raster plots (black dots) of 40 MG MUs intermittently recruited during quiet standing. Red curve represents the global MG electromyogram (EMG) envelope. Note the ballistic-like (phasic) activation of this muscle during postural sway. (C) Raster plots for the population of Ia afferents from the MG muscle. Note the clear modulation in the recruitment of primary afferents. In addition, continuous curves show the instantaneous firing rate (estimated by a Gaussian kernel convolved with the spike trains) for three Ia afferents (4, 16, 28). (D-F) Raster plots for group II excitatory interneurons (INs), Ib inhibitory INs, and Ia inhibitory INs. Continuous curves in panels D and E represent the instantaneous firing rate for two type-specified INs (2 and 94 for group II INs; 24 and 94 for Ib INs). (G-I) Raster plots for type II afferents from MG muscle spindle, Ib afferents from MG muscle spindle, and Ia afferents from TA muscle spindle. Continuous curves in panels G and H represent the instantaneous firing rate of two type-specified afferent fibres (9 and 28 for type-II afferents; 22 and 39 for Ib afferents).

Mentions: Figures 4 and 5 show how the spike trains from spinal MNs, INs, and afferent fibres were modulated during postural sway. An interesting qualitative finding was that MUs from the MG muscle were intermittently recruited/de-recruited as the inverted pendulum swayed forward/backward (Figure 4B). This intermittent pattern of MU recruitment was similar for the LG muscle (not shown), but less evident for the SO muscle (see Figure 5A). The degree of intermittency for the MG and SO MUs was quantified by the activation ratio (see [16] and Methods for details). The median (range) activation ratios calculated for 90 randomly selected MG MUs (30 MUs were chosen per simulation) from Model 1 and Model 2 were 0.69 (0.44–0.80) and 0.65 (0.47–0.81), respectively. For 90 randomly selected SO MUs the activation ratios were 0.97 (0.75–1) and 0.96 (0.79–1) for Model 1 and Model 2, respectively. Because of these results, the MG and LG muscles were considered to have ballistic-like activations (see EMG envelopes in Figures 1D-E and 4B), while the SO muscle was mostly tonically (continuously) active during the maintenance of an upright posture (see Figures 1C and 5A).


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)

Intermittent recruitment of Medial Gastrocnemius (MG) motor units (MUs) and modulation of proprioceptive feedback (typical simulation performed on Model 2).(A) Centre of mass (COM; gray curve) and centre of pressure (COP; black curve) displacements. (B) Raster plots (black dots) of 40 MG MUs intermittently recruited during quiet standing. Red curve represents the global MG electromyogram (EMG) envelope. Note the ballistic-like (phasic) activation of this muscle during postural sway. (C) Raster plots for the population of Ia afferents from the MG muscle. Note the clear modulation in the recruitment of primary afferents. In addition, continuous curves show the instantaneous firing rate (estimated by a Gaussian kernel convolved with the spike trains) for three Ia afferents (4, 16, 28). (D-F) Raster plots for group II excitatory interneurons (INs), Ib inhibitory INs, and Ia inhibitory INs. Continuous curves in panels D and E represent the instantaneous firing rate for two type-specified INs (2 and 94 for group II INs; 24 and 94 for Ib INs). (G-I) Raster plots for type II afferents from MG muscle spindle, Ib afferents from MG muscle spindle, and Ia afferents from TA muscle spindle. Continuous curves in panels G and H represent the instantaneous firing rate of two type-specified afferent fibres (9 and 28 for type-II afferents; 22 and 39 for Ib afferents).
© Copyright Policy
Related In: Results  -  Collection

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

pcbi-1003944-g004: Intermittent recruitment of Medial Gastrocnemius (MG) motor units (MUs) and modulation of proprioceptive feedback (typical simulation performed on Model 2).(A) Centre of mass (COM; gray curve) and centre of pressure (COP; black curve) displacements. (B) Raster plots (black dots) of 40 MG MUs intermittently recruited during quiet standing. Red curve represents the global MG electromyogram (EMG) envelope. Note the ballistic-like (phasic) activation of this muscle during postural sway. (C) Raster plots for the population of Ia afferents from the MG muscle. Note the clear modulation in the recruitment of primary afferents. In addition, continuous curves show the instantaneous firing rate (estimated by a Gaussian kernel convolved with the spike trains) for three Ia afferents (4, 16, 28). (D-F) Raster plots for group II excitatory interneurons (INs), Ib inhibitory INs, and Ia inhibitory INs. Continuous curves in panels D and E represent the instantaneous firing rate for two type-specified INs (2 and 94 for group II INs; 24 and 94 for Ib INs). (G-I) Raster plots for type II afferents from MG muscle spindle, Ib afferents from MG muscle spindle, and Ia afferents from TA muscle spindle. Continuous curves in panels G and H represent the instantaneous firing rate of two type-specified afferent fibres (9 and 28 for type-II afferents; 22 and 39 for Ib afferents).
Mentions: Figures 4 and 5 show how the spike trains from spinal MNs, INs, and afferent fibres were modulated during postural sway. An interesting qualitative finding was that MUs from the MG muscle were intermittently recruited/de-recruited as the inverted pendulum swayed forward/backward (Figure 4B). This intermittent pattern of MU recruitment was similar for the LG muscle (not shown), but less evident for the SO muscle (see Figure 5A). The degree of intermittency for the MG and SO MUs was quantified by the activation ratio (see [16] and Methods for details). The median (range) activation ratios calculated for 90 randomly selected MG MUs (30 MUs were chosen per simulation) from Model 1 and Model 2 were 0.69 (0.44–0.80) and 0.65 (0.47–0.81), respectively. For 90 randomly selected SO MUs the activation ratios were 0.97 (0.75–1) and 0.96 (0.79–1) for Model 1 and Model 2, respectively. Because of these results, the MG and LG muscles were considered to have ballistic-like activations (see EMG envelopes in Figures 1D-E and 4B), while the SO muscle was mostly tonically (continuously) active during the maintenance of an upright posture (see Figures 1C and 5A).

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