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| Author(s): |
Emanuele Moraes Mello
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Instituto de Psicologia (IP/SBD) |
| Defense date: | 2011-07-11 |
| Examining board members: |
Andre Fabio Kohn;
Neri Alves;
José Angelo Barela;
Gilberto Mastrocola Manzano;
Rinaldo André Mezzarane
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| Advisor: | Andre Fabio Kohn |
| Abstract | |
Random oscillations of the center of pressure are an inevitable feature associated with quiet standing (PEQ). This random movement may be attributed to different sources of variability including the sensory inputs at the periphery, the central nervous system and the neuromuscular system that generates the torques. Once the descending commands are generated by the brain, there are sources at the spinal cord and at the motor units that contribute with an added variability to the force generated by synergist muscles acting around a joint. These sources of force variability (generated at the spinal cord and at the motor units) were the focus of the present research, with a specific emphasis on the ankle joint and the triceps surae muscle group, which are directly involved in postural control during PEQ. The methodology consisted in studying the plantarflexion torque variability generated in different conditions together with the analyses of the electromyograms (EMGs) of the three triceps surae muscles. Three sets of experiments were used in the research: 1) subject seated, with either extended knee (JE) or flexed knee (JF), executing isometric contractions; 2) subject seated, with extended knee, comparing force task (isometric contraction, with force control TF) with position task (non-isometric force generation with position control TP); 3) subject standing, either naturally (PEQ) or attached to a fixed structure (PEA), compared with TF and TP exerted while seated, with JE, and with same torque as during PEQ. These conditions were also tested with the soleus H reflex. For experiments 1 and 2 the number of subjects was 13, while for experiment 3 it was 9, all healthy. The quantifiers that were used to characterize the signals were the mean value (), the standard deviation () and the coefficient of variation (CV). These were applied to the torque signal and for the EMG envelope of the muscles soleus (SO), lateral gastrocnemius (GL) and medial gastrocnemius (GM). The results of Experiment 1 are synthesized in what follows. The torque during maximal contraction (CVM) was 58% higher in JE as compared with JF, but the variability ( and CV) was higher in JF than in JE. The variability was also higher in JF for torque levels in the range 10 to 80% CVM. There was a linearly increasing relation between and of the torque generated by the triceps surae in JE and JF ( and normalized with respect to the respective CVMs). The levels of muscle activation (envelope ) were higher in JE than in JF, mainly due to the higher activation of GM and GL in JE when compared with JF. The SO muscle was more activated than GM and GL, both in JE and JF. Both the EMG level and envelope increased as a function of the plantarflexion torque, for SO, GL and GM. The results for Experiment 2 are described in what follows. Torque variability ( e CV) was lower in TP than in TF. There was an increasing relation between and of the torque generated by the triceps surae, and between and of the EMG envelopes of the three muscles as a function of the plantarflexion torque , during TF and TP. The EMG envelope was positively correlated with the plantarflexion torque , both normalized with respect to the respective values at CVM. The results of Experiment 3 are summarized in what follows. Plantarflexion torque variability ( e CV) during PEQ was higher than in the other conditions, the subject sample showing a positive correlation between torque in PEQ and in TF or TP. On the other hand, during 10 PEA, torque was uncorrelated with torque measured in TF and TP. The muscle activation levels in triceps surae were higher in TF than in the other conditions, mainly due to the higher SO activation in TF as compared with the other conditions. The EMG envelope of SO and GM varied with the conditions analyzed, and the GM envelope CV was higher in PEQ than in the other conditions. There were no statistical differences in the values of and CV of the H reflex amplitudes in the different conditions. The results in general may suggest the following: i there is an optimization of the neuromuscular control in position JE with respect to that in JF from the point of view that a lower variability of plantarflexion torque occurs in JE than in JF for a wide force range; ii the relative level of recruitment among the three muscles is sensitive to the knee angle (at least at 90º and 0o); iii the SO muscle is the most activated in the triceps surae, both in JE and JF, for all torque levels analyzed; iv there is less variability in plantarflexion torque in TP than in TF, perhaps due to the action of different proprioceptors, but without significant difference in muscle activation or EMG envelope variability between the two conditions; v plantarflexion torque variability in PEQ is higher than in the other conditions, being similar in PEA and TF and in TP being the smallest of all; vi a subject with a higher torque variability in TF or TP will present a higher variability in PEQ (as suggested by correlation analysis), suggesting that a fraction of the variability in PEQ originates in the spinal cord and neuromuscular system; vii the SO was more activated in TF than in the other conditions (PEQ, PEA and TP), which could be a strategy of the central nervous system to cope with the different tasks; viii in a general sense, the H reflex amplitude does not seem to be a sensitive indicator of the different neural events occurring in the spinal cord during the different experimental conditions (PEQ, PEA, TF and TP) (AU) | |