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Normalized Index of Synergy for Evaluating the Coordination of Motor Commands.

Togo S, Imamizu H - PLoS ONE (2015)

Bottom Line: We hypothesized that a large part of the change in the coordination of motor outputs through learning was because of changes in motor commands.In a motor learning task, subjects tracked a target trajectory of the total torque.We conclude that the normalized index of synergy can be used to evaluate the coordination of motor commands independently from the properties of the musculoskeletal system.

View Article: PubMed Central - PubMed

Affiliation: Cognitive Mechanisms Laboratories, Advanced Telecommunications Research Institute International, Kyoto, Japan; Japan Society for the Promotion of Science, Tokyo, Japan.

ABSTRACT
Humans perform various motor tasks by coordinating the redundant motor elements in their bodies. The coordination of motor outputs is produced by motor commands, as well properties of the musculoskeletal system. The aim of this study was to dissociate the coordination of motor commands from motor outputs. First, we conducted simulation experiments where the total elbow torque was generated by a model of a simple human right and left elbow with redundant muscles. The results demonstrated that muscle tension with signal-dependent noise formed a coordinated structure of trial-to-trial variability of muscle tension. Therefore, the removal of signal-dependent noise effects was required to evaluate the coordination of motor commands. We proposed a method to evaluate the coordination of motor commands, which removed signal-dependent noise from the measured variability of muscle tension. We used uncontrolled manifold analysis to calculate a normalized index of synergy. Simulation experiments confirmed that the proposed method could appropriately represent the coordinated structure of the variability of motor commands. We also conducted experiments in which subjects performed the same task as in the simulation experiments. The normalized index of synergy revealed that the subjects coordinated their motor commands to achieve the task. Finally, the normalized index of synergy was applied to a motor learning task to determine the utility of the proposed method. We hypothesized that a large part of the change in the coordination of motor outputs through learning was because of changes in motor commands. In a motor learning task, subjects tracked a target trajectory of the total torque. The change in the coordination of muscle tension through learning was dominated by that of motor commands, which supported the hypothesis. We conclude that the normalized index of synergy can be used to evaluate the coordination of motor commands independently from the properties of the musculoskeletal system.

No MeSH data available.


Related in: MedlinePlus

Results of the measurement experiments of the target total torque production task.(a) The blue circle indicates individual left, right and total elbow torques of all subjects. (b) The UCM component and the ORT component of all subjects. (c) The index of synergy of all subjects. The asterisk indicates a significant difference (P < 0.05), and n.s. indicates a non-significant difference.
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pone.0140836.g007: Results of the measurement experiments of the target total torque production task.(a) The blue circle indicates individual left, right and total elbow torques of all subjects. (b) The UCM component and the ORT component of all subjects. (c) The index of synergy of all subjects. The asterisk indicates a significant difference (P < 0.05), and n.s. indicates a non-significant difference.

Mentions: In the target total torque production task, subjects performed the same task as in the simulation experiment to evaluate the coordination of motor commands. Fig 7A indicates the left, right, and total elbow torques produced by all subjects. All subjects could accurately produce target total torque (5 N·m). Although all subjects were right-handed, a significant bias between right and left elbow torques was not observed (t(9) = −1.20, P = 0.26), suggesting little effect of hand dominance on the task performance. Fig 7B shows both measured and normalized UCM and ORT components. Two-way ANOVA indicated a significant difference between the UCM and ORT components (F(1, 36) = 32.7, P = 1.65 × 10−6 < 0.05), without a significant interaction (F(1, 36) = 1.2, P = 0.28), and both the measured and normalized UCM components were significantly larger than the measured and normalized ORT components, respectively (measured: t(9) = 4.83, P = 9.37 × 10−4 < 0.05; normalized: t(9) = 4.67, P = 1.20 × 10−3 < 0.05). Then, the measured and normalized indices of synergy were significantly higher than the values of no coordination (measured: t(9) = 8.27, P = 1.70 × 10−5 < 0.05; normalized: t(9) = 5.66, P = 3.10 × 10−4 < 0.05, see UCM = ORT line) as shown in Fig 7C. These results demonstrated that the coordination of muscle tensions was formed during the input step to each muscle, i.e. by the motor commands. Therefore, these results show that the subjects coordinated motor commands to produce the target total elbow torque.


Normalized Index of Synergy for Evaluating the Coordination of Motor Commands.

Togo S, Imamizu H - PLoS ONE (2015)

Results of the measurement experiments of the target total torque production task.(a) The blue circle indicates individual left, right and total elbow torques of all subjects. (b) The UCM component and the ORT component of all subjects. (c) The index of synergy of all subjects. The asterisk indicates a significant difference (P < 0.05), and n.s. indicates a non-significant difference.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4608756&req=5

pone.0140836.g007: Results of the measurement experiments of the target total torque production task.(a) The blue circle indicates individual left, right and total elbow torques of all subjects. (b) The UCM component and the ORT component of all subjects. (c) The index of synergy of all subjects. The asterisk indicates a significant difference (P < 0.05), and n.s. indicates a non-significant difference.
Mentions: In the target total torque production task, subjects performed the same task as in the simulation experiment to evaluate the coordination of motor commands. Fig 7A indicates the left, right, and total elbow torques produced by all subjects. All subjects could accurately produce target total torque (5 N·m). Although all subjects were right-handed, a significant bias between right and left elbow torques was not observed (t(9) = −1.20, P = 0.26), suggesting little effect of hand dominance on the task performance. Fig 7B shows both measured and normalized UCM and ORT components. Two-way ANOVA indicated a significant difference between the UCM and ORT components (F(1, 36) = 32.7, P = 1.65 × 10−6 < 0.05), without a significant interaction (F(1, 36) = 1.2, P = 0.28), and both the measured and normalized UCM components were significantly larger than the measured and normalized ORT components, respectively (measured: t(9) = 4.83, P = 9.37 × 10−4 < 0.05; normalized: t(9) = 4.67, P = 1.20 × 10−3 < 0.05). Then, the measured and normalized indices of synergy were significantly higher than the values of no coordination (measured: t(9) = 8.27, P = 1.70 × 10−5 < 0.05; normalized: t(9) = 5.66, P = 3.10 × 10−4 < 0.05, see UCM = ORT line) as shown in Fig 7C. These results demonstrated that the coordination of muscle tensions was formed during the input step to each muscle, i.e. by the motor commands. Therefore, these results show that the subjects coordinated motor commands to produce the target total elbow torque.

Bottom Line: We hypothesized that a large part of the change in the coordination of motor outputs through learning was because of changes in motor commands.In a motor learning task, subjects tracked a target trajectory of the total torque.We conclude that the normalized index of synergy can be used to evaluate the coordination of motor commands independently from the properties of the musculoskeletal system.

View Article: PubMed Central - PubMed

Affiliation: Cognitive Mechanisms Laboratories, Advanced Telecommunications Research Institute International, Kyoto, Japan; Japan Society for the Promotion of Science, Tokyo, Japan.

ABSTRACT
Humans perform various motor tasks by coordinating the redundant motor elements in their bodies. The coordination of motor outputs is produced by motor commands, as well properties of the musculoskeletal system. The aim of this study was to dissociate the coordination of motor commands from motor outputs. First, we conducted simulation experiments where the total elbow torque was generated by a model of a simple human right and left elbow with redundant muscles. The results demonstrated that muscle tension with signal-dependent noise formed a coordinated structure of trial-to-trial variability of muscle tension. Therefore, the removal of signal-dependent noise effects was required to evaluate the coordination of motor commands. We proposed a method to evaluate the coordination of motor commands, which removed signal-dependent noise from the measured variability of muscle tension. We used uncontrolled manifold analysis to calculate a normalized index of synergy. Simulation experiments confirmed that the proposed method could appropriately represent the coordinated structure of the variability of motor commands. We also conducted experiments in which subjects performed the same task as in the simulation experiments. The normalized index of synergy revealed that the subjects coordinated their motor commands to achieve the task. Finally, the normalized index of synergy was applied to a motor learning task to determine the utility of the proposed method. We hypothesized that a large part of the change in the coordination of motor outputs through learning was because of changes in motor commands. In a motor learning task, subjects tracked a target trajectory of the total torque. The change in the coordination of muscle tension through learning was dominated by that of motor commands, which supported the hypothesis. We conclude that the normalized index of synergy can be used to evaluate the coordination of motor commands independently from the properties of the musculoskeletal system.

No MeSH data available.


Related in: MedlinePlus