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Mapping Muscles Activation to Force Perception during Unloading.

Toma S, Lacquaniti F - PLoS ONE (2016)

Bottom Line: In fact a global measure of the muscles considered was able to predict approximately 60% of the perceptual decisions total variance.Moreover the inter-subjects differences in psychophysical sensitivity showed high correlation with both participants' muscles sensitivity and participants' joint torques.Overall, our findings gave insights into both the role played by the corticospinal motor commands while performing a force detection task and the influence of the gravitational muscular torque on the estimation of vertical forces.

View Article: PubMed Central - PubMed

Affiliation: Centre of Space Bio-medicine, University of Rome Tor Vergata, Rome, Italy.

ABSTRACT
It has been largely proved that while judging a force humans mainly rely on the motor commands produced to interact with that force (i.e., sense of effort). Despite of a large bulk of previous investigations interested in understanding the contributions of the descending and ascending signals in force perception, very few attempts have been made to link a measure of neural output (i.e., EMG) to the psychophysical performance. Indeed, the amount of correlation between EMG activity and perceptual decisions can be interpreted as an estimate of the contribution of central signals involved in the sensation of force. In this study we investigated this correlation by measuring the muscular activity of eight arm muscles while participants performed a quasi-isometric force detection task. Here we showed a method to quantitatively describe muscular activity ("muscle-metric function") that was directly comparable to the description of the participants' psychophysical decisions about the stimulus force. We observed that under our experimental conditions, muscle-metric absolute thresholds and the shape of the muscle-metric curves were closely related to those provided by the psychophysics. In fact a global measure of the muscles considered was able to predict approximately 60% of the perceptual decisions total variance. Moreover the inter-subjects differences in psychophysical sensitivity showed high correlation with both participants' muscles sensitivity and participants' joint torques. Overall, our findings gave insights into both the role played by the corticospinal motor commands while performing a force detection task and the influence of the gravitational muscular torque on the estimation of vertical forces.

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Individual distribution of muscular w coefficients and flexor/extensor ratio for pair of subjects with similar PSE.Each bar represents the w coefficient associated for each muscle. Coefficient value represent an estimate of the sensitivity of each muscle to be modulated by the changes in the external forces. Positive values correspond to joint extensor modulations in response to changes in the upward external force. Negative coefficients quantified the correspondence between joint flexor muscles and external force changes. Flex/Ext w coefficients ratio is a measure of the absolute relation between flexor and extensor joint muscles.
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pone.0152552.g009: Individual distribution of muscular w coefficients and flexor/extensor ratio for pair of subjects with similar PSE.Each bar represents the w coefficient associated for each muscle. Coefficient value represent an estimate of the sensitivity of each muscle to be modulated by the changes in the external forces. Positive values correspond to joint extensor modulations in response to changes in the upward external force. Negative coefficients quantified the correspondence between joint flexor muscles and external force changes. Flex/Ext w coefficients ratio is a measure of the absolute relation between flexor and extensor joint muscles.

Mentions: Apropos of point 1, if we assume that subjects’ biomechanical behavior was exclusively bound up with the upward force applied, we might expect similar muscular behavior (e.g., flexor and extensor w coefficients) for subjects interacting with similar external forces (e.g., PSE), arm posture being negligible different among subjects. On the contrary, inspections of the w coefficients distribution and Flex/Ext ratio of those three pairs of subjects with the most similar PSE values (Subj. 10, 2; Subj. 11, 12; Subj. 4, 5) show that it was not the case (Fig 9). In fact, both relative coefficients distribution and absolute Flex/Ext ratios were observed to be largely different despite of the very similar applied force (PSE), confuting the hypothesis that participants' muscular activity were just a direct consequence of the applied force. Moreover, the Flex/Ext ratios coefficients of all subjects do not show a significant correlation with respect to PSE individual values (r = -0.31, p = 0.27), indicating a not stable relationship between force applied (PSE) modulation and the pattern of muscular activity. Furthermore if the correlation we observed between perceptual decisions and muscular activity modulations was just protocol dependent, such a correlation should appear very high (or at least statistically significant) for each participants. On the contrary, as reported earlier, this was not the case for three subjects (subj.6, 7 and 9). Similarly, if arm muscular activity per se were constrained by the protocol to correlate with force modulation (perceptual decision), the correlation values in Fig 6B (i.e., individual PSE as function of Overall Muscular Activity) should be higher and more similar to that one describing the relation between psycho-metric and muscle-metric individual absolute thresholds (Fig 6C). Taken together these inspections support the hypothesis that despite of the relationship implied in our task between the variables considered, there is not an univocal correlation between the applied force and the modulation of the muscular activity supporting a pure biomechanical hypothesis. Instead we suggest that due to the high level of redundancy of the muscle-skeletal system, participants were able to complete the task with different (across subjects and trials) level of muscles co-activation that contributed partially to take the perceptual decision and consequently to drive the overall applied force distribution.


Mapping Muscles Activation to Force Perception during Unloading.

Toma S, Lacquaniti F - PLoS ONE (2016)

Individual distribution of muscular w coefficients and flexor/extensor ratio for pair of subjects with similar PSE.Each bar represents the w coefficient associated for each muscle. Coefficient value represent an estimate of the sensitivity of each muscle to be modulated by the changes in the external forces. Positive values correspond to joint extensor modulations in response to changes in the upward external force. Negative coefficients quantified the correspondence between joint flexor muscles and external force changes. Flex/Ext w coefficients ratio is a measure of the absolute relation between flexor and extensor joint muscles.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4816335&req=5

pone.0152552.g009: Individual distribution of muscular w coefficients and flexor/extensor ratio for pair of subjects with similar PSE.Each bar represents the w coefficient associated for each muscle. Coefficient value represent an estimate of the sensitivity of each muscle to be modulated by the changes in the external forces. Positive values correspond to joint extensor modulations in response to changes in the upward external force. Negative coefficients quantified the correspondence between joint flexor muscles and external force changes. Flex/Ext w coefficients ratio is a measure of the absolute relation between flexor and extensor joint muscles.
Mentions: Apropos of point 1, if we assume that subjects’ biomechanical behavior was exclusively bound up with the upward force applied, we might expect similar muscular behavior (e.g., flexor and extensor w coefficients) for subjects interacting with similar external forces (e.g., PSE), arm posture being negligible different among subjects. On the contrary, inspections of the w coefficients distribution and Flex/Ext ratio of those three pairs of subjects with the most similar PSE values (Subj. 10, 2; Subj. 11, 12; Subj. 4, 5) show that it was not the case (Fig 9). In fact, both relative coefficients distribution and absolute Flex/Ext ratios were observed to be largely different despite of the very similar applied force (PSE), confuting the hypothesis that participants' muscular activity were just a direct consequence of the applied force. Moreover, the Flex/Ext ratios coefficients of all subjects do not show a significant correlation with respect to PSE individual values (r = -0.31, p = 0.27), indicating a not stable relationship between force applied (PSE) modulation and the pattern of muscular activity. Furthermore if the correlation we observed between perceptual decisions and muscular activity modulations was just protocol dependent, such a correlation should appear very high (or at least statistically significant) for each participants. On the contrary, as reported earlier, this was not the case for three subjects (subj.6, 7 and 9). Similarly, if arm muscular activity per se were constrained by the protocol to correlate with force modulation (perceptual decision), the correlation values in Fig 6B (i.e., individual PSE as function of Overall Muscular Activity) should be higher and more similar to that one describing the relation between psycho-metric and muscle-metric individual absolute thresholds (Fig 6C). Taken together these inspections support the hypothesis that despite of the relationship implied in our task between the variables considered, there is not an univocal correlation between the applied force and the modulation of the muscular activity supporting a pure biomechanical hypothesis. Instead we suggest that due to the high level of redundancy of the muscle-skeletal system, participants were able to complete the task with different (across subjects and trials) level of muscles co-activation that contributed partially to take the perceptual decision and consequently to drive the overall applied force distribution.

Bottom Line: In fact a global measure of the muscles considered was able to predict approximately 60% of the perceptual decisions total variance.Moreover the inter-subjects differences in psychophysical sensitivity showed high correlation with both participants' muscles sensitivity and participants' joint torques.Overall, our findings gave insights into both the role played by the corticospinal motor commands while performing a force detection task and the influence of the gravitational muscular torque on the estimation of vertical forces.

View Article: PubMed Central - PubMed

Affiliation: Centre of Space Bio-medicine, University of Rome Tor Vergata, Rome, Italy.

ABSTRACT
It has been largely proved that while judging a force humans mainly rely on the motor commands produced to interact with that force (i.e., sense of effort). Despite of a large bulk of previous investigations interested in understanding the contributions of the descending and ascending signals in force perception, very few attempts have been made to link a measure of neural output (i.e., EMG) to the psychophysical performance. Indeed, the amount of correlation between EMG activity and perceptual decisions can be interpreted as an estimate of the contribution of central signals involved in the sensation of force. In this study we investigated this correlation by measuring the muscular activity of eight arm muscles while participants performed a quasi-isometric force detection task. Here we showed a method to quantitatively describe muscular activity ("muscle-metric function") that was directly comparable to the description of the participants' psychophysical decisions about the stimulus force. We observed that under our experimental conditions, muscle-metric absolute thresholds and the shape of the muscle-metric curves were closely related to those provided by the psychophysics. In fact a global measure of the muscles considered was able to predict approximately 60% of the perceptual decisions total variance. Moreover the inter-subjects differences in psychophysical sensitivity showed high correlation with both participants' muscles sensitivity and participants' joint torques. Overall, our findings gave insights into both the role played by the corticospinal motor commands while performing a force detection task and the influence of the gravitational muscular torque on the estimation of vertical forces.

Show MeSH
Related in: MedlinePlus