Rate Coding Is Compressed But Variability Is Unaltered for Motor Units in a Hand Muscle of Old Adults

Barry, Benjamin K.; Pascoe, Michael A.; Jesunathadas, Mark; Enoka, Roger M.

doi:10.1152/jn.01280.2006

Cited by 121 publications

(137 citation statements)

References 73 publications

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“…Rate coding occurs over a narrower range of forces in older adults (Barry et al, 2007). In addition, the hand muscles of older adults are disproportionately weaker when compared to young groups and therefore higher levels of muscle activation are required to achieve the same force (Shinohara et al, 2003b).…”

Section: Cihr Author Manuscriptmentioning

confidence: 99%

“…It was hypothesized that the older adults would require greater levels of activation within these motor areas to achieve the same relative force levels as the younger group. It was expected that these differences would be greatest at higher levels of force production due to the extra neural drive that is needed to overcome the neuromuscular changes that occur with aging (Barry et al, 2007;De Luca et al, 1982;Kukulka and Clamann, 1981). A voxel-wise whole brain analysis was also carried out to determine all of the locations of age-related changes in brain activation during this force modulation task.…”

Section: Cihr Author Manuscriptmentioning

confidence: 99%

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Aging effects on the control of grip force magnitude: An fMRI study

Noble

Eng

Kokotilo

et al. 2011

Experimental Gerontology

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Functional neuroimaging techniques have allowed for investigations into the mechanisms of agerelated deterioration in motor control. This study used functional Magnetic Resonance Imaging (fMRI) to investigate age related differences in the control of grip force magnitude. Using an event-related design, fMRI scans were completed on 13 older adults, and 13 gender matched younger adults, while using their dominant hand to squeeze a rubber bulb for 4s at 10%, 40% or 70% of their maximum voluntary contraction. Both groups were able to match the relative force targets, however the older adults produced significantly lower levels of absolute force. fMRI analysis consisted of a 1) region of interest (ROI) approach to detect differences in selected motor areas within brain and 2) a voxel-wise whole brain comparison to find areas of differential activation that were not defined a priori between the older and younger group. The ROI analysis revealed that despite producing lower levels of absolute force, the older adults showed higher levels of activity predominantly in subcortical structures (putamen, thalamus and cerebellum) when compared to the younger group. The older adults also showed higher levels of activity in the ipsilateral ventral premotor cortex. A total of 19 of the 22 ROIs analyzed showed a significant main effect of the required force-level. In the majority of the ROIs that showed a significant force effect there were no significant differences in the magnitude of the blood-oxygen-level-dependent (BOLD) signal between the 10% and 40% conditions but a significantly higher BOLD signal in the 70% condition, suggesting that the modulation of brain activation with grip force may not be controlled in a linear fashion. It was also found that the older adult group demonstrate higher levels of activation in 7 areas during a force production task at higher force levels using a voxelwise analysis. The 7 clusters that showed significant differences tended to be areas that are involved in visual-spatial and executive processing. The results of this study revealed that older adults require significantly higher activation of several areas to perform the same motor task as younger adults. Higher magnitudes of the BOLD signal in older adults may represent a compensatory pattern to counter age related deterioration in motor control systems.Corresponding Author: Janice J. Eng, PT, OT, Ph.D., Professor, Department of Physical Therapy, University of British Columbia, 212-2177 Wesbrook Mall, Vancouver, BC, Canada V6T 1Z3, Janice.Eng@ubc.ca, Phone: IntroductionThe normal aging process leads to several declines of the motor system that are related to changes within the central nervous system, peripheral nervous system and the musculoskeletal system (Seidler et al., 2010). These changes have the potential to lead to decreased motor performance with aging, which has been observed in the forms of decreased coordination, increased movement variability, slower movements and difficulties with balance and gait (Seidler et al., 2...

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Section: Cihr Author Manuscriptmentioning

confidence: 99%

Section: Cihr Author Manuscriptmentioning

confidence: 99%

Aging effects on the control of grip force magnitude: An fMRI study

Noble

Eng

Kokotilo

et al. 2011

Experimental Gerontology

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“…The equations reported previously are not duplicated here; those related to the motor unit and isometric force models can be found in Fuglevand et al (1993a) and Barry et al (2007), and those for the EMG model can be found in Farina et al (2004) and Dimitrova & Dimitrov (2003). and HR (H-reflex amplitude) is DHR(t) = e (t/(3·10 6 /F (t))) 0.4 −1 3 × 10 6 /F (t) , F (t) > 0 and DHR(t) = 0.4 3 × 10 5 , F (t) = 0.…”

Section: Appendix Amentioning

confidence: 99%

Influence of fatigue on the simulated relation between the amplitude of the surface electromyogram and muscle force

Dideriksen

Farina

Enoka

2010

Phil. Trans. R. Soc. A.

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A linear relation between surface electromyogram (EMG) amplitude and muscle force is often assumed and used to estimate the contributions of selected muscles to various tasks. In the presence of muscle fatigue, however, changes in the properties of muscle fibre action potentials and motor unit twitch forces can alter the relation between surface EMG amplitude and force. A novel integrative model of motor neuron control and the generation of muscle fibre action potentials was used to simulate surface EMG signals and muscle force during three fatigue protocols. The change in the simulated relation between surface EMG amplitude and force depended on both the level of fatigue and the details of the fatiguing contraction. In general, surface EMG amplitude overestimated muscle force when fatigue was present. For example, surface EMG amplitudes corresponding to 60 per cent of the amplitude obtained at maximal force without fatigue corresponded to forces in the range 10-40% of the maximal force across three representative fatigue protocols. The results indicate that the surface EMG amplitude cannot be used to predict either the level of muscle activation or the magnitude of muscle force when the muscle exhibits any fatigue.

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“…For example, the first dorsal interosseus muscle comprises an equal number of type I and II fibers, but ϳ84% of the motor units have slow contraction times and are fatigue resistant (7). Fourth, the conduction velocity of muscle fiber action potentials can change by ϳ20% with variation in discharge rate (24) and the range of discharge rates varies with recruitment threshold in a population of motor units (2). Fifth, the conduction velocity of muscle fiber action potentials varies with fiber length (15).…”

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confidence: 99%

Counterpoint: Spectral properties of the surface emg do not provide information about motor unit recruitment and muscle fiber type

Farina

2008

Journal of Applied Physiology

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The surface electromyogram (EMG) comprises the sum of the electrical contributions made by the active motor units to the interference signal detected by electrodes placed on the skin overlying the muscle. Because it provides a global measure of motor unit activity, this signal is a valuable tool for assessing the level of muscle activation.The analysis of the surface EMG in the frequency domain has been used in some instances, such as fatiguing contractions, to provide an indirect measure of the relative changes in average muscle fiber conduction velocity (19) or in the profile of the intracellular action potential (6). In addition to these applications, the spectral characteristics of the surface EMG have been used to infer motor unit recruitment strategies and the fiber-type composition of a muscle. Higher characteristic spectral frequencies are associated with a greater proportion of type II fibers (20, 21) and changes in EMG spectral variables are used to infer the recruitment of faster or slower motor units (3,26).The capacity of surface EMG spectral properties to provide information about motor unit recruitment or the proportion of fiber types is based on the rationale that higher threshold (and type II) motor units produce surface action potentials with larger relative energy at higher frequencies than lower threshold (and type I) motor units.Because in these applications the main determinant of the frequency content of an action potential is assumed to be its conduction velocity (28), the crucial issue in this debate is the validity of two assumptions: 1) average conduction velocity of the active motor units is related to fiber-type proportions, and 2) changes in the spectral properties of the surface EMG are associated with changes in average conduction velocity.Fiber-type composition and average muscle fiber conduction velocity. There are several physiological details that confound this association. First, the two main fiber types do not have distinct conduction velocities in humans, but rather conduction velocity has a continuous distribution with a single peak (25). Second, average conduction velocity of muscle fiber action potentials can differ among populations of motor units due to differences in fiber diameter and independent of changes in fiber-type proportions (4). Third, the number of muscle fibers innervated by a motor unit has a skewed distribution. For example, the first dorsal interosseus muscle comprises an equal number of type I and II fibers, but ϳ84% of the motor units have slow contraction times and are fatigue resistant (7). Fourth, the conduction velocity of muscle fiber action potentials can change by ϳ20% with variation in discharge rate (24) and the range of discharge rates varies with recruitment threshold in a population of motor units (2). Fifth, the conduction velocity of muscle fiber action potentials varies with fiber length (15).Experimental evidence: fiber-type composition vs. muscle fiber conduction velocity. The significance of the physiological limitations discussed in...

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Rate Coding Is Compressed But Variability Is Unaltered for Motor Units in a Hand Muscle of Old Adults

Cited by 121 publications

References 73 publications

Aging effects on the control of grip force magnitude: An fMRI study

Aging effects on the control of grip force magnitude: An fMRI study

Influence of fatigue on the simulated relation between the amplitude of the surface electromyogram and muscle force

Counterpoint: Spectral properties of the surface emg do not provide information about motor unit recruitment and muscle fiber type

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