For at least 20 years, it has been known that the number of functioning motor units declines with age. Although there is some compensation for the decline in motor unit number in the form of collateral sprouting and reinnervation, the net effect is a decrease in motor unit number and in muscle mass but an increase in motor unit size. Little is known about the contribution of this motor unit reorganization to the age-related reduction in manipulative capabilities, the increased hesitancy of goal- directed movements, and the greater kinematic variability of simple movements. The hypothesis of this project is that motor unit reorganization during aging contributes significantly to the age-related decline in some movement capabilities. To test this hypothesis, we propose to identify differences in motor performance between young and elderly subjects that can be attributed to differences in motor unit properties and behavior. We have found evidence of motor unit reorganization in a hand muscle of healthy elderly subjects and a reduced ability to sustain constant submaximal forces. This impairment, which corresponds to an increase in the amplitude of the force fluctuations, appears to be adaptable and can be alleviated with the type of strengthening exercises used in rehabilitation (physical therapy). The current application proposes descriptive and mechanistic experiments that continue to explore a role for motor unit reorganization (decrease in number but increase in size) in the change in motor performance with aging.
The specific aims are: (l) to determine the mechanisms mediating the training-induced reductions in the force fluctuations of constant- force contractions; (2) to characterize motor unit behavior during non- isometric contractions; and (3) to identify the adaptability of neural strategies used during isometric and non-isometric tasks. The experiments will involve single joint tasks performed by one hand muscle and by one arm muscle of human volunteers. The subjects will execute simple isometric and non-isometric contractions that will be characterized by electrophysiological and mechanical measures of muscle activity. The discharge of single motor units will be recorded with intramuscular electrodes so that motor unit behavior can be correlated to the surface- recorded EMG of synergist and antagonist muscles, and to the net muscle force. In addition, images from MRI scans will be used to quantify changes in the activation pattern of muscles following strength training. The data acquired by testing these aims will indicate the extent to which neural activation strategies and muscle function are influenced by the age-related reorganization of motor units. The proposed studies represent novel attempts to characterize the effects of aging on the relationship between neural strategies, motor unit behavior, and motor performance, albeit for simplified motor tasks. The knowledge gained from these studies will have relevance for geriatric, neurologic, and rehabilitation medicine.
| Pascoe, Michael A; Holmes, Matthew R; Stuart, Douglas G et al. (2014) Discharge characteristics of motor units during long-duration contractions. Exp Physiol 99:1387-98 |
| Pascoe, Michael A; Gould, Jeffrey R; Enoka, Roger M (2013) Motor unit activity when young and old adults perform steady contractions while supporting an inertial load. J Neurophysiol 109:1055-64 |
| Heckman, C J; Enoka, Roger M (2012) Motor unit. Compr Physiol 2:2629-82 |
| Jesunathadas, Mark; Aidoor, Sameer S; Keenan, Kevin G et al. (2012) Influence of amplitude cancellation on the accuracy of determining the onset of muscle activity from the surface electromyogram. J Electromyogr Kinesiol 22:494-500 |
| Marmon, Adam R; Pascoe, Michael A; Schwartz, Robert S et al. (2011) Associations among strength, steadiness, and hand function across the adult life span. Med Sci Sports Exerc 43:560-7 |
| Duchateau, Jacques; Enoka, Roger M (2011) Human motor unit recordings: origins and insight into the integrated motor system. Brain Res 1409:42-61 |
| Pascoe, Michael A; Holmes, Matthew R; Enoka, Roger M (2011) Discharge characteristics of biceps brachii motor units at recruitment when older adults sustained an isometric contraction. J Neurophysiol 105:571-81 |
| Marmon, Adam R; Gould, Jeffrey R; Enoka, Roger M (2011) Practicing a functional task improves steadiness with hand muscles in older adults. Med Sci Sports Exerc 43:1531-7 |
| Christou, Evangelos A; Enoka, Roger M (2011) Aging and movement errors when lifting and lowering light loads. Age (Dordr) 33:393-407 |
| Farina, Dario; Holobar, Ales; Merletti, Roberto et al. (2010) Decoding the neural drive to muscles from the surface electromyogram. Clin Neurophysiol 121:1616-23 |
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