Aging movement control
Normal aging movement control in humans is about the changes in the
Force production
For voluntary force production,
Aging is associated with decreases in muscle mass and
In
Compared to the young group, the old group has lower
Sensory function
The detection of a
When subject to a task of
Old adults sway more than young adults while maintaining upright standing posture, especially with eyes closed with a narrow base of support. Young adults show "resourcefulness" by shifting from one sensory input (vision) to another (somatosensory) whereas old adults do not rely on the variety of sensory inputs but rather respond by stiffening their ankles across tasks (wide base of support vs narrow base of support, eyes open vs eyes closed).[8]
Sensory receptors can initiate rapid responses to perturbations thanks to short-latency connections between afferent innervations and motor units. Yet, aging results in decreases in motor conduction velocities. This may be due to losses of the fastest conducting motor units. There is also evidence of slowing of both fast and slow conducting axons which can be explained by decreases in axon diameter through demyelination, by reduction of internodal length. Some studies suggest an overall decrease in the number of myelinated fibers.[9]
Aging results in slowed reaction time in an aiming task for both eye and hand movements. Comparisons between young and old adults who have to follow a target only with their eyes or with a laser in their hand, show that parameters indicative of motor function such as velocity, duration, and amplitude of initial movement are unchanged. However the duration of corrective movement is longer for old adults. It suggests an impairment to sensory system.[10]
Walking gait
When confronted to an unexpected slip or trip during walking, compared to young adults, old adults have a less effective balance strategy: smaller and slower postural muscle responses, altered temporal and spatial organization of the postural response, agonist-antagonist muscles coactivation and greater upper trunk instability. Comparing control and slip conditions, after the perturbation, young adults have a longer stride length, a longer stride duration, and the same walk velocity whereas old adults have a shorter stride length, the same stride duration, and a lower walk velocity.[11]
In an experiment, for a single-task walking, 24% of old adults have gait speed <0.8 m/s but for a dual-task of walking and talking, 62% of old adults have gait speed <0.8 m/s. In practical terms, this means that a large proportion of healthy community-dwelling old adults may not walk fast enough to safely cross the street while simultaneously having a conversation. These findings support the assertion that generating spontaneous speech is highly demanding on cognitive resources and suggest that real world dual-task effects on gait may be underestimated by reaction time tasks.[12]
Fatigue resistance
Compared to young adults, old adults exhibit
For the knee extensors, old adults produce less torque during dynamic or isometric maximal voluntary contractions than young adults. The mechanisms controlling fatigue in the elderly during isometric contractions are not the same as those that influence fatigue during dynamic contractions, while young adults keep the same strategy. The knee extensors of healthy old adults fatigue less during isometric contractions than do those of young adults who had similar levels of habitual physical activity. In contrast, there are no differences between age groups in the fatigue during dynamic contractions.[14]
Speed, dexterity
For old adults, the decreased saccadic accuracy, prolonged latency, and reduced saccadic velocity may be explained by cerebral cortical degeneration with age. Old adults show reduced amplitude of primary saccades and they generally more saccades to reach fixation. Old adults show significant delay of saccades in all conditions (predictable amplitude and time target steps, unpredictable amplitude target steps, unpredictable time target steps). Age-related slowing is only evident for predictable targets; however other studies have shown otherwise but noted higher variance in speed of old adults.[15]
Instructed to look either toward (pro-saccade task) or away from (anti-saccade task) an eccentric target under different conditions of fixation, for young children (5±8 years of age) a long time elapses between the apparition of the target and the onset of the eye movement (Saccadic Reaction Time). Young adults (20±30 years of age) typically have the fastest SRTs. Elderly subjects (60±79 years of age) have slower SRTs and longer duration saccades than any other age groups.[16]
Old adults exhibit reductions in manual dexterity which is observed through changes in fingertip force when gripping and/or lifting. Compared to young adults, old adults show an increase in grip force and safety margins (minimum force necessary to prevent a slip). These increases can be explained by skin slipperiness or it may be the result of declining cutaneous information. Force increases are not associated with impaired capacity to modulate fingertip forces smoothly. There is no evidence that old adults were less able to program fingertips based on the memory of a preceding lift.[17]
The prismatic grasp (4 fingers in opposition to thumb) which is common in everyday activities, involves the organization of the digits into specific tasks and the balance of force/moment production by individual digits. Old adults exhibit an impairment in finger and hand force production. They show excessive grip force which could be related to higher moments produced by antagonist fingers. Both can be viewed as energetically suboptimal but more stable performance.[18]
Old adults often show heightened antagonist muscle coactivation during goal directed movement. Contractions at moderate-to-high force often show activation of other ipsilateral and contralateral muscles. When the intensity of contralateral activity is sufficient to produce movement, this is called "mirror movement". When asked to follow a unilateral task, young and old adults show concurrent activity in contralateral muscle but it is greater in old adults. Contralateral activity is greater for isometric than for anisometric contractions. Contralateral force is greater for eccentric than concentric contractions.[19]
Training consequences
Type I muscle fiber characteristics (area, number of
Neural changes like reduced motor unit discharge rates, increased variability of motor unit discharge activity, altered recruitment and derecruitment behavior mediate modifications in muscle control. On the other hand, physiological deleterious factors including motor unit loss, increased motor unit innervation ratios also affect muscle force. Through strength training, old adults can significantly improve their force control. The rapid adaptation suggests modifications in motor unit activation, increased excitability of motoneuron pool, and decreased antagonist cocontraction.[21]
Heavy resistance and sensorimotor trainings result in increased maximum voluntary contraction and rate force development. But sensorimotor training shows more positive adaptations in postural reflexes, which is likely due to training of sensory reception/processing, central integration of afferent information, transformation of that information into adequate efferent response. The decreased onset latency and increased magnitude of reflex response with sensorimotor training is associated with increased ankle joint stiffness during perturbations.[22]
When asked to reach a given level of force at a certain moment in time without any visual feedback, old adults are less accurate than young adults. With the practice of goal-directed contractions, old adults can improve the accuracy of novel motor tasks (isometric or dynamic) though their strategy differs from the strategy used by young adults. For both age groups, the greatest improvements in accuracy occur at the beginning of practice.[23]
Old adults are able to improve the modulation of grasping forces after motor practice. Unexpectedly, motor practice fails to reduce grasping performance losses under the dual-task conditions but motor practice reduces the decline in cognitive performance under dual-task conditions. Therefore, motor practice seems to free up cognitive resources that were previously monitoring motor performance and old adults seemed to use these resources to improve their cognitive performance under dual-task conditions.[24]
References
- ^ Neuromechanics of human movement, chapter 6 Single-joint system function; Roger M. Enoka.
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