Normal aging movement control in humans is about the changes in the muscles, motor neurons, nerves, sensory functions, gait, fatigue, visual and manual responses, in men and women as they get older but who do not have neurological, muscular (atrophy, dystrophy...) or neuromuscular disorder.
This results in a twitch which properties are driven by two mechanisms: motor unit recruitment and rate coding.
In biceps brachii and brachialis, old adults show decreased strength (by 1/3) correlated with a reduction in the number of motor units (by 1/2).
[2] In first dorsal interosseus, almost all motor units are recruited at moderate rate coding, leading to 30-40% of maximal voluntary contraction (MVC).
Motor unit discharge rates measured at 50% MVC are not significantly different in the young subjects from those observed in the old adults.
Age-related changes are observed in fine structure of spindle nerve innervation in the form of axonal swelling and expanded/abnormal endplates.
Despite a cocontraction strategy, old adults have higher reaction time and also make greater errors in estimating the position of their ankle.
The elderly subjects with greater errors for the dynamic position sense also perform poorly on the single limb stance eyes closed test.
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.
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.
[10] 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.
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.
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).
The capillary supply per unit type II fiber area is not affected by age but is enhanced by training.
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.