Aging movement control
Encyclopedia
Normal aging movement control in human is about the changes on the muscle
s, motor neuron
s, 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. With aging, neuromuscular movements are impaired though with training or practice, some aspects may be prevented.
s occur in the cortex
. They propagate in the spinal cord
, the motor neuron
s and the set of muscle fibers they innervate. This results in a twitch
which properties are driven by 2 mechanisms: motor unit recruitment
and rate coding
. Both mechanisms are affected with aging. For instance, the number of motor unit
s may decrease, the size of the motor units, i.e. the number of muscle fibers they innervate may increase, the frequency at which the action potentials are triggered may be reduced. Consequently, force production is generally impaired in old adults .
Aging is associated with decreases in muscle mass and strength
. These decreases may be partially due to losses of alpha motor neurons. By the age of 70, these losses occur in both proximal and distal muscles. 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). Old adults show evidence that remaining motor units may become larger as motor units innervate collateral muscle fibers .
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. However, for the maximal effort contractions, there is an appreciable difference in discharge rates between the two age groups. Discharge rates obtained at 100% of MVC are 64% smaller in the old adults than in the young subjects: 31.1 ± 11.8 impulses/s in the old subjects, 50.9 ± 19.5 impulses/s in the young subjects .
Isometric strength and physical cross-sectional area of the elbow
flexors
and elbow extensors
are reduced in old compared with young men. The normalized force (maximal voluntary force to the size of the muscle producing the force) of the elbow extensors is the same for old and young people. The normalized force for the elbow flexors is reduced in the old men compared to the young men. The lower normalized force of the elbow flexors may be due to an increase in agonist-antagonist muscles coactivation .
Compared to the young group, the old group has lower dorsiflexors
isometric torque
at all angles, has lower knee
extensors isometric torque at angles >90°. The impairment in force production is muscle specific. During dynamic exercise, the old group requires more time to reach a target velocity and is less able to attain high velocities. The slowing of voluntary contractile speed with age seems to play a role in the loss of dynamic torque .
by a receptor
in the afferent nerve terminals (vs efferent nerve terminals) is useful to protect the body against unexpected disturbances. Studies in post-mortem subjects support that the thickness of muscle spindle
capsules increases with age. There is a slight decrease in the number of intrafusal fibers
in the oldest subjects. Some spindles show changes consistent with denervation associated with grouped denervation atrophy
. Age-related changes are observed in fine structure of spindle innervation in the form of axon
al swelling and expanded/abnormal endplates
.
When subject to a task of proprioception
, the elderly show increased cocontraction
of agonist-antagonist muscles, perhaps to increase gamma drive and spindle sensitivity. It is believed to be used for postural control. 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 .
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) .
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 .
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 .
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..
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 .
(peripheral fatigue) resistance during sustained isometric maximal voluntary contraction, but they show greater supraspinal fatigue at start of sustained task, and during recovery. The first observation reflects changes in fiber type ratio; with aging the proportion of type I muscle fibers which are adapted to long effort becomes greater. The second observation is likely a result of cumulative effects of exercise on the central nervous system
.
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 .
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, unpredictabe time target steps). Age-related slowing is only evident for predictable targets; however other studies have show otherwise, but noted higher variance in speed of old adults .
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 .
Old adults exhibit reductions in manual dexterity which is observed through changes in finger
tip 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 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 .
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 .
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 itensity 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 .
contacts, fiber area/capillary contacts) of the vastus lateralis are unaffected by age. The old men normal fit or trained have smaller type II muscle fiber areas and fewer capillaries surrounding these fibers than do the young men. The capillary supply per unit type II fiber area is not affected by age but is enhanced by training. The old trained men have succinate dehydrogenase activities within their type IIa muscle fibers similar to those in young men and twofold higher than in old normal fit men .
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 .
Heavy resistance and sensorimotor trainings result in increased maximum voluntary contraction and rate force development. But sensorimotor training shows more positive adaptations in postural reflex
es, 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 .
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 .
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 .
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s, motor neuron
Motor neuron
In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles...
s, nerves
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
, sensory functions
Sensory neuron
Sensory neurons are typically classified as the neurons responsible for converting external stimuli from the environment into internal stimuli. They are activated by sensory input , and send projections into the central nervous system that convey sensory information to the brain or spinal cord...
, gait
Gait
Gait is the pattern of movement of the limbs of animals, including humans, during locomotion over a solid substrate. Most animals use a variety of gaits, selecting gait based on speed, terrain, the need to maneuver, and energetic efficiency...
, fatigue, visual and manual responses, in men and women as they get older but who do not have neurological
Neurological disorder
A neurological disorder is a disorder of the body's nervous system. Structural, biochemical or electrical abnormalities in the brain, spinal cord, or in the nerves leading to or from them, can result in symptoms such as paralysis, muscle weakness, poor coordination, loss of sensation, seizures,...
, muscular (atrophy, dystrophy
Muscular dystrophy
Muscular dystrophy is a group of muscle diseases that weaken the musculoskeletal system and hamper locomotion. Muscular dystrophies are characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle cells and tissue.In the 1860s, descriptions of boys who...
...) or neuromuscular
Neuromuscular disease
Neuromuscular disease is a very broad term that encompasses many diseases and ailments that either directly, via intrinsic muscle pathology, or indirectly, via nerve pathology, impair the functioning of the muscles....
disorder. With aging, neuromuscular movements are impaired though with training or practice, some aspects may be prevented.
Force production
For voluntary force production, action potentialAction potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
s occur in the cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
. They propagate in the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
, the motor neuron
Motor neuron
In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles...
s and the set of muscle fibers they innervate. This results in a twitch
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...
which properties are driven by 2 mechanisms: motor unit recruitment
Motor unit recruitment
Motor unit recruitment is the progressive activation of a muscle by successive recruitment of contractile units to accomplish increasing gradations of contractile strength....
and rate coding
Rate coding
The rate coding model of neuronal firing communication states that as the intensity of a stimulus increases the frequency or rate of action potentials, or "spike firing", increases. Rate coding is sometimes called frequency coding....
. Both mechanisms are affected with aging. For instance, the number of motor unit
Motor unit
”A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates; all of these fibers will be of the same type . When a motor unit is activated, all of its fibers contract...
s may decrease, the size of the motor units, i.e. the number of muscle fibers they innervate may increase, the frequency at which the action potentials are triggered may be reduced. Consequently, force production is generally impaired in old adults .
Aging is associated with decreases in muscle mass and strength
Physical strength
Physical strength is the ability of a person or animal to exert force on physical objects using muscles. Increasing physical strength is the goal of strength training.-Overview:...
. These decreases may be partially due to losses of alpha motor neurons. By the age of 70, these losses occur in both proximal and distal muscles. In biceps brachii
Biceps brachii muscle
In human anatomy, the biceps brachii, or simply biceps in common parlance, is, as the name implies, a two-headed muscle located on the upper arm. Both heads arise on the scapula and join to form a single muscle belly which is attached to the upper forearm...
and brachialis
Brachialis muscle
The brachialis is a muscle in the upper arm that flexes the elbow joint. It lies deeper than the biceps brachii, and is a synergist that assists the biceps brachii in flexing at the elbow...
, old adults show decreased strength (by 1/3) correlated with a reduction in the number of motor units (by 1/2). Old adults show evidence that remaining motor units may become larger as motor units innervate collateral muscle fibers .
In first dorsal interosseus
Dorsal interossei of the hand
The dorsal interossei of the hand are muscles that occupy the space between the metacarpals.-Structure:There are four dorsal interossei in each hand...
, almost all motor units are recruited at moderate rate coding, leading to 30-40% of maximal voluntary contraction
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...
(MVC). Motor unit discharge rates
Rate coding
The rate coding model of neuronal firing communication states that as the intensity of a stimulus increases the frequency or rate of action potentials, or "spike firing", increases. Rate coding is sometimes called frequency coding....
measured at 50% MVC are not significantly different in the young subjects from those observed in the old adults. However, for the maximal effort contractions, there is an appreciable difference in discharge rates between the two age groups. Discharge rates obtained at 100% of MVC are 64% smaller in the old adults than in the young subjects: 31.1 ± 11.8 impulses/s in the old subjects, 50.9 ± 19.5 impulses/s in the young subjects .
Isometric strength and physical cross-sectional area of the elbow
Elbow
The human elbow is the region surrounding the elbow-joint—the ginglymus or hinge joint in the middle of the arm. Three bones form the elbow joint: the humerus of the upper arm, and the paired radius and ulna of the forearm....
flexors
Flexion
In anatomy, flexion is a position that is made possible by the joint angle decreasing. The skeletal and muscular systems work together to move the joint into a "flexed" position. For example the elbow is flexed when the hand is brought closer to the shoulder...
and elbow extensors
Extension (kinesiology)
In kinesiology, extension is a movement of a joint that results in increased angle between two bones or body surfaces at a joint. Extension usually results in straightening of the bones or body surfaces involved. For example, extension is produced by extending the flexed elbow. Straightening of...
are reduced in old compared with young men. The normalized force (maximal voluntary force to the size of the muscle producing the force) of the elbow extensors is the same for old and young people. The normalized force for the elbow flexors is reduced in the old men compared to the young men. The lower normalized force of the elbow flexors may be due to an increase in agonist-antagonist muscles coactivation .
Compared to the young group, the old group has lower dorsiflexors
Dorsiflexion
Dorsiflexion is the movement which decreases the angle between the dorsum of the foot and the leg, so that the toes are brought closer to the shin. The movement moving in opposite directions is called plantarflexion...
isometric torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
at all angles, has lower knee
Knee
The knee joint joins the thigh with the leg and consists of two articulations: one between the fibula and tibia, and one between the femur and patella. It is the largest joint in the human body and is very complicated. The knee is a mobile trocho-ginglymus , which permits flexion and extension as...
extensors isometric torque at angles >90°. The impairment in force production is muscle specific. During dynamic exercise, the old group requires more time to reach a target velocity and is less able to attain high velocities. The slowing of voluntary contractile speed with age seems to play a role in the loss of dynamic torque .
Sensory function
The detection of a stimulusStimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....
by a receptor
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...
in the afferent nerve terminals (vs efferent nerve terminals) is useful to protect the body against unexpected disturbances. Studies in post-mortem subjects support that the thickness of muscle spindle
Muscle spindle
Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts...
capsules increases with age. There is a slight decrease in the number of intrafusal fibers
Intrafusal muscle fiber
Intrafusal muscle fibers are skeletal muscle fibers that comprise the muscle spindle and are innervated by gamma motor neurons. These fibers are a proprioceptor that detect the amount and rate of change of length in a muscle. These fibers are walled off from the rest of the muscle by a collagen...
in the oldest subjects. Some spindles show changes consistent with denervation associated with grouped denervation atrophy
Atrophy
Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations , poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply to the target organ, disuse or lack of exercise or disease intrinsic to the tissue itself...
. Age-related changes are observed in fine structure of spindle innervation in the form of axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
al swelling and expanded/abnormal endplates
Joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...
.
When subject to a task of proprioception
Proprioception
Proprioception , from Latin proprius, meaning "one's own" and perception, is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement...
, the elderly show increased cocontraction
Coactivation
Coactivation may mean:* Coactivation , a process by which RNA transcription is increased* Coactivation , a process in which a muscle is activated in tandem with another muscle...
of agonist-antagonist muscles, perhaps to increase gamma drive and spindle sensitivity. It is believed to be used for postural control. Despite a cocontraction strategy, old adults have higher reaction time and also make greater errors in estimating the position of their ankle
Ankle
The ankle joint is formed where the foot and the leg meet. The ankle, or talocrural joint, is a synovial hinge joint that connects the distal ends of the tibia and fibula in the lower limb with the proximal end of the talus bone in the foot...
. The elderly subjects with greater errors for the dynamic position sense also perform poorly on the single limb stance eyes closed test .
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
Visual system
The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world...
) to another (somatosensory
Somatosensory system
The somatosensory system is a diverse sensory system composed of the receptors and processing centres to produce the sensory modalities such as touch, temperature, proprioception , and nociception . The sensory receptors cover the skin and epithelia, skeletal muscles, bones and joints, internal...
) 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) .
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
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...
, by reduction of internodal length
Internodal segment
An internodal segment is the portion of a nerve fiber between two Nodes of Ranvier. The neurolemma or primitive sheath is not interrupted at the nodes, but passes over them as a continuous membrane....
. Some studies suggest an overall decrease in the number of myelinated fibers .
Aging results in slowed reaction time in an aiming task for both eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
and hand
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...
movements. Comparisons between young and old adults who have to follow a target only with their eyes or with a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
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 .
Walking gait
When confronted to an unexpected slip or trip during walking, compared to young adults, old adults have a less effective balanceBalance (ability)
In biomechanics, balance is an ability to maintain the center of gravity of a body within the base of support with minimal postural sway. When exercising the ability to balance, one is said to be balancing....
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..
In an experiment, for a single-task walking, 24% of old adults have gait
Gait
Gait is the pattern of movement of the limbs of animals, including humans, during locomotion over a solid substrate. Most animals use a variety of gaits, selecting gait based on speed, terrain, the need to maneuver, and energetic efficiency...
speed <0.8 m/s but for a dual-task
Dual-task paradigm
A dual-task paradigm is a procedure in experimental psychology that requires an individual to perform two tasks simultaneously, in order to compare performance with single-task conditions.When performance scores on one and/or both tasks are lower when they are done simultaneously compared to...
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
Cognition
In science, cognition refers to mental processes. These processes include attention, remembering, producing and understanding language, solving problems, and making decisions. Cognition is studied in various disciplines such as psychology, philosophy, linguistics, and computer science...
resources and suggest that real world dual-task effects on gait may be underestimated by reaction time tasks .
Fatigue resistance
Compared to young adults, old adults exhibit muscle fatigueMuscle fatigue
Muscle fatigue is the decline in ability of a muscle to generate force. It can be a result of vigorous exercise but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction...
(peripheral fatigue) resistance during sustained isometric maximal voluntary contraction, but they show greater supraspinal fatigue at start of sustained task, and during recovery. The first observation reflects changes in fiber type ratio; with aging the proportion of type I muscle fibers which are adapted to long effort becomes greater. The second observation is likely a result of cumulative effects of exercise on the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
.
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 .
Speed, dexterity
For old adults, the decreased saccadicSaccade
A saccade is a fast movement of an eye, head or other part of an animal's body or device. It can also be a fast shift in frequency of an emitted signal or other quick change. Saccades are quick, simultaneous movements of both eyes in the same direction...
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, unpredictabe time target steps). Age-related slowing is only evident for predictable targets; however other studies have show otherwise, but noted higher variance in speed of old adults .
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 .
Old adults exhibit reductions in manual dexterity which is observed through changes in finger
Finger
A finger is a limb of the human body and a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates....
tip 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 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 .
The prismatic grasp (4 fingers in opposition to thumb) which is common in everyday activities, involves the organization of the digits
Finger
A finger is a limb of the human body and a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates....
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 .
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 itensity 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 .
Training consequences
Type I muscle fiber characteristics (area, number of capillaryCapillary
Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...
contacts, fiber area/capillary contacts) of the vastus lateralis are unaffected by age. The old men normal fit or trained have smaller type II muscle fiber areas and fewer capillaries surrounding these fibers than do the young men. The capillary supply per unit type II fiber area is not affected by age but is enhanced by training. The old trained men have succinate dehydrogenase activities within their type IIa muscle fibers similar to those in young men and twofold higher than in old normal fit men .
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
Strength training
Strength training is the use of resistance to muscular contraction to build the strength, anaerobic endurance, and size of skeletal muscles. There are many different methods of strength training, the most common being the use of gravity or elastic/hydraulic forces to oppose muscle contraction...
, old adults can significantly improve their force control. The rapid adaptation suggests modifications in motor unit activation, increased excitability of motoneuron pool
Motor pool
In neuroscience, a motor pool refers to a group of motor spinal neurons that innervate the same muscle. The biological significance of motor pool organization is in the fact that motor pools with many neurons produce finer movements...
, and decreased antagonist cocontraction .
Heavy resistance and sensorimotor trainings result in increased maximum voluntary contraction and rate force development. But sensorimotor training shows more positive adaptations in postural reflex
Reflex
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...
es, 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
Joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...
stiffness during perturbations .
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 .
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 .