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Aging and Neuromotor Changes: Strength Decline, Postural Impairment, and Behavioral Changes

This lecture discusses the general changes in the neuromotor system with aging, including strength decline, delayed reactions, impaired control of posture and gait, and behavioral changes. It also explores the changes in motor units, reflexes, and sensory function, as well as the effects of training and common subclinical movement signs. Additionally, impairments in posture and gait, and prevalent disorders in the elderly are discussed.

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Aging and Neuromotor Changes: Strength Decline, Postural Impairment, and Behavioral Changes

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  1. Lecture 28: Aging General changes in the neuromotor system: • Strength decline • Longer delays of reactions • Impaired control of posture/gait • Impaired accurate control of force/movement • Unintended force production

  2. Behavioral Changes With Aging • Weakness • Slowness • Higher variability • Larger postural sway; delayed APAs

  3. Movements and Aging Central changes: • Longer RT • Slower movements • Higher antagonist cocontraction • Higher safety margins • Changed synergies

  4. Changes in Motor Units With Age • Decline in the number of alpha-motoneurons • Muscle fiber denervation and atrophy • Reinnervation by surviving motoneurons • Higher innervation ratio • Preferential degeneration of larger motoneurons • Consequence: fewer motoneurons (on average, larger in size and slower)

  5. Changes in Motor Units With Age Number of MUs Number of motor units (extensor digitorum brevis, foot) 300 200 Lower limit 100 20 40 60 80 100 Age (years)

  6. Aging: Denervation and Reinnervation

  7. Consequences of Changes in Motor Units With Age • Smaller MUs are absent; poor control of low forces • Poor smoothness of force production • Inability to develop force quickly (larger twitch contraction time, from 100–150 ms to 125–200 ms)

  8. Changes in Strength With Age • Muscle mass: reduced (partly replaced by fat/connective tissue) • Cross-sectional area: reduced • Normalized force (MVC/cross-sectional area): reduced • Neural activation: depends on the muscle • Coactivation of antagonist muscles: increased by 30% • Not all muscle show similar force losses; typically, more distal muscles are more affected • Consequences for multimuscle synergies

  9. Aging: Parallel Changes in Force and Speed

  10. Changes in Reflexes/Reactions With Age • H-reflex amplitude: slightly reduced (may show a delay) • Tendon tap reflex: slightly reduced (may show a delay) • Polysynaptic reflexes: reduced • Simple reaction time: increased

  11. Accurate Force/Movement Production • Higher variability; larger errors • Large force fluctuations during F = const. • Excessive forces • Excessive grip; larger safety margin

  12. Aging: Increased Variability in Force and MU Firing Rate

  13. Aging: Larger Safety Margin

  14. Changes in Posture/Gait With Age • Postural sway: increased (however, cf. PD) • APAs: loss of asynchronous involvement; delayed • Preprogrammed reactions: decreased and delayed; switch from ankle to hip strategy • Greater coactivation of antagonist muscles • Higher variability (stride to stride)

  15. Aging: Delayed, Smaller APAs

  16. Changes in Fast Movement Production • Slowness in initiation and execution • Longer deceleration • Higher reliance on visual feedback • Higher muscle cocontraction (adaptive?)

  17. Aging: Slower Single-Joint Movements

  18. Aging: Higher Muscle Coactivation

  19. Motor: • Loss of force (more in intrinsic muscles) • Drop in involuntary force production (?) • Poor multidigit synergies in force and moment of force production Sensory: • A decline in the number of receptors • Poor touch discrimination Changes in Hand Function

  20. Aging: Changes Multi-Finger Synergies (Pressing)

  21. Aging: Changes Multi-Finger Synergies (Grasping)

  22. Aging: Changes Multi-Finger Synergies (Grasping)

  23. Effects of Training • Higher forces • Lower antagonist cocontraction • Small changes in cross-sectional area; importance of neural adaptations

  24. Common Subclinical Movement Signs • Bradykinesia • Tone changes (absent in normal aging; paratonia, or a progressive resistance to passive movement) • Cogwheeling phenomenon (rhythmic interruption of attempted passive movement) • Hyperkineses (tremor, in over 25%) • Ophthalmoplegia, particularly limitation of vertical gaze

  25. Impairments in Posture and Gait Posture and postural reflexes: • flexed in neck and trunk, extended in knees and elbows • spontaneous and induced sway are exaggerated • recovery after perturbation is impaired • postural reflexes are impaired in 70% of elderly over 80 years of age Gait abnormality, gait apraxia: • short, slow strides; wide base • 15% of those over 60 have gait problems, reduced arm swing, stiff turns, tendency to fall

  26. Disorders Prevalent in the Elderly • Parkinson’s disease • Drug-induced pakinsonism (neuroleptics) • Essential tremor • Stroke (hemiballismus, dystonia, Parkinson’s-like syndrome) • Tardive dyskinesia (orofacial dyskinesias)

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