Lecture 6 ageing adaptation.pdf

Neuromuscular adaptations to natural adult ageing Michael Paris School of Kinesiology and Health Science York University, Toronto, ON Learning objectives Describe the changes in the neuromuscular system that occur with normal aging. What is sarcopenia and basic causes Explain the process of motor un...

Neuromuscular adaptations to natural adult ageing Michael Paris School of Kinesiology and Health Science York University, Toronto, ON Learning objectives Describe the changes in the neuromuscular system that occur with normal aging. What is sarcopenia and basic causes Explain the process of motor unit remodelling and its functional consequences to motor output What is the role of physical activity and exercise in the remodelling process of natural ageing Loss of strength with age Rice & Cunningham CRC Press 2001 Sarcopenia Loss of muscle mass (and strength & power) associated with biologic aging from maturity to senescence Loss of Muscle Mass - Sarcopenia MR Images of the arm 40% - 50% 22 y 95 y Klein, et al JAP 2001 Loss of muscle mass is due to both loss of fibre area and fibre number Central (CNS) Changes with Aging Reduction in cortex size (white & gray) not a loss of upper motor neurons! BUT significant atrophy/thinning within pre-motor and motor cortex Impairments in the effective connectivity within cortico-networks Disruption in white matter integrity and shorter neurons Decreased levels of dopamine, glutamine, ACh, serotonin & NE Cortical hypo-excitability Decreased ability to activate motor cortex with increasing force requirements Weakness in old age associated with reduced ability to activate muscle No difference in activation between ‘not weak’ old adults and young controls Weak old adults display significant reduction in ability to activate their knee extensors Possible factors causing sarcopenia Modified from: Narici and Maganaris J. Anat. (2006) Age-Related MU Remodeling Evidence in humans? Limited and indirect How to assess? - Anatomically - Nature’s experiments (disease) - Electrophysiologically – indirect Modified from: McComas, 1996 Are all skeletal muscles (MUs) affected equally by adult aging? Influence of Exercise/Activity? Modified from: McComas, 1996 Aging and Motor Unit (MU) Remodeling MU 1 7 fibers MU 2 8 8 fibers Aging and MU remodeling Motoneuron 1 – perhaps a FT/Type II unit Aging and MU remodeling Collateral reinnervation by MU2 of some orphaned fibers of MU 1 – but perhaps not all Aging and MU remodeling MU 2 1 13 fibers Decades of aging = Fewer MUs & larger % of Type I (ST) fiber area Contributes to: Sarcopenia and whole muscle strength loss; and perhaps to overall slowed muscle contractile speed Functional Consequences of Aging Type II fibre size & Type I fibre grouping Slowed contractile response Young Old F = fascicle Lexell and Downham, 1991 10 Hz Twitch Force-Frequency curve? Isometric vs. Dynamic Function Sale and Spriet, 1996 FATIGUABILITY Factors: Task, Muscle, Sex, Activity, Age (55-70y vs 75+y) ISOMETRIC Equal or less fatigue in old Many studies Fewer parameters DYNAMIC Usually more fatigue in old Fewer studies Many parameters Ageing and Motor Units (MU) - Neuromuscular Function Laboratory Measures Age-related decline in number of motor units EBD – small intrinsic foot muscle Well suited for estimating # of motor units Clear age-related loss of MU number with advancing age Accelerated decline after 60 years of age Case-study: 92 year old with a SINGLE motor unit, denervation 5 weeks later Quality of MUs from indwelling EMG recordings Young Old 70 Motor Unit Discharge Rate (Hz) MVCs 60 50 40 N.S. N.S. 30 20 10 0 First Dorsal Interosseous Kamen et al. 1995 Biceps Brachii Triceps Brachii Dalton et al. 2010 Vastus Lateralis Tibialis Anterior Soleus Vastus Medialis Dalton et al. 2009 Connelly et al. 1999 Roos et al. 1999 Kamen & Knight 2004 Motor unit jitter Motor unit jitter is significantly increased in some diseases Also increased with aging compared to healthy young adults Lesser extent compared to disease Overlay many action potentials, aligning based on 1st muscle fibre Two muscle fibres within same motor unit Assess timing variability of 2nd muscle fibre Olga Kotelko 94y Masters Athletes Miyazaki Hidekichi 103y Morita Mitsu 90y Estimates of Numbers of MUs in male tibialis anterior of Young, Older & Masters Runners 150 140 91 27y 66y 64y Power et al 2010 Med Sci Spt Ex Estimates of Numbers of MUs in male and female tibialis anterior of Young, Old & Old Masters athletes TA MUNE 160 140 Number of Motor Units 120 * 150 100 80 60 85 40 20 60 0 Old controls ~82y 1 Masters Athletes ~82y Young controls ~25y Power, at al 2016, JAP Motor Unit Number Estimate Aging 100 75 50 ? 25 ?? 70 y 70 y Take home points Sarcopenia is an ageing-related loss of skeletal muscle mass, strength, and power Central and peripheral components Motor unit death and collateral reinnervation leads to larger and perhaps slower motor units Physical activity can attenuate ageing-related motor unit remodeling, but may be a limit on its capacity in very old age