5. Skeletal changes: age-related changes

Question 1

skeletal muscles
- function?
- composed of what? organized within what?
- 3 categories of muscles fiber types

Answer 1

• allow us to engage in voluntary actions or movement
• composed of a multitude of parallel skeletal muscle fibers (fascicles) organized within a complex arrangement of connective tissues
  1. type 1 slow twitch fibers
  2. type 2 fast twitch fibers
  a) fast oxidative fibers (2A)
  b) fast glycolytic fibers (2X)

Question 2

• which muscle fibers are recruited first?
• do slow oxidative fibers have high or low glycogen stores? vs fast oxidative vs fast glycolytic?
• fiber diameter (small, intermediate, large) for the 3 types?
• myosin ATPase activity (slow or fast for the 3 types?

Answer 2

• slow ox –> fast ox –> fast glycolytic
• slow ox: low –> intermediate –> fast glycolytic has high
• small (slow ox) –> intermediate (fast ox) –> large (fast glyc.)
• slow, fast, fast

Question 3

explain the curve of muscle mass (y) vs age (x) for early life, adult life and older life

• 2 causes?

Answer 3

• early life: growth and development to maximise peak! muscle mass increases!
• adult life (40 yo ish): maintaining peak! muscle mass decreases a bit but those who exercise have a much smaller slope than those who don’t
• older life (70-90 ish): minimizing loss:
• the smaller peak you have, and the more rapid decline you have, the earlier you will cross the disability threshold, after which you won’t be able to maintain function and independence

CAUSES:
- decline in muscle activity (disuse, physical inactivity, immobilization if injured)
- chronic disease

Question 4

• define muscle strength
• essential for what (2 ish) + 4 examples
• higher muscle strength is associated with decreased risk of (7)

Answer 4

• maximal capacity to generate force or tension (ie 1RM or estimated 1-RM, from 10RM or 5RM = safer)
• essential for many activities of daily living and functional independence –> ie carrying groceries, climbing stairs, getting up from a chair, and lifting grandchildren
• CVD, falls, fractures, arthritis, diabetes, depression, and dementia

Question 5

define sarcopenia (from which Greek words?) –> 2 defs ish
- which 3 components are included in sarcopenia?
- define dynapenia

Answer 5

SARCOPENIA:
- Greek words sarx, “flesh” and penia, “loss
- also known as age-related muscle atrophy (muscle wasting, muscle loss)
- Describes the age-related decline in muscle size (mass, area) and function (strength, power, physical performance)
1. low muscle mass and area
2. low muscle strength and power
3. low physical performance

DYNAPENIA:
“poverty of strength”
- describes the natural age-related loss of skeletal muscle strength
*not due to conditions or chronic diseases

Question 6

• when does muscle mass peak? what happens after?
• men vs women have same changes in muscle mass as they age?

Answer 6

• peak in early adulthood, followed by non-linear decline starting around 45-55 (seen in both upper and lower body)
  MEN
• expected to lose 0.7%/year of fat free mass from 60-65 yo + up to 1.3%/year at 75-85 yp
  WOMEN:
• annual relative decline in muscle mass is estimated at 0.53% (for age 60) and 0.7% (for age 75)
• a bit more gradual than men

Question 7

1) overall: muscle mass declines by ___-____% per year vs muscle strength declines by ___-____% per year
2) do age related muscle strength losses occur faster or slower than skeletal muscle mass losses?
3) does upper or lower body muscle mass decline faster?
4) do men or women lower more muscle mass?
5) concentric, isometric and eccentric –> which strength declines faster? why?

Answer 7

1) 1-2% muscle mass loss per year VS 2-4% muscle strength loss per year
2) age-related muscle strength loss = faster!
3) lower body muscle mass declines faster than upper-body –> lower body = weight bearing muscles + more sensitive to disuse
4) men lose more! (probs bc of hormones)
5) concentric (-56% from 20 yo to 90 yo) (goes below the strength needed to rise from chair!), isometric (-46%) and eccentric declines the slowest (25%) –> because of gravity component of muscle lengthening?

Question 8

with aging, decrease in muscle strength and power are characterized by (2)
- predominantly which fibers?
- at what age does muscle area start to decrease?
- at what age does muscle area reach the clinical threshold?

Answer 8

1) loss in size of all muscle fibers (size aka fiber cross-sectional area decreases)
2) loss in number of muscle fibers (controversial bc some ppl think the number of myonuclei doesnt change)
- reductions in fiber size and number occur predominantly in type 2 muscle fibers! BUT type 1 also decreases
- 40yo
- 80 yo

Question 9

there is a much larger decline in cross-sectional area of type ___ fibers (__-__% decline) than type ___ fibers
- explained by (3)

Answer 9

type II fibers (25-50% decline) than type I fibers (slow twitch)
*figure: age not correlated with type 1 fiber! vs age negatively correlated with type 2A and 2B fibers

1) inactivity of immobilization
2) changes in functional demands (force, velocity, duration)
3) selective loss of motor neurons (see this a bit with age but mostly pathological)

Question 10

what is the consequence of the shift to greater proportion of type 1 (slow oxidative) fibers?
- even more pronounced in who?

Answer 10

• reduces muscle’s ability to produce strength and power!
• even more pronounced in older, inactive individuals!!
• old inactive have a lot less type 2a than old active counterparts

Question 11

SKELTAL CHANGES
aging is associated with an increase in WHAT infiltration btw (2)
- results in what?

Answer 11

• fat infiltration btw muscle groups and fascicles
• RESULT: reduced muscle strength caused by reduced force and contractile properties of the skeletal muscle (bc decreased muscle quality)

Question 12

what are 3 ish morphologic changes in muscles as one ages?
- 3 consequences ish

Answer 12

1. mechanical disruption of thick and thin filaments or Z-disk attachments
2. inflammatory response: free radical damage –> leads to muscle soreness (delayed onset muscle soreness)
3. evidence of a reduced number of satellite cells in aged human muscle, predominantly type 2 fibers –> impairment in satellite cell proliferation and differentiation due to inflammation and fibrosis (IMPORTANT!)

leads to greater susceptibility to injury and delayed recovery! –> cannot repair and regenerate fibers anymore :(
* decrease muscle regenerative capacity + increase satellite cell differentiation, muscle inflammation and fibrosis! –> can lead to thrombosis (?)

Question 13

• what happens to central (voluntary) activation capacity throughout aging process? statistic of older adults vs younger adults
• differs btw _________ ________, likely due to differences in (2)
• explained by (2)

Answer 13

• decreases! 11% lower in older adults vs younger adults
• differs btw muscle groups (and its fct) –> likely due to differences in motor unit innervations and fiber type characteristics

1) decreased motor unit recruitment
2) decreased rate of motor unit firing

Question 14

• changes in motor unit discharge with aging alters what?
• what exactly happens with aging? (2)

Answer 14

• alters functional properties of skeletal muscle at older ages

1) decrease number and maximal firing rates of motor unit
2) denervation of type 2 fibers (die off ish) and reinnervation of type 1 fibers

Question 15

• disruption in key events of excitation-contraction coupling process with aging decreases what?
• what exactly happens with aging? (3)

Answer 15

• decreases intrinsic force capacity

1) interference in calcium channels (decrease Ca2+ release)
2) changes in ratio of myosin to actin content (decrease number of active cross-bridges)
3) slow twitch contraction speed

Question 16

what happens to which hormones that can explain decrease in (2) par rapport à skeletal muscles
- describe relative change of 5 hormones

Answer 16

• declines in anabolic hormone concentrations –> may explain decrease hypertrophic capacity and decreased muscle strength
• insulin levels: increase or stay same
• insulin sensitivity, insulin-like growth factor-1, growth hormone and testosterone –> all decline (bc decrease receptors)

Question 17

• define muscle power
• age-related decline in muscle power plays an important role in overall health and function –> associated with WHAT? + HOW can it limit older adults?

Answer 17

• ability of muscle to generate force rapidly (force * velocity / time) (ie isokinetic dynamometer, vertical jump on force platform, stair climb test (used clinically!)
• clearly associated with lower physical performance + can limit older adults if activities of daily living bc ability to generate power is necessary for movement

Question 18

does age-related decline in muscle power or age-related decline in muscle strength occur at a faster rate?
- which exercise shown to correlate with all physical performance measures in frail, older residents with multiple chronic conditions?
- mobility limited older adults exhibit greater deficits in ____-________ muscle power compared to healthy middle-aged and older adults –> what about rate of muscle power loss?

Answer 18

• age-related decline in muscle power occurs faster!
  -leg extensor power! (ie from chair rising speed or stair climbing speed)
• in lower-extremity muscle power –> BUT rate of muscle power loss is similar across groups (mobility-limited group start with lower power peak, and end with lower power peak

Question 19

__________ factors explain loss of maximal muscle power –> 5

Answer 19

NEUROMUSCULAR factors!
1) Loss of motor neurons and demyelination of axons
2) Loss of type II muscle fibers –> decrease contraction velocity
3) decrease muscle strength, especially at fast contraction velocities
4) Impairment in neuromuscular activation –> decrease movement velocity and skeletal muscle coordination
5) decrease in maximal force development and longer time to reach maximal force

Question 20

what happens to mitochondrial function with age? starting around what age?
WHY? (3)
- contributes to decrease (2) during exercise!

Answer 20

decreases with age! starting around 20-25 yo
1) accumulation of DNA damage and mutations
2) oxidative stress (increase reactive oxidative species, lipid/protein damage)
3) reduced muscle protein synthesis in response to anabolic stimuli and feeing (anabolic resistance) (one of the big factors!!)

• contributes to decrease aerobic and neuromuscular capacity during exercise!

Question 21

what happens to metabolic enzyme activity with aging?
- give examples

Answer 21

decrease metabolic enzymes involved in glycolysis, Kreb’s cycle and aerobic metabolism! –> leads to decrease neuromuscular performance

• increase or same or decrease phosphofructokinase
• decrease creatine phosphokinase, lactate dehydrogenase, citrate synthase, hexokinase (only males), succinate dehydrogenase (SDH)

Question 22

CASE STUDY:
Max is an 80-year-old man living alone at home. Previously highly functional and independent, in the last year, he fell and fractured his left femur. After he was discharged from the hospital, he was transferred to a nursing home for rehabilitation. He received physical therapy and has now returned home after a 3-week stay. He has lost 12 lbs (8% of his body weight) in the past year and reports that he is more easily fatigued. He walks slowly, at 0.6 m/s, and is unsteady without his walker. He has lost a significant amount of muscle mass, and his grip strength is significantly lower than age-, sex-, and BMI-matched values.
1) What age-related changes are happening at the whole muscle level? At the muscle fiber level?
2) What age-related changes are happening to muscle strength and muscle power? Why?

Answer 22

1) *immobilization –> leads to decrease muscle mass from disuse + 80 yo = probably already age-related decrease in muscle mass 1-2% per year
WHOLE MUSCLE
- decrease cross-sectional area (more type 2 than type 1)
- increase fat infiltration
- change in body composition
- mechanical disruption of thick and thin filaments
MUSCLE FIBER:
- loss of type 2 –> shift to more type 1
- decrease motor neuron
- decrease size + decrease firing rate

2) decrease strength and power due to fat infiltration, decrease cross-sectional size and decrease motor neuron/firing rate
- shown by needing his walker, increased fatigue, decreased grip strengthù

Question 23

SUMMARY:
- describe the 7 changes that happen to muscles when aging + sub

Answer 23

1. muscle fiber: decrease in size and in number
2. fiber type shift: from fast to slow
3. myofilaments: reduced single fiber maximal force + reduction in myosin protein content and function
4. mitochondria: reduced volume and function + lower enzyme activity (across all E systems)
5. adipocyte infiltration: increase inter and intramuscular adipose tissue
6. excitation-contraction coupling: disruption or uncoupling (myosin actin bridge) + deficits in calcium release
7. satellite cells: reduced number + less responsive to injury –> greater inflammation + more pronounced muscle soreness

Question 24

SUMMARY age-related muscle changes:
- progressive age-related decline in skeletal muscle _____ leads to decrease in (3)
- sarcopenia is reflective of (4) changes associated with (2) (as well as what?)
- muscle strength and power are affected by aging through (2) systems
- evidence suggests that muscle _______ is more critical to physical performance than muscle _________

Answer 24

• size leads to decrease in strength, power and physical performance
• morphological, neurological, hormonal and metabolic changes associated with aging and disuse (as well as chronic disease)
• muscular system (loss of type 2 fibers) and nervous system (loss of motor units, demyelination)
• muscle POWER is more critical to physical performance than muscle strength