in Skeletal Muscle
Dr. Michael Banoub
Skeletal muscle undergoes major structural and
functional adaptations in response to physical
inactivity, just as it does in response to disease,
nutritional status, and obesity.
Age-related reductions in muscle mass are a
direct cause of declines in muscle strength and
disability in the older adult
Reductions in Muscle Mass
Aging is associated with decreases in total
muscle cross sectional area, to approximately
40% between the ages of 20 and 80 years
This accompanied by increases in noncontractile
structures such as fat and connective tissue.
The maintenance of muscle mass occurs as a result
of the balance between protein synthesis and
The rates of skeletal muscle protein synthesis
decline with age and may also contribute to
Reduction in Muscle Fiber Number
The decline in muscle cross sectional area is due to
decreases in total fiber number, especially type II
fast-twitch fibers. The loss of muscle fibers is
followed by a replacement with fat and fibrous
Changes in Muscle Fiber Size
The size of the individual fast-twitch type II
fibers decreases with age whereas the slow-
twitch type I fiber size does not change.
For example, in the third or fourth decade of
life, the mean cross-sectional area of individual
fast-twitch type II fibers in the quadriceps
femoris muscle exceeds that of . slow-twitch
type I fibers by approximately 20%. By the age
of 85 the area of individual fast-twitch type II
fibers is less than 50% of that of slow-twitch
type I fibers.
Motor Unit Number and Size
There is a decrease in total number of motor
units with age. The average motor neuron loss
from the second to tenth decade is
The decrease in motor unit number is
accompanied by an increase in size or
innervation ratio, each motor neuron innervates
more muscle fibers in the older adult.
The increase in size of the motor unit is
found primarily in the muscles of the lower
limb, particularly in persons 60 years old,
and more in distal than in proximal
There is an age-related reduction in the
numbers of large and intermediate
myelinated fibers but no significant
reduction of the small nerve fibers.
Age-Related Changes in Motor
Aging beyond 60 years is associated with a reduction in
number of lumbosacral spinal cord motor neurons.
These decreases appear to be caused by losses of the
largest alpha motor neurons and their myelinated axons
in lumbar ventral roots
The surviving segmental neurons increase in branching
complexity and exhibit additional collateral growth
increased load due to increased innervation ratios to the
muscle fibers they innervate.
The collected evidence strongly suggests that as
age increases beyond 60 years, muscle undergoes
continuous denervation and reinnervation, due
to an accelerating reduction of functioning
motor units Initially, reinnervation can
compensate for this denervation. However, as
this neurogenic process progresses, more and
more muscle fibers become permanently
denervated and subsequently replaced by fat and
Age-Related Changes in Muscle
Arm, leg, and back strength decline at an overall
rate of 8% per decade, starting in the third
decade of life.
Healthy men and women in their seventh and
eighth decades of life demonstrate average
reductions of 20% to 40% in maximal isometric
strength in various muscles.
Loss of muscle strength in leg muscles is greater
than loss in arm muscles between the ages of 30
and 80 (40% compared with 30%). Weight-
bearing muscles showed greater changes than
The age-associated reduction of quadriceps
muscle strength is such that the average 80-year-
old is at or near the minimum level of strength
required to rise from a chair.
Metabolic Pathways: Glucose Uptake
Aging is associated with decreased glucose
tolerance and a greatly increased incidence of
noninsulin-dependent diabetes mellitus
That is linked to the age-associated changes in
body composition and activity levels. Improved
fitness as a result of aerobic exercise improves
glucose tolerance, and exercise prevents the
onset of NIDDM.
Insulin resistance is primarily due to defects in skeletal
muscle with age. there is an age-associated decline in
the insulin-receptor signaling system at the cellular level
which has a significant impact on function.
Blood Flow and Capillarity
Sustained muscular performance requires a
proper balance between energy supply and
Capillary density decreases with age and appears
to be due to an actual reduction in the total
number of blood capillaries, as both capillary-to-
fiber ratio and the number of capillaries in
contact with each muscle cell is lower in the