Exam 3 Skeletal Muscle Adaptations Flashcards
Muscle fibers recruited first
In general, during any muscular activity, slow fibers are recruited first
Requirement for stronger contractions results in recruitment of fast fibers
Clinical Correlation of recruitment order of muscle fibers
Repetitive, low-intensity exercise recruits predominately slow twitch fibers (type I)
Rapid power movements (or strong isometric contractions) recruit all three fiber types (Type I, Type IIa, and Type IIb)
Motorneurons to type I fibers
Are smaller and innervate 100-180 muscle fibers
Motorneurons to type II fibers
are larger and innervate 300-800 muscle fibers
Type I and Type II fibers individually create very _____ amounts of force
similar
The main difference in force development between types is
motor unit size (and muscle fiber twitch velocity)
Why are motor units recruited such that only 1/3-1/2 of muscle fibers are contracting at one time?
This prevents injury that could occur if all fibers contracted at once
Muscle fatigue
Inability of contractile and metabolic processes of muscle fibers to continue supplying the same work output; usually the result of repetitive stimulation
Fatigue of muscle is based on
its maximal force output
Any decrement in maximal force is considered fatigue
what generates fatigue?
Any reaction in the sequence from motorneuron AP generation to force generation by actin and myosin could be impaired to generate fatigue
Muscle fatigue is associated with that factors
Depletion of muscle ATP, glycogen, and creatine phosphate
Delivery of nutrients and oxygen by the cardiovascular system
Decrease in motorneuron activity (AP signaling)
Build up of lactic acid (H+ ions) - reduces fiber pH from 7.0-6.0 (in fast twitch), lower pH inhibits the metabolic mechanisms of muscle fiber contraction
Cause of fatigue in muscle fibers
depletion of ATP; rate of ATP utilization is greater than the rate of ATP formation (via glycolysis, oxidative phosphorylation, creatine phosphate)
What is utilized at initiation of contraction
ATP
increased concentration of ADP + Pi stimulate glycolysis and oxidative phosphorylation; creatine phosphate maintains ATP production while glycolysis and oxidative phosphorylation produce “new” ATP
At Moderate exercise rates ATP can be supplied by
Oxidative phosphorylation
Why does glycolysis augment ATP production during intense exercise rates?
Delivery of oxygen by the circulation is overwhelmed; oxidative phosphorylation cannot produce ATP fast enough
(Glycolysis produces ATPs very quickly, but is an inefficient use of stored glycogen - 2 ATP vs 38 ATP)
In regards to muscle fatigue and recovery, after intense exercise
creatine phosphate is depleted, glycogen is depleted, both must be newly synthesized to return muscle to normal state
Why does increased pulmonary respiration continue for some time after intense exercise
to provide ATPs necessary for synthesis of glycogen and creatine phosphate
20-45% of actual energy used during muscle activity is utilized for contraction, what happens to the rest of it?
the majority of the energy is lost as heat
muscle adaptations occur throughout life in response to
work demands
muscle is affected by
frequency, duration and intensity of work demands
Remodeling of muscles consists of alterations in muscle characteristics:
fiber diameter, fiber length, vascular supply, fiber type, and sometimes fiber number
Observable adaptations can occur in
as little as 2 weeks of training or disuse
Hypertrophy
increase in size of muscle due to increase in the size of cells (increase of myofibrils)
Atrophy
Decrease in size of muscle due to decrease in the size of cells (loss of myofibrils)
Hyperplasia
increase int he size of a tissue due to increase in number of cells; believed to be due to fiber splitting
Fiber transformation
alteration in muscle fiber type due to work demand (change in muscle metabolic capabilities - i.e. glycolytic to oxidative)
Immobility (casting, bed rest, microgravity) imposes load restrictions of muscle that result in
atrophy of unused muscle tissue
loss of muscle mass, strength, and endurance
increase in fat content of muscles
reduction in diameter of muscle fibers
reduction in contractile proteins of muscle (decrease synthesis and increased degradation)
Effects of disuse atrophy
- reduced oxidative enzymes decreases muscle endurance (disuse has minimal effect on the glycolytic process, fast twitch fibers are less affected by disuse atrophy)
- Depletion of glycogen and creatine phosphate stores in muscle fibers
- Disuse atrophy is reversible (longer period of disuse requires a longer recovery period; muscle fibers that are not innervated do not typically recover from disuse atrophy - e.g. nerve damage, SCI)
Type of fibers most affected by disuse atrophy
Slow twitch fibers
Can lose 50% ______ in 6-8 weeks with complete disuse
muscle mass
Can lose 50% _____ in 3-5 weeks of disuse
strength
Prolonged disuse (2 or more years) results in
Non-functional muscle with fibers replaced by collagen and fatty tissue (typical in complete nerve transection or spinal cord injury)
Disuse atrophy: denervation
loss of neural connection to muscle induces atrophy and fibrosis of muscle (trophic factor(s) from neuron to muscle fiber have been lost; more rapid decline in muscle atrophy with denervation compared to disuse atrophy, can result in muscle with 5-10% its original volume)
Denervation > 1-2 years renders muscles
completely non-function
denervated muscles/fibers are composed of
fat, fibrous tissue with few, if any, contractile proteins
immobilization preferentially affects
type I fibers
i.e. - muscles typically lose their endurance (fatigue resistant) much more rapidly than their strength (fatigue-able)
Following immobilization , type I fibers shoe
greater reduction of contractile function compared to type II fibers
For elite athletes, immobilization affects the
fiber type affected by the chose sport; i.e. sprinter -> type II fibers affected first
Prevention of atrophy
atrophy can be prevented with early motion (AROM)
Electrical stimulation following immobilization can prevent type I atrophy and loss of oxidative capacity
Contracture
restricted ROM across a joint
Contractures are related to
shortening of muscle and periarticulare joint structures (connective tissue of joints)
How does muscle shorten
by removal of terminal sarcomeres in fibers; occurs when muscles are held in shortened positions for prolonged periods (weeks to months)
Significant disabilities associated with contractures include
ADLs, transfers, ambulation, endurance
Muscle hypertrophy
increased use (training) of muscle fibers results in hypertrophy of those fibers
A few strong (maximal) contractions per day creates hypertrophy in 6-10 weeks
Increased number of myofibrils (increased muscle diameter)
Anaerobic training induces
- increased protein synthesis (actin and myosin filaments)
- Increases glycolytic and aerobic enzyme machinery
- increased glycogen, creatine, ATP content of fibers
Connective tissue elements of muscle fibers also are increased via what process
collagen production by fibroblasts
CT elements hypertrophy allows for
increase in size/strength of tendons and ligaments
fiber-specific training
fibers trained are those that will hypertrophy
order of fiber recruitment has a role in fiber hypertrophy
order of fiber recruitment
slow (type I) > Fast (type IIa) > fast (type IIb)
Strength training
short duration, high resistance exercise creates increases in muscle mass and strength
Hypertrophy is greatest in strength training when
muscle operates at or near maximum force-generating capacity (4-6 reps at near max force in sets of 3)
Strength training results in increases of
actin and myosin proteins
SR and T-tubule systems
glycogen, glycolytic enzymes, creatine, and ATP
Aerobic training
low intensity, long duration training results in enhancement of fiber fatigue-resistance
aerobic training results in increases of
- number of mitochondria and mitochondrial enzymes
- amount of fat sores and enzyme for their metabolism
- increase in capillary network and blood flow (to deliver oxygen and remove metabolites)
Aerobic training also creates enhanced
respiratory and circulatory systems
improved/increase oxygen and fuel delivery- e.g. more efficient pulmonary ventilation and delivery of oxygenated blood
Overall, minimal actual hypertrophy occurs in __________ with aerobic training
slow twitch fibers
The small amount of slow twitch fiber hypertrophy is balanced by fast fiber atrophy = minimal net gain in muscle size or power
Fiber transformation
Change in fiber type classification due to training activities (e.g. heavy endurance training can transform type II fibers (fast) into slow twitch type I fibers)
Scientific evidence exists for fiber transformation following intense, prolonged training suggesting:
that aerobic (endurance) training leads to conversion of type IIb to IIa and type IIa to type I fibers
resistive exercise does not appear to transform fibers from slow to fast types
Fiber transformation accounts for _______ of overall muscular adaptations to training
very little
most alterations due to training are
fiber specific
hypertrophy of type II fibers =
increased force generation
increased fatigue resistance to type I fibers =
increased muscular endurance
For endurance and power athletes, the high proportion of slow twitch fibers and fast twitch fibers, respectively, is believed to be
inherited (not trained)
