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Training
Adaptations of skeletal
muscle to exercise training
By Michelle Kienholz.
What Happens When You Train?
You can open almost any cycling or other fitness
magazine to get the latest and greatest training regimen guaranteed to produce results in
30 days or less. How do any of these programs work? More importantly, will any of these
programs help you improve your cycling performance?
Any type of physical training affects your
skeletal muscles, your cardiovascular system, your oxygen intake and use, and your energy
use (type and amount). Once you understand how specific workouts change the way your body
functions during exercise, you'll be able to put together your own personalized schedule
customized to your needs.
In this article, we introduce you to the
adaptations of skeletal muscle to exercise training. You may find more detail than you
care to know, but even this is fairly simplified, and we've suggested some links to
additional resources on the Web for more in-depth coverage.
Muscle Structure and Physiology
Although you might think of a specific muscle,
such as the hamstring, as a single unit, skeletal muscle is highly compartmentalized,
which makes it well-designed to generate force and produce movement. When looked at as a
whole, muscles are comprised of individual cells or fibers embedded in a matrix of
collagen. This matrix forms the tendon at either end that connects muscle to bone.
Individual muscle cells are roughly cylindrical
in shape and can be as long as two to three centimeters. Most fibers are somewhat oblique
to the muscle's line of action. The size and number of fibers (which contributes to the
muscle cross-sectional area) determine the peak amount of force that can be generated by a
muscle. The speed and range of contraction are related to muscle fiber length.
Muscle cells are densely packed with contractile
proteins, energy stores, and signaling mechanisms. Each muscle cell or myofiber contains
hundreds or thousands of threadlike structures called myofibrils. Each myofibril contains
thousands of actin and myosin microfilaments plus some smaller protein structures (these
regulate the organization, length, and activity of the actin and myosin). The organized
stacking of actin and myosin creates the stripes that are visible under a microscope and
also allows the two components to slide across each other.
Actin looks like two thin strings of beads
twisted together. Along the actin filaments are active spots that are strongly attracted
to a specific part of myosin. These active spots remain covered when the muscle is
relaxed.
Myosin like a thick stalk (looks a bit like
broccoli) with cross-bridges and globular heads that are attracted to the active sites on
actin. As part of its structure, myosin has an energy source, adenosine triphosphate
(ATP), bound on.
When the active spots on actin are uncovered, the
myosin globular heads fit into the actin much like spokes fit into a wheel rim. The ATP is
hydrolyzed or split to release energy from the chemical bonds (adenosine diphosphate or
ADP and inorganic phosphate remain) During this release of energy, the cross-bridges
rotate slightly to pull the actin filaments across each other toward the central myosin
stalk (much like the oars of a rowboat pull the across the water). The motion of the actin
displaces the ADP and phosphate from the myosin cross-bridge.
Fig. 1
The blue knobs of the myosin pull the green
strands of actin together, which brings the z-lines (the black lines on the outer edges)
in toward the middle of the sarcomere. This process occurs in each sarcomere of each
myofibril, causing the entire muscle to contract and generate force.
Muscle Contractions
Muscle contractions are triggered by nerve
impulses from the brain. Higher motor nerves carry the signal to the spinal cord, and from
there lower motor nerves conduct the impulses directly to muscle fibers. Each muscle fiber
maintains an electrical potential difference across its cell membrane, just as a battery
creates an electrical potential difference by having different concentrations of ions at
its two poles (the + and - signs marked on either end).
The nerve impulses alter the potential difference
of muscle cell membranes, allowing positively charged sodium ions to rush in, reversing
the charge in a wavelike pattern across the muscle fibers. The change in polarity is
carried deeper into the cell by special tubules. The depolarization signal causes certain
sacs (known as the sarcoplasmic reticulum) to release calcium, which eventually causes the
active spots on the actin to be exposed.
As noted above, then, during muscle contraction,
actin combines with myosin and ATP to produce force (movement), ADP, and inorganic
phosphate. The calcium must be pumped back into the sacs (ATP is again split to provide
the energy to do this) before the actin will disconnect from the myosin, allowing the
muscle fiber to relax again. Otherwise, as in rigor mortis, actin and myosin interact to
form a very stiff, permanent connection. This entire complex coupling and decoupling
process repeats as often as 100 times per second.
(By the way, the manner in which energy is
generated in the muscle cell—mainly by burning or oxidizing glucose, fatty acids,
and/or creatine phosphate and ATP—will be discussed in detail in another article. The
mysteries and myths of sports nutrition should be cleared up for you then.)
When muscles contract or shorten, energy is
dissipated, causing a build up of heat, water, and carbon dioxide in the tissue. During
heavy continuous exercise, not enough oxygen is available to prevent the formation and
build up of lactic acid, which results in aching, heavy limbs. In addition, if heat is
allowed to increase within the muscle, performance will go down.
Muscle Fiber Types
You've probably heard terms like fast-twitch and
slow-twitch muscles discussed with regard to athletes who are good in certain events (such
as the 100-meter sprint versus a 40-kilometer race). The three major types of muscle
fibers have been identified through laboratory techniques used in staining fibers for
viewing under a microscope.
Slow muscle fibers, the most distinct type are
also known as Type I fibers. They are red (because they are fed by many capillaries), have
long twitch times, low peak force, and high resistance to fatigue. They have a high
mitochondrial content that are especially good at burning fatty acids for energy during
long-distance events. In biochemical terms, these fibers have high levels of oxidative
enzymes, low levels of glycolytic markers, and little ATPase activity (the enzyme used to
split ATP for energy).
Fast muscle fibers are white, store high levels
of glycogen and ATP for energy, and show fast contraction times. Fast muscle fibers are
further divided into two groups: intermediate fatigue-resistant or fast oxidative
glycolytic (Type IIa) and fast fatiguable or fast glycolytic (Type IIb). The Type IIa
fibers can maintain their force production even after a large number of contractions.
These fibers tend to be rich in oxidative (fat-burning) and glycolytic
(carbohydrate-burning) enzymes as well as ATPase activity. The Type IIb fibers show very
fast contraction and very large force production but cannot sustain this effort for more
than a few contractions without rest. These fibers rely on ATPase and glycolytic activity
rather than oxidative enzymes for energy.
As a side note, muscle fiber type may also play a
role in some disease, such as diabetes and obesity. Researchers have discovered that
patients with diabetes and/or who are obese tend to have mainly Type IIb fibers. With
regular exercise, these patients show improvement in glucose control and better weight
control (versus with dietary changes alone). Whether these patients are genetically
predispositioned to have predominantly Type IIb fibers (which in turn contribute to the
development of diabetes and to weight gain) or whether their disease causes the muscle
fibers to switch to Type IIb is under study.
Applying This Knowledge to Training
If your eyes glazed over back there, you might
want to take the time to read it through again when you're all done here. Understanding
this basic muscle biology will help you understand the rationale for various training
recommendations. At this point, though, you should recognize that your training will
alter:
muscle mass, the amount of actin
and myosin that form cross-bridges within each muscle fiber;
muscle efficiency, through the
faster and stronger transmission of nerve impulses throughout muscle fibers and improved
coordination; and
muscle metabolism, by targeting a
specific fiber type, depending on whether you want to improve endurance, strength, or
both.
Hypertrophy
Hypertrophy, an increase in mass or girth of a
muscle, is the most widely recognized result of regular exercise. It is also the slowest
result of training (it can take as along as two months for actual hypertrophy to begin).
You may think that you are growing new muscle cells, but you are mainly enlarging
individual fibers (in fact, hypertrophy refers only to an increase in size; when cells
multiply in number, this is called hyperplasia). The number of fibers and their propensity
to enlarge in response to training is dictated mainly by your genes. Additional
contractile proteins are incorporated into existing myofibrils. However, if a myofibril
becomes sufficiently large, a separate population of cells known as adult myoblasts will
divide and fuse with existing fibers, resulting in hyperplasia.
Neuromotor Training
As you learned earlier, muscle contractions are
triggered by nerve impulses. Each muscle fiber is fed by one nerve cell axon. These axons
are part of a single motoneuron or nerve cell. A single motoneuron may control hundreds of
individual fibers. When viewed together, they form a motor unit. As the signal for
contraction increases, more motor units are recruited, and these motor units are
stimulated to fire more frequently.
Even during maximal effort, you are not likely to
activate all the motor units (and hence muscle fibers) for a specific muscle. However,
through regular training, you can increase the number of motor units recruited to
stimulate muscle contraction. In fact, this effect accounts for any immediate strength
gains you experience. In other words, you're training the muscle to be used more and more
completely through the available physiologic mechanisms.
In fact, from a neuromotor standpoint, your
training is more like practice, and your fitness more like skill level. By improving the
proficiency in neural activity and motor coordination, you lower the amount of energy
expenditure required to perform the desired exercise or event. Improved coordination also
lessens the risk of accidents and injury and reduces the stress placed on the
cardiovascular system.
In addition, maintaining neuromotor drive can
actually overcome feelings of fatigue. Elite athletes may be neurologically better able to
shut out the strong desire (but not necessarily physiologic need) to reduce intensity of
effort in response to sensations of fatigue. A fatigue-induced reflex mechanism seems to
keep the motoneuron firing rates at the minimal level necessary to maintain the task,
which also minimizes the rate at which strength and function is impaired.
Muscle Fiber Type
Although your genes determine your basic muscle
structure and physiology, you can, through training, alter their method of energy
metabolism, size, and vascularization. Muscle fibers adapt to the stresses placed on them.
The type of training you select will affect the muscle fiber types involved in the
activity. Further, within the recruited fibers (recruited by the chosen physical activity,
whether strength or endurance training), only the organelles, enzymes, and molecules
stimulated beyond a threshold for adaptation will undergo significant changes. For
example, mitochondria (the organelle responsible for oxidative or fat-burning energy) will
be stimulated to increase in number and size only when placed under sufficient aerobic
stress.
With consistent and appropriate endurance
training, both slow and fast muscle fibers increase their aerobic capacity through more
and larger mitochondria, higher levels of oxidative (fat-burning) enzymes, increased fat
stores within the muscles (for faster access to energy), lower production of lactate, and
higher resting energy levels. With consistent strength training, fibers increase their
anaerobic capacity and become more efficient at burning glycogen (a form of stored
carbohydrate), ATP, and creatine phosphate; in addition, fast-twitch fibers, rather than
slow-twitch fibers, are preferentially increased in size (through increased number of
myofibrils). However, research to date does not support the common belief that you can,
through training, actually convert one fiber type to another.
If you engage in endurance training, you will
become better at burning fat, conserving carbohydrates, and delaying lactic acid build-up
in the muscle tissue. In two to three months of training, an increase in the density and
use of capillaries increases blood flow to Type I fibers (slow twitch) and decreases blood
flow to Type IIb fibers (fast glycolytic fibers).
If you engage in moderate strength training, the
cross-sectional size of both Type I and Type II fibers will increase, which in turn
translates into increases in force production. If you engage in high-intensity power
training, you will selectively enlarge Type II fibers and reduce mitochondrial size and
number.
Recovery
Muscle soreness that develops immediately
following intense activity is caused by poor blood flow and lack of oxygen. With
consistent training, your muscles will grow additional capillaries to increase blood flow
and hence oxygen and nutrient delivery (remember, slow twitch muscles are red because they
are fed by more capillaries).
Delayed muscle soreness (one or two days after
intense activity) is more likely due to damage to the muscles and/or connective tissues
and will be discussed in a future article. Younger athletes experienced delayed or
residual muscle soreness sooner (as soon as 12 hours after activity) than older athletes
(may take as long as 36 hours), possibly due to a decrease in the permeability of muscle
cell membranes that occurs with age.
You now know how your skeletal muscles respond to
endurance and strength training. In the next segment, you'll learn how your cardiovascular
and respiratory systems adapt to regular exercise and improve your athletic performance.
From there we'll move on to nutritional metabolism in exercise. With this basic
understanding of exercise physiology, you should get more out of our specific training
recommendations since you'll understand how your workouts affect your performance_and
you'll know more than just to blindly and robotically do X activity for Y minutes at Z
intensity on this and that day of the week.
Resources for Additional Information:
Muscle Biochemistry
http://web.indstate.edu/thcme/mwking/muscle.html
Comprehensive, well-illustrated review (fairly advanced level) that covers: Introduction;
Organization of the Sarcomere; Proteins of the Myofilament, Organization of Actin Thin
Filaments; Myosin and the Power Stroke of Contraction; Regulation of Sarcoplasmic Calcium;
Muscle Relaxation; Tetany and Rigor Mortis; Smooth Muscle; and Red Oxidative and White
Glycolytic Muscle.
Muscle Composition
http://www.biofitness.com/composit.html
Thorough, text-only review of muscle structure with links to glossary for scientific
terms.
Muscle Fiber Structure
http://ortho84-13.ucsd.edu/MusIntro/Myofiber.html
Part of a reasonably simplified and well-illustrated scientific tutorial of muscle
structure & physiology.
Skeletal Muscle Structure
http://www.cs.rpi.edu/~parkej/MUSCLE/mus_intro.html
Basic overview of muscle structure & contraction (some illustrations, some images).
Sport, movement and molecules
http://nimnet51.nimr.mrc.ac.uk:8000/mhe95/muscle.htm
Essay by Mike Ferenczi on muscle physiology during exercise (and many other
exercise-related physiological explanations) written at a level most athletes will
appreciate (thorough and detailed but not overly technical).
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