This is intended to be a bare-bones review of physiology of muscle function.
There are numerous sources on the internet for those who are interested in a more in-depth exploration of
skeletal muscle physiology. The concepts here have direct application
to understanding how specific training improves (or decreases) endurance performance capacity.
Basic Architecture
A single muscle fiber is a cyclindrical, elongated cell. Muscle cells can
be extremely short, or long. The sartorious muscle contains single fibers
that are at least 30 cm long. Each fiber is surrounded by a thin layer of
connective tissue called endomysium. Organizationally, thousands of muscle
fibers are wrapped by a thin layer of connective tissue called the
perimysium to form a muscle bundle. Groups of muscle bundles that join
into a tendon at each end are called muscle groups, or simply muscles.
The biceps muscle is an example. The entire muscle is surrounded by a
protective sheath called the epimysium. Between and within the muscle
cells is a complex latticework of connective tissue, resembling struts
and crossbeams that help to maintain the integrity of the muscle during
contraction and strain. It is an amazing cellular system even before it
contracts!
Interior Components
Every muscle cell contains a series of common components that are directly
associated with contraction in some way, and influenced by training.
I will briefly describe these. For now we will not worry about the rest
(like the nucleus, ribosomes etc.).
The Cell Membrane - Controls what enters and leaves the cell. Contains regulatory proteins
that are influenced by hormones like epinephrine (adrenalin) and insulin. The blood concentration of these
hormones greatly influences fuel utilization by the muscle cell.
Contractile Proteins - The contractile machinery of a muscle fiber is organized into structural units called sarcomeres.
Muscle length is determined by how many sarcomeres are lined up in series, one next to the other. Muscle thickness ultimately depends on
how many sarcomeres line up in parallel (one on top of the other). The sarcomere structures consist of two important proteins, actin and myosin
(about 85% by volume). Several other important proteins called troponin and tropomyosin, and proteins with cool names like titin, nebulin, and
desmin help to hold these units together. The sarcomeres are organized as many thin myofibrils. A single muscle fiber will contain 5 to 10,000 myofibrils.
Each myofibril in turn contains about 4500 sarcomeres. Multiply the number of muscles in the body by the number of muscle fibers per muscle by the number
of myofibrils per fiber by the number of sarcomeres per myofibril and well, the numbers become pretty staggering. It is the individual myofibrils, long chains
of sarcomeres, which actually produce force in the muscle cell. All of the rest of the machinery plays a supporting or repair function.
The Cytosol. This is the aqueous fluid of the cell. It provides a medium for diffusion and movement of oxygen, new proteins,
and ATP within the cell’s interior. The cytoplasm also contains glycogen, lipid droplets, phosphocreatine, various chemical ions like magnesium, potassium
and chloride, and numerous enzymes.
Mitochondria - 1. The organelles in each muscle cell that contain oxidative enzymes consume oxygen during exercise.
Recent research suggests that mitochondria may look more like an interconnected network than little isolated oval “powerhouses” shown in most old textbooks.
Mitochondria convert the chemical energy contained in fat and carbohydrate to ATP, the only energy source that can be used directly by the cell to
support contraction. Ultimately, glucose and fat molecules (and certain amino acids) break down and combine with oxygen to form ATP, carbon dioxide,
water, and heat energy. This occurs via enzymatic processes occurring first in the cytosol and then the mitochondria. The carbon dioxide and excess water
leave the body through our breath. The ATP generated provides a usable energy source for muscle contraction and other cell functions. Heat removal
occurs by sweating and as radiant heat transfer from the skin to the surrounding air. Clearly, each by-product of energy metabolism has significance to
the exercising athlete.
Capillaries - 1. These microscopic size blood vessels are not actually part of the muscle cell.
Instead, capillaries physically link the muscle with the cardiovascular system. Each muscle cell may have from 3 to as many as 8 capillaries directly in
contact with it, depending on fiber-type and training. One square inch of muscle cross-section contains 125,000 to 250,000 capillaries!
The volume of blood forced through the heart’s aorta (about the diameter of a heavy duty garden hose) is spread so thin among the billions
of capillaries that red blood cells must squeeze through in single file like soldiers marching along a path. Distributing the blood flow through
such an immense network of vessels is critical so every individual cell maintains a supply line and waste removal system. This and other
“infrastructural challenges” are the price multi-celled organisms (we humans) pay for our complex organization. Endurance exercise increases the
demands on nutrient supply and waste removal, but also stimulates the growth of more capillaries. Endurance training improves the delivery and
removal function of this fantastic network of vessels. The total number of capillaries per muscle in endurance-trained athletes is about 40% higher
than in untrained persons. Interestingly, this is about the same as the difference in VO2max between well-trained and untrained people. In contrast,
strength training tends to decrease the capillary to muscle fiber diameter ratio. This occurs because muscle fibers grow in diameter, but the number
of capillaries essentially remains unaltered.
The Motor Unit
A motor unit is the name given to a single alpha motor neuron and all the muscle fibers it activates (neurophysiologists use the term innervates).
With 250 million skeletal muscle fibers in the body (give or take a few million), and about 420,000 motor neurons, the average motor neuron
branches out to stimulate about 600 muscle fibers. Interestingly, large muscles may have as many as 2000 fibers per motor unit, while the
tiny eye muscles may have only 10 or so fibers per motor unit. The size of a motor unit varies considerably according to the muscle’s function.
Muscles with high force demands but low fine control demands (like a quadriceps muscle) are organized into larger motor units.
Muscles controlling high precision movements like those required in the fingers or the eyes are organized into smaller motor units.
The motor neuron branches into many terminals, and each terminal innervates a specific muscle fiber. The motor unit is the brain’s smallest functional unit
of force development control; if a motor unit comprising 600 muscle fibers in the left biceps is stimulated, than all 600 of those fibers
will contract simultaneously and contribute to the total force produced by the biceps. The brain cannot stimulate individual fibers one at a time.
Even for our sophisticated nervous system, that would require far too much wiring.
Regulation of Muscular Force
The brain combines two control mechanisms to regulate the force a single
muscle produces. The first is RECRUITMENT. The motor units that
make up a muscle are not recruited in a random fashion. Motor units are
recruited according to the Size Principle. Smaller motor units
(fewer muscle fibers) have a small motor neuron and a low threshold for
activation. These units are recruited first. As more force is demanded
by an activity, progressively larger motor units are recruited. This has
great functional significance. When requirements for force are low, but
control demands are high (writing, playing the piano) the ability to
recruit only a few muscle fibers gives the possibility of fine control.
As more force is needed the impact of each new motor unit on total force
production becomes greater. It is also important to know that the smaller
motor units are generally slow units, while the larger motor units are
composed of fast twitch fibers.
The second method of force regulation is called RATE CODING. Within
a given motor unit there is a range of firing frequencies. Slow units
operate at a lower frequency range than faster units. Within that range,
the force generated by a motor unit increases with increasing firing
frequency. If an action potential reaches a muscle fiber before it has
completely relaxed from a previous impulse, then force summation will
occur. By this method, firing frequency affects muscular force generated
by each motor unit.
Firing Pattern
If we try and relate firing pattern to exercise intensity, we see this
pattern. At low exercise intensities, like walking or slow running, slow
twitch fibers are selectively utilized because they have the lowest
threshold for recruitment. If we suddenly increase the pace to a sprint,
the larger fast units will be recruited. In general, as the intensity of
exercise increases in any muscle, the contribution of the fast fibers will
increase.
For the muscle, intensity translates to force per contraction
and contraction frequency/minute. Motor unit recruitment is regulated by
required force. In the unfatigued muscle, a sufficient number of motor units
will be recruited to supply the desired force. Initially desired force
may be accomplished with little or no involvement of fast motor units.
However, as slow units become fatigued and fail to produce force,
fast units will be recruited as the brain attempts to maintain desired force
production by recruiting more motor units. Consequently, the same force
production in fatigued muscle will require a greater number of motor units.
This additional recruitment brings in fast, fatiguable motor units.
Consequently, fatigue will be accelerated toward the end of long or
severe bouts due to the increased lactate produced by the late
recruitment of fast units.
Specific athletic groups may differ in the control of the motor units.
Top athletes in the explosive sports like Olympic weightlifting or the high
jump appear to have the ability to recruit nearly all of their motor units in a
simultaneous or synchronous fashion. In contrast, the firing pattern
of endurance athletes becomes more asynchronous.
During continuous contractions, some units are firing while others
recover, providing a built in recovery period. Inital gains in strength
associated with a weight training program are due to improved recruitment,
not muscle hypertrophy.
