Distribution of Fiber Types

Distribution of Fiber Types

The amount of Type I and Type II muscle fibers in an individual is genetically predetermined,
and all normal skeletal muscle contains all fiber types. However, the
proportion or distribution of these fiber types within and across individuals differs.
Moreover, within an individual, the distribution of fibers in various muscles can vary
widely. Physical training may transform muscle fiber type, and the metabolic capacity
of both type I and type II muscle fibers can be modified by endurance and power
training. It should be noted that performance depends not only on your fiber type
composition, but also on the interplay between a variety of factors such as training, and
diet, etc. Figure 1-1 represents the average percentage of slow twitch (Type I) fibers found
in various subgroups of the population. As can be seen, persons whose activities are
primarily endurance-related have a higher proportion of Type I fibers as compared to
sprinters or wrestlers. Also in Figure 1-1 are the maximal aerobic capacities of these
groups: the greater the proportion of Type I fibers, the higher the maximal aerobic capacity.
Note the wide variability in the group of “recreational athletes”. Specific types of training
can induce changes in muscle fiber composition and characteristics.

Type II Muscle Fibers

Type II Muscle Fibers

Type II fibers can be categorized into at least two types: Type IIa and Type IIb.
These fibers are adapted for strength and power activities. The Type IIa fiber is a cross
between a slow twitch and a fast twitch fiber in that it is both aerobic (oxidative) and
anaerobic (glycolytic). Whereas, it is “faster” than the slow twitch fiber, it is not as well
suited for endurance activities. Its color is also reddish, a result of the myoglobin content.
In contrast, the Type IIb fiber is truly a fast twitch fiber, with very high contraction
speeds. These fibers are almost exclusively anaerobic and have minimal capacity for
aerobic production of energy. They rely primarily on glycogen within the muscle for
energy and are therefore very susceptible to fatigue. Their color is pale, and some consider
it “white” because it lacks myoglobin. Type IIb fibers tend to accumulate lactate, which
ultimately leads to rapid fatigue if the lactate is not removed.
It should be noted that each of the fiber types has different recruitment patterns, and
typically the Type IIb fiber is only recruited for use during maximal effort. The other fibers
contract during light as well as moderate activity. Moreover, physical training can lead
to changes in the characteristics of the fibers. Thus, endurance training would lead to
changes in the Type IIa fiber such that they take on more characteristics of the Type I, or
slow twitch fiber. Table 1-1 presents the distinguishing characteristics of the various fiber
types.

Table 1-1

Type Muscle Fibers

Type Muscle Fibers

Type I muscle fibers are involved in endurance activities. These fibers, also called
slow twitch fibers, are noted for their ability to produce energy in the presence of oxygen.
Thus, they are primarily aerobic. The main fuel source for this fiber is fats (fatty acids),
which allow the muscle to work at a steady rate with noticeable resistance to fatigue. Their
color is typically quite red, a result of the high content of “myoglobin”, an iron-containing
protein that stores and delivers oxygen. Slow twitch fibers are not noted for their speed,
their anaerobic capacity, or their ability to contract at a fast rate repeatedly, but rather their
indefatigability and aerobic capacity.

Muscle Structure and Function

Muscle Structure and Function

The three major types of muscle are:

•  Cardiac muscle
• Skeletal muscle
• Smooth muscle

This discussion will be limited to skeletal muscles which, by converting chemical
energy to mechanical energy, produce movement. We will present a description of the
subtypes of skeletal muscles, including their characteristics and distribution. Although
there may be many new subtypes of skeletal muscle fibers, generally skeletal muscle can
be characterized into three basic types:

• Slow Twitch Oxidative (Type I)
• Fast Twitch Oxidative-Glycolytic (Type IIa)
• Fast Twitch Glycolytic (Type IIb)

Exercise Physiology Definitions and Terminology

Exercise Physiology Definitions and Terminology 

Following are some commonly used exercise physiology terms and their
definitions. You will come across many of these terms in subsequent chapters.
Aerobic: A process of producing energy that requires oxygen.
Aerobic Capacity: Total or maximal amount of aerobic work that can be done.
4 Overview of Physical Fitness
Aerobic Metabolism: Most of the energy needed to support exercise that goes beyond 3
minutes is provided by aerobic or oxidative energy metabolism. In other words oxygen is
required to produce energy.
Agility: Ability to change physical position with speed and accuracy.
Anaerobic: A process of producing energy that does not require the presence of oxygen.
Anaerobic Capacity: Total or maximal amount of anaerobic work that can be done.
Anaerobic Glycolysis: A process of breaking down glycogen stores without oxygen; lactate
(lactic acid) is the by-product of this process.
Anaerobic Metabolism: A type of energy metabolism that does not require oxygen.
Anaerobic Threshold: Transition point when aerobic metabolism can no longer meet the
energetic demands, and energy from sources independent of oxygen are required. This is
also the work rate at which blood lactate concentrations start to increase during graded
exercise.
Adenosine Triphosphate (ATP): Energy released from food is stored in the muscle in the
form of ATP. When ATP is broken down energy is released.
Balance: Ability to maintain equilibrium when stationary or while moving.
Cardiorespiratory Fitness: Ability of the heart, lung and blood vessels to transport oxygen
and to remove waste products from the exercising muscle.
Concentric Contraction: Shortening of the muscle as it develops tension. This type of
exercise is sometimes known as “positive exercise”.
Coordination: Ability to use the senses, such as sight, along with the functioning of a set
of muscle groups to complete an activity accurately. For example: hand-eye coordination
during rifle shooting.
Dynamic Exercise: Alternate contraction and relaxation of a skeletal muscle or muscles
causing partial or complete range of movement through a joint.
The Navy SEAL Physical Fitness Guide 5
Eccentric Contraction: Involves the lengthening of a muscle as it develops tension and is
also known as “negative exercise”. Eccentric contractions are used when resisting gravity
as is the case in walking down hill or down stairs.
Electrocardiogram (ECG): A tracing that shows the electrical activity of the heart.
Ergometer: Instrument used to measure work and power.
Ergometry: Measurement of work and power during exercise.
Exercise: Planned, structured, and repetitive movements performed to improve or maintain
components of physical fitness. The components include cardiorespiratory fitness,
muscle strength and endurance, flexibility and body composition (see Introduction).
Fartlek Training: An unstructured type of interval training for speed-work.
Flexibility: Controlled range of motion of a specific joint. The range is a function of elasticity
of the tendons, ligaments, and surrounding soft tissue. Control is a function of
strength at each degree of motion, especially at the end ranges.
Glycogen: A form of carbohydrates that is stored in the liver and in muscles for energy.
Glycolysis: Breaking down of simple sugars into simpler compounds (chiefly pyruvate
and lactate) for energy. This process is anaerobic.
Glycolytic: Pertaining to or promoting glycolysis.
Heart Rate: Number of heart beats per minute.
Interval Training: Very intense exercise bouts are alternated with rest or periods of low
intensity exercise. Exercise during intervals is typically anaerobic.
Isokinetic: Contraction of a muscle or muscle group which results in joint movement at a
constant angular velocity. For example: the arm stroke during free style swimming.
Isometric (Static): Muscle contracts without shortening or lengthening such that tension
is developed but no muscular work is performed; energy is lost as heat. There is no joint
movement during this type of exercise.
Isotonic (Dynamic): Muscle contracts and maintains constant tension by lengthening or
shortening.
Lactic Acid (lactate): A by-product of anaerobic metabolism.
Ligament: A band of fibrous tissue that connects bone to bone or bone to cartilage so as to
strengthen joints.
Maximal Oxygen Uptake (VO2max): A measure of aerobic fitness: the maximal rate of
oxygen uptake, and therefore aerobic energy utilization during exercise. Typically
expressed as liters per minute or milliliters per kilogram (kg) body weight per minute (ml/
min/kg).
6 Overview of Physical Fitness
MET (Metabolic Equivalent Unit): A unit used to estimate the metabolic cost of physical
activity. One MET is the energy used by an individual at rest. This is equivalent to 3.5
ml of oxygen consumed per kg body weight per minute.
Metabolism: Physical and chemical processes that maintain life.
Minute Ventilation: Volume of air breathed per minute.
Muscular Endurance: Ability of a muscle or muscle group to contract at a submaximal
force, usually against 50 to 60% of maximal resistance, over a period of time. Measured as
the number of repetitions completed.
Muscular Strength: Maximal force or tension generated by a muscle or muscle group.
Myoglobin: An iron containing muscle protein that is responsible for the reddish color of
various muscle fiber types.
Physical Activity: Movement by skeletal muscles that results in energy expenditure.
Physical Fitness: Ability to perform physical activity.
Plyometrics: Also known as explosive jump training. Muscles are rapidly stretched prior
to contraction. Examples include standing jumps, multiple jumps, etc.
Power: Ability of a muscle to quickly generate force over a very short period of time.
Examples include sprint starts, vertical jumps, kicks and throwing a punch.
Rating of Perceived Exertion (RPE): Measured using the Borg Category RPE Scale. As
exercise intensity increases, the RPE increases and in general it is closely associated with
physiological measures such as heart rate and oxygen consumption.
Reaction Time: Time taken between receiving a signal and reacting to it.
Respiratory Exchange Ratio: Ratio of carbon dioxide produced to oxygen consumed.
An indication of the primary energy source used during exercise.
Speed: Ability to perform a movement in a short period of time.
Strength: Ability of a muscle to contract against resistance and provide control throughout
the full range of motion.
Stroke Volume: Volume of blood pumped from the heart with each beat.
Tendon: A fibrous cord in which the fibers of a muscle end and by which the muscle is
attached to a bone or other structure.
Tidal Volume: Volume of air moved during one breathing cycle while inhaling or exhaling.

Exercise Physiology

Exercise Physiology

Exercise physiology is a branch of science which studies how the body responds
and adapts physically to exercise training or to an acute bout of physical exertion. Such
information is used for designing physical education, fitness and athletic programs.
Physical fitness includes cardiopulmonary endurance, body composition, muscular
strength and endurance, and flexibility.

Overview of Physical Fitness

Overview of Physical Fitness

This chapter will introduce terms and definitions
commonly used to measure or define fitness levels, and other terms associated with athletic
training. Terms such as aerobic and anaerobic will be appearing throughout this fitness
guide and various training techniques to optimize fitness will be discussed in greater detail
in subsequent chapters. We suggest that you take some time to familiarize yourself with the
terms and concepts in this chapter as this will greatly enhance your ability to apply the
information provided in the remaining chapters.