Sports Medicine: Just the Facts

Rapid Glycolysis (Lactic Acid System)

• Glycolysis uses carbohydrate, primarily muscle
  glycogen as a fuel source. When glycolysis is rapid, it
  is capable of producing only a few ATP without
  involvement of oxygen. Lactic acid is also produced
  as a by-product of this reaction. The accumulation of
  excessive amounts of lactic acid in muscle tissue is
  associated with fatigue. The lactic acid system pro-
  duces enough energy to last approximately 1–2 min
  before the accumulation of excessive lactic acid
  would produce fatigue. It would fuel activities such as
  middle distance sprints (400-, 600-, and 800-m runs).
  Although glycolysis is considered an anaerobic path-
  way, it can readily participate in aerobic metabolism
  when oxygen is available, and it is considered the first
  step in the aerobic metabolism of carbohydrate
  (Demaree et al, 2001; Rupp, 2001).

Aerobic Oxidation System

• The final metabolic pathway for ATP production com-
  bines two complex metabolic processes, the Krebs cycle
  and the electron transport chain. This system resides in
  the mitochondria. It is capable of using carbohydrates,
  fat, and small amounts of protein to produce energy
  (ATP) during exercise through a process called oxida-
  tive phosphorylation. During exercise this pathway uses
  oxygen to completely metabolize the carbohydrates to
  produce energy (ATP) leaving only carbon dioxide and
  water as by-products. The aerobic oxidation system is
  complex, and thus requires 2–3 min to adjust to a
  change in exercise intensity; however, it has an almost
  unlimited ability to regenerate ATP, limited only by the
  amount of fuel and oxygen that is available to the cell.
  Maximal oxygen consumption,also known as VO 2 max,
  is a measure of the power of the aerobic energy system
  and is generally regarded as the best indicator of aerobic
  fitness (Demaree et al, 2001; Rupp, 2001).

FUELUSAGEDURINGEXERCISE

• All the energy-producing pathways are active during
  most exercise; however, different types of exercise
  place greater demands on different pathways. The con-
  tribution of the anaerobic pathways (CP system and
  glycolysis) to exercise energy metabolism is inversely
  related to the duration and intensity of the activity. The
  shorter and more intense the activity, the greater the
  contribution of anaerobic energy production; whereas
  the longer the activity and the lower the intensity, the
  greater the contribution of aerobic energy production.
  In general, carbohydrates are used as the primary fuel
  at the onset of exercise and during high-intensity work;
  however, during prolonged exercise of low to moderate
  intensity (longer than 30 min), a gradual shift from
  carbohydrate toward an increasing reliance on fat as a
  substrate occurs. The greatest amount of fat use occurs

at about 60% of maximal aerobic capacity (VO2max)

(Demaree et al, 2001; Rupp, 2001).

MUSCLE PHYSIOLOGY

CLASSIFICATION OFMUSCLEFIBERS

• Muscle fibers possess certain characteristics that result
  in relative specialization and can be classified broadly
  as Type I (slow-twitch) or Type II (fast-twitch) with
  differing functional and metabolic characteristics. The
  type of muscle fiber recruited to perform a specific
  activity depends on intensity and duration of exercise.
  Most muscles contain both fast-twitch and slow-twitch
  muscle fibers; however, the ratio of fast-twitch to slow-
  twitch muscle fibers varies in an individual. The ratio
  also differs within the same muscle from one individ-
  ual to another (Rupp, 2001; Humphrey, 2001).

Type I (Slow-Twitch) Muscle Fibers

•Type I fibers are those that resist fatigue and thus are

recruited for lower intensity, longer duration activities.

Sedentary persons have approximately 50% Type I,

and this distribution is generally equal throughout the

major muscle groups of the body. Endurance athletes

have a greater percentage of Type I fibers thought to

be the result of geneticpredisposition (Rupp, 2001;

Humphrey, 2001).

Type II (Fast-Twitch) Muscle Fibers

•Type II fibers are muscle fibers that can generally gen-

erate a great deal of force very rapidly. These fibers are

recruited when a person is performing high-intensity

activities. These fibers can produce large amounts of

tension in a very short time period, but the accumula-

tion of lactic acid from anaerobic glycolysis causes

them to fatigue quickly. Type II fibers are subdivided

into Type IIa and IIb fibers.

• Type IIa fibers:Type IIa fibers represent a transition
  type of fiber. While these fibers are capable of generat-
  ing a moderately large amount of force, they also have
  some aerobic capacity, although not as much as the
  Type I fibers. These fibers represent a logical and nec-
  essary bridge between the two types of muscle fibers
  allowing one to meet the energy demands for a variety
  of physical tasks (Rupp, 2001; Humphrey, 2001).


• Type IIb fibers:Type IIb fibers are the classic fast-
  twitch fibers that are predominately anaerobic since
  they rely on energy sources intrinsic to the muscle.

CARDIORESPIRATORY PHYSIOLOGY

• The cardiorespiratory system consists of the heart,
  lungs, and blood vessels. The purpose of this system
  is for the delivery of oxygen and nutrients to the cells

76 SECTION 1 • GENERAL CONSIDERATIONS IN SPORTS MEDICINE