SENSORYRECEPTORS
- Muscle spindles:These specialized intrafusal muscle
fibers are located between and among extrafusal
fibers deep within the interior of the muscle. As pro-
prioceptors, they sense and relay the length or veloc-
ity of a muscle’s movement (e.g., patellar reflex). - Golgi tendon organs:These sensory organs detect
differences in tension and muscle length. Although
quiescent during periods of slow to moderate muscle
activity, they increase discharge when muscle load is
excessive. Golgi tendon organs(GTO) act to protect
against injury due to overloading.
SKELETAL MUSCLE FIBER TYPES
•Skeletal muscle is extremely dynamic, with various
fiber types characterized by differences in morphol-
ogy, histochemistry, enzyme activity, surface charac-
teristics, and functional capacity (Pette and Staron,
2001; Staron, 1997). Table 8-1 presents selected char-
acteristics of muscle fiber types as currently desig-
nated in human skeletal muscle.
•Within the normal population, the distribution of spe-
cific fiber types depends on a variety of factors, and
shows extraordinary adaptive potential in response to
innervation/neuronal activity, hormones, neural sig-
nalling, training/functional demands, and aging (Pette
and Staron, 2001). Muscle fiber type appears to
change in response to these effectors in a sequential
manner from either slow to fast or fast to slow
(Demirel et al, 1999; Pette and Staron, 2001; Zhen-He
et al, 2000).
- Recent advances in fiber typing have brought about a
change in nomenclature for human muscle fiber types:
Type IIb is now referred to as Type IId(x) due to the
expression of the myosin heavy chain(MHC) form
IId(x). MHC IIb is expressed in rodents and other
species, but does not appear as a distinct fiber type in
humans (Pette and Staron, 2001; Staron, 1997; Zhen-
He et al, 2000).
ENERGY METABOLISMENERGY SYSTEMS- Energy for muscular activity can be derived from
three specific systems (Table 8-2, Fig. 8-2). The
immediate, or phosphagen, system consists of adeno-
sine triphosphate (ATP) and creatine phosphate(PC
or phosphocreatine); it allows for very short bursts of
maximal power. - The short term, or glycogen-lactic acid, system con-
sists of glucose entering the glycolytic pathway and
the subsequent oxidation of pyruvic acid to lactic acid.
The short term system is often classified as anaerobic
because it can occur in the absence (or presence)
of oxygen (O 2 ). The energy formed allows for only
1–1.6 min of maximal muscle activity at a reduced
power output as compared to the phosphagen system. - The aerobic, or long term, system involves glucose
entering the glycolytic pathway through to pyruvic acid
and its subsequent oxidation to acetyl coenzyme A
(CoA) for entry into the tricarboxylic acid (TCA)
36 SECTION 1 • GENERAL CONSIDERATIONS IN SPORTS MEDICINE
TABLE 8-1 Characteristics of Major Skeletal Muscle Fiber Types*
FIBER CHARACTERISTICS SLOW TWITCH FAST TWITCH
Other terminology Type I Type IIa Type IId(x)
Slow Oxidative (SO) Fast/Oxidative/Glycolytic (FOG) Fast/Oxidative/Glycolytic (FOG)
Aerobic capacity HIGH MED/HIGH MED
Myoglobin content HIGH MED LOW
Color RED RED PINK/WHITE
Fatigue resistance HIGH MED/HIGH MED
Glycolytic capacity LOW MED MED/HIGH
Glycogen content LOW MED HIGH
Triglyceride content HIGH MED MED/LOW
Time to peak tension SLOW MED MED/HIGH
Myosin ATPase activity LOW MED MED
Myosin heavy chain (MHC) MHCIb MHCIIa MHCIId(x)
Tension cost† LOW MED MED
ATP/ADP LOW MED MED
*This chart represents current nomenclature for human skeletal muscle fiber types. Former Type IIb human muscle fibers are currently referred to as
Type IId(x) because of MHC types found in human muscle. Type IIb muscle fibers and Type IIb MHC are found in rodents and other species (Pette and
Staron, 2001; Staron, 1997; Zhen-He et al, 2000).
†Tension cost = ATP ase activity to isometric tension ratio.