Covering, Support, and Movement of the Body
Check Your Understanding
List two factors that influence contractile force and two that
influence velocity of contraction.
Jim called several friends to help him move. Would he
prefer to have those with more slow oxidative muscle
fibers or those with more fast glycolytic fibers as his
helpers? Why?
For answers, see Appendix H.
Adaptations to Exercise
Compare and contrast the effects of aerobic and
resistance exercise on skeletal muscles and on other
body systems.
Te amount of work a muscle does is reflected in changes in
the muscle itself. When used actively or strenuously, muscles
may become larger or stronger, or more efficient and fatigue
resistant. Inactivity, on the other hand,
leads to muscle
weakness and wasting.
Aerobic (Endurance) Exercise
, or
such as swimming, jogging,
fast walking, and biking results in several recognizable changes
in skeletal muscles:
Te number of capillaries surrounding the muscle fibers
Te number of mitochondria within the muscle fibers also
Te fibers synthesize more myoglobin.
Tese changes occur in all fiber types, but are most dramatic
in slow oxidative fibers, which depend primarily on aerobic
pathways. Te changes result in more efficient muscle metabo-
lism and in greater endurance, strength, and resistance to fa-
tigue. Additionally, regular endurance exercise may convert fast
glycolytic fibers into fast oxidative fibers.
Finally, consider the less common intermediate muscle fiber
types, called
fast oxidative fibers
(±able 9.2, middle column). Tey
have many characteristics intermediate between the other two types
(fiber diameter and power, for example). Like fast glycolytic fibers,
they contract quickly, but like slow oxidative fibers, they are oxygen
dependent and have a rich supply of myoglobin and capillaries.
Some muscles have a predominance of one fiber type, but
most contain a mixture of fiber types, which gives them a range
of contractile speeds and fatigue resistance. For example, a calf
muscle can propel us in a sprint (using its white fast glycolytic
fibers) or a long-distance race (making good use of its slow and
fast oxidative fibers). But, as might be expected, all muscle fibers
in a particular
motor unit
are of the same type.
Although everyone’s muscles contain mixtures of the three
fiber types, some people have relatively more of one kind. Tese
differences are genetically initiated, but can be modified by ex-
ercise and no doubt determine athletic capabilities, such as en-
durance versus strength, to a large extent. For example, muscles
of marathon runners have a high percentage of slow oxidative
fibers (about 80%), while those of sprinters contain a higher
percentage (about 60%) of fast oxidative and glycolytic fibers.
Interconversion between the “fast” fiber types occurs as a result
of specific exercise regimes, as we’ll describe below.
Because muscles are attached to bones, they are always pitted
against some resistance, or load, when they contract. As you might
expect, they contract fastest when there is no added load on them.
A greater load results in a longer latent period, a slower contrac-
tion, and a shorter duration of contraction
(Figure 9.24)
. If the
load exceeds the muscle’s maximum tension, the speed of shorten-
ing is zero and the contraction is isometric (see Figure 9.18b).
In the same way that many hands on a project can get a job
done more quickly and also can keep working longer, the more
motor units that are contracting, the faster and more prolonged
the contraction.
(a) The greater the load, the less the muscle shortens
and the shorter the duration of contraction
(b) The greater the load, the
slower the contraction
Time (ms)
Distance shortened
Intermediate load
Light load
Heavy load
Increasing load
Velocity of shortening
Figure 9.24
Influence of load on duration and velocity of muscle contraction.
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