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
For answers, see Appendix H.
Adaptations to Exercise
Compare and contrast the effects of aerobic and
resistance exercise on skeletal muscles and on other
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
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
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
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
. 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
(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
Velocity of shortening
Inﬂuence of load on duration and velocity of muscle contraction.