Chapter 9
Muscles and Muscle Tissue
297
9
kicking a ball. Concentric contractions are probably more fa-
miliar, but
eccentric contractions
, in which the muscle gener-
ates force as it lengthens, are equally important for coordination
and purposeful movements.
Eccentric contractions occur in your calf muscle, for example,
as you walk up a steep hill. Eccentric contractions are about 50%
more forceful than concentric ones at the same load and more of-
ten cause delayed-onset muscle soreness. (Consider how your calf
muscles
feel
the day aFer hiking up that hill.) Te reason is unclear,
Isotonic and Isometric Contractions
As noted earlier, there are two main categories of contractions—
isotonic
and
isometric
. In
isotonic contractions
(
iso
5
same;
ton
5
tension), muscle length changes and moves a load. Once
sufficient tension has developed to move the load, the tension
remains relatively constant through the rest of the contractile
period
(Figure 9.18a)
.
Isotonic contractions come in two “flavors”—
concentric
and
eccentric
.
Concentric contractions
are those in which the
muscle shortens and does work, such as picking up a book or
(a) Isotonic contraction (concentric)
On stimulation, muscle develops enough tension (force)
to lift the load (weight). Once the resistance is overcome,
the muscle shortens, and the tension remains constant for
the rest of the contraction.
(b) Isometric contraction
Muscle is attached to a weight that exceeds the muscle’s
peak tension-developing capabilities. When stimulated, the
tension increases to the muscle’s peak tension-developing
capability, but the muscle does not shorten.
Amount of resistance
Amount of
resistance
Peak tension
developed
Peak tension
developed
Resting length
Muscle
stimulus
Resting length
Muscle
stimulus
Time (ms)
Time (ms)
Muscle length (percent
of resting length)
Tension developed
(kg)
Muscle length (percent
of resting length)
Tension developed
(kg)
0
100
90
80
70
6 kg
3 kg
3 kg
6 kg
Muscle
contracts
(isometric
contraction)
Muscle
contracts
(isotonic
contraction)
Tendon
3 kg
3 kg
6 kg
6 kg
Tendon
Muscle
relaxes
Muscle
relaxes
2
4
6
8
0
100
90
80
70
2
4
6
8
Figure 9.18
Isotonic (concentric) and isometric contractions.
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