Human Physiology, 14th edition (2016)

Muscle 377

In the body, this normal resting length is maintained by reflex
contractions in response to passive stretching (described in
section 12.5).
When the sarcomere lengths are greater than about 2.2  m m,
the tension produced by the muscle contraction decreases with
increasing sarcomere length. This is because there are fewer
interactions of myosin cross bridges with actin. When the sar-
comeres reach a length of about 3.6  m m, there is no overlap of
thick and thin filaments, and no interactions can occur between
myosin and actin. Therefore the muscle produces zero tension
( fig. 12.21 ).
When the sarcomere length is shorter than 2.0  m m, the
force generated by muscle contraction declines with decreas-
ing sarcomere length ( fig. 12.21 ). This is because the cross-
bridge action becomes less effective as the muscle fiber gets
shorter and thicker due to: (1) the development of oppos-
ing forces (such as the fluid pressure of the sarcoplasm) as
the muscle fiber gets shorter; and (2) the increasing distance
between thick and thin filaments as the muscle fiber gets
thicker. The double overlapping of thin filaments (see the left
sarcomere in fig. 12.21 ) may further interfere with the action
of cross bridges. The force of muscle contraction declines still
further when the thick filaments abut against the Z discs at a
sarcomere length of 1.7  m m. This may be due to deformation
of the myosin. At a sarcomere length of 1.25  m m, the muscle
produces zero force ( fig. 12.21 ).

Figure 12.21 The length-tension relationship in skeletal muscles. Maximum relative tension (1.0 on the y -axis) is
achieved when the muscle is 100% to 120% of its resting length (sarcomere lengths from 2.0 to 2.25  m m). Increases or decreases in
muscle (and sarcomere) lengths result in rapid decreases in tension.

1.65 μm 2.25 μm 3.65 μm

Relative tension

Percentage rest length

0

0.5

1.0

60 80 100 120 140 160

1.25 μm

1.65 μm

2.0 μm 2.25 μm

3.65 μm

| CHECKPOINT

6a. Explain how graded contractions and smooth,

sustained contractions can be produced in vitro and

in vivo.

6b. Distinguish among isotonic, isometric, concentric,

and eccentric contractions, and describe what

factors determine if a contraction will be isometric or

isotonic.

7a. Identify the nature and physiological significance of

the series-elastic component of muscle contraction.

7b. Describe the relationship between the resting muscle

length and the strength of its contraction.

12.4 ENERGY REQUIREMENTS OF

SKELETAL MUSCLES

Skeletal muscles generate ATP through cell respiration

and through the use of phosphate groups donated by cre-

atine phosphate. The aerobic abilities of skeletal muscle

fibers differ according to muscle fiber type, which are dis-

tinguished by their speed of contraction, color, and major

mode of energy metabolism.