Chapter 9
Muscles and Muscle Tissue
301
9
ATP-generating pathway have his working muscles been using
that makes him breathless? What metabolic products might
account for his sore muscles and muscle weakness?
For answers, see Appendix H.
Force of Muscle Contraction
Describe factors that influence the force, velocity, and
duration of skeletal muscle contraction.
Describe three types of skeletal muscle fibers and explain
the relative value of each type.
Te force of muscle contraction depends on the number of my-
osin cross bridges that are attached. Tis in turn is affected by four
factors
(Figure 9.21)
: (1) number of muscle fibers stimulated,
(2) relative size of the fibers, (3) frequency of stimulation, and
(4) degree of muscle stretch. Let’s examine the role of each factor.
Number of Muscle Fibers Recruited
As already discussed, the more motor units that are recruited,
the greater the muscle force.
Size of Muscle Fibers
Te bulkier the muscle (the greater its cross-sectional area), the
more tension it can develop and the greater its strength, but
there is more to it than this. As noted earlier, the large fibers
of large motor units produce the most powerful movements.
Regular resistance exercise increases muscle force by causing
muscle cells to
hypertrophy
(hi-per
9
tro-fe), or increase in size.
Frequency of Stimulation
As a muscle begins to contract, the force generated by the cross
bridges—the
internal tension
—stretches the connective tissue
sheaths (noncontractile components). Tese in turn become taut
and transfer their tension, called the
external tension
, to the load
normally regulated within normal limits in all but the greatest
degree of exertion. Additionally, lactic acid actually counteracts
high K
1
levels, which do lead to muscle fatigue (as noted above).
In general, intense exercise of short duration produces fa-
tigue rapidly via ionic disturbances that alter E-C coupling, but
recovery is also rapid. In contrast, the slow-developing fatigue
of prolonged low-intensity exercise may require several hours
for complete recovery. It appears that this type of exercise dam-
ages the SR, interfering with Ca
2
1
regulation and release, and
therefore with muscle activation.
Excess Postexercise Oxygen Consumption (EPOC)
Whether or not fatigue occurs, vigorous exercise alters a mus-
cle’s chemistry dramatically. For a muscle to return to its resting
state, all the following must occur:
Its oxygen reserves in myoglobin must be replenished.
Te accumulated lactic acid must be reconverted to pyruvic acid.
Glycogen stores must be replaced.
A±P and creatine phosphate reserves must be resynthesized.
Te use of these muscle stores during anaerobic exercise simply
defers when the oxygen is consumed, because replacing them
requires oxygen uptake and aerobic metabolism a²er exercise
ends. Additionally, the liver must convert any lactic acid persist-
ing in blood to glucose or glycogen. Once exercise stops, the
repayment process begins.
Te extra amount of oxygen that the body must take in for
these restorative processes is called the
excess postexercise oxy-
gen consumption (EPOC)
, formerly called the oxygen debt.
EPOC represents the difference between the amount of oxygen
needed for totally aerobic muscle activity and the amount ac-
tually used. All anaerobic sources of A±P used during muscle
activity contribute to EPOC.
Heat Production During Muscle Activity
Only about 40% of the energy released during muscle contraction
is converted to useful work (still, this percentage is significantly
higher than that of many mechanical devices). Te rest is given off
as heat, which has to be dealt with to maintain body homeostasis.
When you exercise vigorously, you start to feel hot as the
liberated heat warms your blood. Like a car’s cooling system
that dissipates heat, several homeostatic processes prevent heat
in the body from building to dangerous levels. Tese processes
include sweating and radiating heat from the skin surface.
Shivering represents the opposite side of homeostatic bal-
ance. In this case the body is too cold and muscle contractions
are used to produce more heat.
Check Your Understanding
13.
When Eric returned from jogging, he was breathing heavily,
sweating profusely, and complained that his legs ached and felt
weak. His wife poured him a sports drink and urged him to take
it easy until he could “catch his breath.” On the basis of what
you have learned about muscle energy metabolism, respond to
the following questions: Why is Eric breathing heavily? Which
Large
number of
muscle
fibers
recruited
Contractile force
(more cross bridges attached)
High
frequency of
stimulation
(wave
summation
and tetanus)
Large
muscle
fibers
Muscle and
sarcomere
stretched to
slightly over 100%
of resting length
Figure 9.21
Factors that increase the force of skeletal muscle
contraction.
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