285
CHAPTER NINE
Muscular System
THE MUSCULAR MOVEMENTS BEHIND “TEXTING”
tion and movements of the hand as the F
nger pressed the pad. They saw two
clearly di±
erent patterns of muscle activation, indicating two di±
erent types of
movement—light tapping from an angle versus direct downward pressure on
one key. The act of texting entails a key-locating “tap” followed by a more direct
push (static force). The switch from one type of movement to another is so fast
and ²
uid that we usually are not aware of it.
Understanding the complexity of these dual tasks helps to explain why it
takes years for children to master F
ne-hand coordination, as well as why these
skills are often the F
rst to be noticeably lost in neuromuscular disease. Our F
nger
dexterity enabled our distant ancestors to live in the trees and then come down
from them.
Practical applications of the F
ndings include guidance of prosthetic
design, suggesting physical therapy techniques, and assisting the design of
machines and electronic devices to be compatible with our natural F
nger and
hand movements.
O
ur musculoskeletal systems can rapidly adapt to new chal-
lenges. Consider sending text messages on a handheld
device (“texting”) or other movements that require the
F
ngers to rapidly press precise sequences of very small but-
tons. Texting is similar to other challenges to dexterity, such
as manipulating buttons on clothing or slicing or dicing foods. Loss of this
dexterity may be an early sign of a disease that a±
ects the muscles, such as
amyotrophic lateral sclerosis (Lou Gehrig’s disease).
³ingertip dexterity and hand movements are more complex than it may
seem, altogether involving more than 30 muscles. To track the exact move-
ments required for sending a text message, researchers recorded the electri-
cal activity (using a measure called an electromyogram) and F
ngertip force in
seven muscles of the index F
ngers of volunteers as they pushed their F
ngers
against a surface. The researchers used an algorithm to assess the coordina-
9.1
INTRODUCTION
Talking and walking, breathing and sneezing—all movements—
require muscles. Muscles are organs composed of special-
ized cells that use the chemical energy stored in nutrients to
exert a pulling force on structures to which they are attached.
Muscular actions also provide muscle tone, propel body fl uids
and food, generate the heartbeat, and distribute heat.
Muscles are of three types—skeletal muscle, smooth
muscle, and cardiac muscle, as described in chapter 5
(pp. 163–164). This chapter focuses mostly on skeletal mus-
cle, which attaches to bones and to the skin of the face and is
under conscious control. Smooth muscle and cardiac muscle
are discussed briefl
y.
9.2
STRUCTURE OF A
SKELETAL MUSCLE
A skeletal muscle is an organ of the muscular system. It is
composed primarily of skeletal muscle tissue, nervous tis-
sue, blood and other connective tissues.
Connective Tissue Coverings
An individual skeletal muscle is separated from adjacent
muscles and held in position by layers of dense connective
tissue called
fascia
(fash
e-ah). This connective tissue sur-
rounds each muscle and may project beyond the ends of its
muscle F
bers, forming a cordlike
tendon.
±ibers in a ten-
don may intertwine with those in the periosteum of a bone,
attaching the muscle to the bone. Or, the connective tissues
associated with a muscle form broad, F
brous sheets called
aponeuroses
(ap
o-nu-ro
se
¯z), which may attach to bone or
the coverings of adjacent muscles
(f
gs. 9.1
and
9.2)
.
Aponeuroses
Skeletal muscles
Tendons
FIGURE 9.1
Tendons attach muscles to bones, whereas
aponeuroses attach muscles to other muscles.
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