Civil Engineering Reference
In-Depth Information
Physiology topics published until the middle of the twentieth century often divided muscle activities
into either dynamic efforts lasting for minutes or hours, with work, energy, and endurance typical topics;
or short bursts of contractile exertion. Much research on muscle effort concerned the “isometric” con-
dition in which muscle length (and hence body segment position) did not change. Consequently, much
information on muscle strength applies to such static exertion. All other muscle activities were typically
called “anisometric,” often even falsely labeled “isotonic” or “kinetic,” meant to cover all the many poss-
ible dynamic muscle uses. Chaffin et al. (1999), Marras et al. (1993), Kroemer (1999), and Kumar (2004)
discuss proper terminology: Table 10.1 lists and explains terms that correctly describe muscular events.
For the engineer, skeletal muscles are of primary interest since they pull on segments of the human
body and generate energy for exertion to outside objects. Skeletal muscles connect two body links
across their joint, as shown in Figure 10.1; in some cases muscles cross even two joints. Muscles are
usually arranged in “functional pairs” so that contracting muscles counteract each other. The muscle,
or the group of synergistic muscles, pulling in the intended direction is the agonist (also called protago-
nist) and the opposite is the antagonist. Cocontraction, the simultaneous activation of paired opposing
muscles, serves to control speed and strength exertion.
There are several hundred skeletal muscles in the human body, known by their Latin names. Connec-
tive tissue (fascia) enwraps them; it imbeds nerves and blood vessels. At the ends of the muscle, the tissues
combine to form tendons, which usually attach to bones.
Thousands of individual muscle fibers run, more or less parallel to the length of the muscle. Seen via a
microscope, skeletal muscle fibers appear striped (striated) crosswise: thin and thick, light and dark
bands run across the fiber in regular patterns, which repeat along the length of the fiber. One such
thick dark stripe appears to penetrate the fiber like a membrane or disc: this is the so-called z-disk
(from the German zwischen, between). The distance between two adjacent z-lines defines the sarcomere.
Its length at rest is approximately 250
˚
(1
˚
10
2
10
m), meaning that there are about 40,000 sarco-
meres in series within 1 mm of muscle fiber length.
Within each muscle fiber, thread-like myofibrils (from the Greek mys, muscle) lie in parallel by the
hundreds or thousands. Each of these, in turn, consists of bundles of myofilaments. A network of
tubular channels, sacs and cisterns, which connect with a larger tubular system in the z-disks, fill the
spaces between the filaments. All of this is part of the networks of blood vessels and nerves in the fascia.
This is the “plumbing and control” system of the muscle, the sarcoplasmic reticulum. It provides fluid
transport between the cells inside and outside the muscle and carries chemical and electrical messages.
Two of the myofibrils, myosin and actin, have the ability to slide along each other; this is the source of
muscular contraction. Small projections, called cross-bridges, protrude fromthemyosin filaments towards
neighboring actins. The actin filaments are twisted double-stranded protein molecules, wrapped in a
double helix around the myosin molecules. This is the “contracting microstructure” of the muscle.
The only active action that a muscle can take is contraction; external forces that stretch the muscle
bring about passive elongation. According the “sliding filament theory,” the heads of adjacent actin
rods moving toward each other cause contraction. This pulls the z-disks closer together: sarcomeres
in series (and those parallel) shorten, and as a result, the whole muscle shortens. After a contraction,
the muscle returns to its resting length, primarily through a recoiling of its shortened filaments,
fibrils, fibers, and other connective tissues. Force external to the muscle can stretch the muscle beyond
its resting length, either by gravity or other force acting from outside the body, or by the action of antag-
onistic muscle. (Refer to texts by Asimov, 1963; Astrand and Rodahl, 1977, 1986; Chaffin et al., 1999;
Enoka, 2002; Kroemer et al., 1997, 2001; Schneck 1990, 1992; and Winter, 1990, among others, for
more information.)
¼
10.3 Relation Between Muscle Length and Tension
Stimulation from the central nervous system (CNS) causes the muscle to contract to its smallest possible
length, which is about half the resting length with no external load. In this condition, the actin proteins
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