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Static Contraction


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									In This Chapter:
Types of Muscle Contraction 76
      Static Contraction 76
      Dynamic Contraction 78
      Isometric Contraction 78
      Auxotonic Contraction 78
      Isotonic Contraction 79
      Isokinetic Contraction 80
      Concentric and Eccentric Contractions 81
      Plyocentric Contraction 81

Factors Influencing Muscle Contraction 81
      Joint Angle 81
      Muscle Cross-sectional Area 82
      Speed of Movement 84
      Muscle Fiber Type 88
      Age 89
      Sex 89

Summary 91
             Let’s explore muscle contraction....
                                                         CHAPTER 4

Muscles at Work
A fter completing this chapter you should be able to:
n   differentiate between the various types of muscle contractions;

n   describe the factors that influence strength development;

n   identify the components of strength;

n   discuss the relationships between the various components of strength.

 76                                                                                  Foundations of Kinesiology

M uscle is an organ that creates movement. Its                            Did You Know?
structure and function presented in the previous
                                                             It is interesting to note that the terminology for
chapter can adapt according to some important                major muscle contractions has been developed
training principles. These principles are designed           from the Greek language (Figure 4.1).
to improve general and specific fitness, which is an
important component of overall health. However,
before we present the various principles of training
in Chapter 9, you must become familiar with the          Static Contraction
various types of muscle contraction. You must also
understand the concept of muscular strength, its         Static contraction refers to a contraction in which
components, and the interrelationships between           the muscle tension or force exerted against an ex-
these components, which provide the basis of             ternal load is equal or weaker, so no visible move-
training for fitness and athletic performance.           ment of the load occurs. Consider an athlete who
                                                         attempts to flex an arm against the resistance of a
                                                         fixed bar. Even if all energy and strength are mo-
          Types of Muscle                                bilized, the athlete will not succeed in moving the
            Contraction                                  arm or the bar. Nonetheless, the exerted muscle
                                                         force is substantial.
Several types of muscle contraction are relevant to           In most sports, maximal static tension is
a fitness and strength training program. The first       rare. It may occur, however, in gymnastics (in
distinction is made between static and dynamic           the iron cross and hanging scale) or in wrestling
contraction.                                             and judo (in floor grips, holding techniques, and
     Static and dynamic work involves five types of      bridges). In general, most sport activities require
muscle contraction: isometric, isotonic, auxotonic,      low to submaximal static contraction. Sailing in
isokinetic, and plyocentric. Each of these types of      close contact with the wind, shooting, and alpine
contractions is further divided into two forms of        downhill events often require static work over
movement: concentric and eccentric.                      extended periods (Figure 4.2).

               Static                                                     Dynamic

            Isometric                             Isotonic        Auxotonic        Isokinetic      Plyocentric

  Concentric            Eccentric                                                           Eccentric

 Figure 4.1 Types of muscle contractions.
Studying Human Movement and Health                                                                        77

                      A                                                           B

 Figure 4.2 Static or isometric contractions. A. Activities requiring maximal static muscle tension. B.
 Activities requiring submaximal static muscle tension.
 78                                                                              Foundations of Kinesiology

Dynamic Contraction                                    based on auxotonic contraction (auxo = increased,
                                                       tonos = tension). Because of the continual change
The neuromuscular system is said to work dy-           in joint angle and speed that occurs during
namically if internal and external forces are un-      dynamic work, the muscle needs to contract at
balanced. For instance, an athlete may be able to      either increasing or decreasing tension; the term
exert enough force to lift a weight through the        auxotonic literally means “increased tension,”
full range of an exercise. When the external force     although here it is used in the sense of “variable
(gravity of a weight or object) is smaller than the    tension.” The constant addition or subtraction of
internal force generated by the athlete, the latter    motor units recruited causes the muscle to adapt
will be able to resist, and the result will be move-   to constantly changing tension requirements.
ment. Thus, a dynamic contraction involves             Auxotonic contraction is a dynamic contraction.
movement.                                                   When an athlete bends the arms while holding
                                                       a barbell, the mass of the barbell obviously remains
Isometric Contraction                                  unchanged during the entire range of movement.
                                                       The strength needed to perform this movement
An isometric contraction (iso = same, metric =         is not, however, constant, but depends upon the
length) is one in which there is no visible change     physique of the athlete, the athlete’s leverage,
in muscle length, even though the muscle has           the angle position of the limbs, and the speed of
undergone muscle contraction. In this case, the        movement (Figure 4.3). Also see Figure 3.5 on
contraction is against a load that is beyond the       page 66 in Chapter 3.
capability of the muscle(s) to move, and therefore,         Lateral arm raises, too, require greater strength
no movement of the load occurs. We also know           initially, reaching a maximum at 90 degrees and
that considerable force has been produced by the       then dropping constantly. When lifting the trunk
tiredness that one feels. The issue here is that       from a horizontal position, an athlete needs to
no external movement is registered. Isometric          mobilize maximal strength at the beginning of
contraction is a static contraction.                   the movement, gradually reach peak values, and
    Strictly speaking, no work is performed during     then decline continuously toward zero.
an isometric contraction (work = force x distance);         The issue of changing muscle force or tension
nonetheless, a relatively high amount of tension       throughout a movement also poses a problem
is developed and energy is used. Therefore, an         to those using free weights to train. What often
isometric contraction is not defined by the work       happens is one of two scenarios. If the chosen
performed but by the rate of tension developed         load can be lifted throughout the complete range
and by the duration over which the tension lasts.      of motion, it provides adequate stress for training
    When two individuals of equal strength compete     in the initial and final stages of the movement but
in arm wrestling, an isometric contraction occurs.     does not stress the muscle as much in the area of
There will be no movement of the hands until           movement corresponding to optimal cross bridge
one individual fatigues (loses some of the cross       formation. It is often in this area that the athlete
bridges) and therefore can no longer maintain the      wants to train, whether it is for the development of
status quo. For more on cross bridge formation, see    additional strength or the building of muscle bulk.
Chapter 3.                                             If the load is chosen to provide training stress to
    Can you name other activities that are based       the muscle in that part of the range of movement
on isometric contraction?                              corresponding to optimal cross bridge formation,
                                                       then the load is often too great for the individual
Auxotonic Contraction                                  to be able to move at either end of the complete
                                                       range of movement. In this case, the individual
Under normal circumstances, dynamic work is            often gets the bar moving by “bumping” it with
Studying Human Movement and Health                                                                         79

  A                                                       B


                                                            Figure 4.3 Muscle tension during elbow
                                                            flexion varies according to the joint angle.

his thighs, and, at the end of the curl, lets it fall   as you pass the region where optimal cross bridge
onto the shoulders. When lowering the bar, the          formation occurs, a difficult task to say the least
first movement is to drop the bar until sufficient      when training with free weights.
cross bridges can form to stop its falling, and the
movement ends with the bar falling onto the             Isotonic Contraction
thighs. Most often, the latter course of action is
taken, and the result is that the individual does       Dynamic contraction is based on isotonic
not train throughout the full range of motion,          contraction (iso = same or constant and tonos =
often resulting in the appearance that the arms         tension) only in exceptional cases. In an isotonic
can’t be straightened! What you would like to           contraction, the muscle changes length but not
do is optimally stress the muscle throughout the        its tension. For instance, an athlete achieves
range of motion. To do this, the load must be           (an approximately) isotonic contraction when
increased as the lift is made and then decreased        lowering an extremely heavy barbell at a slow and
 80                                                                             Foundations of Kinesiology

constant speed against maximal resistance.             of movement (despite the constantly changing
    In its pure form, however, this type of            leverage or torque) against a preset high resistance.
contraction is rarely encountered in sports and        This allows the working muscles and muscle
athletic events. The movement of any load involves     groups to release high tension over each section
continually changing joint angles, and these place     of the movement range. This type of contraction
changing demands on the level of tension required      is effective for strengthening the musculature
to move the load.                                      uniformly at all angles of motion.
                                                           As in the auxotonic contraction, however, the
Isokinetic Contraction                                 precise amount of muscle tension release is always
                                                       dependent upon the corresponding joint angle and
In isokinetic contraction (iso = same or constant      the velocity of movement. This is accomplished,
and kinetic = motion), the neuromuscular system        with varying degrees of success, by a number of
can work at a constant speed during each phase         expensive dynamometers, including the Cybex,

 Figure 4.4 Isokinetic contractions are generated by a variety of very expensive dynamometers.
Studying Human Movement and Health                                                                      81

                                                       from a stretched position. The “prestretching”
            Terminology Alert!                         of the muscle is achieved by jumping off an
  In strength training, coaching, and exercise         object (box) from a height of 10 to 15 inches,
  science literature, the word isotonic has            or 25 to 40 cm (depth jumping). This not only
  been imprecisely used as auxotonic muscle            prestretches the muscles but also sets off the Golgi
                                                       tendon organ reflex, which functions to protect
                                                       the muscle from too much stretch. The reflex
                                                       causes the muscles to contract. Activities that
the Kin-Com, and the Lido, which keep the speed        utilize this type of contraction to train jumping
of movement constant electronically, and the           ability include leaping and bounding, such as in
HydraGym and Nautilus, which use mechanical            plyometric training. Research has shown that
means to produce movements that are “isokinetic”       this type of strength and power training leads
in nature (Figure 4.4).                                to a greater increase in jump height than that
    The relatively constant speeds involved            developed by strength training alone.
in swimming and rowing are similar to those
produced in isokinetic exercise forms. For this
reason, these sports use isokinetic training to               Factors Influencing
increase performance levels. However, the majority
of sports contain few pure isokinetic movements               Muscle Contraction
as they continuously require changes in velocity
and force application throughout movements.            There are numerous factors that can affect the
   Isokinetic contractions are classified as dynamic   force and power output of a muscle, including the
contractions.                                          individual’s state of health and training status.
                                                       Other factors that affect force and power output
Concentric and Eccentric                               include: (1) joint angle; (2) muscle cross-sectional
                                                       area; (3) speed of movement; (4) muscle fiber
                                                       type; (5) age; and (6) sex.
A concentric contraction is one in which the
muscle shortens as it goes through the range           Joint Angle
of motion; this is usually termed flexion. An
eccentric contraction is one in which the              Let us again consider the example of elbow flexion
muscle lengthens during the movement, usually          during a barbell lift (Figure 4.3). Contraction of
termed extension. Again, let’s use the arm curl        the elbow flexors is initially isometric (static). The
with free weights as an example (Figure 4.3).          muscle does not visibly shorten until after the
The movement of the bar from the thighs to             internal forces generated by muscle flexing exceed
the shoulder region (i.e., flexion of the biceps       the external forces of the barbell. As the arms flex
brachii) is an auxotonic concentric contraction.       at the elbow joints, the barbell is moved toward the
The movement from the shoulder area back to            shoulders. The barbell accelerates in proportion
the thighs (i.e., extension of biceps brachii) is an   to the degree that the internal forces exceed the
isotonic eccentric contraction.                        load of the barbell. A short phase of static work,
                                                       often lasting only a few hundredths of a second,
Plyocentric Contraction                                occurs between the dynamic work of concentric
                                                       contraction and the lowering of the barbell. On
The last type of muscle contraction, a plyocentric     lowering the barbell into its initial position,
contraction, is a hybrid contraction in that the       external forces exceed internal ones. The same
muscle performs an isotonic concentric contraction     muscles that previously were involved in lifting

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