Externally Imposed Conditions Also Affect Contraction

Mechanical factors external to the muscle also influence the force and speed of contraction. For example, if a muscle is not allowed to shorten when it is stimulated, it will develop more force than it would if its length were allowed to change. If a muscle is in the process of lifting a load, its force of contraction is determined by the size of the load, not by the capabilities of the muscle. The speed with which a muscle shortens is likewise determined, at least in part, by external conditions.

Speed Contraction Muscle

Motor unit summation. Two units are shown above, their motor nerve action potentials and muscle twitches are shown below. In the first contraction, there is a simple summation of two twitches, in the second, a brief tetanus in one motor unit sums with a twitch in the other.

Isometric Contraction. If a muscle is prevented from shortening when activated, the muscle will express its contractile activity by pulling against its attachments and developing force. This type of contraction is termed isometric (meaning "same length"). The forces developed during an isometric contraction can be studied by attaching a dissected muscle to an apparatus similar to that shown in Figure 9.7. This arrangement provides for setting the length of the muscle and tracing a record of force versus time. In a twitch, isometric force develops relatively rapidly, and subsequent isometric relaxation is somewhat slower. The durations of both contraction time and relaxation time are related to the rate at which calcium ions can be delivered to and removed from the region of the crossbridges, the actual sites of force development. During an isometric contraction, no actual physical work is done on the external environment because no movement takes place while the force is developed. The muscle, however, still consumes energy to fuel the processes that generate and maintain force.

Isotonic Contraction. When conditions are arranged so the muscle can shorten and exert a constant force while doing so, the contraction is called isotonic (meaning "same force"). In the simplest conditions, this constant force is

Isotonic Contraction Graph

A simple apparatus for recording isometric contractions. The length of the muscle (marked on the graph by the pen attached near its lower end) is adjustable at rest but is held constant during contraction. The force transducer provides a record of the isometric force response to a single stimulus at a fixed length (isometric by definition). (Force, length, and time units are arbitrary.)

provided by the load a muscle lifts. This load is called an af-terload, since its magnitude and presence are not apparent to the muscle until after it has begun to shorten.

Recording an isotonic contraction requires modification of the apparatus used to study isometric contraction (Fig. 9.8). Here the muscle is allowed to shorten while lifting an afterload, which is provided by the attached weight. This weight is chosen to present somewhat less than the peak force capability of the muscle. When the muscle is stimulated, it will begin to develop force without shortening, since it takes some time to build up enough force to begin to lift the weight. This means that early on, the contraction is isometric (phase 1,- Fig. 9.8). After sufficient force has been generated, the muscle will begin to shorten and lift the load (phase 2). The contraction then becomes isotonic because the force exerted by the muscle exactly matches that of the weight, and the mass of the weight does not vary. Therefore, the upper tracing in Figure 9.8 shows a flat line representing constant force, while the muscle length (lower tracing) is free to change. As relaxation begins (phase 3), the muscle lengthens at constant force because it is still supporting the load, this phase of relaxation is isotonic, and the muscle is reextended by the weight. When the muscle has been extended sufficiently to return to its original length, conditions again become isometric (phase 4), and the remaining force in the muscle declines as it would in a purely isometric twitch. In almost all situations encountered in daily life, isotonic contraction is preceded by isometric force development, such contractions are called mixed contractions (isometric-isotonic-isometric).

The duration of the early isometric portion of the contraction varies, depending on the afterload. At low after-

Isotonic Contraction Graph

^nQGUQEM^^ A modified apparatus showing the recording of a single isotonic switch. The pen at the lower end of the muscle marks its length, and the weight attached to the muscle provides the afterload, while the platform beneath the weight prevents the muscle from being overstretched at rest. The first part of the contraction, until sufficient force has developed to lift the weight, is isometric. During shortening and isotonic relaxation the force is constant (isotonic conditions), and during the final relaxation, conditions are again isometric because the muscle no longer lifts the weight. The dotted lines in the force and length traces show the isometric twitch that would have resulted if the force had been too large (greater than 3 units) for the muscle to lift. (Force, length, and time units are arbitrary.) (See text for details.)

^nQGUQEM^^ A modified apparatus showing the recording of a single isotonic switch. The pen at the lower end of the muscle marks its length, and the weight attached to the muscle provides the afterload, while the platform beneath the weight prevents the muscle from being overstretched at rest. The first part of the contraction, until sufficient force has developed to lift the weight, is isometric. During shortening and isotonic relaxation the force is constant (isotonic conditions), and during the final relaxation, conditions are again isometric because the muscle no longer lifts the weight. The dotted lines in the force and length traces show the isometric twitch that would have resulted if the force had been too large (greater than 3 units) for the muscle to lift. (Force, length, and time units are arbitrary.) (See text for details.)

loads, the muscle requires little time to develop sufficient force to begin to shorten, and conditions will be isotonic for a longer time. Figure 9.9 presents a series of three twitches. At the lowest afterload (weight A only), the isometric phase is the briefest and the isotonic phase is the longest with the lowest force. With the addition of weight B, the afterload is doubled and the isometric phase is longer, while the isotonic phase is shorter with twice the force. If weight C is added, the combined afterload represents more force that the muscle can exert, and the contraction is isometric for its entire duration. The speed and extent of shortening depend on the afterload in unique ways described shortly.

Other Types of Contraction. Other physical situations are sometimes encountered that modify the type of muscle contraction. When the force exerted by a shortening muscle continuously increases as it shortens, the contrac tion is said to be auxotonic. Drawing back a bowstring is an example of this type of contraction. If the force of contraction decreases as the muscle shortens, the contraction is called meiotonic.

In the body, a concentric contraction is one in which shortening (not necessarily isotonic) takes place. In an eccentric contraction, a muscle is extended (while active) by an external force. Activities such as descending stairs or landing from a jump utilize this type of contraction. Such contractions are potentially dangerous because the muscle can experience forces that are larger than it could develop on its own, and tearing (strain) injuries can result. A static contraction results in no movement, but this may be due to partial activation (fewer motor units active) opposing a load that is not maximal. (This is different from a true isometric contraction, in which shortening is physically impossible regardless of the degree of activation.)

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Responses

  • katja
    What is a muscle contraction sufficient to lift a load?
    7 years ago
  • aden
    Are isotonic muscle contractions preceded by isometric muscle contractions?
    7 years ago
  • Primula
    What external things affect the rate of contraction of muscles?
    7 years ago
  • livia
    Is the longest twitch duration for isotonic muscle contraction with light weight?
    6 years ago
  • luca
    How to know isotonic and isometric contraction from graph of simple muscle twitch ?
    6 years ago

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