Isometric Contractions
Work involves the distance through which a load is moved. In an isometric contraction the muscle does not shorten and so no work is done. The contractile tissue (muscle myofibrils) is contained within connective tissue which is elastic and tethered at both ends by another slightly elastic component (tendon). When the muscle is not stimulated to contract, muscle tension is mostly caused by the resistance in the tendons but to some extent by the connective tissue (passive tension).
Little resistance exists in the myofibrils, the components of which (actin/myosin) can slide freely along each other. Force is only generated in the myofibrils when stimulated (electrically triggered) to contract. The speed of contraction of a muscle allowed to shorten depends on the load it is moving. A light load is easier to move than a heavy one because there is less force opposing the shortening. Initially, there is no load (the limb is not moved) opposing the contraction of the myofibrils so tension is zero and the muscle contracts at maximum velocity. However, when no shortening is allowed or possible, the slack is taken up by the tendons and connective tissues, which then start to stretch offering resistance.
The speed of contraction is reduced to zero and tension becomes maximal. The total tension in muscle is made up of two components, the active tension developed by the contractile component and the passive tension caused by the elastic connective tissue resisting shortening. The contractile tension is the same for all muscle, but the passive tension depends on the amount of connective tissue which varies between different muscles. As the muscle length increases there comes a point, approximately at resting length, where the active tension begins to decline because the overlap between the actin and myosin filaments lessens. No active cross-bridges between these two proteins can develop. More tension is due to the connective tissue alone.
This corresponds to passive stretching where no contractile tension exists (at any length). The connective tissue resisting contraction becomes stretched causing tension. The sarcomeres in the myofibrils lengthen to counter this developing tension. The overall length of the myofibril increases. Isometric contraction at full length of muscle becomes an isometric stretch which results in lengthening the muscle.
Why does an isometric contraction cause muscle strengthening at only one length?
In an isometric contraction, the tension is constant in the muscle spindle at its current length for a particular load . The speed of contraction is zero. The Golgi organs should only be moderately active if there is no excessive stretch or contraction. The muscle is not allowed to shorten so the tension is taken up in the connective tissue as the load pulls against the contraction of the muscle. No movement occurs and no mechanical work is done. As the muscle gets fatigued, more muscle fibres must be recruited to maintain the load. For a light load the neuromuscular efficiency may be adequate for it to be unnecessary to recruit new muscle fibres in this way and the support of the load is sustained by those already available but not yet used. A heavier load will ensure that more of the available muscle fibres are recruited early on with the result that the muscle will tire quicker. In either case there will come a point when the muscle can no longer maintain the position of the load and a negative isotonic contraction will be initiated. It is at this very point that more muscle fibres are needed to sustain the maximum possible load. They are not available yet but the neuromuscular efficiency will increase as the brain learns to recruit more for the next time. The same argument applies to the muscle fibre recruitment for any isotonic contraction but for this isometric contraction it is at a fixed muscle length. The strength increases are for that length only.
Little resistance exists in the myofibrils, the components of which (actin/myosin) can slide freely along each other. Force is only generated in the myofibrils when stimulated (electrically triggered) to contract. The speed of contraction of a muscle allowed to shorten depends on the load it is moving. A light load is easier to move than a heavy one because there is less force opposing the shortening. Initially, there is no load (the limb is not moved) opposing the contraction of the myofibrils so tension is zero and the muscle contracts at maximum velocity. However, when no shortening is allowed or possible, the slack is taken up by the tendons and connective tissues, which then start to stretch offering resistance.
The speed of contraction is reduced to zero and tension becomes maximal. The total tension in muscle is made up of two components, the active tension developed by the contractile component and the passive tension caused by the elastic connective tissue resisting shortening. The contractile tension is the same for all muscle, but the passive tension depends on the amount of connective tissue which varies between different muscles. As the muscle length increases there comes a point, approximately at resting length, where the active tension begins to decline because the overlap between the actin and myosin filaments lessens. No active cross-bridges between these two proteins can develop. More tension is due to the connective tissue alone.
This corresponds to passive stretching where no contractile tension exists (at any length). The connective tissue resisting contraction becomes stretched causing tension. The sarcomeres in the myofibrils lengthen to counter this developing tension. The overall length of the myofibril increases. Isometric contraction at full length of muscle becomes an isometric stretch which results in lengthening the muscle.
Why does an isometric contraction cause muscle strengthening at only one length?
In an isometric contraction, the tension is constant in the muscle spindle at its current length for a particular load . The speed of contraction is zero. The Golgi organs should only be moderately active if there is no excessive stretch or contraction. The muscle is not allowed to shorten so the tension is taken up in the connective tissue as the load pulls against the contraction of the muscle. No movement occurs and no mechanical work is done. As the muscle gets fatigued, more muscle fibres must be recruited to maintain the load. For a light load the neuromuscular efficiency may be adequate for it to be unnecessary to recruit new muscle fibres in this way and the support of the load is sustained by those already available but not yet used. A heavier load will ensure that more of the available muscle fibres are recruited early on with the result that the muscle will tire quicker. In either case there will come a point when the muscle can no longer maintain the position of the load and a negative isotonic contraction will be initiated. It is at this very point that more muscle fibres are needed to sustain the maximum possible load. They are not available yet but the neuromuscular efficiency will increase as the brain learns to recruit more for the next time. The same argument applies to the muscle fibre recruitment for any isotonic contraction but for this isometric contraction it is at a fixed muscle length. The strength increases are for that length only.
posted by Louis Brothnias @ 9:46 AM
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