Negative Isotonic Contraction

Soreness is generally accepted as being a result of negative isotonic contractions especially by placing a great load upon the muscle causing damage to connective tissue. If the load is greater than can be moved positively then the strain upon the muscle is more likely to cause discomfort. The reason that this type of work is done is that it promotes strength gains and muscle fibre recruitment is implicated again. A good picture to keep in mind is of an elastic band but held under some tension. This corresponds to the normal resting state for a limb at the current (training). If the limb is extended beyond the normal limit for range of motion (stretched) then the tension increases until the limb can be moved no further. Shortening the muscle by flexion will cause the tension in the connective tissue to be reduced as its elastic nature allows it to take up the slack - the connective tissue spans a shorter distance. In the positive concentric movement an increasing number of muscle fibres become involved in moving the load and this ensures sufficient contractile force is generated up to the maximum possible, again for the current state of training. This is the type of work to increase the neuromuscular efficiency and hence strength gains.

The muscle working in this eccentric phase (negative contraction) is still the prime mover. Initially the mechanism involves selectively switching off muscle fibres to reduce the contractile force. When alpha-motorneurons stimulate muscle fibres, the fibres contract in an "all or nothing" fashion. Some fibres may become fatigued during the slow movement and others are recruited to sustain the contractile power. This recruitment explains why negative contractions promote strength gains. This reduction in force reaches a point where there are no longer sufficient fibres to overcome the load and the muscle extends, pulled down by gravity acting on the load. The tension within the muscle and the surrounding connective tissue begins to rise as the muscle lengthens. Under normal circumstances, a load which has been moved in the positive phase is the same which is moved in the negative phase. Fibres still participating in the contractile mode are attempting, and failing, to resist the load and they become overloaded. Contraction is in the opposite direction to the movement of the limb as the muscle lengthens.

The tension in the connective tissue in the contracting fibres is increased because a greater number of fibres are not participating and the connective tissue around these fibres is pulling in the opposite direction. The stress receptors, Golgi organs in tendons and muscle spindles within muscle, monitor this rising tension and inhibit the alpha-motorneurons, those responsible for contraction. The muscle contracts less strongly and continues to lengthen. The antagonist muscle becomes increasingly involved to control the limb as the alpha-motorneurons are inhibited. Remember the agonisVantagonist muscles work together and in the positive phase as the agonist contracts the antagonist is inhibited. The reverse occurs in the negative phase - the agonist contraction is reduced in intensity so the inhibition of the antagonist is similarly reduced.

The tension reached depends on the load used. The greatest stress possible is to use a load which cannot be moved positively at all and this is where most of the problems, and strength gains, occur. For example, a press up can normally be performed using the body weight as the load without a problem. If, however, with the arms aimost in the fully extended position (elbows not locked out), a heavy weight was placed upon your back - just too much that a press up to this position would not be possible - then the triceps and chest would not have sufficient strength to stay in this position and the elbows would begin to bend. Gravity is pulling you down and a slow downward movement would occur in the negative contraction of the triceps. The point is that you do not have enough strength to stop it completely but can resist and slow it down if the weight is correct. At the full range of motion the antagonist will become the prime mover as you attempt to stretch the muscle beyond its normal range. The roles reverse. It is at this point where the tension receptors adjust the stress by initiating contraction against the stretch and inhibit the contraction of the muscle causing the stretch beyond the limit of the range of motion. To a large extent this is where training can overcome the amount of reflexive resistance against the stretch and allow further lengthening of the muscle. If a load is used especially in a negative contraction exercise where this load is too much to move positively then great care is needed. Injury could occur.