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1 Department of Physiology, Monash University, Melbourne, Victoria, Australia
2 Department of Electrical and Computer Systems Engineering, Monash University, Melbourne, Victoria, Australia
* To whom correspondence should be addressed. E-mail: uwe.proske{at}med.monash.edu.au.
This is a report of experiments carried out on the medial gastrocnemius muscle of the anaesthetised cat, investigating the effects of eccentric contractions carried out at different muscle lengths on the passive and active length-tension relationships. In one series of experiments the motor supply to the muscle was divided into three approximately equal parts, in the other, whole muscles were used. Fifty eccentric contractions were carried out over different regions of the active length-tension curve for each part-muscle or whole-muscle. Active and passive length-tension curves were measured before and after the eccentric contractions. When eccentric contractions were carried out at longer lengths, there was a larger shift of the optimum length for active tension in the direction of longer muscle lengths and a larger fall in peak isometric tension. Passive tension was higher immediately after the eccentric contractions and if the muscle was left undisturbed for 40 minutes, it increased further to higher values, particularly after contractions at longer lengths. A series of 20 passive stretches of the same speed and amplitude and covering the same length range as the active stretches, reduced the passive tension which re-developed over a subsequent 40 minute period. It is hypothesised that there are two factors influencing the level of passive tension in a muscle after a series of eccentric contractions. One is injury contractures in damaged muscle fibres tending to raise passive tension, the other is disruption of overextended sarcomeres tending to reduce it.
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