When a muscle has used up all of the ATP available then it is said to be in rigor. In terms of the physiology of this state then this is only ever achieved after death, and this is what is known as rigor mortis. The cause of this state is that, during the ATPase cycle, the ATP is required for the myosin to dissociate from the actin. As there is no more ATP available this is therefore impossible and so the myosin remains locked in a strong binding state.
Due to the stability of this state it is frequently used to study the mechanics and kinetics of muscle systems. These are very difficult to examine in other cases because only 10-25% of the myosin heads are actually attached to the actin at any one point. Therefore creating a rigor attachment greatly increases this number.
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An
historical look at muscle contraction