1. Isometric contractions of tenotomized rabbit solei have been compared with a group of unoperated control muscles. A large decrease in the tension developed by the tenotomized muscle was accompanied by a slight shortening of the twitch contraction time, and a larger, progressive, reduction in the time to half relaxation.2. A comparison of motor units obtained from muscles which had been tenotomized for six weeks with those of the control muscles showed a large reduction in the range of contraction and half relaxation times of the units from the tenotomized muscles.3. The mean motor unit tension (expressed as a percentage of the whole muscle tension) was similar for both those units from the control muscles and those from muscles which had been tenotomized for six weeks, indicating a uniform atrophy of motor units within the tenotomized muscles.4. It was concluded that the change in the pattern of motor unit contraction times was not the result of a process of differential atrophy favouring the preservation of the faster contracting motor units.5. A correlation between axon conduction velocity and both the speed of contraction and the size (tension) of the motor units was demonstrated in the control muscles. Following tenotomy the relationship between axon conduction velocity and motor unit tension was lost.
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