Age-related differences in diaphragm muscle injury after lengthening activations

The susceptibility of postnatal day 15 and adult rat diaphragms (DIAs) to acute injury after repetitive isovelocity lengthening activations was examined in vitro. Forces were measured during two phases of each stimulation protocol: 1) isometric phase: during the first 300 ms of each 500-ms train, DIA length was not changed; and 2) isovelocity lengthening phase: during the remaining 200 ms of each stimulus train, DIA was lengthened at a constant velocity from 90 to 110% of optimal length. At maximal activation (75 Hz and a lengthening velocity of 1.0 optimal length per second), the maximum force measured during the isometric phase and that measured during the isovelocity lengthening phase were both greater in adult DIAs than in day 15 DIAs but both declined to a greater extent in adults with repetitive activation. Ultrastructural analysis showed that after lengthening activations muscle fiber injury was very evident in adult but much less prevalent in day 15 DIAs. This difference in susceptibility between the adult and day 15 DIAs did not depend on differences in peak force or absolute velocity of lengthening. We conclude that lengthening activations result in DIA injury and that the adult is more susceptible than its younger counterpart.