The fibre type composition showed a higher proportion of the oxidative type IIBd fibres in the middle-aged group [5 months old; 39.8 +/- 6.8 vs. 23.6 +/- 4.2% of the fibre area in the young rats (1.3 months old)] in contrast to the type IIBm fibres (52.9 vs. 67.9%, respectively), while the old group (22 months old) was not different from the middle-aged group. Work output in the last contraction (relative to the first contraction) was not different between the age-groups (53.1 +/- 18.1; 48.0 +/- 6.5 and 61.1 +/- 6.2%, respectively). High-energy phosphate utilization was not different between the groups (150.6 +/- 11.2; 154.6 +/- 15.6 and 157.2 +/- 7.0 mumol g-1 dry wt, respectively). However, the efficiency was approximately 30% lower in the muscles of the youngest group, which corresponds with a lower specific power and specific tension. Since the change in fibre type composition is unlikely to be the cause of the low efficiency in the young animals, the causes remain unclear, but may be related to the rapid growth of the young rats in our study.
This research carried out in the Department of Anthony Sargeant in Amsterdam demonstrated how during rapid growth the muscle of young animals is weaker and less efficient compared with older animals.
Acta Physiologica Scandanavica
Acta Physiol Scand. 1993 Apr;147(4):347-55