Calculated fatigue envelope parameters. Compared to the 10-12 months old retired breeders (RB), the aged subgroups show a graded Fmax reduction, while reductions of slope T50 and F AUC T50 are more pronounced. No differences were observed between adult rats and all aged rat subgroups for T50. Error bars indicate mean ± SEM. P values are given compared to 10-12 month old RB.