Oxygen consumption during maximal exercise in Fischer 344 x Brown Norway F1 hybrid rats.

We characterized O2 consumption (VO2) during treadmill exercise in 12-, 24-, and 35-month-old Fischer 344 x Brown Norway F1 hybrid (F344BNF1) rats. When accounting for differences in body mass (Mb), (O2)peak decreased by 10% and 33% in 24- and 35-month-old rats, respectively, compared with rats at 12 months (analysis of covariance, p < .01). O2 cost per unit work at VO2peak (i.e., VO2peak/work) was greater in 35-month-old rats compared with 12- and 24-month-old rats (p < .001). During submaximal exercise, the O2 cost was greater in 24- and 35-month-old than 12-month-old rats (p < .01). Analysis of covariance revealed similar patterns irrespective of differences in Mb or lean Mb as covariates. The underlying mechanism responsible for increasing O2 consumption in aged F344BNF1 rats during exercise, although partly explained by mechanical inefficiencies of locomotion, still remains to be determined.