Recent evidence demonstrates that aging results in an increase in fast (type IIB) myosin heavy chain (MHC) in the rat diaphragm. It is unknown whether this age-related change in fast MHC influences the diaphragmatic maximal shortening velocity (V-max). Therefore, we tested the hypothesis that aging is associated with an increase in the diaphragmatic V-max and that the increase in the V-max is highly correlated with the percentage of type IIb MHC. In vitro contractile properties were measured with costal diaphragm strips obtained from young (4 mo old; n = 8) and old (24 mo old; n = 8) male Fischer-344 rats. Diaphragmatic maximal tetanic specific force production was 14.5% lower in the old compared with the young animals (23.0 +/- 0.4 vs. 19.7 +/- 0.8 N/cm(2); P < 0.05). In contrast, the diaphragmatic V,, was significantly higher in the old compared with the young animals (5.5 +/- 0.1 vs. 4.4 +/- 0.3 lengths/s; P < 0.05). Although the percent type IIb MHC was significantly higher (approximately +14%; P < 0.05) in the old compared with the young animals, the correlation between V-max and percent type IIb MHC was relatively low (r = 0.50; P > 0.05). These data support the hypothesis that an age-related increase in diaphragmatic V-max occurs; however, factors in addition to type IIb MHC are involved in regulating diaphragmatic V-max. Interestingly, although aging resulted in a decrease in diaphragmatic maximal specific force production, power output at all muscle loads was maintained in the old animals due to the increase in diaphragmatic shortening velocity.