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1 Faculty of Kinesiology
2 Faculty of Medicine, University of Calgary, Alberta, Canada.
Address correspondence to Russell T. Hepple, PhD, Faculty of Kinesiology, University of Calgary, 2500 University Dr. NW, Calgary, AB, Canada T2N 1N4. E-mail: hepple{at}ucalgary.ca
We used a pump-perfused rat hind-limb preparation to compare young adult (YA: 8–9- month-old), late middle-aged (LMA: 28–29-month-old), and senescent (SEN: 36-month-old) rats at similar rates of convective O2 delivery during a 4-minute contraction bout. We hypothesized that not only would 
O2 and lactate production be reduced, but also that contractile economy would be altered with aging. Peak tension was lower in LMA (42%) and SEN (71%) versus YA. O2 and lactate efflux was progressively lower with increasing age. Estimated adenosine triphosphate per N of force was increased in LMA (35%) and reduced in SEN (31%) versus YA. Myosin heavy chain (MHC) analysis by sodium dodecyl sulphate–polyacrylamide gel electrophoresis showed a lower MHC type IIb and higher MHC type IIa/IIx in SEN versus YA. Therefore, whereas contractile economy is impaired in LMA, it is improved in SEN, and this latter effect may be due in part to reduced type IIb MHC.
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