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In this study we characterize the impact of aging on the spontaneous running performance of the Tgα q *44 mice (transgenic murine model of chronic heart failure) as compared to the wild-type FVB mice. In 166 mice we have recorded the following parameters of their physical activities in the running wheels: the total distance covered during the experiment (D sum ), the maximal distance covered in single-effort (D max ), mean time spent on running per 24 h (T mean ), mean running speed (ν mean ), the maximum instantaneous speed of run (ν max ) and the number of efforts (i.e. the number of running events undertaken by the mice) during 54 days, in four age groups ~4, ~10, ~12 and ≥12.5 months of age. The level of spontaneous running performance of the FVB mice remained essentially unchanged, but a strong impact of aging in the Tgα q *44 mice on their running performance was found. Namely, the D sum , D max , T mean and ν mean in the Tgα q *44 mice at the age of ≥12.5 months decreased by ~50%, when compared to its level corresponding level at the age of ~4 months, with far lesser effect of aging on their V max . Surprisingly, the number of attempts to perform running by the Tgαq*44 mice at the age of 4 - 12 months remained essentially unchanged. This suggests that the exercise intolerance of the aging heart failure (HF) mice seems to be more dependent on deterioration of heart and muscles function linked to HF than on a possible ageing-related impairment of the 'willngness' to initiate running, generated by the central nervous system.

In this study we examined the relationship between fast myosin heavy chain (MyHC2) content in the vastus lateralis and the rate of oxygen uptake (VO2) and heart rate (HR) increase during an incremental exercise in 38, young, healthy men. Prior to the exercise test, muscle biopsies were taken in order to evaluate the MyHC composition. It was found that during cycling performed below the lactate threshold (LT), a positive relationship between MyHC2 and the intercept of the oxygen uptake and power output (VO2-PO) relationship existed (r=0.49, P=0.002), despite no correlation between MyHC2 and the slope value of the VO2-PO relationship (r= -0.18, P=0.29). During cycling performed above the LT, MyHC2 correlated positively with the magnitude of the nonlinearity in the VO2-PO relationship; i.e. with the accumulated VO2'excess' (r=0.44, P=0.006) and peak VO2'excess' (r=0.44, P=0.006), as well as with the slope of the HR-PO relationship (r=0.49, P=0.002). We have concluded that a greater MyHC2 content in the vastus lateralis is accompanied by a higher oxygen cost of cycling during exercise performed below the LT. This seems to be related to the higher energy cost of the non-cross-bridge activities in the muscles possessing a greater proportion of MyHC2 content. In the case of heavy-intensity exercise, a higher MyHC2 content in the vastus lateralis is accompanied by greater non-linearity in the VO2-PO relationship, as well as a steeper increase in HR in the function of an increase of PO. This relationship can be explained by greater disturbances in metabolic stability in type II muscle fibres during exercise, resulting in a decrease of muscle mechanical efficiency and greater increase of heart rate at a given power output. Therefore, MyHC composition has an impact on the oxygen cost of cycling both below and above the LT.