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The accuracy of using the HR/VO2 relation determined in running to predict VO2 from HR in tasks involving static and combined static/dynamic exercise was examined in a group of 8 healthy subjects (age 20-27 years). The HR measured in weight-holding tasks (with static exercise) and weight-carrying tasks (with combined static/dynamic exercise) with weights varying from 4-12 kg was inserted into the linear relation between HR and VO2 in the running task (dynamic exercise). The predicted VO2 was compared with measured value. The conclusions were as follows. It is not accurate to use a simple dynamic task to predict VO2 from a measured HR in static work. The percentage differences vary between 78 and 186%. In combined static-dynamic work a simple dynamic task can be accurately used to predict VO2 from measured HR, while carrying small weights (4, 8 and 10 kg). However, more static work (12 kg) makes the estimations poorer with a significant percentage difference in VO2 of 38%.

The time course was examined of the energy-rich phosphate usage and exerted isometric tetanic force in electrically stimulated rat quadriceps muscle. The maximal rate of energy-rich phosphate usage was calculated from the changes in the intramuscular concentrations of phosphocreatine, lactate, ATP and inosine monophosphate (IMP) and was somewhat higher than those calculated on the basis of exercise in vivo. The IMP concentration increased directly from the onset of the contraction until after about 11 s it remained constant. The increase in the IMP concentration coincided with a decrease in the ATP concentration. The relationship between mechanical output and energy usage was examined in two ways (i) by calculating the ratio time integral of the force (FTI) and the total energy-rich usage (Ptot) and (ii) by calculating the ratio Force (Ft) to the energy flux (dPtot/dt) at a certain time t. Whereas the ratio FTI/Ptot showed a hyperbolic relationship, the ratio Ft/(dPtot/dt) showed a parabolic relationship. From the latter finding and from the results described in the literature it is concluded that the ratio mechanical output/energy-rich phosphate usage depends on the conditions under which exercise is carried out. Recovery under aerobic conditions from a maximal tetanic isometric contraction sustained for 15 s was slow compared to results of experiments in vivo.

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