ABSTRACT
Oxygen uptake (V̇O2) is measured most conveniently across the mouth which permits characterization of pulmonary V̇O2 (pV̇O2) kinetics during transitions in metabolic rate such as following the onset of exercise. However, as pV̇O2 reflects the culmination of muscle V̇O2 (mV̇O2) and V̇O2 from the rest of the body as well as convective O2 transport and changes in lung gas stores (Barstow and Molé, 1987), inferences to processes occurring within specific systems, for example muscle are indirect and interpretations regarding mV̇O2 must be made with caution. Because of transit time delays and the interposition of O2 stores between the sites of O2 exchange and measurement, determination of mV̇O2 directly at the site of capillary-myocyte O2 exchange is likely to be a closer representation of ‘true’ muscle O2 utilization than that made at more remote sites. Therefore, to validate pV̇O2 measurements as a true reflection of muscle metabolism, it is crucial to understand the fidelity with which pV̇O2 dynamics reflect those actually occurring within the muscle. Ultimately, the muscle itself or the mitochondrial matrix needs to be studied directly to justify statements regarding mV̇O2. Whereas it has been possible to determine V̇O2 across the exercising limbs (Grassi et al., 1996; Bangsbo et al., 2000), single muscles (Behnke et al., 2002a; Kindig et al., 2002) and even individual muscle fibres (Kindig et al., 2003), technical limitations have so far precluded in vivo measurements at the mitochondria. It is also pertinent that the invasive steps required to determine mV̇O2 run the risk of altering the V̇O2 response itself. Moreover, when electrically-induced muscle contractions are utilized which is usually necessary to elicit muscle contractions in anaesthetized animals or individual muscles/muscle fibres, the intact cardiovascular responses as well as the physiological fibre recruitment patterns are altered. Thus, whereas measurements made in such preparations can provide useful insights into metabolic control, it must be accepted that a certain ‘ecological validity’ is lost when compared to the intact individual performing voluntary muscular exercise.
