Patients with mitochondrial myopathies (MM) or myophosphorylase deficiency (McArdle's disease, McA) show impaired capacity for O(2) extraction, low maximal aerobic power, and reduced exercise tolerance. Non-invasive tools are needed to quantify the metabolic impairment. Six patients with MM, 6 with McA, 25 with symptoms of metabolic myopathy but negative biopsy (patient-controls, P-CTRL) and 20 controls (CTRL) underwent an incremental cycloergometric test. Pulmonary O(2) uptake (VO(2)) and vastus lateralis oxygenation indices (by near-infrared spectroscopy, NIRS) were determined. Concentration changes of deoxygenated hemoglobin and myoglobin (Delta[deoxy(Hb + Mb)]) were considered an index of O(2) extraction. Delta[deoxy(Hb + Mb)] peak (percent limb ischemia) was lower in MM (25.3 +/- 12.0%) and McA (18.7 +/- 7.3) than in P-CTRL (62.4 +/- 3.9) and CTRL (71.3 +/- 3.9) subjects. VO(2) peak and Delta[deoxy(Hb + Mb)] peak were linearly related (r(2) = 0.83). In these patients, NIRS is a tool to detect and quantify non-invasively the metabolic impairment, which may be useful in the follow-up of patients and in the assessment of therapies and interventions.

During exercises with relatively small muscle masses, limitations to exercise performance by the cardiovascular system should be significantly reduced, allowing one to fully-test the "oxidative potential" of the investigated muscles. Ten elderly males (E, 77.8 +/- 2.9 years [x +/- SD]) and eight young controls (Y, 26.6 +/- 3.0) underwent incremental exercises to voluntary exhaustion on a dynamic leg-extension (dominant limb) machine (knee-extension, KE) and on a cycloergometer (CYCLO). During KE the load was increased every 3 min to loads corresponding to 20, 40 and 60% of the force of one-repetition maximum (1RM). The following variables were determined (vastus lateralis muscle): concentration changes of deoxygenated haemoglobin and myoglobin (Delta[deoxy(Hb + Mb)]) by near-infrared spectroscopy (NIRS), expressed as percentage of the maximal value obtained during transient limb ischemia, and taken as an index of O2 extraction; root mean square (RMS) and median power frequency (MDF) by electromyography. The total lifted load during KE and peak workload during CYCLO were lower in E versus Y (620.4 +/- 321.9 kg vs. 1347.4 +/- 458.7; 113.5 +/- 23.9 W vs. 224.3 +/- 41.0, respectively). During CYCLO Delta[deoxy(Hb + Mb)] peak (i.e. the value determined at exhaustion) was lower in E (44.5 +/- 17.7%) versus Y (67.1 +/- 22.9), whereas during KE Delta[deoxy(Hb + Mb)] peak was higher in E (56.8 +/- 20.9%) versus Y (38.6 +/- 15.8). "Thresholds", that is abrupt increases in RMS slopes, were detected in Y but not in E, suggesting less recruitment or a preferential atrophy of type 2 fibers in the elderly. These findings, associated with the preserved capacity of O2 extraction, suggest a shift towards oxidative metabolism in skeletal muscles of 78 year-old subjects, which could preserve, at least in part, their capacity to carry out exercise.

The tolerable duration (tlim) of high intensity exercise is well described by a hyperbolic function of work‐rate (WR), where only a fixed amount of work (W′) can be performed above the WR asymptote (CP). We have shown that supra‐CP WRs are not tolerated at the tlim of constant WR (CW) exercise (Coats et al. JAP 95: 483–490, 2003), and so aimed to determine whether this was also the case for incremental‐ramp (IR) exercise. Twelve males performed repeated CW cycle ergometry to estimate CP, W′ and determine VO2max (breath‐by‐breath measurement). Subjects then completed IR and CW exercise to tlim, immediately after which they attempted to sustain a lower WR (110% CP). The amount of supra‐CP work done at tlim was lower (p < 0.01) during IR (15.5 ± 3.6 kJ) than CW and also W′ (18.8 ± 3.4 kJ and 19.8 ± 2.9 kJ, respectively; N.S.), with VO2max (4.2 ± 0.6 l/min) evidence of maximum effort in all tests. tlim at 110% CP was greater (p < 0.01) following IR (range 23–297 s) than CW (6–168 s); the former manifest with no fall in VO2 (4.1 ± 0.7 l/min) despite an 84 ± 15 W reduction in WR and presumably time for it to do so. If the metabolic equivalent of CP is the same during IR and CW, these data suggest that tlim in IR is achieved at VO2max without W′ depletion. We conclude therefore that the ‘profile’ by which intolerance is achieved significantly influences the tlim of subsequent exercise by increasing the rate of W′ depletion and/or accumulation of fatigue‐inducing metabolites.