Single-leg Knee-extension Exercise Research Articles

Muscle metabolic responses to exercise above and below the “critical power” assessed using31P-MRS

We tested the hypothesis that the asymptote of the hyperbolic relationship between work rate and time to exhaustion during muscular exercise, the "critical power" (CP), represents the highest constant work rate that can be sustained without a progressive loss of homeostasis [as assessed using (31)P magnetic resonance spectroscopy (MRS) measurements of muscle metabolites]. Six healthy male subjects initially completed single-leg knee-extension exercise at three to four different constant work rates to the limit of tolerance (range 3-18 min) for estimation of the CP (mean +/- SD, 20 +/- 2 W). Subsequently, the subjects exercised at work rates 10% below CP (<CP) for 20 min and 10% above CP (>CP) for as long as possible, while the metabolic responses in the contracting quadriceps muscle, i.e., phosphorylcreatine concentration ([PCr]), P(i) concentration ([P(i)]), and pH, were estimated using (31)P-MRS. All subjects completed 20 min of <CP exercise without duress, whereas the limit of tolerance during >CP exercise was 14.7 +/- 7.1 min. During <CP exercise, stable values for [PCr], [P(i)], and pH were attained within 3 min after the onset of exercise, and there were no further significant changes in these variables (end-exercise values = 68 +/- 11% of baseline [PCr], 314 +/- 216% of baseline [P(i)], and pH 7.01 +/- 0.03). During >CP exercise, however, [PCr] continued to fall to the point of exhaustion and [P(i)] and pH changed precipitously to values that are typically observed at the termination of high-intensity exhaustive exercise (end-exercise values = 26 +/- 16% of baseline [PCr], 564 +/- 167% of baseline [P(i)], and pH 6.87 +/- 0.10, all P < 0.05 vs. <CP exercise). These data support the hypothesis that the CP represents the highest constant work rate that can be sustained without a progressive depletion of muscle high-energy phosphates and a rapid accumulation of metabolites (i.e., H(+) concentration and [P(i)]), which have been associated with the fatigue process.

EPR Spectroscopic Detection of Semiquinone Radicals in Human Skeletal Muscle; Focus on Exercise and Age

1824 PURPOSE: To examine the potential effects of exercise and chronological age on free radical generation in human skeletal muscle using electron paramagnetic resonance (EPR) spectroscopy. METHODS: Following ethical approval, twelve males were assigned to either a YOUNG [n = 6, aged 26 (mean) ± 6 (SD) years] or OLD (n = 6, aged 71 ± 6 years) group and performed graded, single-leg knee-extension (KE) exercise to determine maximum work rate (WRMAX). On a separate occasion, needle biopsies were obtained from the vastus lateralis (1% lidocaine) of a rested leg (REST) and from the contra-lateral leg after 2 mins of KE exercise at 50% WRMAX followed by 3 mins at 100% WRMAX (EXERCISE). Samples were immediately frozen in liquid nitrogen and placed in a quartz dewar cell following minimal tissue fracturing prior to EPR spectroscopy at 77° K (Varian 104). Data were analyzed with a two-way mixed ANOVA and expressed in arbitrary units (AU) normalized for tissue mass. RESULTS: Samples exhibited a prominent isotropic EPR signal at g = 2.004 with a weaker broadline component detected at g = 1.960, consistent with a semiquinone radical (CoQ-·) and iron-sulfur center respectively. Molecular identification could not be further clarified with electron-nuclear double resonance spectroscopy due to insufficient signal intensity. Despite the absence of any age effect, exercise (pooled YOUNG + OLD) increased the signal intensity of the g = 2.004 component (P< 0.05 vs. REST): YOUNG REST: 3.760 ± 1.602 vs. YOUNG EXERCISE: 6.897 ± 2.906 AU/g and OLD REST: 2.566 ± 0.939 vs. OLD EXERCISE: 4.873 ± 3.271 AU/g whereas changes in the g = 1.960 signal were unremarkable. Importantly, exercise did not alter the line- width of either signal. CONCLUSIONS: These data are the first to provide direct evidence for increased mitochondrial CoQ-· formation in contracting human skeletal muscle that preliminary indications suggest is age-independent. The extracellular formation of lipid-derived radicals previously detected in peripheral blood using this model (Bailey, D.M., et al. J. Appl. Physiol. 94: 1714–1718, 2003) may be initiated by mitochondrial superoxide subsequent to an exercise-induced increase in the univalent reduction rate of coenzyme Q. Support: NIH HL17731 and the Stein Institute for Research on Aging.

THE EFFECT OF HYPOXIA ON O2 UPTAKE KINETICS, MEAN BLOOD VELOCITY AND MUSCLE DEOXYGENATION WITH SINGLE-LEG KNEE-EXTENSION EXERCISE.

The effect of hypoxic breathing on phase 2 pulmonary O2 uptake (V˙O2 p) kinetics, leg mean blood velocity (MBV) and deoxygenation of the vastus laterials muscle was examined during constant-load single-leg knee-extension exercise. Seven subjects (24 ± 4 yr; mean ± SD) performed two transitions from passive to moderate exercise (21W) under normoxic (N) and hypoxic (H) (PETO2 = 55 mmHg) conditions. Breath-by-breath V˙O2 p was measured by mass spectrometry and volume turbine, beat-by-beat femoral artery MBV by Doppler ultrasound (Vingmed, CFM 750), and continuous oxy-(HbO2), deoxy-(HHb) and total hemoglobin/myoglobin (Hbtot) by near-infrared spectroscopy (NIRS; Hamamatsu NIRO-300). V˙O2 p data were filtered and averaged to 5 s bins, MBV data to 2 s bins (1 contraction cycle) and HHb-NIRS data to 5 s bins. V˙O2 p MBV and HHb data were fit with a single-component exponential model. V˙O2 p was not affected by hypoxic breathing throughout the protocol; the phase 2 τ at exercise onset showed inter-individual variability and was similar in H (38 ± 27 s) and N (33 ± 29 s). MBV and blood flow were similar in H and N during passive and steady state exercise, as was the on-transient τ for MBV (H: 48 ± 27 s, N: 35 ± 28 s). NIRS showed a greater (p < 0.05) muscle deoxygenation during passive exercise in H, and although the increase in HHb with exercise and the steady-state HHb were not different, five of seven subjects had greater HHb in H (by 40%). The time prior to the onset of muscle deoxygenation (H: 11 ± 6 s, N: 12 ± 3 s) and HHb-NIRS τ (H: 20 ± 11 s, N: 14 ± 4 s) were not affected by H. Thus, with hypoxic breathing the V˙O2 p requirement of knee-extension exercise was met primarily by a greater deoxygenation, not increased blood flow. Supported in part by NSERC, Canada

EFFECT OF HYPERPNEA ON LEG BLOOD FLOW

We have previously reported that leg VO2 and blood flow decreased with respiratory muscle loading and increased with respiratory unloading during maximal exercise but not during submaximal exercise in highly fit cyclists. We hypothesized that the normal hyperpnea of heavy exercise and its associated work of breathing is sufficient by itself to elicit sympatho-excitatory reflex effects from the respiratory muscles and decrease leg blood flow in subjects of moderate fitness levels. Five healthy subjects (age: 27 ± 2 yrs, VO2maz: 45.9 ± 1.4 ml/kg/min) performed eight randomized upright, single leg knee extension exercise (KE) at a work rate of 14.2 ± 0.8% of VO2maz. Subjects mimicked ventilation (VE) > 80% VEmaz (83.4 ± 5.3%) for all trials while hypocapnia was prevented. Breathing frequency, tidal volume, and inspiratory time was maintained constant under both normal (N; esophageal pressure (Pes) 19.1 ± 2.4 cmH2O) and inspiratory loaded breathing trials (via mesh screens) (L; Pes:123.8 ± 3.5% N). Leg blood velocity (MBV; via Doppler sonography) and blood pressure were measured continuously. KE exercise increased MBV approximately 2x resting levels. MBV was not significantly different, however between KE (23.0 ± 3.4 cm/s), KE+N (23.5 ± 3.6 cm/s) and KE+L (22.5 ± 3.7 cm/s). No significant difference was noted in heart rate between conditions although VO2 increased by 36 ± 3% with increased VE. In summary, we postulate that even with ventilatory work of 80% VEmaz under normal or loaded conditions, the respiratory system makes insufficient demands for VO2 and cardiac output to require any cardiovascular adjustment and is too small to activate sympathetic vasoconstrictor efferent output in moderately fit subjects. Support from American Heart Association, Parker B. Francis Foundation

ARTERIAL DIAMETER AND MEAN BLOOD VELOCITY MEASUREMENTS

The purpose of this study was to determine: 1) inter-observer difference in arterial diameter (AD) measures; 2) femoral AD changes during exercise; and 3) variability of mean blood velocity (MBV) measurements. Seven subjects (23 ± 2 yr) performed 4–6 repeats on different days of constant-load single-leg knee extension exercise transitions from unloaded exercise to a work rate eliciting 80% of VO2 peak, followed by loadless recovery. Doppler ultrasound was used to image the femoral artery during one trial for each subject, and to measure MBV on each trial. AD measurements were made independently by two investigators at rest and every 2 min throughout the test. MBV data were averaged over 2 s intervals (1 contraction cycle) and 10 s averages were taken during the first and last min of each phase of the trial to test for day-to-day variability. Mean AD measurements across all subjects and conditions were highly correlated (r = 0.93, r2 = 0.86) and were not significantly different between the two observers (8.2 ± 1.0 mm vs 8.4 ± 1.2 mm, mean ± SD). AD did not change significantly from rest values at any time point. The coefficient of variation (CV) for MBV day-to-day variability at any one time point ranged from 7.1 to 38.5% and the mean CV across all subjects and time points was 20.7%. Variability was largest during unloaded exercise, but similar during heavy exercise and recovery. These data demonstrate that femoral artery diameter does not change significantly from rest through to heavy exercise and any rest-exercise difference is similar in magnitude to that of inter-observer differences. Furthermore, because of the variability in femoral artery MBV, we recommend the use of repeated trials to obtain an accurate MBV. Supported in part by NSERC, Canada

Creatine supplementation and age influence muscle metabolism during exercise.

Young [n = 5, 30 +/- 5 (SD) yr] and middle-aged (n = 4, 58 +/- 4 yr) men and women performed single-leg knee-extension exercise inside a whole body magnetic resonance system. Two trials were performed 7 days apart and consisted of two 2-min bouts and a third bout continued to exhaustion, all separated by 3 min of recovery. 31P spectra were used to determine pH and relative concentrations of Pi, phosphocreatine (PCr), and beta-ATP every 10 s. The subjects consumed 0.3 g . kg-1 . day-1 of a placebo (trial 1) or creatine (trial 2) for 5 days before each trial. During the placebo trial, the middle-aged group had a lower resting PCr compared with the young group (35.0 +/- 5.2 vs. 39.5 +/- 5.1 mmol/kg, P < 0.05) and a lower mean initial PCr resynthesis rate (18.1 +/- 3.5 vs. 23.2 +/- 6.0 mmol . kg-1 . min-1, P < 0.05). After creatine supplementation, resting PCr increased 15% (P < 0.05) in the young group and 30% (P < 0.05) in the middle-aged group to 45.7 +/- 7.5 vs. 45.7 +/- 5.5 mmol/kg, respectively. Mean initial PCr resynthesis rate also increased in the middle-aged group (P < 0.05) to a level not different from the young group (24.3 +/- 3.8 vs. 24.2 +/- 3.2 mmol . kg-1 . min-1). Time to exhaustion was increased in both groups combined after creatine supplementation (118 +/- 34 vs. 154 +/- 70 s, P < 0.05). In conclusion, creatine supplementation has a greater effect on PCr availability and resynthesis rate in middle-aged compared with younger persons.

Muscle acetyl group availability is a major determinant of oxygen deficit in humans during submaximal exercise.

The delay in skeletal muscle mitochondrial ATP production at the onset of exercise is thought to be a function of a limited oxygen supply. The delay, termed the oxygen deficit, can be quantified by assessing the above baseline oxygen consumption during the first few minutes of recovery from exercise. During submaximal exercise, the oxygen deficit is reflected by the extent of muscle phosphocreatine (PCr) breakdown. In the present study, nine male subjects performed 8 min of submaximal, single leg knee extension exercise after saline (Control) and dichloroacetate (DCA) infusion on two separate occasions. Administration of DCA increased resting skeletal muscle pyruvate dehydrogenase complex activation status threefold (Control = 0.4 +/- 0.1 vs. DCA = 1.3 +/- 0.1 mmol acetyl-CoA.min-1.kg wet muscle-1 at 37 degrees C, P < 0.01) and elevated acetylcarnitine concentration fivefold (Control = 2.2 +/- 0.5 vs. DCA = 10.9 +/- 1.2 mmol/kg dry mass, P < 0.01). During exercise, PCr degradation was reduced by approximately 50% after DCA (Control = 33.2 +/- 7.1 vs. DCA = 18.4 +/- 7.1 mmol/kg dry mass, P < 0.05). It would appear, therefore, that in humans acetyl group availability is a major determinant of the rate of increase in mitochondrial respiration at the onset of exercise and hence the oxygen deficit.

Substrate availability limits human skeletal muscle oxidative ATP regeneration at the onset of ischemic exercise.

We have demonstrated previously that dichloroacetate can attenuate skeletal muscle fatigue by up to 35% in a canine model of peripheral ischemia (Timmons, J.A., S.M. Poucher, D. Constantin-Teodosiu, V. Worrall, I.A. Macdonald, and P.L. Greenhaff. 1996. J. Clin. Invest. 97:879-883). This was thought to be a consequence of dichloroacetate increasing acetyl group availability early during contraction. In this study we characterized the metabolic effects of dichloroacetate in a human model of peripheral muscle ischemia. On two separate occasions (control-saline or dichloroacetate infusion), nine subjects performed 8 min of single-leg knee extension exercise at an intensity aimed at achieving volitional exhaustion in approximately 8 min. During exercise each subject's lower limbs were exposed to 50 mmHg of positive pressure, which reduces blood flow by approximately 20%. Dichloroacetate increased resting muscle pyruvate dehydrogenase complex activation status by threefold and elevated acetylcarnitine concentration by fivefold. After 3 min of exercise, phosphocreatine degradation and lactate accumulation were both reduced by approximately 50% after dichloroacetate pretreatment, when compared with control conditions. However, after 8 min of exercise no differences existed between treatments. Therefore, it would appear that dichloroacetate can delay the accumulation of metabolites which lead to the development of skeletal muscle fatigue during ischemia but does not alter the metabolic profile when a maximal effort is approached.

A Single Bout of Concentric Resistance Exercise Increases Basal Metabolic Rate 48 Hours After Exercise in Healthy 59-77-year-old Men

It has been shown that basal metabolic rate (BMR) decreases with age. The extent to which some of the decrease can be reversed by exercise in older men and women is unclear. Resistance exercise has been shown to significantly increase muscle mass in older individuals, and because muscle is a highly active metabolic tissue there is potential to increase BMR as a secondary outcome to the training adaptation. Twelve healthy men aged 59-77 years performed single-leg knee extension exercise (right and left leg) and bench press lifts (16 sets, 10 reps/set with timed recovery between sets) at 75% of the individual's 3RM. Subjects only performed the concentric phase of the lift. BMR was measured on two separate occasions, once after a nonexercise control period and again 48 hrs after a bout of resistance exercise. BMR was significantly increased (p < .006) 48 hrs after exercise (EX) compared to control (CON) (284.0 +/- 34.0 vs 274.9 +/- 34.0 kJ/hr, respectively). Calculated over a 24-hour period, the energy expenditure corresponded to 1570 +/- 193 and 1627 +/- 193 kcal/24 hr (p < .0002) for the CON and EX measures, respectively. VO2 (L/min) was higher (p < .0002) 48 hrs after the EX bout compared to 48 hrs post-CON (0.232 +/- 0.03 vs 0.225 +/- 0.03 L/min, respectively). We conclude that in healthy 59-77-year-old men, an acute bout of resistance exercise causes a sustained increase in BMR that persists for up to 48 hours after exercise.