Noninvasive estimation of oxygen consumption in human calf muscle through combined NMR measurements of ASL perfusion and T2 oxymetry.

The objective of this work was to demonstrate the feasibility of measuring muscle O2 consumption (V˙O2) noninvasively with a combination of functional nuclear magnetic resonance (NMR) imaging methods, and to verify that changes in muscle V˙O2 can be detected with a temporal resolution compatible with physiological investigation and patient ease. T2-based oxymetry of arterial and venous blood was combined with the arterial-spin labeling (ASL)-based determination of muscle perfusion. These measurements were performed on 8 healthy volunteers under normoxic and hypoxic conditions in order to assess the sensitivity of measurements over a range of saturation values. Blood samples were drawn simultaneously and used to titrate blood T2 measurements versus hemoglobin O2 saturation (%HbO2) in vitro. The in vitro calibration curve of blood T2 fitted very well with the %HbO2 (r(2): 0.95). The in vivo venous T2 measurements agreed well with the in vitro measurements (intraclass correlation coefficient 0.82, 95% confidence interval 0.61-0.91). Oxygen extraction at rest decreased in the calf muscles subjected to hypoxia (p = 0.031). The combination of unaltered muscle perfusion and pinched arteriovenous O2 difference (p = 0.038) pointed towards a reduced calf muscle V˙O2 during transient hypoxia (p = 0.018). The results of this pilot study confirmed that muscle O2 extraction and V˙O2 can be estimated noninvasively using a combination of functional NMR techniques. Further studies are needed to confirm the usefulness in a larger sample of volunteers and patients.

Effects of coil characteristics for femoral nerve magnetic stimulation.

The aim of this study was to compare the efficiency of two coils used for femoral nerve magnetic stimulation and to compare them with electrical stimulation in inducing maximal response of the quadriceps. The mechanical and electromyographic (EMG) responses were dependent on the coil used. The 45-mm double coil showed greater efficiency to elicit a maximal quadriceps response, which was similar to electrical stimulation.

Comparison of electrical and magnetic stimulations to assess quadriceps muscle function.

This study aimed to 1) compare electrical and magnetic stimulations for quadriceps muscle function assessment, and 2) ascertain whether the ratios of the second twitch elicited by supramaximal electrical and magnetic femoral nerve stimulation at 10 and 100 Hz (T2(10:100)) and the total twitch force elicited by the same types of stimulations (Fpaired(10:100)) are equivalent to the standard low- to high-frequency force ratio associated with submaximal electrical tetanic stimulations (Ftet(10:100)). Quadriceps force and vastus lateralis EMG were recorded at rest (n = 21 subjects), immediately after, and 30 min after a 30-min downhill run (n = 10) when 1) supramaximal electrical nerve stimulation (ENS), 2) magnetic nerve stimulation (MNS) and 3) submaximal electrical muscle stimulation (EMS) were delivered in random order at 1 (single stimulation), 10, and 100 Hz (paired stimulations). Ten- and 100-Hz 500-ms tetani were also evoked with EMS to determine Ftet(10:100). Before exercise, contractile properties with single and paired stimulations were similar for ENS and MNS (all intraclass correlation coefficients k > 0.90), but smaller for EMS (P < 0.001). M-wave characteristics were also similar for ENS and MNS (all k > 0.90). After exercise, changes in all parameters did not differ between methods. With fatigue, the changes in Ftet(10:100) were inconsistently correlated with the changes in T2(10:100) (r(2) = 0.24-0.73, P = 0.002-0.15) but better correlated with the changes in Fpaired(10:100) (immediately after exercise: r(2) = 0.80-0.83, P < 0.001; 30 min after exercise: r(2) = 0.46-0.82, P = 0.001-0.03). We conclude that ENS and MNS provide similar quadriceps muscle function assessment, while Fpaired(10:100) is a better index than T2(10:100) of low- to high-frequency fatigue of the quadriceps in vivo.

Effects of acute salbutamol inhalation on quadriceps force and fatigability.

INTRODUCTION: Oral beta2-agonist administration improves muscle function in persons without asthma. We performed a double-blind, randomized, controlled crossover study to assess whether acute inhaled salbutamol administration improves muscle strength and fatigability in healthy moderately trained subjects. METHODS: Quadriceps muscle strength was measured during maximal voluntary contraction (MVC) and femoral nerve magnetic stimulation (potentiated single twitch, TwQpeak) before and after (i) a maximal incremental cycling test (n = 10) and (ii) 50 maximal isometric one-leg extensions (n = 9). Each exercise test was performed on three occasions, after salbutamol (200 and 800 microg) or placebo inhalation. RESULTS: Before exercise, treatments had no significant effect on MVC [(placebo) 597 +/- 146 N vs (200 microg) 629 +/- 151 N vs (800 microg) 610 +/- 148 N] and TwQpeak [(placebo) 215 +/- 83 N vs (200 microg) 227 +/- 69 N vs (800 microg) 250 +/- 84 N]. Maximal power during cycling and maximal force during leg extensions did not differ between treatments. Treatments had no effect on MVC and TwQpeak reductions at 30 min [MVC: (placebo) -8 +/- 9% vs (200 microg) -9 +/- 7% vs (800 microg) -8 +/- 5%; TwQpeak: (placebo) -29 +/- 13% vs (200 microg) -23 +/- 15% vs (800 microg) -20 +/- 8%] and 60 min [MVC: (placebo) -12 +/- 17% vs (200 microg) -6 +/- 9% vs (800 microg) -8 +/- 8%; TwQpeak: (placebo) -20 +/- 21% vs (200 microg) -19 +/- 23% vs (800 microg) -8 +/- 7%] after cycling. Similarly, reductions in MVC and TwQpeak were not significantly different between treatments at 30 [MVC: (placebo) -11 +/- 9% vs (200 microg) -12 +/- 7% vs (800 microg) -8+/- 16%; TwQpeak: (placebo) -37 +/- 12% vs (200 microg) -33 +/- 20% vs (800 microg) -32 +/- 16%] and 60 min [MVC: (placebo) -10 +/- 11% vs (200microg) -11 +/- 6% vs (800 microg) -8 +/- 20%; TwQpeak: (placebo) -30 +/- 11% vs (200 microg) -28 +/- 24% vs (800 microg) -27 +/- 15%] after leg extensions. Treatments did not modify maximal voluntary activation at any time of the protocol. CONCLUSION: Acute therapeutic or supratherapeutic doses of inhaled salbutamol have no effect on quadriceps strength, fatigue, and recovery in men without asthma.

Original Research: Central and peripheral quadriceps fatigue in young and middle-aged untrained and endurance-trained men: A comparative study.

This study aimed to compare quadriceps function (i.e. strength, endurance, central, and peripheral fatigue) of young (Young-UnTr) and middle-aged (MidAge-UnTr) untrained men and young endurance-trained men (Young-Tr). Twenty-four male subjects (eight Young-UnTr (26 ± 4 yr), eight Young-Tr (29 ± 3 yr), and eight MidAge-UnTr (56 ± 4 yr) performed a maximal cycling test to assess their fitness level. On a separate visit, subjects performed sets of 10 intermittent (5-s on/5-s off) isometric contractions starting at 10% maximum voluntary contraction (MVC), with 10% MVC increments from one set to another until exhaustion. Electrophysiological and mechanical (e.g. twitch) evoked responses elicited with magnetic femoral nerve stimulation in the relaxed muscle and during MVC (i.e. estimation of voluntary activation using the interpolated twitch technique) were measured at baseline and after each set to assess peripheral and central fatigue, respectively. Endurance (= total number of contractions) was also evaluated. Young-UnTr exhibited larger reductions in evoked quadriceps mechanical responses than MidAge-UnTr and Young-Tr after identical standardized muscle loading (e.g. after the 50% MVC set, reduction in single potentiated twitch was -36 ± 9%, -21±16%, and -2 ± 4%, respectively). At both 50% MVC set and exhaustion, MidAge-UnTr exhibited similar reduction in maximal voluntary activation and displayed similar endurance compared to Young-UnTr. Young-Tr exhibited greater endurance than Young-UnTr without significant changes in maximal voluntary activation throughout the test. This study provides robust comparative data regarding the influence of chronic exposure to endurance training and middle-aged on central and peripheral quadriceps fatigability and endurance. Endurance-trained subjects showed smaller level of peripheral fatigue and displayed no significant central fatigue, even at exhaustion and despite greater endurance performance. Our findings also demonstrate that men in the sixth decade exhibit significant alterations in quadriceps function typically observed in much older subjects. These data emphasize the need for developing normative data for both central and peripheral quadriceps fatigability.

Skeletal muscle contractility and fatigability in adults with cystic fibrosis.

BACKGROUND: Recent discovery of cystic fibrosis transmembrane conductance regulator expression in human skeletal muscle suggests that CF patients may have intrinsic skeletal muscle abnormalities potentially leading to functional impairments. The aim of the present study was to determine whether CF patients with mild to moderate lung disease have altered skeletal muscle contractility and greater muscle fatigability compared to healthy controls. METHODS: Thirty adults (15 CF and 15 controls) performed a quadriceps neuromuscular evaluation using single and paired femoral nerve magnetic stimulations. Electromyographic and mechanical parameters during voluntary and magnetically-evoked contractions were recorded at rest, during and after a fatiguing isometric task. Quadriceps cross-sectional area was determined by magnetic resonance imaging. RESULTS: Some indexes of muscle contractility tended to be reduced at rest in CF compared to controls (e.g., mechanical response to doublets stimulation at 100 Hz: 74±30 Nm vs. 97±28 Nm, P=0.06) but all tendencies disappeared when expressed relative to quadriceps cross-sectional area (P>0.5 for all parameters). CF and controls had similar alterations in muscle contractility with fatigue, similar endurance and post exercise recovery. CONCLUSIONS: We found similar skeletal muscle endurance and fatigability in CF adults and controls and only trends for reduced muscle strength in CF which disappeared when normalized to muscle cross-sectional area. These results indicate small quantitative (reduced muscle mass) rather than qualitative (intrinsic skeletal muscle abnormalities) muscle alterations in CF with mild to moderate lung disease.

Quadriceps function assessment using an incremental test and magnetic neurostimulation: a reliability study.

We investigated the reliability of a test assessing quadriceps strength, endurance and fatigability in a single session. We used femoral nerve magnetic stimulation (FMNS) to distinguish central and peripheral factors of neuromuscular fatigue. We used a progressive incremental loading with multiple assessments to limit the influence of subject's cooperation and motivation. Twenty healthy subjects (10 men and 10 women) performed the test on two different days. Maximal voluntary strength and evoked quadriceps responses via FMNS were measured before, after each set of 10 submaximal isometric contractions (5-s on/5-s off; starting at 10% of maximal voluntary strength with 10% increments), immediately and 30min after task failure. The test induced progressive peripheral (41±13% reduction in single twitch at task failure) and central fatigue (3±7% reduction in voluntary activation at task failure). Good inter-day reliability was found for the total number of submaximal contractions achieved (i.e. endurance index: ICC=0.83), for reductions in maximal voluntary strength (ICC>0.81) and evoked muscular responses (i.e. fatigue index: ICC>0.85). Significant sex-differences were also detected. This test shows good reliability for strength, endurance and fatigability assessments. Further studies should be conducted to evaluate its feasibility and reliability in patients.

Effect of salbutamol on neuromuscular function in endurance athletes.

PURPOSE: The potential ergogenic effects of therapeutic inhaled salbutamol doses in endurance athletes have been controversially discussed for decades. We hypothesized that salbutamol inhalation may increase peripheral muscle contractility, reduce fatigability, and improve force recovery after a localized exercise in endurance athletes. METHODS: Eleven healthy, nonasthmatic male athletes with high aerobic capacities were recruited to be compared in a double-blinded, randomized crossover study of two dose levels of salbutamol (200 and 800 µg) and a placebo administered by inhalation before a quadriceps fatigue test. Subjects performed an incremental exercise protocol consisting in sets of 10 intermittent isometric contractions starting at 20% of maximum voluntary contraction (MVC) with 10% MVC increment until exhaustion. Femoral nerve magnetic stimulation was used during and after MVC to evaluate neuromuscular fatigue after each set, at task failure, and after 10 and 30 min of recovery. RESULTS: Initial MVC and evoked muscular responses were not modified with salbutamol (P > 0.05). The total number of submaximal contractions until task failure significantly differed between treatments (placebo, 72 ± 7; 200 µg, 78 ± 8; and 800 µg, 82 ± 7; P < 0.01). MVC and evoked muscular responses were similarly reduced with all treatments during the fatiguing task (all P > 0.05). Voluntary activation was unaffected by the fatiguing task and treatments (P > 0.05). CONCLUSION: Supratherapeutic inhaled doses of ß2-agonists increased quadriceps endurance during an incremental and localized fatiguing task in healthy endurance-trained athletes without significant effect on neuromuscular fatigue. Further studies are needed to clarify the underlying mechanisms.

Benefits of neuromuscular electrical stimulation prior to endurance training in patients with cystic fibrosis and severe pulmonary dysfunction.

BACKGROUND: We investigated the effect of neuromuscular electrical stimulation (NMES) training prior to endurance training in patients with cystic fibrosis (CF) and severe pulmonary obstruction. METHODS: Fourteen patients with CF (FEV(1) = 35% ± 11% predicted) were prospectively randomized to either a 6-week NMES training program (n = 7) or a 6-week control period (n = 7) both followed by ergocycle (ERGO) training (8 weeks) (NMES + ERGO and control + ERGO groups). Measurements were pulmonary function, mid-thigh circumference, quadriceps strength, 6-min walk distance, maximal exercise capacity on a cycloergometer, plasma biomarkers, insulin resistance (homeostasis model assessment indexes), and quality of life (CF questionnaire for adults and teenagers > 14 years of age [CFQ14 + ], Baseline Dyspnea Index-Transition Dyspnea Index). RESULTS: NMES + ERGO training greatly improved mid-thigh circumference ( + 2.6 ± 0.9 cm vs - 0.4 ± 1.4 cm), quadriceps strength ( + 6 ± 5 kg vs - 2 ± 2 kg), and BMI ( + 0.6 ± 0.6 kg/m(2) vs - 0.5 ± 0.7 kg/m(2) ) compared with control + ERGO training ( P < .05). No differences between groups were found in exercise-induced changes in 6-min walk distance and maximal exercise capacity. However, dyspnea after the 6-minute walk test, the fasting glucose/insulin ratio (calculated as an index of insulin resistance), and physical function and health perception domains of the CFQ14 + improved after NMES + ERGO training compared with control + ERGO training ( P < .05). Significant correlations were found between changes in mid-thigh circumference and muscle strength, ventilation requirements during exercise, insulin sensibility, and the physical function section of CFQ14 + ( P < .05). CONCLUSIONS: NMES training performed prior to endurance training is useful for strengthening peripheral muscles, which in turn may augment gains in body weight and quality of life, further reductions in ventilation requirements during exercise, and retard insulin resistance in patients with CF with severe pulmonary obstruction.

Fat tissue alters quadriceps response to femoral nerve magnetic stimulation.

OBJECTIVE: We investigate the influence that inguinal fat has on maximal quadriceps response to femoral nerve magnetic stimulation. METHODS: Vastus lateralis M-wave amplitude and quadriceps twitch peak torque were measured (i) in lean and overweight subjects and (ii) when slices of pig fat ranging from 2 to 18 mm in thickness were placed between the coil and the nerve. RESULTS: In overweight group, the maximal response could not be elicited when intensity was ≤90% and ≤85% of maximal power for twitch torque and M-wave amplitude, respectively. The maximal response was obtained at 80% of maximal power in the lean group. Negative correlations between relative twitch torque changes and inguinal thickness were observed. Fat thickness altered quadriceps response at 14 mm and 8 mm in the lean and overweight groups, respectively. CONCLUSIONS: The capacity of femoral nerve magnetic stimulation to deliver supramaximal stimulation is altered when fat thickness below the coil increases. SIGNIFICANCE: Special caution should be taken by clinicians when overweight or obese subjects are tested using femoral nerve magnetic stimulation.