EMG changes in respiratory and skeletal muscles during isometric contraction under normoxic, hypoxemic, or ischemic conditions

Low-Level Laser Therapy Facilitates Postcontraction Recovery with Ischemic Preconditioning

PURPOSE

Despite early development of muscle fatigue, ischemic preconditioning is gaining popularity for strength training combined with low-load resistance exercise. This study investigated the effect of low-level laser (LLL) on postcontraction recovery with ischemic preconditioning.

METHODS

Forty healthy adults (22.9 ± 3.5 yr) were allocated into sham (11 men, 9 women) and LLL (11 men, 9 women) groups. With ischemic preconditioning, they were trained with three bouts of intermittent wrist extension of 40% maximal voluntary contraction (MVC). During the recovery period, the LLL group received LLL (wavelength of 808 nm, 60 J) on the working muscle, whereas the sham group received no sham therapy. MVC, force fluctuations, and discharge variables of motor units (MU) for a trapezoidal contraction were compared between groups at baseline (T0), postcontraction (T1), and after-recovery (T2).

RESULTS

At T2, the LLL group exhibited a higher normalized MVC (T2/T0; 86.22% ± 12.59%) than that of the sham group (71.70% ± 13.56%; P = 0.001). The LLL group had smaller normalized force fluctuations (LLL, 94.76% ± 21.95%; sham, 121.37% ± 29.02%; P = 0.002) with greater normalized electromyography amplitude (LLL, 94.33% ± 14.69%; sham, 73.57% ± 14.94%; P < 0.001) during trapezoidal contraction. In the LLL group, the smaller force fluctuations were associated with lower coefficients of variation of interspike intervals of MUs (LLL, 0.202 ± 0.053; sham, 0.208 ± 0.048; P = 0.004) with higher recruitment thresholds (LLL, 11.61 ± 12.68 %MVC; sham, 10.27 ± 12.73 %MVC; P = 0.003).

CONCLUSIONS

LLL expedites postcontraction recovery with ischemic preconditioning, manifesting as superior force generation capacity and force precision control for activation of MU with a higher recruitment threshold and lower discharge variability.

The effects of hypoxia on muscle deoxygenation and recruitment in the flexor digitorum superficialis during submaximal intermittent handgrip exercise

Background

Decreased oxygenation of muscle may be accentuated during exercise at high altitude. Monitoring the oxygen saturation of muscle (SmO₂) during hand grip exercise using near infrared spectroscopy during acute exposure to hypoxia could provide a model for a test of muscle performance without the competing cardiovascular stresses that occur during a cycle ergometer or treadmill test. The purpose of this study was to examine and compare acute exposure to normobaric hypoxia versus normoxia on deoxygenation and recruitment of the flexor digitorum superficialis (FDS) during submaximal intermittent handgrip exercise (HGE) in healthy adults.

Methods

Twenty subjects (11 M/9 F) performed HGE at 50% of maximum voluntary contraction, with a duty cycle of 2 s:1 s until task failure on two occasions one week apart, randomly assigned to normobaric hypoxia (FiO₂ = 12%) or normoxia (FiO₂ = 21%). Near-infrared spectroscopy monitored SmO₂, oxygenated (O₂Hb), deoxygenated (HHb), and total hemoglobin (tHb) over the FDS. Surface electromyography derived root mean square and mean power frequency of the FDS.

Results

Hypoxic compared to normoxic HGE induced a lower FDS SmO₂ (63.8 ± 2.2 vs. 69.0 ± 1.5, p = 0.001) and both protocols decreased FDS SmO₂ from baseline to task failure. FDS mean power frequency was lower during hypoxic compared to normoxic HGE (64.0 ± 1.4 vs. 68.2 ± 2.0 Hz, p = 0.04) and both decreased mean power frequency from the first contractions to task failure (p = 0.000). Under both hypoxia and normoxia, HHb, tHb and root mean square increased from baseline to task failure whereas O₂Hb decreased and then increased during HGE. Arterial oxygen saturation via pulse oximetry (SpO₂) was lower during hypoxia compared to normoxia conditions (p = 0.000) and heart rate and diastolic blood pressure only demonstrated small increases. Task durations and the tension-time index of HGE did not differ between normoxic and hypoxic trials.

Conclusion

Hypoxic compared to normoxic HGE decreased SmO₂ and induced lower mean power frequency in the FDS, during repetitive hand grip exercise however did not result in differences in task durations or tension-time indices. The fiber type composition of FDS, and high duty cycle and intensity may have contributed greater dependence on anaerobiosis.

Exercise-Induced Hypoxaemia Developed at Sea-Level Influences Responses to Exercise at Moderate Altitude

Purpose

The aim of this study was to investigate the impact of exercise-induced hypoxaemia (EIH) developed at sea-level on exercise responses at moderate acute altitude.

Methods

Twenty three subjects divided in three groups of individuals: highly trained with EIH (n = 7); highly trained without EIH (n = 8) and untrained participants (n = 8) performed two maximal incremental tests at sea-level and at 2,150 m. Haemoglobin O₂ saturation (SpO₂), heart rate, oxygen uptake (VO₂) and several ventilatory parameters were measured continuously during the tests.

Results

EIH athletes had a drop in SpO₂ from 99 ± 0.8% to 91 ± 1.2% from rest to maximal exercise at sea-level, while the other groups did not exhibit a similar decrease. EIH athletes had a greater decrease in VO_2max at altitude compared to non-EIH and untrained groups (-22 ± 7.9%, -16 ± 5.3% and -13 ± 9.4%, respectively). At altitude, non-EIH athletes had a similar drop in SpO₂ as EIH athletes (13 ± 0.8%) but greater than untrained participants (6 ± 1.0%). EIH athletes showed greater decrease in maximal heart rate than non-EIH athletes at altitude (8 ± 3.3 bpm and 5 ± 2.9 bpm, respectively).

Conclusion

EIH athletes demonstrated specific cardiorespiratory response to exercise at moderate altitude compared to non-EIH athletes with a higher decrease in VO_2max certainly due to the lower ventilator and HR_max responses. Thus EIH phenomenon developed at sea-level negatively impact performance and cardiorespiratory responses at acute moderate altitude despite no potentiated O₂ desaturation.

The influence of acute hypoxic exposure on isokinetic muscle force production

To investigated whether an acute hypoxic stimulus affects muscle strength development assessed by isokinetic dynamometry during maximal knee extension. A total of 15 healthy young men participated in this study (61.9 ± 6.1 kg; 1.72 ± 0.08 m; 20.9 ± 2.6 years). We evaluated knee extension and flexion isokinetic dynamometer performance in normoxic and hypoxic conditions. The analyzed parameters, for concentric contraction, were peak torque and total work measured at 1.05 and 5.23 rad/s; and fatigue index measured at 5.23 rad/s. During isokinetic testing, heart rate and oxygen saturation (SpO2) were monitored. Hypoxic conditions (3,600 m) were simulated, via a mixing chamber, with the dilution being constantly controlled by a PO2 probe. Test reproducibility results (test-retest) for all isokinetic knee parameters were classified as moderate to almost perfect (ICC = 0.694 to 0.932). SpO2 was 88.4 ± 3.4% in the hypoxic condition and 97.1 ± 0.7% in the normoxic condition (p = 0.000, effect size = 0.87). Heart rate was not significantly different between normoxic and hypoxic conditions at the end of the test. There were no significant differences in isokinetic variables evaluated for the extensor and flexor muscles at concentric contraction between the normoxic and hypoxic conditions. Our findings indicate that reduced arterial oxygenation per se has no effect on the muscular isokinetic strength of the knee extensors.

Altitude-induced changes in muscle contractile properties

Because of its high energetic demand, skeletal muscle is sensitive to changes in the partial pressure of oxygen. Most human studies on in vivo skeletal muscle function during hypoxia were performed with voluntary contractions. However, skeletal muscle function is not only characterized by voluntary maximal or repeated force- generating capacity, but also by force generated by evoked muscle contractions (i.e., force-frequency properties). This mini-review reports on the effects of acute or prolonged exposure to hypoxia on human skeletal muscle performance and contractile properties. The latter depend on both the amount and type of contractile proteins and the efficiency of the cellular mechanism of excitation-contraction coupling. Observations on humans indicate that hypoxia (during simulated ascent or brief exposure) exerts modest influences on the membrane propagation of the muscle action potentials during voluntary contractions. Overall in humans, in physiological conditions, including that of climbing Mt. Everest, there is extraordinarily little that changes with regard to maximal force-generating capacity. Interestingly, it appears that the adaptations to chronic hypoxia minimize the effects on skeletal muscle dysfunction (i.e., impairment during fatigue resistance exercise and in muscle contractile properties) that may occur during acute hypoxia for some isolated muscle exercises. Only sustained isometric exercise exceeding a certain intensity (30% MVC) and causing substantial and sustained ischemia is not affected by acute hypoxia.

Nervous system function during exercise in hypoxia

Aerobic exercise capacity decreases with exposure to hypoxia. This article focuses on the effects of hypoxia on nervous system function and the potential consequences for the exercising human. Emphasis is put on somatosensory muscle afferents due to their crucial role in the reflex inhibition of muscle activation and in cardiorespiratory reflex control during exercise. We review the evidence of hypoxia influences on muscle afferents and discuss important consequences for exercise performance. Efferent (motor) nerves are less affected at altitude and are thought to stay fairly functional even in severe levels of arterial hypoxemia. Altitude also alters autonomic nervous system functions, which are thought to play an important role in the regulation of cardiac output and ventilation. Finally, the consequences of hypoxia-induced cortical adaptations and dysfunctions are evaluated in terms of neurotransmitter turnover, brain electrical activity, and cortical excitability. Even though the cessation of exercise or the reduction of exercise intensity, when reaching maximum performance, implies reduced motor recruitment by the nervous system, the mechanisms that lead to the de-recruitment of active muscle are still not well understood. In moderate hypoxia, muscle afferents appear to play an important role, whereas in severe hypoxia brain oxygenation may play a more important role.

Skeletal muscle dysfunction in COPD: clinical and laboratory observations

COPD (chronic obstructive pulmonary disease), although primarily a disease of the lungs, exhibits secondary systemic manifestations. The skeletal muscles are of particular interest because their function (or dysfunction) not only influences the symptoms that limit exercise, but may contribute directly to poor exercise performance. Furthermore, skeletal muscle weakness is of great clinical importance in COPD as it is recognized to contribute independently to poor health status, increased healthcare utilization and even mortality. The present review describes the current knowledge of the structural and functional abnormalities of skeletal muscles in COPD and the possible aetiological factors. Increasing knowledge of the molecular pathways of muscle wasting will lead to the development of new therapeutic agents and strategies to combat COPD muscle dysfunction.

Consequences of prolonged total thermoneutral immersion on muscle performance and EMG activity

We hypothesized that the changes in muscle temperature and interstitial pressure during thermoneutral immersion may affect the reflex adaptation of the motor drive during static contraction, assessed by the decrease in median frequency (MF) of electromyogram (EMG) power spectrum. Ten subjects were totally immersed for 6 h at 35 degrees C and repeated maximal voluntary contraction (MVC) and submaximal (60% MVC) leg extensions sustained until exhaustion. In vastus lateralis (VL) and soleus (SOL) muscles, the compound muscle potential evoked by muscle stimulation with single shocks (M-wave) was recorded at rest, and MF of surface EMG was calculated during 60% MVCs. We measured lactic acid and potassium venous blood concentrations and calculated plasma volume changes. Data were compared to those obtained in the same individuals exercising at 35 degrees C under dry conditions where the MF decrease during 60% MVCs was modest (-4 to-5%). During immersion, the rectal temperature remained stable, but the thigh and calf surface temperatures significantly increased. Lactic acid and potassium concentrations did not vary, but plasma volume decreased from the 180th min of immersion. The M-wave did not vary in VL but was prolonged in SOL from the 30th min of immersion. From the 220th min of immersion, the maximal MF decrease was majored in both muscles (-18 to -22%). Thus, compared to the dry condition, total body thermoneutral immersion enhances fatigue-induced EMG changes in leg muscles, perhaps through the activation of warm-sensitive muscle endings and/or the changes in interstitial pressure because of vasodilatation.

Effect of acute severe hypoxia on peripheral fatigue and endurance capacity in healthy humans

We hypothesized that severe hypoxia limits exercise performance via decreased contractility of limb locomotor muscles. Nine male subjects [mean ± SE maximum O2 uptake (V̇o2 max) = 56.5 ± 2.7 ml·kg−1·min−1] cycled at ≥90% V̇o2 max to exhaustion in normoxia [NORM-EXH; inspired O2 fraction (FiO2) = 0.21, arterial O2 saturation (SpO2) = 93 ± 1%] and hypoxia (HYPOX-EXH; FiO2 = 0.13, SpO2 = 76 ± 1%). The subjects also exercised in normoxia for a time equal to that achieved in hypoxia (NORM-CTRL; SpO2 = 96 ± 1%). Quadriceps twitch force, in response to supramaximal single (nonpotentiated and potentiated 1 Hz) and paired magnetic stimuli of the femoral nerve (10–100 Hz), was assessed pre- and at 2.5, 35, and 70 min postexercise. Hypoxia exacerbated exercise-induced peripheral fatigue, as evidenced by a greater decrease in potentiated twitch force in HYPOX-EXH vs. NORM-CTRL (−39 ± 4 vs. −24 ± 3%, P < 0.01). Time to exhaustion was reduced by more than two-thirds in HYPOX-EXH vs. NORM-EXH (4.2 ± 0.5 vs. 13.4 ± 0.8 min, P < 0.01); however, peripheral fatigue was not different in HYPOX-EXH vs. NORM-EXH (−34 ± 4 vs. −39 ± 4%, P > 0.05). Blood lactate concentration and perceptions of limb discomfort were higher throughout HYPOX-EXH vs. NORM-CTRL but were not different at end-exercise in HYPOX-EXH vs. NORM-EXH. We conclude that severe hypoxia exacerbates peripheral fatigue of limb locomotor muscles and that this effect may contribute, in part, to the early termination of exercise.

The effects of short-term hypoxia on motor cortex excitability and neuromuscular activation

The effects of acute hypoxia on motor cortex excitability, force production, and voluntary activation were studied using single- and double-pulse transcranial magnetic stimulation techniques in 14 healthy male subjects. Electrical supramaximal stimulations of the right ulnar nerve were performed, and transcranial magnetic stimulations were delivered to the first dorsal interosseus motor cortex area during short-term hypoxic (HX) and normoxic (NX) condition. M waves, voluntary activation, F waves, resting motor threshold (rMT), recruitment curves (100-140% of rMT), and short-interval intracortical inhibition and intracortical facilitation were measured. Moreover, motor-evoked potentials (MEPs) and cortical silent periods were determined during brief isometric maximum right index finger abductions. Hypoxia was induced by breathing a fraction of inspired oxygen of 12% via a face mask. M waves, voluntary activation, and F waves did not differ between NX and HX. The rMT was significantly lower in HX (55.79 +/- 9.40%) than in NX (57.50 +/- 10.48%) (P < 0.01), whereas MEP recruitment curve, short-interval intracortical inhibition, intracortical facilitation, maximum right index finger abduction, and MEPs were unaffected by HX. In contrast, the cortical silent periods in HX (158.21 +/- 33.96 ms) was significantly shortened compared with NX (169.42 +/- 39.69 ms) (P < 0.05). These data demonstrate that acute hypoxia results in increased cortical excitability and suggest that acute hypoxia alters motor cortical ion-channel function and GABAergic transmission.

Reliability of different blood indices to explore the oxidative stress in response to maximal cycling and static exercises

This study compares the changes in four blood markers of exercise-induced oxidative stress in response to exercise protocols commonly used to explore the global muscle performance at work (maximal incremental cycle) and endurance to fatigue of selected muscles (static handgrip and thumb adduction). Cycling and static exercises allow the muscle to work in aerobic and anaerobic conditions, respectively. Healthy adults performed an incremental cycling exercise until volitional exhaustion and, on separated days, executed infra-maximal static thumb adduction and handgrip until exhaustion. Exercise-induced oxidative stress was assessed by the increased plasma concentration of thiobarbituric acid reactive substances (TBARS), the consumption of plasma reduced ascorbic acid (RAA), and erythrocyte reduced glutathione (GSH) antioxidants, and the changes in the total antioxidant status (TAS) of plasma. Five minutes after the end of the incremental cycling exercise, we measured a peak increase in TBARS level, maximal consumption of GSH and RAA, and a modest but significant decrease in TAS concentration. In response to both static thumb adduction and handgrip, significant variations of TBARS, GSH and RAA occurred but we did not measure any significant change in TAS level throughout the 20-min recovery period of both exercise bouts. The present study shows that only the changes in TBARS, GSH and RAA explore both dynamic and static exercises. In addition, TAS measurement does not seem to represent a reliable and unique tool to explore exercise-induced oxidative stress, at least during isometric efforts that allow the muscle to work under anaerobic condition.

Consequences of prolonged total body immersion in cold water on muscle performance and EMG activity

The consequences of a prolonged total body immersion in cold water on the muscle function have not been documented yet, and they are the object of this French Navy research program. Ten elite divers were totally immerged and stayed immobile during 6 h in cold (18 and 10 degrees C) water. We measured the maximal voluntary leg extension (maximal voluntary contraction, MVC) and evoked compound muscle potential (M wave) in vastus lateralis and soleus muscles at rest, after a submaximal (60% MVC) isometric extension allowing the measurement of the endurance time (Tlim). The power spectrum of surface electromyograms (EMG) was computed during 60% MVCs. MVCs and 60% MVC maneuvers were repeated four times during the immersion. Data were compared with those obtained in a control group studied in dry air condition during a 6-h session. Total body cooling did not affect MVC nor Tlim. The M wave duration increased in the coolest muscle (soleus), but only at 10 degrees C at rest. There were no further fatigue-induced M wave alterations in both muscles. During 60% the MVCs, a time-dependant increase in the leftward shift of the EMG spectrum occurred at the two temperatures. These EMG changes were absent in the control group of subjects studied in dry air. The plasma lactate concentration was elevated throughout the 18 and mostly the 10 degrees C immersion conditions. Throughout the 18 degrees C immersion study, the resting potassium level did not significantly vary, whereas at 10 degrees C, a significant potassium increase occurred soon and persisted throughout the study. Thus, total body immersion in cold water did not affect the global contractile properties of leg muscles during static efforts but elicited significant alterations in electromyographic events which may be related to the variations of interstitial fluid composition.

Respiratory muscle strength may explain hypoxia-induced decrease in vital capacity

PURPOSE

High altitude exposure has consistently been reported to decrease forced vital capacity (FVC), but the mechanisms accounting for this observation remain incompletely understood. We investigated the possible contribution of a hypoxia-related decrease in respiratory muscle strength.

METHODS

Maximal inspiratory and expiratory pressures (MIP and MEP), sniff nasal inspiratory pressure (SNIP), FVC, peak expiratory flow rate (PEF), and forced expiratory volume in 1 s (FEV1) were measured in 15 healthy subjects before and after 1, 6, and 12 h of exposure to an equivalent altitude of 4267 m in a hypobaric chamber.

RESULTS

Hypoxia was associated with a progressive decrease in FVC (5.59 +/- 0.24 to 5.24 +/- 0.26 L, mean +/- SEM, P < 0.001), MIP (130 +/- 10 to 114 +/- 8 cm H2O, P < 0.01), MEP (201 +/- 12 to 171 +/- 11 cm H2O, P < 0.001), and SNIP (125 +/- 7 to 98 +/- 7 cm H2O, P < 0.001). MIP, MEP, and SNIP were strongly correlated to FVC (r ranging from 0.77 to 0.92). FEV1 didn't change, and PEF increased less than predicted by the reduction in air density (11-20% of sea-level value compared with 32% predicted).

CONCLUSION

We conclude that a decrease in respiratory muscle strength may contribute to the decrease in FVC observed at high altitude.

Effect of acute hyperoxia during exercise on quadriceps electrical activity in active COPD patients

AIMS

This study investigated whether acute hyperoxia improves electrical muscle activity in active chronic obstructive pulmonary disease (COPD) patients with mild hypoxemia (rest PaO(2) = 9.1 +/- 0.4 kPa).

METHODS

Two identical incremental exercise tests were performed by nine patients while breathing either air or 30% oxygen. Pulmonary gas exchanges, venous concentrations of lactate and pyruvate, and the electromyographic signal of the quadriceps muscle (vastus lateralis and vastus medialis) were sampled each minute.

RESULTS

Peak working capacity increased significantly in hyperoxia (94.4 +/- 5.2W) compared with normoxia (85.4 +/- 5.8W, P < 0.01). During hyperoxic exercise and for a given work load, oxygen uptake was increased (P < 0.001) and ventilation decreased (P < 0.05). Lactate concentration was significantly decreased (P < 0.01) at isowork level and during recovery (respectively - 26% and at least - 15%). In the quadriceps muscle, M-wave amplitude (P < 0.05), root mean square (P < 0.01) and root mean square/oxygen uptake ratio (P < 0.001) were significantly increased during hyperoxic exercise compared with room air. Although median frequency values did not differ between conditions, the median frequency was significantly decreased for higher exercise intensity in hyperoxic condition. These modifications reflected better aerobic metabolism, later emergence of muscle fatigue, and greater muscle excitability and activation for the same level of exercise under hyperoxic condition.

CONCLUSION

These data suggest that the acute addition of oxygen in active COPD patients improves their muscle electrical activity during dynamic exercise. Hypoxemia-induced skeletal muscle dysfunction most probably acts through mechanisms based on oxygen availability.

Increased diaphragmatic strength and tolerance to fatigue after bilateral lung transplantation: an electromyographic study

We evaluated the diaphragmatic function of seven patients with severe chronic respiratory failure before and after a bilateral lung transplantation (BLT), with follow-up at one year of pulmonary function tests, maximal inspiratory mouth pressure (MIP) and surface diaphragmatic electromyogram (Edi). The patients were asked to sustain target inspiratory pressures at -15, -30, and -50 cmH(2)O. We measured the endurance time (Tlim) to sustain inspiratory efforts and the power spectrum density function of Edi at each inspiratory maneuver. The Edi power spectra was analysed in terms of median frequency (MF), total power (TP) and energies in high-and low-frequency bands (EL and EH). Before BLT, a defect of the diaphragmatic function was evident: MIP was 62+/-7% of the predicted value and the Tlim measured at each inspiratory effort was very short ( 13+/-1 s, 10+/-1 s and 8+/-1 s at pressures of -15, -30, and -50 cmH(2)O, respectively). One month after BLT, the Tlim began to increase at all target inspiratory pressures and at 6 months MIP recovered to normal values. One month after BLT, there was a significant decrease in TP measured at the beginning of each inspiratory efforts and also an increase in the concomitant MF value. BLT markedly accentuated the maximal variations of TP, MF and low-frequency Edi energy. Some hypotheses are raised to explain this dramatic improvement in diaphragmatic function after BLT.

Influence of chronic hypoxemia on peripheral muscle function and oxidative stress in humans

Transient re-oxygenation of humans suffering from chronic obstructive pulmonary disease (COPD) allows the assessment of the consequences of chronic hypoxemia on peripheral muscle and metabolism apart from the effects of de-conditioning. The subjects performed maximal voluntary contractions (MVC) of flexor digitorum and vastus lateralis muscles and sustained infra-maximal contractions. COPD patients repeated the whole challenge during a 50-min oxygen breathing period and after recovery to baseline hypoxemia. We measured the compound evoked muscle mass action potential (M-wave) and the medium frequency (MF) of surface electromyography (EMG) power spectrum. Blood lactate (LA) and potassium (K+), erythrocyte-reduced glutathione (GSH), and plasma thiobarbituric acid reactive substances (TBARS) were also measured. Compared with a control group, COPD patients had lower MVCs, an attenuated decrease in MF during exercise, lower resting level of GSH, no posthandgrip TBARS increase and no GSH consumption. Reoxygenation (1) increased MVCs, (2) accentuated the MF decline and (3) elicited a posthandgrip TBARS increase and GSH consumption. Thus, we conclude that chronic hypoxemia exerts specific muscular effects: a reduced force production, an attenuated 'muscle wisdom', and the suppression of the exercise oxidative stress.

Effects of exhaustive incremental treadmill exercise on diaphragm and quadriceps motor potentials evoked by transcranial magnetic stimulation

It is unknown whether changes in corticomotor excitability follow exercise in healthy humans. We hypothesized that a fall in the diaphragm and quadriceps motor-evoked potential (MEP) amplitude elicited by transcranial magnetic stimulation of the motor cortex would occur after an incremental exercise task. In 11 healthy subjects, we measured transdiaphragmatic pressure and isometric quadriceps tension in response to supramaximal peripheral magnetic nerve stimulation. MEPs were recorded from these muscles in response to transcranial magnetic stimulation. After baseline measurements, subjects performed a period of submaximal exercise (gentle walking). Measurements were repeated 5 and 20 min after this. The subjects then exercised on a treadmill with an incremental protocol to exhaustion. Transcranial magnetic stimulation was performed at baseline and at 5, 20, 40, and 60 min after exhaustive exercise, and force measurements were obtained at baseline, 20 min, and 60 min. Mean exercise duration was 18 +/- 4 min, and mean maximum heart rate was 172 +/- 10 beats/min. Twitch transdiaphragmatic pressure and twitch isometric quadriceps tension were not different from baseline after exercise, but a significant decrease was observed in diaphragm MEP amplitude 5 and 20 min after exercise (60 +/- 38 and 45 +/- 24%, respectively, of baseline, P = 0.0001). At the same times, the mean quadriceps MEPs were 59 +/- 39 and 74 +/- 32% of baseline (P < 0.0001 and P < 0.01, respectively). Studies using paired stimuli confirmed a likely intracortical mechanism for this depression. Our data confirm significant depression of both diaphragm and quadriceps MEPs after incremental treadmill exercise.

Changes in neuromuscular function after training by functional electrical stimulation

We examined whether the neuromuscular function of rectus femoris (RF) and flexor digitorum brevis (FDB) in humans was modified after a 6-week training period of functional electrical stimulation (FES), and whether any effects persisted at the end of a 6-week post-FES recovery period. In both the stimulated and contralateral nonstimulated muscles, we recorded the muscle force, surface electromyogram, and M wave, and also measured the root mean square (RMS) and the median frequency (MF) during static contraction sustained until exhaustion at 60% of maximal voluntary contraction (MVC). FES was performed with symmetric biphasic pulses, with a ramp modulation of both the stimulation frequency and pulse duration. No changes in MCV and endurance time to exhaustion occurred in nonstimulated muscles, whereas a significant MVC increase occurred immediately after FES in RF (+14 +/- 5%) and FDB (+13 +/- 5%), these effects persisting 6 weeks after the end of FES. In FDB, FES also elicited a significant increase in endurance time to exhaustion (+18 +/- 7%). The M-wave characteristics never varied after FES, but a marked attenuation occurred in the MF decrease and the RMS increase measured at endurance time to sustained 60% MVC, especially in FDB, which contains the higher proportion of type II fibers. These data indicate that FES improves muscle function and elicits changes in central muscle activation. The benefits of FES were greater in FDB, which is highly fatigable, and persisted for at least a 6-week period.

Electrophysiologic changes during exercise testing in patients with chronic obstructive pulmonary disease

To determine whether skeletal muscle is involved in the exercise limitation of chronic obstructive pulmonary disease (COPD), we investigated electrical adaptations in muscle during incremental cycling exercise testing. Changes in quadriceps activity were compared using surface electromyography (SEMG) and motor point stimulation in ten COPD patients and ten healthy subjects. Patients showed significantly lower exercise capacity, and M-wave duration was increased from exercise onset (P < 0.05) with a parallel decrease in amplitude (P < 0.05). The SEMG power spectrum median frequency was always higher (P < 0.04) in patients and its decline was earlier (P < 0.01). The ratio of the root mean square of the SEMG to oxygen uptake was decreased (P < 0.001) during exercise in patients, although it remained constant in controls. Electromyographic parameters were significantly more involved in the exercise limitation than ventilatory factors. Thus, modified electrical activity in muscle appeared in COPD patients from exercise onset, indicating that skeletal muscle function is clearly implicated in the exercise intolerance of these patients.

Interactions between endogenous nitric oxide and hypoxemia in activation of group IV muscle afferents

It has previously been shown that both hypoxemia and nitric oxide (NO) synthase blockade depress the activation of group IV muscle afferents after muscle stimulation (MS). In the present study, we questioned whether hypoxemia exerts a specific inhibitory influence, independently from its effects on endogenous NO formation. This hypothesis was tested in two groups of anesthetized rabbits in which we examined the effects of hypoxemia, and then of subsequent NO synthase blockade by N(G)-nitro-L-arginine methyl ester (L-NAME), and vice versa. In each protocol, group IV afferent activity was recorded from the resting tibialis anterior muscle and after 3-min periods of MS that elicited a significant decrease in muscle force. NO synthase blockade in normoxemia suppressed the group IV afferent response to MS, and hypoxemia alone significantly reduced the post-MS activation of these nerve afferents (+18% vs. +28% in normoxemia). In hypoxemic rabbits, further NO synthase blockade abolished the post-MS activation of group IV afferents. Moreover, when hypoxemia followed the NO synthase blockade, MS significantly reduced the discharge of group IV afferents (-28%). Thus, while these muscle afferents are activated after fatiguing muscle contractions when the endogenous NO production is present, they are deactivated by hypoxemia when NO production is blocked. We conclude that endogenous NO production and hypoxemia exert opposite effects on the activation of the group IV afferents. Our data anticipate the neuromuscular side effects of treatments using exogenous NO or drugs acting on endogenous NO production.

Morphological, histochemical, and interstitial pressure changes in the tibialis anterior muscle before and after aortofemoral bypass in patients with peripheral arterial occlusive disease

BACKGROUND

Morphological and electrophysiological studies of ischemic muscles in peripheral arterial disease disclosed evidence of denervation and fibre atrophy. The purpose of the present study is to describe morphological changes in ischemic muscles before and after reperfusion surgery in patients with peripheral occlusive arterial disease, and to provide an insight into the effect of reperfusion on the histochemistry of the reperfused muscle.

METHODS

Muscle biopsies were obtained from the tibialis anterior of 9 patients with chronic peripheral arterial occlusive disease of the lower extremities, before and after aortofemoral bypass, in order to evaluate the extent and type of muscle fibre changes during ischemia and after revascularization. Fibre type content and muscle fibre areas were quantified using standard histological and histochemical methods and morphometric analysis. Each patient underwent concentric needle electromyography, nerve conduction velocity studies, and interstitial pressure measurements.

RESULTS

Preoperatively all patients showed muscle fibre atrophy of both types, type II fibre area being more affected. The mean fibre cross sectional area of type I was 3,745 microm2 and of type II 4,654 microm2. Fibre-type grouping, great variation in fibre size and angular fibres were indicative of chronic dennervation-reinnervation, in the absence of any clinical evidence of a neuropathic process. Seven days after the reperfusion the areas of both fibre types were even more reduced, being 3,086 microm2 for type I and 4,009 microm2 for type II, the proportion of type I fibres, and the interstitial pressure of tibialis anterior were increased.

CONCLUSIONS

The findings suggest that chronic ischemia of the leg muscles causes compensatory histochemical changes in muscle fibres resulting from muscle hypoxia, and chronic dennervation-reinnervation changes, resulting possibly from ischemic neuropathy. Reperfusion seems to bring the oxidative capacity of the previously ischemic muscle closer to normal.

Effects of chronic hypoxemia on the afferent nerve activities from skeletal muscle

An acute reduction of the oxygen supply to contracting muscles not only affects their metabolism but also modifies their sensorimotor control through changes in afferent discharge of the group I and group III-IV nerve fibers, the latter playing a pivotal role in the protective mechanisms against muscle fatigue. The effects of chronic hypoxemia on the muscle sensitivity are totally unknown. In the present study, group I fibers (mechanosensory afferents) and group III-IV fibers (mechanosensory and chemosensory afferents) from the anterior tibial muscle were recorded in normoxemic and chronic hypoxemic rats. Hypoxemic rats breathed for 45 d a gas mixture containing 9.5 to 10% O(2) in N(2). The data were compared with those obtained in normoxemic animals of the same age. To activate the different muscle afferents, we used different test agents, including electrically induced fatigue (EIF), KCl, lactic acid injections, as well as tendon vibrations. The conduction velocity of all nerve fibers was significantly (p < 0.01) higher in hypoxemic rats than in the normoxemic group. Chronic hypoxemia significantly depressed the response of the group III-IV muscle afferents to KCl injections and even abolished their response to lactic acid and EIF. However, the response to tendon vibrations of the group I afferents was similar in hypoxemic and normoxemic rats. These results suggest that chronic hypoxemia markedly alters the chemosensitivity of the group III-IV muscle afferents, which may explain the higher fatigability of hypoxemic subjects.

Effects of endogenous nitric oxide in activation of group IV muscle afferents

Based on previous observations that acute hypoxemia, which enhances nitric oxide (NO) production, depresses the activation of group IV afferents after repetitive low-frequency muscle stimulation (MS), we hypothesized that endogenous NO modulates the response of these nerve endings to their specific stimuli. The present study in rabbits examined the effects of a blocker of NO synthase (NG-nitro-L-arginine methyl ester L, L-NAME) and an exogenous NO donor (3-morpholinosydnonimine, SIN-1) on the group IV afferents of tibialis anterior. The efficacy of the two test agents was judged by their effects on systemic blood pressure. L-NAME markedly elevated (+46%) the resting discharge rate of group IV afferents but abolished their activation after repetitive MS. After SIN-1 injection, there was a transient decrease in blood pressure, which correlated well with a lowered resting discharge rate of group IV afferents. SIN-1 infusion caused a stable reduction of blood pressure; the resting afferent nerve discharge rate began first to decrease but then recovered control mean values. SIN-1 infusion abolished the activation of group IV afferents after MS. This study indicates that endogenous NO production in a resting or contracting muscle attenuates the baseline activity of group IV muscle afferents and their activation after repetitive muscle contractions.

Effects of acute hypoxemia on force and surface EMG during sustained handgrip

Data on the consequences of acute hypoxemia on the strength of contraction are often contradictory. In healthy subjects, we tested the effects of hypoxemia (PaO(2) = 56 mmHg), maintained for a 30-min period, on static handgrip elicited by voluntary effort or direct electrical muscle stimulation, in order to separate the consequences of hypoxemia on central or peripheral factors, respectively. Force was measured during maximal voluntary contractions (MVCs), 60% MVCs sustained until exhaustion, and 1-min periods of electrical muscle stimulation at 60 HZ. The evoked compound muscle action potential (M wave) was recorded in resting muscle and after each period of 60-HZ stimulation or sustained 60% MVC. Power spectrum analysis of surface electromyogram (EMG) was performed during sustained 60% MVC. Compared to normoxemia, acute hypoxemia lowered MVC (-12%, P < 0.01) but enhanced (+38%, P < 0.01) the peak force elicited by electrical muscle stimulation. In resting muscle, hypoxemia had no influence on the M-wave amplitude but lengthened the neuromuscular transmission time(+740 micros, P < 0.05). Hypoxemia did not alter the M wave measured after 60 HZ stimulation and 60% MVC. During sustained 60% MVC, hypoxemia markedly depressed the EMG changes, abolishing the leftward shift of power spectra. These data show that acute hypoxemia reduces MVC through depression of the central drive, whereas it improves the peripheral muscle response to electrical stimulation. In addition, hypoxemia reduces the recruitment of slow firing motor unit, which are highly oxygen-dependent. This could constitute an adaptative muscle response to a reduced oxygen supply.

Influence of oxygen supply on activation of group IV muscle afferents after low-frequency muscle stimulation

Anaerobic muscle metabolism and local release of inflammatory mediators play key roles in the mechanism of postfatigue-induced activation of group IV muscle afferents. The present study focused on activation of these muscle afferents after a 3-min period of low-frequency muscle stimulation (LFMS) in different conditions of muscle oxygenation, such as occur in patients with respiratory insufficiency and subjects living at high altitude. In anesthetized rabbits, spontaneous activity of group IV afferents (conduction velocity = 1.52 +/- 0.13 m.s(-1)) from the tibialis anterior muscle was recorded at rest (baseline) and then after LFMS under normoxic (PaO(2) = 113 mmHg), hyperoxic (PaO(2) = 186 mmHg), or hypoxic (PaO(2) = 35 mmHg) conditions. The maximal force decay at the end of LFMS did not differ significantly with respect to conditions of muscle oxygenation. Compared with normoxia, hypoxia significantly increased the baseline activity of group IV muscle afferents, whereas no effect was noted when hypoxia followed a period of hyperoxia. LFMS-induced activation of group IV afferents occurred in all circumstances, except when hypoxia was first tested. The activation of group IV muscle afferents after LFMS was markedly reduced when hypoxia followed normoxia (+14% versus +27%) or hyperoxia (+55% versus +144%), whereas it was accentuated when hyperoxia followed hypoxia (+25% versus +8%). We concluded that the sensorimotor control of skeletal muscles may be altered during acute hypoxia but facilitated after reoxygenation.

Functional consequences of acute and chronic hypoxia on respiratory and skeletal muscles in mammals

Reduced oxygen supply to contracting muscles affects not only the metabolic paths but also modifies the gain of sensorimotor reflex loops initiated from the activation of specialized nervous endings that detect the changes in muscle metabolism and membrane outflow of potassium. Large differences are found between skeletal muscles and the diaphragm with respect to their sensitivity to acute or chronic hypoxia. The diaphragm tolerates much more hypoxemia than do skeletal muscles, namely those constituted by a large proportion of slow twitch oxidative fibers. Acute hypoxemia or ischemia accentuates the inhibitory influences exerted by the afferent paths from muscle metaboreceptors. This adaptative response may be responsible for enhanced muscle wisdom phenomenon during fatiguing contractions under hypoxic conditions. Prolonged and severe chronic hypoxemia markedly reduces muscle force generation by skeletal muscles and their endurance to fatigue. Restoration of normal PaO2 levels in these individuals immediately improves maximal muscle performance, perhaps through more efficient excitation-contraction coupling. Recent data on the consequences of hypoxia on muscle metabolism and the associated changes in sensorimotor control strongly suggest that local acidosis cannot entirely explain all electromyogram changes found during and after fatiguing exercise.

Resistive loaded breathing changes the motor drive to arm and leg muscles in man

Breathing through inspiratory or expiratory resistive loads activates respiratory afferents. In healthy individuals, we explored the recruitment of motor units in arm (adductor pollicis, AP and biceps branchialis, BB) and leg (vastus lateralis, VL) muscle groups during voluntary contractions sustained at 80% of maximal force. Quantitative EMG analysis consisted of measurement of energies in high (EH) and low (EL) frequency bands. EH and EL changes were measured at constant time, i.e. 10 and 20 s after the onset of plateau contraction. The resistive load was added to the inspiratory or the expiratory circuit for 10-min periods. Its value was high but not enough to induce changes in blood gases and blood pressure. Compared to muscle contractions performed during non-loaded breathing periods, inspiratory loading did not affect BB and VL contractions, whereas it induced significant changes in AP contraction, characterized by enhanced variations in EL value measured at 10 s. Expiratory loading affected solely the VP contraction. Then, EH decreased at 10 and 20 s while it increased always when VP contractions were executed during non-loaded breathing. Expiratory loading elevated the functional residual capacity (FRC), but the load-induced changes in VL contraction persisted when subjects adjusted their FRC to the control level. These data suggest that respiratory afferents influence the skeleto-motor drive. Thus, viscero-somatic reflex may be present in patients with severe obstructive pulmonary disease.

Maximal force and endurance to fatigue of respiratory and skeletal muscles in chronic hypoxemic patients: the effects of oxygen breathing

The consequences of chronic hypoxemia on maximal force and endurance time to sustained 80% of maximal isometric contraction of two skeletal muscles (adductor pollicis and vastus lateralis) and the diaphragm were studied in patients with chronic obstructive pulmonary disease (COPD). Compared to normal subjects, COPD patients have lower values of Fmax for the two skeletal muscle groups and Pmax (diaphragm). Endurance time was also shorter for the diaphragm and adductor pollicis. Chronic hypoxemia was associated with an accentuation in integrated EMG changes in both low and high frequency bands for adductor pollicis and diaphragm. Inhalation of oxygen enriched gas mixture for a 15-min period increased markedly Fmax and PImax values, prolonged the endurance time to sustained thumb adduction, and reduced the EMG changes in the low frequency band for adductor pollicis. The present observations provide evidence for altered maximal performances of skeletal muscles in chronic hypoxemic patients and also point out the virtues of oxygen breathing in these patients.