Effects of prolonged hypobaric hypoxia on human ... - Semantic Scholar

Clinical Science (2000) 98, 329–337 (Printed in Great Britain)

Effects of prolonged hypobaric hypoxia on human skeletal muscle function and electromyographic events F. CAQUELARD*, H. BURNET*, F. TAGLIARINI*, E. CAUCHY†, J. P. RICHALET† and Y. JAMMES* *Laboratoire de Physiopathologie Respiratoire (EA 2201), Institut Jean Roche, Faculte! de Me! decine, Universite! de la Me! diterrane! e, 13916 Marseille, France, and †ARPE – Laboratoire des Re! ponses Cellulaires et Fonctionnelles a' l’Hypoxie, Faculte! de Me! decine de Bobigny, Universite! Paris XIII, Paris, France

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This study tested the hypothesis that a prolonged decrease in arterial oxygen pressure in resting or contracting skeletal muscles alters their ability to develop force through an impairment of energy-dependent metabolic processes and also through an alteration of electrophysiological events. The experiment was conducted during a 32-day simulated ascent of Mt. Everest (8848 m altitude) (Everest III Comex ‘97), which also allowed testing of the effects of re-oxygenation on muscle function. Maximal voluntary contractions (MVCs) of the flexor digitorum, and static handgrips sustained at 60 % of MVC, were performed by eight subjects before the ascent (control), then during the stays at simulated altitudes of 5000 m, 6000 m and 7000 m, and finally 1 day after the return to 0 m. The evoked muscle compound action potential (M-wave) was recorded at rest and during the manoeuvres at 60 % of MVC. The changes in median frequency of electromyographic (EMG) power spectra were also studied during the contraction at 60 % of MVC. In four individuals, transient re-oxygenation during the ascent allowed us to test the reversibility of hypoxia-induced MVC and M-wave changes. At rest, a significant decrease in Mwave amplitude was noted at 5000 m. This effect was associated with a prolonged M-wave conduction time at 6000 m and an increased M-wave duration at 7000 m, and persisted after the return to 0 m. Re-oxygenation did not modify the changes in M-wave characteristics. A significant decrease in MVC was measured only during the ascent (k10 to k24 %) in the nondominant forearm of subjects who underwent re-oxygenation ; this intervention slightly improved muscle strength at 6000 m and 7000 m. During the ascent and after the return to 0 m, there was a significant reduction of the median frequency decrease throughout contraction at 60 % of MVC compared with the EMG changes measured before the ascent. It is concluded that prolonged exposure to hypoxia slows the propagation of myopotentials and alters sensorimotor control during sustained effort. Re-oxygenation did not affect the hypoxia-induced EMG changes and had a modest influence on muscle strength.

INTRODUCTION Data published previously indicate that the oxygen supply to skeletal muscles plays a key role in energy-

dependent metabolic processes [1], and some human studies [2–4] suggest that hypoxia also affects the neurophysiological events associated with the development of force during fatiguing efforts. In humans,

Key words : electromyogram, high altitude, hypoxia, M-wave, skeletal muscle. Abbreviations : EMG, electromyographic ; E and E , EMG energies in two separate equal bandwidths of low (60–110 Hz) and high L H (150–200 Hz) frequency respectively ; MF, median frequency ; MVC, maximal voluntary contraction ; M-wave, muscle compound action potential ; PSD, power spectrum density distribution ; RMS, root mean square. Correspondence : Professor Y. Jammes (e-mail jammes.y!jean-roche.univ-mrs.fr).

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acute hypoxia produced by breathing a low-oxygen gas mixture reduces the maximal voluntary contraction (MVC) [2,3,5,6] and shortens work time [7], whereas the effects of chronic hypoxia on MVC are still debated. Reduced MVC was barely or not observed in normal subjects exposed for several days to hypobaric hypoxia [6,8–11]. By contrast, we showed [12] in hypoxic patients with severe respiratory insufficiency that the maximal strength developed by arm and leg muscles was less than in an age-matched group of normal individuals. Few studies can be found on the consequences of hypoxia on neuromuscular events in skeletal muscles that might provide an insight into altered mechanical events. Indeed, the impaired muscle force generation under hypoxic condition may result from the limitation of energy-dependent metabolic processes, as well as from the reduced activation of excitable structures involved in muscle contraction. Hypoxia may affect neuromuscular transmission and\or the muscle membrane itself, thereby slowing down the propagation of myopotentials. It may also alter the recruitment of motor units, which depends physiologically on both the central drive to motoneurons and the feedback mechanisms elicited by the activation of muscle afferents [13]. The computation of the median frequency (MF) of the electromyographic (EMG) power spectrum density function has been used in previous studies [13–16] to approach the changes in motor unit recruitment during contractions. These studies reported that the normal response was a progressive decline in MF during fatiguing contractions, attributed to the reduced recruitment of high-frequency motor units. This interpretation of variations in MF is valid on condition that there are no simultaneous alterations in the evoked muscle compound action potential (M-wave), because the decline in MF may also result from a slowed propagation of myopotentials in the sarcolemma, measured by a decreased M-wave amplitude, a prolonged M-wave duration and, maximally, a lengthened conduction time, which may also indicate an altered neuromuscular transmission [13]. Data on hypoxia-induced M-wave alterations in humans are sparse and contradictory. The M-wave amplitude was decreased by acute normobaric hypoxia [17], but not by prolonged exposure to high altitude [9]. The consequences of hypoxia on the EMG changes during fatiguing contractions are less debated. Previous studies [2,3] have showed that acute normobaric hypoxia reduced the integrated EMG variations during sustained static contractions and also markedly attenuated the decline in MF. An inhibitory influence on the integrated EMG changes has also been found during chronic exposure to hypobaric hypoxia [10], but in that protocol the maximal altitude was limited to 5050 m. In a primary interpretation, it was suggested that the altered muscle function under hypoxic conditions results from impaired oxidative processes. However, there were # 2000 The Biochemical Society and the Medical Research Society

marked discrepancies between the observations in isolated mitochondria, which showed that oxidative phosphorylation became limited when the partial pressure of O was as low as 0.5 mmHg [18], and those obtained in # working muscles in situ [19] and in isolated muscle fibres [20], which indicated that fatigue occurred under hypoxic conditions, but at a much higher arterial partial pressure of O . If the impairment of oxidative processes was # responsible for at least a part of hypoxia-induced muscle failure, then both force generation and electrophysiological neuromuscular events should be restored after reoxygenation. An improved MVC during re-oxygenation was reported in chronic hypoxic patients [12] and also in healthy subjects at high altitude (5050 m) [21]. However, no data were found in the literature on the benefits of reoxygenation on the hypoxia-induced alterations in neuromuscular transmission and the propagation of myopotentials. During a 32-day simulated ascent of Mount Everest (Everest III Comex ‘97), we analysed in healthy subjects the changes in MVC, M-wave and the EMG power spectrum at rest and during sustained static contractions. This multiple approach allowed us to address three questions. (1) Were there any hypoxia-induced deteriorations in M-wave at rest, as well as in exercising muscle ? (2) Was there any reduced motor drive to contracting muscles during prolonged hypoxia ? This was explored by measuring MVC and analysing the EMG changes during sustained efforts. (3) Was there any benefit of reoxygenation on MVC and M-wave in chronically hypoxic individuals ?

MATERIALS AND METHODS Subjects Eight male subjects (mean age 26 years ; median age 30 years) participated in the study. All were trained in mountain climbing and athletics. Procedures involved in the study and possible risks were explained to the subjects, whose written consent was obtained. The whole protocol was approved by the Ethics Committee of the University of Marseille.

Protocol The study was part of a larger investigation examining a number of physiological responses during prolonged exposure to simulated high altitude up to 8000 m, with a brief ascent to 8848 m. No physiological measurements could be performed at 8848 m and our team was not involved in the explorations at 8000 m. The subjects lived at sea level in hypobaric chambers at the Experimental Hyperbaric Center of Comex SA in Marseille. After basal sea-level measurements performed in the open hypobaric

Effects of hypoxia on neuromuscular drive

Table 1

Body weight and arterialized blood gases of the eight subjects taking part in Operation Everest III Comex ‘97

Abbreviations : PaO2, arterial partial pressure of O2 ; PaCO2, arterial partial pressure of CO2 ; pHa, arterial pH. Data were collected on the same days as those on which muscle studies were performed. Values are meanspS.E.M. Significant differences compared with control values at 0 m : *P 0.05 ; **P 0.01 ; ***P 0.001.

Body weight (kg) PaO2 (mmHg) PaCO2 (mmHg) pHa

Control (0 m)

5000 m

6000 m

7000 m

Return to 0 m

74p2 107p3 39p1 7.43p0.01

72p2* 53p1*** 24p1*** 7.47p0.01***

71p2* 49p1*** 20p1*** 7.51p0.01***

70p2** 40p2*** 18p1*** 7.49p0.01***

71p2** 111p2 31p1** 7.42p0.01

chambers, the eight subjects reached Chamonix within 1 day and flew by helicopter from Chamonix to the Vallot Observatory (4350 m) for a 7-day stay, allowing preacclimatization. Then they flew down to Chamonix and reached the Comex Center by car the same day, where they were decompressed in 3 h to reach a simulated altitude of 5000 m. During the progressive decompression the subjects underwent 3-day stays at simulated altitudes of 5000 m, 6000 m, 7000 m and 8000 m ; at the end of the experiment, repetitive brief ascents to a simulated altitudes of 8848 m were performed. Physiological studies on muscular performance were carried out at sea level before the ascent, after 2 days ’ stay at each of 5000 m, 6000 m and 7000 m, and on the day after the 32-day exposure to hypoxia, when the subjects had returned to ‘ sea level ’. In companion investigations, body weight loss during the ascent was measured, along with arterial blood gases. Samples of 100 µl of arterialized capillary blood from the ear were collected 1 h before our interventions began for determination of the arterial partial pressures of O and CO and of arterial pH # # (Chiron Diagnostics model 348). Table 1 shows mean values for body weight and arterial blood gases.

MVC and 60 % of MVC trials The dominant forearm was chosen to compare muscle performance at sea level, at altitude and on return to sea level. However, during exposure to altitude the effects of re-oxygenation on MVC were studied in the nondominant forearm. The device used to maintain the position of the forearm during MVC and to sustain fatiguing contractions has already been described [3,22]. The forearm of the subject was maintained in a prone position in the anatomical device. To help the subject to maintain the pre-set torque level constant at 60 % of MVC, visual feedback was given from an isometric dynamometer connected to the strain gauge transducer (SCAIME-model ZF ; linear from 0 to 100 kg). After practising the task, the subject was able to maintain the requested force within p5 % of the target value during contractions by means of the visual feedback. The subjects were first studied before the simulated ascent. MVC was determined in the dominant and nondominant forearms, and was defined as the highest torque

production achieved in three trials, executed with 60 s intervals. Then the subjects were asked to use their dominant forearm and to sustain static handgrips at 60 % of MVC. They had to maintain contraction for 3 min despite muscle failure in order to maintain the target force determined previously. Endurance time was determined from chart recordings of the torque from the onset of sustained contractions at 60 % of MVC to the first decline in torque. For experiments conducted at simulated altitude, as well as after the return to 0 m, the handgrip was sustained at 60 % of the initial sea-level MVC.

EMG studies These were performed on both forearms before the simulated ascent. The surface EMG signal from the flexor digitorum muscle group was recorded by a pair of Ag\AgCl electrodes (Dantec 13 L20) placed over the belly of the muscle on positions marked on the skin. The inter-electrode distance was 2 cm and the inter-electrode impedance was kept below 2000 Ω by careful skin shaving and abrasion. The EMG signal was amplified (Nihon Kohden : common mode rejection ratio, 90 dB ; input impedance, 100 mΩ ; gain, 1000–5000) with a frequency band ranging from 10 to 3000 Hz, and recorded on a numeric data magnetic tape recorder (TEAC RD-120T) for further analyses. The EMG signal was also fed on to a numeric oscilloscope (Gould DSO 400), which allowed us to average the compound muscle action potentials (Mwave) evoked by direct muscle stimulation with 0.1 mslong rectangular pulses delivered by two electrodes (inter-electrode distance 2 cm) fixed on the cranial part of the muscle. Averaging allowed the separation of the muscle response to electrical stimulation from action potentials occurring during voluntary contractions. A GRASS S88 neurostimulator delivered supramaximal shocks through an isolation unit. The stimulation voltage was adjusted day-to-day to produce the maximal Mwave amplitude. Peak amplitude, duration of the M-wave and conduction time (i.e. the time between the stimulus artifact and peak) were calculated from eight averaged potentials. M-waves were recorded at rest, at the end of 1, 2 and 3 min of the 60 % MVC handgrips, and 1, 5 and 10 min into the recovery period. # 2000 The Biochemical Society and the Medical Research Society

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As in previous studies [2,3,17,22], the EMG analysis was performed in a time and frequency domain by calculating the root mean square (RMS) and the power spectrum density distribution (PSD) respectively. Whatever the simulated altitude, the EMG signal was always recorded during static efforts sustained at 60 % of MVC measured at sea level. The EMG was digitized with a sampling frequency of 1200 Hz using a data acquisition card mounted on a 486 PC computer. Both RMS and PSD analyses were started at the beginning of muscle contraction and covered all of the muscle contraction period. The MF was calculated from averaged power spectra obtained from 128-ms window epochs, overlapping each other by half their length, for a total period of 20 s. PSD analyses also allowed us to calculate the EMG energies in two separate equal bandwidths of low (EL ; 60–110 Hz) and high (EH ; 150–200 Hz) frequency. Indeed, we have already shown [2,3] that changes in MF under acute hypoxaemic conditions may result from independent variations in EH or EL. EMG analyses were performed at the beginning of the contraction (first 20 s), and then for 20-s periods at 1, 2 and 3 min during trials at 60 % of MVC. All changes in RMS, MF, EH and EL during the 60 % MVC manoeuvres were expressed as a percentage of levels measured during the first 20 s of contraction. Potential changes in the baseline MF value during the experiment were examined during the first 20 s of contraction.

titative EMG analyses (RMS, MF, EH and EL) existed among testing days before and during the simulated ascent and on return to sea level, and also before and during re-oxygenation periods. When a statistically significant F ratio was calculated, differences between individual means were tested for significance with Tukey’s post hoc test. A difference was accepted as statistically significant at P 0.05. Data are presented as meanspS.E.M.

RESULTS Hypoxia-induced M-wave changes in resting muscle As shown in Figure 1, a significant decrease in M-wave amplitude was noted in the eight subjects during the stay at a simulated altitude of 5000 m, but no further significant effect was noted during the remainder of the simulated ascent. M-wave conduction time was sig-

Re-oxygenation experiments In four individuals, the consequences on MVC and Mwave of transient re-oxygenation for a 30-min period were studied during each stay at simulated high altitude ; these studies were carried out in the non-dominant forearm in order to avoid any consequences of the persistence of fatigue in the dominant forearm that had exercised 30 min before. The subject inhaled through a mouthpiece a mixture of oxygen and nitrogen, which restored the sea-level inspired partial pressure of O # (149 mmHg). The inspired gas mixture contained 39.8 % O in nitrogen at 5000 m, 46.3 % O at 6000 m and 53.9 % # # O at 7000 m. M-wave recordings at rest and MVC were # measured 15 min after the beginning of re-oxygenation. During the stays at 5000 m, 6000 m and 7000 m, one research worker entered into the hypobaric chamber, where he inhaled an enriched oxygen mixture. He supervised the manoeuvres of re-oxygenation as well as the fixation of the skin electrodes for stimulation and EMG recordings.

Statistical treatment Analysis of variance with repeated measures (one- or two-way) was used to determine whether differences in MVC, endurance time, M-wave characteristics and quan# 2000 The Biochemical Society and the Medical Research Society

Figure 1 M-wave changes before and during the simulated ascent and after return to sea level

Shown are mean values (pS.E.M.) of the amplitude (A), conduction time (B) and duration (C) of the M-wave recorded in the flexor digitorum muscle in eight individuals at rest before the ascent (0 m), during the stays at simulated altitudes of 5000, 6000 and 7000 m, and 24 h after the return to sea level (0 m). Significant differences compared with values at 0 m : *P 0.05 ; **P 0.01.


Figure 3 Hypoxia-induced changes in maximal muscle performance in the eight subjects (dominant forearm)

Figure 2 Effects of re-oxygenation on M-wave characteristics (non-dominant forearm)

Shown are mean values (pS.E.M.) of MVC for static handgrip contraction (A) and endurance time to sustain contraction at 60 % of MVC (B) before (0 m) and during the simulated ascent, and after the return to 0 m. Significant difference compared with values at 0 m : **P 0.01.

Shown are mean values (pS.E.M.) of amplitude (A), conduction time (B) and duration (C) of the M-wave measured in four resting subjects before the ascent (0 m ; normoxia), during the stays at simulated altitudes of 5000, 6000 and 7000 m [hypobaric hypoxia and re-oxygenation (l normoxia)] and after the subjects had returned to 0 m (normoxia). Significant differences compared with values at 0 m : *P 0.05 ; **P 0.01. No significant differences were noted between data obtained under hypoxic and then normoxic (re-oxygenation) conditions at each altitude.

nificantly increased at 6000 m and 7000 m, and M-wave duration was prolonged at 7000 m. The changes in Mwave characteristics persisted when the subjects returned to 0 m. In the four subjects who underwent reoxygenation during the stays at high altitude, there was no effect of the periods of normoxia on the hypoxiainduced M-wave changes (Figure 2).

Effects of high altitude on the contracting muscle Maximal force In the eight subjects, the mean MVC value measured in the dominant forearm tended to decrease during the stays at simulated high altitude (Figure 3). The absence of significant variation was due to a large scatter of MVC changes within individuals. However, when the nondominant forearm was tested in four subjects, the effects

Figure 4 Effects of re-oxygenation on MVC (non-dominant forearm in four subjects)

Shown are mean values (pS.E.M.) of MVC measured in the same muscle under normoxic conditions (i.e. before the ascent and after the return to 0 m) and during the stays at simulated altitudes of 5000, 6000 and 7000 m [hypobaric hypoxia and re-oxygenation (l normoxia)]. Significant differences compared with values at 0 m : *P 0.05 ; **P 0.01. Significant differences compared with value obtained under hypoxic conditions : jP 0.05. of hypoxia on MVC were significant (Figure 4). In this case, MVC began to decrease from the 5000 m stay, and there was no further significant accentuation of limited # 2000 The Biochemical Society and the Medical Research Society

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Figure 5

Hypoxia-induced changes in EMG during contractions at 60 % of MVC

Shown are changes in RMS (A), MF of power spectra (B), EH (C) and EL (D) during sustained handgrip contractions. Values are meanspS.E.M. Significant differences between values obtained throughout the sustained contraction at 60 % of MVC and those measured during the first 20-s period of contraction : *P 0.05 ; **P 0.01. muscle strength at the higher altitudes. When the four subjects returned to 0 m, MVC recovered the control value measured before the simulated ascent. Transient reoxygenation slightly but significantly enhanced MVC at 6000 m (j10 %) and 7000 m (j11 %), whereas there was no effect at 5000 m (Figure 4).

Sustained contraction at 60 % of MVC In the eight subjects, endurance time was markedly (k46 %) and significantly (P 0.01) shortened at 7000 m, and returned to the control sea-level value when the subjects were studied 1 day after the end of the experiment (Figure 3). No further significant changes in M-wave characteristics were measured during handgrip efforts sustained at 60 % of MVC, whatever the conditions of oxygenation (hypoxia or re-oxygenation at high altitude). In fact, even before the ascent, significant M-wave changes were not found during the period of contraction at 60 % of MVC. The changes in RMS, MF, EH and EL measured during contraction at 60 % of MVC are reported in Figure 5. Absolute RMS values (in mV#) were nearly the same and remained stable under the different experimental conditions, except at 7000 m, when a significant (P 0.05) increase in RMS was noted after 2 and 3 min of contraction. Before the ascent, MF decreased markedly throughout the trial at 60 % of MVC (maximal MF decrease at 3 min of contraction lk30 Hz, i.e. 79 % of the MF value measured during the first 20 s). Hypoxia as well as the final re-oxygenation (return to 0 m) significantly attenu# 2000 The Biochemical Society and the Medical Research Society

ated the decrease in maximal MF (e.g. the leftward shift in the EMG power spectrum during fatiguing contractions) compared with that measured under control conditions (at 5000 m, ∆MFmax lk2 Hz, P 0.001 ; at 6000 m, ∆MFmax lk17 Hz, P 0.01 ; at 7000 m, ∆MFmax l k23 Hz, P 0.01 ; at return to 0 m, ∆MFmax lk10 Hz, P 0.001). This was the result of a smaller increase in EL throughout the sustained efforts under these conditions (Figure 5).

DISCUSSION The present study has demonstrated modest alterations in skeletal muscle strength during a 32-day simulated ascent of Mt. Everest. MVC changes were only significant when we tested the non-dominant forearm. Moreover, we only measured a decreased endurance time to sustained fatiguing contractions at 7000 m. By contrast, the modifications of electrophysiological events were more notable, and were found in the dominant forearm of all individuals. Such effects were also present in the nondominant forearm in the four subjects in which this was tested. In resting muscle there was a slowed propagation of myopotentials (decreased amplitude and lengthened duration of M-waves), which may be associated with altered neuromuscular transmission (prolonged conduction time), the effects being maximal at the highest simulated altitude explored (7000 m) and persisting at least for 1 day when the subjects returned to 0 m. During


the stays at simulated high altitude there was an attenuation of the fall in MF of the EMG power spectrum during sustained effort. This was due to a reduced EMG signal which prevailed over the effect of EL. A reduced decline in MF during fatiguing contractions occurred despite the fact that the endurance time during which the target force was sustained was lower at 7000 m. Compared with previous data from normoxic subjects [2,3,15,16], force failure should have been associated with an accentuated decline in MF providing that the neurophysiological events that control motoneuron recruitment were unaltered. In the present study, the hypoxiainduced M-wave changes were not reduced during reoxygenation. By contrast, re-oxygenation tended to attenuate the decrease in muscle strength measured at high altitude. As the subjects had already completed the stay at 8000 m and the final ascent to 8848 m, we found it difficult to interpret the persistence of altered M-wave configuration and reduced MF decline after the return to 0 m. Indeed, no measurements of muscle function were planned at the highest altitudes, and the persistence of altered muscle function after the return to 0 m may correspond to a progressive recovery of the physiological variables, which could be affected maximally at 8000 m and 8848 m. Previous studies in normal subjects exposed to prolonged hypobaric hypoxia [4,6,21] did not find any significant decrease in MVC in larger skeletal muscles. Moreover, no significant decrease in MVC or shortening of endurance time to sustained handgrips were noted in these protocols ; however, the altitude was limited to 3700 m [8] and 4572 m [11]. In the present study a significant decrease in MVC was measured at higher altitudes in the non-dominant forearm, and a decrease in the endurance time occurred only at 7000 m. As in a previous study of chronic hypoxic patients [12], improvement of muscle strength with re-oxygenation occurred within less than 15 min, and the benefits rapidly disappeared after the inhalation of the oxygen-enriched mixture had stopped. The rapidity of these effects did not allow us to postulate any effect of re-oxygenation on the enzymic processes. As we have suggested previously [12], re-oxygenation may have improved excitation– contraction coupling. The present study clearly indicated a dissociation between the effects of re-oxygenation on the mechanical and EMG muscle events, as the latter were not rapidly influenced by the restoration of a normal partial pressure of O . It is therefore likely that # membrane excitability was not impaired by oxygen substrate limitation. One explanation for the altered M-wave configuration at simulated high altitude and its persistence after the return to sea level could be the existence of muscle membrane damage caused by lipoperoxidation due to an increased release of oxygen free radicals by hypoxic tissues. In a companion study during Everest III [23], we

showed that the baseline plasma levels of two indexes of lipid peroxidation, i.e. thiobarbituric acid reactive compounds and true peroxide groups, were significantly increased at 6000 m (j14 % and j20 % respectively), at 8000 m (j74 % and j81 % respectively) and 1 day after the subjects had returned to sea level (j79 % and j83 % respectively). These observations corroborated data reported by Nakanishi et al. [24] in rats exposed to severe hypoxia. Increased plasma levels of peroxide groups may result from an enhanced release of free radicals, not only by the muscles but also by the different organs exposed to prolonged severe hypoxia. Whatever the origin of free radicals, they may alter the function of excitable membranes, including the sarcolemma. We noted that hypoxia-induced changes in electrophysiological events were characterized by a decreased amplitude and increased duration of the M-wave, which suggested an alteration of muscle membrane properties. It is necessary to discard the possibility of recording artifacts during the simulated ascent and their persistence after recompression to sea level. Because electrode impedence was measured when the subjects were out of the hypobaric chamber (before the ascent and after the return to 0 m), and not during the simulated ascent, impedence variations may have occurred during the experiment. Such variations cannot result from the changes in the positions of skin electrodes, which were marked on the skin before the ascent. However, changes in skinfold thickness at the recording sites may be responsible for some variations in the configuration of myopotentials. It is well known that increased subcutaneous tissue thickness, due mainly to enhanced body fat, markedly affects EMG activity. Up to 80 % of the lowered EMG signal may result from variations in the amount of subcutaneous tissue [25]. However, a companion investigation [26] demonstrated that, after the 32day exposure to hypoxia, there were no significant changes in subcutaneous adipose tissue mass or in fat-free muscle mass. Our subjects lost weight throughout the simulated ascent (Table 1). This effect was already significant at 5000 m, and persisted for at least 1 day after the return to 0 m. The progressive loss of weight could partly explain the depression of EMG signals and its persistence 24 h after recompression. In addition, an increased proportion of slow-oxidative compared with fast-glycolytic fibres may explain the slowing of the Mwave. However, earlier studies did not show any significant difference in the proportion of slow-oxidative muscle fibres in sea-level natives after a 18-day residence at 4300 m [27], during the simulated Everest II operation [28] or in high-altitude (3600 m) natives [29]. Thus variations in fibre type composition of the forearm muscle group did not influence EMG variations in the present study. The hypoxia-induced M-wave changes were not useful for interpretation of the attenuation of the decline in MF # 2000 The Biochemical Society and the Medical Research Society

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during sustained contraction. First, M-wave slowing should have accentuated, and not reduced, the leftward shift of the EMG spectrum. Secondly, the altered Mwave configuration was not accentuated throughout the trials at 60 % of MVC performed under hypoxic conditions. Thus it was speculated that the physiological mechanisms involved in motoneuron control (reduced recruitment of high-frequency motor units) may be affected by hypoxia. In support of this hypothesis, animal studies have shown that fatigue elicited a long-lasting activation of the group IV muscle afferents [14,30,31], their stimulation exerting in turn a marked inhibition of motoneuron discharge [14,16]. There are no published observations that the fatigue-induced stimulation of group III\IV muscle afferents is modified by hypoxia. However, a recent study by our team (S. Arbogast, J. L. Darques and Y. Jammes, unpublished work) showed that acute hypoxia in rabbits markedly attenuated or even suppressed the activation of these muscle afferents by electrically induced muscle fatigue. This may be one explanation for the attenuated decline in MF observed during contraction at 60 % of MVC in human subjects exposed to simulated high altitude. The aim of the present study was to transform healthy subjects into hypoxic individuals and to observe the hypoxia-induced alterations in muscle function throughout the period of exposure to simulated high altitude. Only a part of this protocol could be easily repeated during re-oxygenation of hypoxic patients with chronic respiratory insufficiency. Since the number of such patients is much larger than that of alpinists or subjects living at high altitude, it may be opportune to study whether chronic hypoxia alters the myoelectrical events in these individuals, who are unable to produce the same muscle strength as normoxic subjects [12].

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ACKNOWLEDGMENTS This study was supported by a grant from the Conseil Regional Provence-Alpes-Co# te d ’Azur and by a grant from A. P. R. E. S. We acknowledge Mrs. S. Pacull for linguistic improvements to the manuscript.

REFERENCES 1 Hochachka, P. W. (1994) Muscles as Molecular and Metabolic Machines, CRC Press, Boca Raton, Ann Arbor, London and Tokyo 2 Badier, M., Guillot, Ch., Lagier-Tessonnier, F. and Jammes, Y. (1994) EMG changes in respiratory and skeletal muscles during isometric contraction under normoxic, hypoxemic or ischemic conditions. Muscle Nerve 17, 500–508 3 Bendahan, D., Badier, M., Jammes, Y. et al. (1998) Metabolic and myoelectrical effects of acute hypoxaemia during isometric contraction of forearm muscles in


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