Strenuous endurance training in humans reduces oxidative stress following exhausting exercise

The aim of this study was to evaluate whether high-intensity endurance training would alleviate exercise-induced oxidative stress. Nine untrained male subjects (aged 19–21 years) participated in a 12-week training programme, and performed an acute period of exhausting exercise on a cycle ergometer before and after training. The training programme consisted of running at 80% maximal exercise heart rate for 60 min · day⁻¹, 5 days · week⁻¹ for 12 weeks. Blood samples were collected at rest and immediately after exhausting exercise for measurements of indices of oxidative stress, and antioxidant enzyme activities [superoxide dismutase (SOD), glutathione peroxidase (GPX), and catalase (CAT)] in the erythrocytes. Maximal oxygen uptake (V˙O_2max) increased significantly (P < 0.001) after training, indicating an improvement in aerobic capacity. A period of exhausting exercise caused an increase (P < 0.01) in the ability to produce neutrophil superoxide anion (O₂ ^•−) both before and after endurance training, but the magnitude of the increase was smaller after training (P < 0.05). There was a significant increase in lipid peroxidation in the erythrocyte membrane, but not in oxidative protein, after exhausting exercise, however training attenuated this effect. At rest, SOD and GPX activities were increased after training. However, there was no evidence that exhausting exercise enhanced the levels of any antioxidant enzyme activity. The CAT activity was unchanged either by training or by exhausting exercise. These results indicate that high-intensity endurance training can elevate antioxidant enzyme activities in erythrocytes, and decrease neutrophil O₂ ^•− production in response to exhausting exercise. Furthermore, this up-regulation in antioxidant defences was accompanied by a reduction in exercise-induced lipid peroxidation in erythrocyte membrane. Accepted: 26 September 2000     

膀胱压与腹内压相关性的临床研究

目的　研究膀胱压与腹内压相关性 .方法　选取腹腔镜胆囊切除术患者 30名 .排空膀胱后经Foley导管注入 5 0ml生理盐水 .置入 18G气腹穿刺针后 ,连接自动电子充气连续测压仪 ,选取腹内压为 0、5、10、15、2 0、2 5mmHg时记录相应的膀胱压 .结果　当膀胱注入 5 0ml的生理盐水后 ,腹内压由 0增至 2 5mmHg时 ,膀胱压与腹内压存在直线相关 ,相关系数r=0 .939(p<0 .0 1) ,决定系数r2 =0 .882 .结论　膀胱压能准确估计腹内压     

Central command increases cardiac output during static exercise in humans

Neural control of the circulation during static two-leg exercise was evaluated in 10 subjects. External compression of the legs was employed to assess muscle mechano-receptor influence by achieving the same intramuscular pressure (80 mmHg) as developed during exercise. The muscle metabo-reflex contribution was assessed by post-exercise muscle ischaemia, and the influence from higher centres in the central nervous system (‘central command’) was taken as the part of the response that could not be accounted for by the two reflex contributions. During static exercise, mean arterial pressure was higher (26±3 mmHg; P<0.01) as compared with leg compression (10±2 mmHg) and with post-exercise muscle ischaemia (11±2 mmHg). Heart rate (25±4 b.p.m.) and cardiac output (0.8±0.3 L min^-1) were increased only during static exercise (P<0.05). Increase in total peripheral resistance were similar during static exercise, post-exercise muscle ischaemia and leg compression. The pressor response to static exercise with a large muscle group was equally attributable to mechanical and metabolic stimulation of afferent nerves; and the two influences were redundant in their effect on total peripheral resistance. In contrast, the influence from central command was directed to the heart with elevation of its rate and minute volume.     

The effect of ageing in spinal cord injured humans on the blood pressure and heart rate responses during fatiguing isometric exercise

Groups of 50 healthy male controls and 50 subjects suffering from paraplegia (aged 20–65 years) were examined as to the inter-relationships between age, paraplegia and the strength, endurance, blood pressure and heart rate responses to fatiguing isometric exercise. Contractions were maintained in both groups under voluntary effort and through a contraction induced by electrical stimulation in the paraplegic group. All contractions were maintained to fatigue at a tension of 40% of the maximal muscle strength in either the handgrip or quadriceps muscles. Muscle strength of the handgrip was higher in the paraplegic subjects than in the controls, averaging 589 N and 463 N, respectively for the two groups. In contrast, quadriceps leg extension strength averaged 696 N in the controls and 190 N in the paraplegic groups; for both groups, ageing was associated with a reduction in muscle strength. While leg endurance was less in the paraplegic group than the control group, handgrip endurance was similar in the two groups, endurance increasing with ageing in both the controls and paraplegics. Both systolic and diastolic blood pressures increased at rest in paraplegic and control subjects with age. The magnitude of the pressor response to exercise also increased with age. This was true during both voluntary exercise and exercise induced through electrical stimulation in the paraplegic groups. The heart rate response (change in heart rate during exercise) to a fatiguing isometric handgrip contraction decreased by about 50% between the ages of 20 and 60 years in both the controls and paraplegics for isometric handgrip exercise. In contrast, heart rate changed little with age during contractions of the quadriceps muscle in paraplegics which were induced by electrical stimulation. Electronic Publication     

Intra-abdominal pressure changes during natural movements in man

The weight of the upper part of the trunk is partially transmitted to the pelvis via the vertebral column. If the muscle walls around the abdominal cavity are contracted, a high pressure can be generated within the cavity (>200 mmHg). The abdominal space can then transmit part of the weight to, e.g., the upper part of the body. Intra-abdominal pressure recordings have been performed during locomotion and other natural movements with intragastric pressure recordings. With each step, there is a phasic variation in pressure, with its peak coinciding with that of the peak vertical force exerted by the leg against the ground. The peak values increase progressively with the speed of walking/running up to a mean of 38 mmHg and with trough values of 16 mmHg. The phasic variations with each step is due to a phasic activation of the abdominal muscles, with an EMG activity starting 50 ms or more before foot contact. If an extra load is put on the back, the posture changes and at the highest speed of running the pressure values are significantly higher than without this additional load. After a jump down from a moderate height of 0.4 m, the average increase is 89 mmHg and can often exceed 100 mmHg. These pressure changes are large and will presumably act to unload the spine under the prevailing biomechanical conditions and, in addition, there will no doubt be an effect on the circulatory system.     

Time-courses in renin and blood pressure during sleep in humans

We previously described a strong concordance between nocturnal oscillations in plasma renin activity (PRA) and the rapid eye movement (REM) and non-REM (NREM) sleep cycles, but the mechanisms inducing PRA oscillations remain to be identified. This study was designed to examine whether they are linked to sleep stage-related changes in arterial blood pressure (ABP). Analysis of sleep electroencephalographic (EEG) activity in the delta frequency band, intra-arterial pressure, and PRA measured every 10 min was performed in eight healthy subjects. Simultaneously, the ratio of low frequency power to low frequency power + high frequency power [LF/(LF + HF)] was calculated using spectral analysis of R--R intervals. The cascade of physiological events that led to increased renin release during NREM sleep could be characterized. First, the LF/(LF + HF) ratio significantly (P < 10(-4) decreased, indicating a reduction in sympathetic tone, concomitantly to a significant (P < 10(- 3) decrease in mean arterial pressure (MAP). Delta wave activity increased (P < 10(-4) 10-20 min later and was associated with a lag of 0-10 min with a significant rise in PRA (P < 10(-4) . Rapid eye movement sleep was characterized by a significant increase (P < 10(-4) in the LF/(LF + HF) ratio and a decrease (P < 10(-4) in delta wave activity and PRA, whereas MAP levels were highly variable. Overnight cross-correlation analysis revealed that MAP was inversely correlated with delta wave activity and with PRA (P < 0.01 in all subjects but one). These results suggest that pressure-dependent mechanisms elicit the nocturnal PRA oscillations rather than common central processes controlling both the generation of slow waves and the release of renin from the kidney.     

Respiratory muscle endurance training in humans increases cycling endurance without affecting blood gas concentrations

Isolated respiratory muscle endurance training (RMT) can prolong constant-intensity cycling performance. We tested whether RMT affects O₂ supply during exercise, i.e. whether the partial pressure of oxygen in arterial blood (P _aO₂) and/or its oxygen saturation (S _aO₂) are higher during exercise after RMT than before. A group of 28 sedentary subjects were randomly assigned to either an RMT (n=13) or a control group (n=15). The RMT consisted of 40×30 min sessions of normocapnic hyperpnoea. The control group did not perform any training. Breathing and cycling endurance time as well as P _aO₂ and S _aO₂ during cycling at a constant intensity of 70% maximum power output were measured before and after the RMT or the control period. Mean breathing endurance increased significantly after RMT compared to control [RMT 5.2 (SD 2.9) vs 38.1 (SD 6.8) min, control 6.5 (SD 5.7) vs 6.4 (SD 7.6) min; P<0.01], as did mean cycling endurance [RMT 35.6 (SD 11.9) vs 44.0 (SD 17.2) min, control 32.8 (SD 11.6) vs 31.4 (SD 14.4) min; P<0.05]. The RMT did not affect P _aO₂ which ranged from 11.6 to 12.3 kPa (87–92 mmHg), and S _aO₂ which ranged from 96% to 98% throughout all tests. In conclusion, RMT substantially increased breathing and cycling endurance in sedentary subjects. These changes, however, cannot be attributed to increased O₂ supply, as neither P _aO₂ nor S _aO₂ were increased during exercise after RMT. Electronic Publication     

Intra-abdominal pressure changes during natural movements in man.

The weight of the upper part of the trunk is partially transmitted to the pelvis via the vertebral column. If the muscle walls around the abdominal cavity are contracted, a high pressure can be generated within the cavity (greater than 200 mmHg). The abdominal space can them transmit part of weight to, e.g., the upper part of the body, Intra-abdominal pressure recordings have been performed during locomotion and other natural movements with intragastric pressure recordings. With each step, there is a phasic variation in pressure, with its peak coinciding with that of the peak vertical force exerted by the leg against the ground. The peak values increase progressively with the speed of walking/running up to a mean of 38 mmHg and with trough values of 16 mmHg. The phasic variations with each step is due to a phasic activation of the abdominal muscles, with an EMG activity starting 50 ms or more before foot contact. If an extra load is put on the back, the posture changes and at the highest speed of running the pressure values are significantly higher than without this additional load. After a jump down from a moderate height of 0.4 m, the average increase is 89 mmHg and can often exceed 100 mmHg. These pressure changes are large and will presumably act to unload the spine under the prevailing biomechanical conditions and, in addition, there will no doubt be an effect on the circulatory system.     

Changes in intra-abdominal pressure,trunk muscle activation and force during isokinetic lifting and lowering

Intra-abdominal pressure (IAP), force and electromyographic (EMG) activity from the abdominal (intra-muscular) and trunk extensor (surface) muscles were measured in seven male subjects during maximal and sub-maximal sagittal lifting and lowering with straight arms and legs. An isokinetic dynamometer was used to provide five constant velocities (0.12–0.96 m·s^–1) of lifting (pulling against the resistance of the motor) and lowering (resisting the downward pull of the motor). For the maximal efforts, position-specific lowering force was greater than lifting force at each respective velocity. In contrast, corresponding IAPs during lowering were less than those during lifting. Highest mean force occurred during slow lowering (1547 N at 0.24 m·s^–1) while highest IAP occurred during the fastest lifts (17.8 kPa at 0.48–0.96 m·s^–1). Among the abdominal muscles, the highest level of activity and the best correlation to variations in IAP (r=0.970 over velocities) was demonstrated by the transversus abdominis muscle. At each velocity the EMG activity of the primary trunk and hip extensors was less during lowering (eccentric muscle action) than lifting (concentric muscle action) despite higher levels of force (r between –0.896 and –0.851). Sub-maximal efforts resulted in IAP increasing linearly with increasing lifting or lowering force (r=0.918 and 0.882, respectively). However, at any given force IAP was less during lowering than lifting. This difference was negated if force and IAP were expressed relative to their respective lifting and lowering maxima. It appears that the IAP increase primarily accomplished by the activation of the transversus abdominis muscle can have the dual function of stabilising the trunk and reducing compression forces in the lumbar spine via its extensor moment. The neural mechanisms involved in sensing and regulating both IAP and trunk extensor activity in relation to the type of muscle action, velocity and effort during the maximal and sub-maximal loading tasks are unknown.     

Sympathetic nerve regulation of cochlear blood flow during increases in blood pressure in humans

The purpose of this work was to show that regulation of the blood flow to the cochlea by the sympathetic nervous system occurs in humans at the level of the cochlear microcirculation during increases in blood pressure and that its involvement depends on the pressure level. Eight anaesthetized patients undergoing tympanoplasty for hearing disease took part in a pharmacological protocol of stimulation and inhibition of the autonomic nervous system (ANS) to provide variations in systolic blood pressure (BP_S) and cochlear blood flow (CBF). The CBF was measured by laser-Doppler flowmetry. Changes in autonomic nerve activity were brought about by changes in baroreceptor activity (BR) initiated by the injection of an α adrenergic agent before and after sympathetic and parasympathetic blockade. The CBF variations (δCBF) were plotted against BP_S increases at each stage of the ANS inhibition. The BR diminished significantly after α blockade, after α and β blockade, and after α and β blockade and atropine, by 50% (P?P?P?S increased significantly (P?S exhibited two opposing variations in the patients: CBF decreased significantly in one group, and increased significantly in the other group. In both groups, δCBF decrease and δCBF increase, respectively, were significant after ANS blockade; even so the decrease and increase, respectively, levelled off at BP_S around 160 mmHg before ANS blockade. For BP_S below 160 mmHg, correlations between δCBF and BP_S were significant before inhibition and after inhibition of ANS. For BP_S above 160 mmHg, BP_S and δCBF were not correlated before inhibition of ANS, and were significantly correlated after inhibition of ANS. For BP_S below 160 mmHg, CBF response to the BP_S increase was the same before and after ANS blockade, i.e. ANS control did not predominate; even so, for BP_S above 160 mmHg, the CBF response to BP_S increase was different before and after ANS blockade: CBF varied significantly after ANS blockade as it varied for BP_S below 160 mmHg, while it remained constant before ANS blockade that elicited ANS control of CBF. In conclusion, sympathetic nerve regulation via its vasomotor tone at the level of cochlear microcirculation occurred markedly when the blood pressure was above 160 mmHg; the autonomic nervous system would appear to control the cochlear blood flow against large variations in blood flow in response to hypertensive phenomena.     

Regulation of arterial blood pressure in humans during isometric muscle contraction and lower body negative pressure

Previous studies have shown that the blood pressure response to isometric handgrip remains unchanged during reductions in preload induced by lower body negative pressure (LBNP). The purpose of the present study was to assess the beat-by-beat haemodynamic mechanisms allowing for precise control of mean arterial pressure (MAP). We have followed the cardiovascular variables involved in the regulation of MAP during isometric handgrip with and without additional application of LBNP during defined periods of the ongoing contraction. Sixteen subjects participated. Mean arterial blood pressure (MAP), heart rate (HR), stroke volume (SV), cardiac output (CO), blood flow velocity in the brachial artery, acral skin blood flow, as well as total (TPR) and local (LPR) peripheral resistance were continuously recorded/calculated before, during and after 2 min of handgrip both with and without concomitant LBNP. The main finding was that MAP increased at the same rate and to the same absolute level whether or not LBNP was applied. A uniform increase in MAP was observed even though the cardiovascular variables evolved differently in the periods with and without LBNP. At the onset of LBNP at –20 mmHg, there was a transient drop in MAP and a transient increase in HR, but within seconds, MAP was regulated back to the slope caused by the isometric handgrip proper. CO and SV, which were declining gradually, showed an additional marked but gradual reduction upon LBNP application. At the same time, both LPR and TPR increased markedly and continuously. In summary, the increase in MAP during isometric handgrip remained essentially unchanged by LBNP-induced alterations in preload. The increase in MAP was caused by a marked increase in peripheral resistance. This supports the concept of a central set point, continuously regulated upwards as long as the isometric handgrip persists. Furthermore, it reveals a considerable flexibility in the cardiovascular control mechanisms used to achieve the desired arterial pressure.     

Postural control of the trunk in response to lateral support surface translations during trunk movement and loading

The postural response to translation of the support surface may be influenced by the performance of an ongoing voluntary task. This study was designed to test this proposal by applying lateral perturbations while subjects handled a load in the frontal plane. Measurements were made of medio-lateral displacement of the centre of pressure, angular displacement of the trunk and thigh in the frontal plane and intra-abdominal pressure. Subjects were translated randomly to the left and right in a variety of conditions that involved standing either quietly or with a 5 kg load in their left hand, which they were required either to hold statically or to lift or lower. The results indicate that when the perturbation occurred towards the loaded left side the subjects were able to return their centre of pressure, trunk and thigh rapidly and accurately to the initial position. However, when the perturbation occurred towards the right (away from the load) this correction was delayed and associated with multiple changes in direction of movement, suggesting decreased efficiency of the postural response. This reduced efficiency can be explained by a conflict between the motor commands for the ongoing voluntary task and the postural response, and/or by the mechanical effect of the asymmetrical addition of load to the trunk.     

Adhesion formation in intubated rabbits increases with high insufflation pressure during endoscopic surgery

The aim of the study was to test the hypothesis that the increase in adhesion formation by CO(2) pneumoperitoneum is caused by mesothelial hypoxaemia. Therefore the effect of the intra-abdominal pressure together with the flow rate upon adhesion formation was evaluated in rabbits following laser and bipolar lesions during endoscopic surgery using humidified CO(2) at 35 +/- 1 degrees C. The intra-abdominal pressure and flow rate were 5 mmHg and 1 l/min in group 1 (n = 5), 5 mmHg and 10 l/min in group 2 (n = 4), 20 mmHg and 1 l/min in group 3 (n = 5) and 20 mmHg and 10 l/min in group 4 (n = 4) respectively. A rapid and reliable intubation method for rabbits was developed to permit high insufflation pressure. By two-way analysis of variance, total adhesion scores following a laser lesion increased with flow rate (P = 0.0003) and insufflation pressure (P = 0.002). Total adhesion scores of bipolar lesions increased with pressure (P = 0.02) but not with flow rate (P = 0.1). The total adhesion scores of laser and bipolar lesions together increased with flow rate (P = 0.005) and with insufflation pressure (P = 0.004). There was no statistical interaction between flow rate and insufflation pressure. In conclusion, the insufflation pressure in endoscopic surgery with CO(2) pneumoperitoneum is a co-factor in adhesion formation, together with desiccation.     

Near-infrared monitoring of tissue oxygenation during application of lower body pressure at rest and during dynamical exercise in humans

During the application of a wide range of graded lower body pressures (LBP) (–50 to 50 mmHg), we examined how (1) the tissue oxygenation in the lower and upper parts of the body changes at rest, and (2) how tissue oxygenation changes in the lower extremities during dynamical leg exercise. We used near-infrared spectroscopy (NIRS) to measure the changes induced by LBP in total Hb content and Hb oxygenation in seven subjects. At rest, total Hb increased and Hb oxygenation decreased in the thigh muscles during –25 and –50 mmHg LBP, while both decreased during +25 and +50 mmHg LBP. However, in the forearm muscles during graded LBP, the pattern of change in total Hb was the reverse of that in the thigh. Measurements from the forehead showed changes only during +50 mmHg LBP. These results demonstrated that the pattern of change in total Hb and Hb oxygenation differed between upper and lower parts with graded LBP at rest. During dynamical leg exercise, total Hb and Hb oxygenation in the thigh muscles decreased during stepwise increases in LBP above –25 mmHg, Hb oxygenation decreasing markedly during +50 mmHg LBP. These results suggest that during dynamical exercise (i) LBP at +25 mmHg or more causes a graded decline in blood volume and/or flow in the thigh muscles, and (ii) especially at +50 mmHg LBP, the O₂ content may decrease markedly in active muscles. Our results suggest that NIRS can be used to monitor in a non-invasive and continuous fashion the changes in oxygenation occurring in human skeletal muscles and head during the graded changes in blood flow and/or volume caused by changes in external pressure and secondary reflexes both at rest and during dynamical exercise.     

Effects of intra-abdominal pressure on adrenal gland function and morphology in rats

Intra-abdominal hypertension and abdominal compartment syndrome (IAH/ACS) are life-threatening conditions and caused by several clinical status. Although there is insufficient data regarding its effects on adrenal glands. This study aimed to identify whether elevated intra-abdominal pressure (IAP) caused any alteration on the morphology and function of adrenal glands in a rat model. Twenty four Sprague-Dawley male rats were included in the study. Animals were allocated into 4 groups. IAP was elevated to 15 mmHg for one hour and four hours in group 2 and 4. Group 1 and 3 were sham groups. Blood samples were taken for the assessment of plasma adrenaline, noradrenaline, and corticosterone levels and adrenalectomies were performed to evaluate apoptosis. Blood adrenaline, noradrenaline and corticosterone levels were significantly higher in the study groups compared with the sham groups. However, there were no significant changes in apoptotic index scores in the study groups as compared to sham groups. These results support that increased IAH leads to discharge of catecholamine and corticosterone from the adrenal glands. Failure to demonstrate similar changes in apoptotic index score may be concluded as apoptosis is not a leading pathway for impairment of adrenal glands during IAH period.     

Atrial plasma ANP and NH2-terminal proANP during right atrial pressure increase in humans

To compare plasma NT-proANP, a stable and biologically inactive N-terminal portion of ANP prohormone, with the known plasma ANP response to increased right atrial pressure a Swan–Ganz catheter was inserted into the right atrium of five normal healthy male volunteers. The elevation of right atrial pressure was produced by a head-down tilt after a hypertonic saline infusion. Blood samples were drawn from the lumen of the right atrium. After 5 min of starting the tilt the right atrial pressure had increased from 7.0±1.0 to 11.6±0.9 mmHg (P<0.05) and then began to normalize in spite of the constant tilt. Atrial plasma ANP increased in relation to the pressure increase and peaked at 15 min after the start of the tilt. The change was from 27.9±6.5 to 53.9±9.7 pmol L^-1 (P<0.05). Atrial plasma NT-proANP increased significantly from 357±91.2 to 529.1±116.0 pmol L^-1 (P<0.05) at 10 min and remained high throughout the experiment. The molar ratio of NT-proANP to ANP varied in atrial plasma from 9.5±1.2 to 13.9±2.7 showing that the plasma clearance of ANP from plasma was much higher than that of NT-proANP.     

Changes in motor planning of feedforward postural responses of the trunk muscles in low back pain

Changes in trunk muscle recruitment have been identified in people with low-back pain (LBP). These differences may be due to changes in the planning of the motor response or due to delayed transmission of the descending motor command in the nervous system. These two possibilities were investigated by comparison of the effect of task complexity on the feedforward postural response of the trunk muscles associated with rapid arm movement in people with and without LBP. Task complexity was increased by variation of the expectation for a command to either abduct or flex the upper limb. The onsets of electromyographic activity (EMG) of the abdominal and deltoid muscles were measured. In control subjects, while the reaction time of deltoid and the superficial abdominal muscles increased with task complexity, the reaction time of transversus abdominis (TrA) was constant. However, in subjects with LBP, the reaction time of TrA increased along with the other muscles as task complexity was increased. While inhibition of the descending motor command cannot be excluded, it is more likely that the change in recruitment of TrA represents a more complex change in organisation of the postural response.     

Chronic hypoxia increases blood pressure and noradrenaline spillover in healthy humans

Chronic hypoxia is associated with elevated sympathetic activity and hypertension in patients with chronic pulmonary obstructive disease. However, the effect of chronic hypoxia on systemic and regional sympathetic activity in healthy humans remains unknown. To determine if chronic hypoxia in healthy humans is associated with hyperactivity of the sympathetic system, we measured intra-arterial blood pressure, arterial blood gases, systemic and skeletal muscle noradrenaline (norepinephrine) spillover and vascular conductances in nine Danish lowlanders at sea level and after 9 weeks of exposure at 5260 m. Mean blood pressure was 28% higher at altitude ( P < 0.01) due to increases in both systolic (18% higher, P < 0.05) and diastolic (41% higher, P < 0.001) blood pressures. Cardiac output and leg blood flow were not altered by chronic hypoxia, but systemic vascular conductance was reduced by 30 % ( P < 0.05). Plasma arterial noradrenaline (NA) and adrenaline concentrations were 3.7- and 2.4-fold higher at altitude, respectively ( P < 0.05). The elevation of plasma arterial NA concentration was caused by a 3.8-fold higher whole-body NA release ( P < 0.001) since whole-body noradrenaline clearance was similar in both conditions. Leg NA spillover was increased similarly (× 3.2, P < 0.05). These changes occurred despite the fact that systemic O₂ delivery was greater after altitude acclimatisation than at sea level, due to 37 % higher blood haemoglobin concentration. In summary, this study shows that chronic hypoxia causes marked activation of the sympathetic nervous system in healthy humans and increased systemic arterial pressure, despite normalisation of the arterial O₂ content with acclimatisation.     

Prediction of endurance capacity of quadriceps muscles in humans using surface electromyogram spectrum analysis during submaximal voluntary isometric contractions

The measurement of endurance time (t _lim) is the procedure commonly used to quantify the ability of a muscle to maintain force. The relationship between surface electromyographic (sEMG) manifestations of localised muscle fatigue and t _lim during an effort at 50% of maximal voluntary isometric torque of the knee extensors (vastus lateralis and vastus medialis) until exhaustion was studied in 14 healthy volunteers. It was carried out to test whether changes in sEMG computed over shorter periods than expected t _lim could be used to predict t _lim. Changes in mean muscle fibre conduction velocity, mean power frequency , median frequency , root mean square ), in the relative power in the 6–30 Hz and 30–60 Hz frequency bands were monitored using linear slope and area ratio index as statistical indicators. These indicators were computed over fixed periods shorter than t _lim. The subjects were able to maintain the required force level for [mean (SD)] 78.8 (9.5) s. During the fatigue trial, it was the greatest of the increases in the 6–30 Hz frequency band, recorded for either of the two muscles investigated, that was the only variable which correlated with t _lim. Significant relationships between t _lim and changes in this low frequency band were observed as early as the first 15–30 s of the contraction. These results suggest that sEMG frequency banding may predict mechanical endurance without the need to maintain the contraction until exhaustion. From a clinical perspective, this could be an advantage for patients who might not be able to tolerate contractions to exhaustion. Electronic Publication