Showing papers in "European Journal of Applied Physiology in 1988"

Muscle blood flow during isometric activity and its relation to muscle fatigue.

Abstract: The effect of isometric exercise on blood flow, blood pressure, intramuscular pressure as well as lactate and potassium efflux from exercising muscle was examined. The contractions performed were continuous or intermittent (5 s on, 5 s off) and varied between 5% and 50% maximal voluntary contraction (MVC). A knee-extensor and a hand-grip protocol were used. Evidence is presented that blood flow through the muscle is sufficient during low-level sustained contractions (<10% MVC). Despite this muscle fatigue occurs during prolonged contractions. One mechanism for this fatigue may be the disturbance of the potassium homeostasis. Such changes may also play a role in the development of fatigue during intermittent isometric contractions and even more so in the recovery from such exercise. In addition the role of impaired transport of substances within the muscle, due to longlasting daily oedema formation, is discussed in relation to fatigue in highly repetitive, monotonous jobs.

Fibre conduction velocity and fibre composition in human vastus lateralis

Abstract: The relationship between muscle fibre composition and fibre conduction velocity was investigated in 19 male track athletes, 12 sprinters and 7 distance runners, aged 20–24 years, using needle biopsy samples from vastus lateralis. Cross sectional areas of the fast twitch (FT) and slow twitch (ST) fibres were determined by histochemical analysis. The percentage of FT fibre areas ranged from 22.6 to 93.6%. Sprinters had a higher percentage of FT fibres than distance runners. Muscle fibre conduction velocity was measured with a surface electrode array placed along the muscle fibres, and calculated from the time delay between 2 myoelectric signals recorded during a maximal voluntary contraction. The conduction velocity ranged from 4.13 to 5.20 m·s−1. A linear correlation between conduction velocity and the relative area of FT fibres was statistically significant (r = 0.84,p<0.01). This correlation indicates that muscle fibre composition can be estimated from muscle fibre conduction velocity measured noninvasively with surface electrodes.

Eccentric and concentric torque-velocity characteristics of the quadriceps femoris in man.

Abstract: The primary purpose of this investigation was to study the eccentric and concentric torque-velocity characteristics of the quadriceps femoris in man using a recently developed combined isometric, concentric and eccentric controlled velocity dynamometer (the SPARK System). A secondary purpose was to compare the method error associated with maximal voluntary concentric and eccentric torque output over a range of testing velocities. 21 males (21-32 years) performed on two separate days maximal voluntary isometric, concentric and eccentric contractions of the quadriceps femoris at 4 isokinetic lever arm velocities of 0 degree.s-1 (isometric), 30 degrees.s-1, 120 degrees.s-1 and 270 degrees.s-1. Eccentric peak torque and angle-specific torques (measured every 10 degrees from 30 degrees to 70 degrees) did not significantly change from 0 degrees.s-1 to 270 degrees.s-1 (p greater than 0.005) with the exception of angle-specific 40 degrees torque, which significantly increased; p less than 0.05). The mean method error was significantly higher for the eccentric tests (10.6% +/- 1.6%) than for the concentric tests (8.1% +/- 1.7%) (p less than 0.05). The mean method error decreased slightly with increasing concentric velocity (p greater than 0.05), and increased slightly with increasing eccentric velocity (p greater than 0.05). A tension restricting neural mechanism, if active during maximal eccentric contractions, could possibly account for the large difference seen between the present eccentric torque-velocity results and the classic results obtained from isolated animal muscle.

Oxygen uptake as related to work rate increment during cycle ergometer exercise.

Abstract: We postulated that the commonly observed constant linear relationship between VO2 and work rate during cycle ergometry to exhaustion is fortuitous and not due to an unchanging cost of external work. Therefore we measured VO2 continuously in 10 healthy men during such exercise while varying the rate of work incrementation and analyzed by linear regression techniques the relationship between VO2 and work rate (delta VO2/delta wr). After excluding the first and last portions of each test we found the mean +/- SD of the delta VO2/delta wr in ml.min-1.W-1 to be 11.2 +/- 0.15, 10.2 +/- 0.16, and 8.8 +/- 0.15 for the 15, 30, and 60 W.min-1 tests, respectively, expressed as ml.J-1 the values were 0.187 +/- 0.0025, 0.170 +/- 0.0027 and 0.147 +/- 0.0025. The slopes of the lower halves of the 15 and 30 W.min-1 tests were 9.9 +/- 0.2 ml.min-1.W-1 similar to the values for aerobic work reported by others. However the upper halves of the 15, 30, and 60 W.min-1 tests demonstrated significant differences: 12.4 +/- 0.36 vs 10.5 +/- 0.31 vs 8.7 +/- 0.23 ml.min-1.W-1 respectively. We postulate that these systematic differences are due to two opposing influences: 1) the fraction of energy from anaerobic sources is larger in the brief 60 W.min-1 tests and 2) the increased energy requirement per W of heavy work is evident especially in the long 15 W.min-1 tests.

Electromyography and fatigue during prolonged, low-level static contractions

Abstract: Findings from five separate studies of EMG changes and muscle fatigue during prolonged low-level static contractions are summarized, and the possible mechanisms behind the changes are briefly discussed. Sustained static contractions (10%, 7% and 5% MVC) of up to 1 h duration were performed by finger flexors, elbow flexors and extensors, and knee extensors. In one experiment, intermittent static arm pulling (triceps) (10 s contraction and 5 s rest, average work load 14% and 10% MVC) was performed for 7 h. The endurance time for the sustained contractions was around one hour for 10% MVC, and it was shown--all in all--that the concept of "indefinite" endurance times at contractions below 15-20% MVC cannot be maintained. After 5% MVC sustained contractions for one hour a 12% reduction in MVC was seen, and significant increases in EMG amplitude and decreases in the mean spectral frequency of the EMG-power spectrum were found. Marked differences were also seen in the EMG changes in the elbow flexors and extensors, and transcutaneous electrical stimulation of the knee extensors showed that low frequency fatigue was present after the contraction. With intermittent contractions similar changes in the EMG parameters were seen after 2-3 h of contractions at 14% MVC. On average, during contractions of 10% MVC no EMG changes were detected. Increased extracellular potassium concentration in the contracting muscles is suggested as a possible explanation of these findings.

The static load component in muscle work.

Abstract: By citing examples from actual work situations and discussing the concept of muscular endurance and fatigue this paper is intended to provide an account of past and current research on the "static" component of muscular load during work. By amplitude probability distribution analysis of electromyographic signals it is possible to estimate the "static level" of muscular load during work. Electromyographic studies of job rotation between different assembly tasks in electronic industries often show that there are quantitatively and qualitatively only small differences in muscular load between different tasks.

Effects of training at simulated altitude on performance and muscle metabolic capacity in competitive road cyclists.

Abstract: Differences between the effects of training at sea level and at simulated altitude on performance and muscle structural and biochemical properties were investigated in 8 competitive cyclists who trained for 3–4 weeks, 4–5 sessions/week, each session consisting of cycling for 60–90 min continuously and 45–60 min intermittently. Four subjects, the altitude group (AG), trained in a hypobaric chamber (574 torr=2300 m above sea level), and the other four at sea level (SLG). Before and after training work capacity was tested both at simulated altitude (574 torr) and at sea level, by an incremental cycle ergometer test until exhaustion. Work capacity was expressed as total amount of work performed. Venous blood samples were taken during the tests. Leg muscle biopsies were taken at rest before and after the training period. AG exhibited an increase of 33% in both sea level and altitude performance, while SLG increased 22% at sea level and 14% at altitude. Blood lactate concentration at a given submaximal load at altitude was significantly more reduced by training in AG than SLG. Muscle phosphofructokinase (PFK) activity decreased with training in AG but increased in SLG. All AG subjects showed increases in capillary density. In conclusion, work capacity at altitude was increased more by training at altitude than at sea level. Work capacity at sea level was at least as much improved by altitude as by sea level training. The improved work capacity by training at altitude was paralleled by decreased exercise blood lactate concentration, increased capillarization and decreased glycolytic capacity in leg muscle.

Biochemical correlates of fatigue. A brief review.

Abstract: Muscle fatigue, defined as a decreased force generating capacity, develops gradually during exercise and is distinct from exhaustion, which occurs when the required force or exercise intensity can no longer be maintained. We have reviewed several biochemical and ionic changes reported to occur in exercising muscle, and analysed the possible effects these changes may have on the electrical and contractile properties of the muscle. There is no evidence that substrate depletion can account for the decreased force generating capacity, but this factor may be important for the rate of energy turnover and be a major determinant for endurance. Increased concentration of inorganic phosphate and hydrogen ions will depress the force generating capacity, but since fatigue can develop gradually without accumulation of these ions they can only be important when aerobic ATP production is insufficient to support the contractions. Evidence is presented showing that a disturbed balance of K+ alone might cause depolarisation block at high stimulation frequencies, but extracellular K+ accumulation does not increase gradually during prolonged dynamic or static exercise, and is therefore not closely related to fatigue. The repeated release of Ca2+ from the sarcoplasmic reticulum (SR) during muscular activity is suggested of Ca2+ by the mitochondria, increasing with stimulation frequency and duration and possibly also deteriorating mitochondrial function. We therefore speculate that decreased Ca2+ availability for release from SR might contribute to a gradual decline in force generating capacity during all types of exercise.

Incidence of exercise induced hypoxemia in elite endurance athletes at sea level.

Abstract: Recent evidence suggests that exercise-induced hypoxemia (EIH) may occur in healthy trained endurance athletes. However, at present, no data exist to describe the regularity of EIH in athletes or non-athletes. Therefore, the purpose of the present investigation was to determine the incidence of EIH during exercise in healthy subjects varying in physical fitness. Subjects (N=68) performed an incremental cycle ergometer test to volitional fatigue with percent arterial oxyhemoglobin saturation (%SaO2) measured min-by-min. For the purpose of data analysis subjects were divided into three groups according to their level of physical training: 1) untrained (N=16), 2) moderately trained (N=27), and 3) elite highly trained endurance athletes (N=25). EIH was defined as a %SaO2 of ≤91% during exercise. EIH did not occur in any of the untrained subjects or the moderately trained subjects. However, EIH occurred in 52% of the highly trained endurance athletes tested and was highly reproducible (r=0.95; P<0.05). These findings further confirm the existence of EIH in healthy highly trained endurance athletes and suggests a rather high incidence of EIH in this healthy population. Hence, it is important that the clinician or physiologist performing exercise testing in elite endurance athletes recognize that EIH can and does occur in the elite endurance athlete in the absence of lung disease.

Muscle stretching and motoneuron excitability.

Abstract: Change of motoneuron excitability has been studied during the three basic modalities of slow or static stretching of the human soleus muscle. Tendon (T) and Hoffmann (H) reflexes were analyzed during static stretching (SS). The H response was compared in SS, in SS preceded by a maximal isometric contraction of the muscle or contraction-relaxation (CR) and during stretching of the muscle by contracting the antagonistic muscles (AC). During progressive dorsiflexion of the foot there is a significant difference (p less than 0.05) between T and H reflexes during SS, although the amplitude of direct motor (M) response, evoked by a maximal stimulation of the motor nerve, is not changed. The maximal joint mobilization during SS, CR and AC modalities appears to be closely related to the decrease in the H response during stretching. This decrease is significantly (p less than 0.05) smaller in SS than in AC or CR. In this last method, the duration of the maximal isometric contraction does not affect the results. In these three basic stretching procedures, the H reflex quickly recovers as soon as the manoeuvre is interrupted. It is suggested that changes in muscle motoneuron pool excitability closely control joint mobilization during slow or static stretching. The inhibition of the motoneurons observed during SS, CR and AC modalities is limited to the duration of the stretching manoeuvre.

Serum creatine kinase and lactate dehydrogenase changes following an eighty kilometer race. Relationship to lipid peroxidation.

Abstract: Pre and post race serum malondialdehyde (MDA), creatine kinase (CK) and lactate dehydrogenase (LDH) levels were studied in runners following an 80 km (50 mile) race. MDA is an indicator of lipid peroxidation. Subjects averaged 47.4 years (range 35-60), had a mean maximal oxygen uptake (VO2max) of 48.2 ml/kg, and averaged 121 km (75 miles) per week in training. Throughout the race, runners maintained a pace approximating 72% of VO2max. Previous data from our laboratory indicated a high correlation between resting MDA and total CK and CK-MB. Present resting data confirms prior results (r = 0.84 and 0.69 respectively). In addition, the relationship established at rest persisted following exercise (r = 0.62 and 0.85 respectively). Post race CK, CK-MB, LDH and MDA values for all subjects were significantly greater than resting values (p less than 0.01). Mean post CK and CK-MB levels were nearly 10 and 4 times lower, respectively, than prior values from our laboratory in subjects following a 100 km (62 mile) race. It was concluded that post exercise serum enzyme elevations, universally accepted as a marker of tissue damage, correlate well and may be related to an exercise induced lipid peroxidation.

The consequences of eccentric contractions and their relationship to delayed onset muscle pain.

Abstract: Exercise can cause muscle pain for a number of reasons. Usually the pain is experienced during the exercise and recovers rapidly afterwards. There is one type of muscle pain that has a very different and characteristic time course. In this situation the exercise itself, and the immediate post-exercise period are painfree. The pain is not felt for about eight hours and is maximal 1 or 2 days later. Delayed onset muscle pain occurs after unaccustomed, high force contractions and is particularly associated with eccentric contractions. The concensus of opinion is that the pain is caused by some form of damage, but the mechanism for the pain is not known. This review summarises the literature on the consequences of eccentric contractions and relates them to delayed onset muscle pain. There is clear evidence of damage to the muscle fibres themselves, their membranes and, at a later stage, mononuclear cell infiltration, but all these have very different time courses and none are the same as the pain. Intramuscular pressures are raised in some, but not all, painful compartments and even when raised follow a different time course to the pain. Anti-inflammatory agents do not affect the pain, but due to the incomplete understanding of the action of these drugs, the role of inflammation in delayed onset muscle pain is uncertain. Despite the considerable evidence of damage after eccentric contractions, the cause of delayed onset muscle pain is still unknown.

Hypotheses of peripheral and central mechanisms underlying occupational muscle pain and injury.

Abstract: In an overview of the problem of occupational muscle pain the evidence indicates that injury is more common the greater the load and the worse the posture in which the work is performed. The commonest are backstrains or ligament or joint damage due to overuse. Fatigue is associated with alterations in energy metabolites in muscle while pain is often due to microscopical damage to the cellular architecture. The progress of pathological changes in muscle following occupational injury may be similar to those seen in primary fibromyalgia (fibrositis) because of a final common pathway involving calcium-induced secondary damage. Occupational muscle pain frequently occurs in the muscles supporting the upper limb girdle and head in workers engaged in repetitively performing skilled manipulations or activities requiring high or sustained mental concentration. It is suggested that both occupational myalgia of this kind may be due to an imbalance in the use of muscles for postural activity (holding or supporting fine movements) compared to phasic use in dynamic work. While there are undoubtedly muscular indications of damage these may be secondary to alterations in (unconscious) central motor control mechanisms.

Psychological factors and self-reports of muscle pain

Abstract: Factorial analyses of subjectively felt health complaints in a population of 400 males and 74 females revealed nine orthogonal (independent) factors. One factor (Factor 4) involved pain in the neck, pain in the back, pain in arms and shoulders, and migraine. This type of complaint did not relate to anxiety and depression. The prevalence of muscle pains varied between the sexes, and the types of occupations. Shiftwork was also important. 54% of the women and 40% of the men in the total population had some forms of muscle pain, but only 8% of the women and 3% of the men felt this to be a really serious problem. Psychological factors explained only moderate amounts of variance of muscle pain when the population was taken as a whole. However, within each type of occupation, psychological factors explained a considerable amount of the variance.

Measurement and evaluation of postural load in occupational work situations.

Abstract: This paper describes methodological considerations and problems experienced when quantifying muscle load in occupational work situations. A system for the quantification of the health effect of prolonged muscle load on the shoulder muscles is also described. Combined measurements of postural load and health effects can be used in a quantitative evaluation of postural load as a risk factor for the development of musculo-skeletal injury. Postural muscle load may be quantified by electromyography or by biomechanical methods. Problems associated with quantitative electromyography are described, including selective inhibition of functional compartments in a muscle. This phenomenon results in other compartments coming under proportionally higher strain, disturbing the force-EMG calibration curves. It is suggested that fatigue measurements, indicated by a shift in the centre frequency of the EMG frequency spectrum, are not easily used for evaluation of vocational EMG recordings if the purpose is to indicate the risk of occupational muscle injury. Load measurements using biomechanical methods may provide an acceptable alternative to electromyography, but more work is required before these methods can be used on a routine basis.

Neuromuscular and hormonal responses in elite athletes to two successive strength training sessions in one day.

Abstract: Acute neuromuscular and endocrine adaptations to weight-lifting were investigated during two successive high intensity training sessions in the same day. Both the morning (I) (from 9.00 to 11.00 hours) and the afternoon (II) (from 15.00 hours to 17.00 hours) training sessions resulted in decreases in maximal isometric strength (p less than 0.01 and less than 0.05), shifts (worsening) in the force-time curve in the absolute scale (p less than 0.05 and ns.) and in decreases in the maximal integrated EMG (p less than 0.01 and less than 0.05) of the selected leg extensor muscles. Increases in serum total (p less than 0.05) and free testosterone (p less than 0.01) and in cortisol (p less than 0.01) concentrations were found during training session II. These were followed by decreases (p less than 0.001 and p less than 0.01 and ns.) in the levels of these hormones one hour after the termination of the session. The responses during the morning training session were different with regard to the decreases in serum total testosterone (p less than 0.05), free testosterone (ns.) and cortisol (p less than 0.05). Only slight changes were observed in the levels of luteinizing hormone and sex hormone-binding globulin during the training sessions. Increases (p less than 0.01) took place in somatotropin during both training sessions. The present findings suggest that high intensity strengthening exercises may result in acute adaptive responses in both the neuromuscular and endocrine systems. The diurnal variations may, however, partly mask the exercise-induced acute endocrinological adaptations in the morning.(ABSTRACT TRUNCATED AT 250 WORDS)

Training induced effects on blood volume, erythrocyte turnover and haemoglobin oxygen binding properties

Abstract: The effect of three weeks ergometer training (Tr) 5 times a week for 45 min at 70% VO2max by 6 subjects on erythrocyte turnover and haemoglobin O2 affinity has been studied. Increased reticulocytosis could be observed from the second day after beginning Tr until a few days after its end, probably caused by increased erythropoietin release by the kidney. Erythrocyte destruction was most pronounced in the first and markedly reduced in the third week of Tr. Elevated glutamate oxalacetate transaminase activity and creatine as well as lowered mean corpuscular haemoglobin indicate a younger erythrocyte population in the first week of recovery. Total blood volume increased during the course of Tr by 700 ml, mainly caused by a raised plasma volume (74%). Red cell volume increased later with maximal values one week after Tr (+280 ml). In this week the standard oxygen dissociation curve was most shifted to the right (P50 increased from 3.77 +/- 0.05 kPa to 3.99 +/- 0.07 kPa) and the Bohr coefficients had their lowest values. Both effects are completely explainable by the haemoglobin O2 binding properties of young erythrocytes. After training, all parameters of physical performance (VO2max, maximal workload, heart rate during rest and exercise) were markedly improved, indicating fast adaptation mechanisms. The increased erythrocyte turnover, including higher erythropoiesis, seems to be one important part of these effects.

Muscle fibre conduction velocity, mean power frequency, mean EMG voltage and force during submaximal fatiguing contractions of human quadriceps

Abstract: In 5 healthy male subjects the changes in muscle fibre conduction velocity, mean power frequency, mean EMG voltage, and force in the quadriceps were measured during submaximal fatiguing contractions, ranging from 60 to 100% of maximum force. Surface EMG was recorded from vastus lateralis. The analysis was divided into 2 parts — before and after force had begun to decline (endurance point). The conduction velocity and mean power frequency declined less before the endurance point than after this point. For all forces, the mean EMG voltage increased up to the endurance point. After the endurance point, the effect of fatigue had a decreasing influence on mean EMG voltage.

Force of knee extensor and flexor muscles and cross-sectional area determined by nuclear magnetic resonance imaging

Abstract: The maximal strengths of knee extensor (E) and flexor (F) muscles were compared in a group of 6 male subjects aged 24–31 years.

Changes in strength and cross sectional area of the elbow flexors as a result of isometric strength training.

Abstract: Changes in strength and size of the elbow flexor muscles have been compared during six weeks of isometric strength training in six male and six female subjects. Isometric training of one arm resulted in a significant increase in isometric force (14.5±5.1%, mean±SD,n = 12). No differences were seen in the response of male and female subjects. The extent of the change was similar to that reported for training studies of other muscles, thus refuting the suggestion that the elbow flexors may be especially amenable to training. Biceps and brachialis cross-sectional area (CSA) was measured from mid-upper arm X-ray computerized tomography before and after training. Muscles increased in area (5.4±3.8%) but this was smaller than, and not correlated with, the increase in strength. The main change in the first six weeks of strength training was therefore an increase in the force generated per unit cross-sectional area of muscle. The arrangement of fibres in the biceps is nearly parallel to the action of the muscle and it is argued that the increase in force per unit cross-sectional area is unlikely to be due to changes in the pennation of the muscle fibres as has been suggested for other muscles.

Control of heat-induced cutaneous vasodilatation in relation to age

Abstract: Well matched unacclimatised older (age 55–68, 4 women, 2 men) and younger (age 19–30, 4 women, 2 men) subjects performed 75 min cycle exercise (∼40%\(\dot V_{{\text{O}}_{{\text{2max}}} } \)) in a hot environment (37°C, 60% rh). Rectal temperature (Tre), mean skin temperature (¯Tsk), arm blood flow (ABF, strain gauge plethysmography), and cardiac output (Q, CO2 rebreathing) were measured to examine age-related differences in heat-induced vasodilatation.Tre and¯Tsk rose to the same extent in each group during the exposure. There was no significant intergroup difference in sweat rate (older: 332±43 ml · m−2 · h−1, younger: 435±49 ml · m−2 · h−1; mean±SEM). However, the older subjects responded to exercise in the heat with a lower ABF response which could be attributed to a lower\(\dot Q\) for the same exercise intensity. The slope of the ABF-Tre relationship was attenuated in the older subjects (9.3±1.3 vs 17.9±3.3 ml · 100 ml−1 · min−1 · °C−1,p <0.05), but theTre threshold for vasodilatation was about 37.0°C for both groups. These results suggest an altered control of skin vasodilatation during exercise in the heat in older individuals. This attenuated ABF response appears to be unrelated to\(\dot V_{{\text{O}}_{{\text{2max}}} } \), and may reflect an age-related change in thermoregulatory cardiovascular function.

Tocopherol mobilization during intensive exercise.

Abstract: This work shows that the level of plasma tocopherol (vitamin E) which has free radical scavenging properties rises significantly during intensive exercise. It is proposed that mobilization of tocopherol could help to prevent lipoperoxidation phenomena occurring in exercising skeletal muscle. A hypothetical mechanism relating to a lipolysis effect is discussed to explain this mobilization.

Induced metabolic alkalosis and its effects on 400-m racing time

Abstract: Six trained male athletes who competed regularly in 400 metre races, were studied under control, alkalotic (NaHCO3) and placebo (CaCO3) conditions to study the effect of induced metabolic alkalosis on 400 m racing time. Pre and post exercise blood samples in the three conditions were analysed for pH, bicarbonate and base excess. Following ingestion of NaHCO3, pre-exercise pH, bicarbonate and base excess levels were significantly higher than either control or placebo conditions. In the alkalotic condition the subjects ran significantly (p<0.005) faster (1.52 s) than either the control of placebo conditions. The post-exercise pH, bicarbonate and base excess levels were all lower in the alkalotic condition than in the others. The results suggest that NaHCO3 can be used as an effective ergogenic aid and support the speculation that the increased extracellular buffering afforded by NaHCO3 ingestion facilitated efflux of H+ from the working tissues, thus decreasing intracellular pH and hence offsetting fatigue.

The danger of an inadequate water intake during prolonged exercise. A novel concept re-visited.

Abstract: To prevent thermal injuries during distance running, the American College of Sports Medicine proposes that between 0.83 and 1.65 l of water should be ingested each hour during prolonged exercise. Yet such high rates of fluid intake have been reported to cause water intoxication. To establish the freely-chosen rates of fluid intake during prolonged competitive exercise, we measured fluid intake during, body weight before and after, and rectal temperature after competition in a total of 102 runners and 91 canoeists competing in events lasting from 170-340 min. Fluid intakes during competition ranged from 0.29-0.62 l.h-1; rates of water loss ranged from 0.69-1.27 l.h-1 in the runners; values were lower in the canoeists. Mean post-race rectal temperatures ranged from 38.0-39.0 degrees C. There was no relationship between the degree of dehydration and post-race rectal temperature. We conclude that hyperthermia is uncommon in prolonged competitive events held in mild environmental conditions, and that exercise intensity, not the level of dehydration, is probably the most important factor determining the postexercise rectal temperature. During prolonged exercise in mild environmental conditions, a fluid intake of 0.5 l.h-1 will prevent significant dehydration in the majority of athletes.

Intramuscular pressure and muscle blood flow in supraspinatus.

Abstract: Intramuscular pressure and muscle blood flow was measured in the supraspinatus muscle in 6 healthy subjects. The recordings were performed at rest, during isometric exercise, during an isometric muscle contraction of 5.6 kPa (42 mm Hg) and 10.4 kPa (78 mm Hg), and at rest after the contraction. Intramuscular pressure was measured by the microcapillary infusion technique, and muscle blood flow by the Xenon-133 washout technique. Intramuscular pressure was 38.2 kPa (SD 12.0) (287 mm Hg) during maximal voluntary contraction. A muscle contraction pressure of 5.6 kPa (42 mm Hg), which is 16% of maximal voluntary contraction, reduces local muscle blood flow significantly. It is concluded that the high intramuscular pressures found in supraspinatus during work with the arms elevated impedes local muscle blood flow.

The influence of force and circulation on average muscle fibre conduction velocity during local muscle fatigue

Abstract: Two series of experiments were performed to examine the relationship between force and change in average musle fibre conduction velocity (MFCV) during local muscle fatigue. The average MFCV was estimated using the cross-correlation method. In the first experiment this relationship was studied with surface EMG of vastus lateralis at force levels from 10 to 100% of maximal voluntary contraction (MVC) with and without occluded circulation. The product of relative force and time was held constant. At 10–20% MVC, MFCV increased slightly under the 2 conditions. Between 30–40% MVC, MFCV decreased, this decline in conduction velocity being significantly greater with occluded circulation. Above 40% MVC the decline in MFCV was larger at higher forces, but without any difference between the ischaemic and non-ischaemic conditions. In the second experiment the relationship between change in force and MFCV was studied during sustained maximal voluntary contractions of biceps brachii. MFCV declined during the first 26–39 s of the contraction, followed by an increase. Since this increase occurred when the force had dropped to 30–50% of the initial maximal force, a partial restoration of blood flow is thought to be responsible for this phenomenon. Because an increase in MFCV was noted, despite a further decline in force, this implies that at high force levels the change in MFCV during fatigue could partly be caused by mechanisms different from those accounting for the force loss. It is concluded that above 40% MVC intramuscular pressure is sufficiently high to cause ischaemia, and MFCV is found to be very sensitive to changes in intramuscular blood flow.

External load can alter the energy cost of prolonged exercise

Abstract: The present study was undertaken to examine the energy cost of prolonged walking while carrying a backpack load. Six trained subjects were tested while walking for 120 min on a treadmill at a speed of 1.25 m · s−1 and 5% elevation with a well fitted backpack load of 25 and 40 kg alternately. Carrying 40 kg elicited a significantly higher (p<0.01) enery cost than 25 kg. Furthermore, whereas carrying 25 kg resulted in a constant energy cost, 40 kg yielded a highly significant (p<0.05) increase in energy cost over time. The study implies that increase in load causes physical fatigue, once work intensity is higher than 50% maximal work capacity. This is probably due to altered locomotion biomechanics which in turn lead to the increase in energy cost. Finally, the prediction model which estimates energy cost while carrying loads should be used with some caution when applied to heavy loads and long duration of exercise, since it might underestimate the acutal enery cost.

Myoelectric reactions to ultra-low frequency and low-frequency whole body vibration.

Abstract: 5 healthy males were exposed to vertical sinusoidal whole body vibration (WBV) at 5 frequencies (F1=0.315 Hz, F2=0.63 Hz, F3=1.25 Hz, F4=2.5 Hz, F5=5.0 Hz) and 2 intensities (11=1.2 ms−2 rms, F1-F5; I2=2.0 ms−2 rms, F2–F5). Erector spinae EMGs were derived at the levels of the first thoracic (T1) and third lumbar (L3) spinous processes, rectified and synchronously averaged, as were the accelerations of the seat and the head. WBV induced vibration-synchronous EMG activity (T1 and L3) which exceeded the activity without WBV during enhanced gravitation and decreased during lowered gravitation from F1 to F3. At F4 and F5, these phase relations changed drastically, thus suggesting a different trigger mechanism. The extreme average EMG-amplitudes remained nearly constant at F1 to F3 and increased at higher frequencies. Maximum EMG activity was higher at I2 than at I1. WBV from F1 to F3 is supposed to cause tonic muscular activity triggered by the otoliths; at higher frequencies, stretch reflexes probably gain additional importance. The results hint at an increasing sensory conflict with decreasing frequency of WBV and are interpreted within the theoretical framework of different modes of motor control. Relations between transmissibility and muscle activity suggest the usefulness of including time-variant spring-characteristics into biomechanical models.

Alteration of period and amplitude of circadian rhythms in shift workers. With special reference to temperature, right and left hand grip strength.

Abstract: 48 male shift workers in various industries volunteered to document circadian rhythms in sleeping and working, oral temperature, grip strength of both hands, peak expiratory flow and heart rate. All physiological variables were selfmeasured 4 to 5 times a day for 2 to 4 weeks. Individual time series were analyzed according to several statistical methods (power spectrum, cosinor, chi squares, ANOVA, correlation, etc.) in order to estimate rhythm parameters such as circadian period (τ) and amplitude (A), and to evaluate subgroup differences with regard to tolerance to shift work, age, duration of shift work, speed of rotation and type of industry. The present study confirms for oral temperature and extends to other variables (grip strength of both hands, heart rate) that intolerance to shift work is frequently associated with both internal desynchronization and small circadian amplitude. The internal desynchronization among several circadian rhythms supports the hypothesis that these latter are driven by several oscillators. Many differences were observed between circadian rhythms in right and left hand grip strength: circadian τ in oral temperature was correlated with that in the grip strength of the dominant hand but not with that of the other hand; changes in τs of the non-dominant hand were age-related but did not correlate with temperature τ; only the circadian A of the non-dominant hand was associated with a desynchronization. Thus, circadian rhythms in oral temperature and dominant hand grip strength may be driven by the same oscillator while that of the non-dominant hand may be governed by a different one. Internal desynchronization between both hand grip rhythms as well as desynchronization of performance rhythms reported by others provide indirect evidence that circadian oscillator(s) may be located in the human cerebral cortex.

Sublethal muscle fibre injuries after high-tension anaerobic exercise.

Abstract: The vastus lateralis muscles of eleven male elite sprinters (17-28 years) were investigated in order to examine the impact of high tension anaerobic muscular work on muscle fibre fine structure. In an attempt to reproduce the training regimen six subjects ran 20 repetitions of 25 s on a treadmill with 2 min 35 s in between, at a speed corresponding to 86% of their personal best 200 m time. PAS-stained sections of biopsies taken approximately 2 h after training generally indicated glycogen depletion in type 1 and type 2B fibres. At the light microscopic level, no signs of inflammation or fibre rupture were observed. However, at the ultrastructural level, frequent abnormalities of the contractile material and the cytoplasmic organelles were detected. Z-band streaming, autophagic vacuoles and abnormal mitochondria were the most conspicuous observations. Control specimens from sprinters who did not perform the acute exercise routine also displayed structural deviations, although to a lesser degree. It is hypothesized that during sprint training the leg musculature is put under great mechanical and metabolic stress which causes the degenerative response reported here.