Elastic strain energy in the low back muscles during human walking

Summary A simple model of the thorax, pelvis and three columns of the intrinsic lumbar back muscles (=ILBM) was constructed. The model was used to study the length of the ILBM during the different stages of the walking cycle. The length of the right ILBM (especially the lateral column) was largest at right toe off, exactly the stage of the walking cycle in which most force was needed to prevent the torso from falling forwards and laterally.     

光化学诱导的血栓形成性脑缺血及其代谢改变

本文用光化学法激发大鼠局部脑血栓形成，以组织病理学、局部脑血流量、皮层水含量、三磷酸腺苷、二磷酸腺苷，一磷酸腺苷、能量负荷、肌酸及乳酸为指标，探讨局部脑血栓形成后５天ｒＣＢＦ明显增高的可能机理。结果表明，血栓形成后局部脑组织ＡＴＰ水平进行性减少（Ｐ＜０．０１）、ＡＤＰ／ＡＴＰ及ＡＭＰ／ＡＴＰ比值明显升高（Ｐ均＜０．０１），ＬＡ及脑水含量显著增加（Ｐ＜０．０１）。这些因素既是能量衰竭、无氧酵解的结果     

External load can alter the energy cost of prolonged exercise

Summary 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.     

Identification of the three-element windkessel model incorporating a pressure-dependent compliance

A new one-step computational procedure is presented for estimating the parameters of the nonlinear three-element windkessel model of the arterial system incorporating a pressure-dependent compliance. The data required are pulsatile aortic pressure and flow. The basic assumptions are a steadystate periodic regime and a purely elastic compliant element. By stating two conditions, zero mean flow and zero mean power in the compliant element, peripheral and characteristic resistances are determined through simple closed form formulas as functions of mean values of the square of aortic pressure, the square of aortic flow, and the product of aortic pressure with aortic flow. The pressure across as well as the flow through the compliant element can be then obtained so allowing the calculation of volume variation and compliance as functions of pressure. The feasibility of this method is studied by applying it to both simulated and experimental data relative to different circulatory conditions and comparing the results with those obtained by an iterative parameter optimization algorithm and with the actual values when available. The conclusion is that the proposed method appears to be effective in identifying the three-element windkessel even in the case of nonlinear compliance.     

Does feeding regime affect physiologic and thermal responses during exposure to 8, 20, and 27° C?

Summary When the loss of body heat is accelerated by exposure to low environmental temperatures, additional substrates must be oxidized to provide energy to sustain temperature homeostasis. Therefore, the present investigation examined the relation between feeding regime [pre-experimental carbohydrate feeding (FED) vs a fast (FAST)], during 120 min of exposure to 8, 20, and 27° C in well-nourished men. The following were examined: tissue insulation (I; °C · m² · W^–1), rectal temperature (T _re; °C), and oxygen consumption ( O₂; ml · kg^–1 · min^–1). O₂, T _re, and I revealed no significant differences between treatments (FED vs FAST) at any temperature. At 27° C, I was less (P < 0.05) than at 20 and 8° C, and decreased (P < 0.05) as exposure time increased. At 8° C, O₂was higher (P < 0.5) than at 20 or 27°C, and O₂increased as time increased (P < 0.05). T _re decreased (P < 0.05) as time increased for all conditions. Respiratory exchange ratio (R) differed (P < 0.05) between treatments (FED vs FAST), temperature (8 vs 20° C), and across time. Values for R suggests that carbohydrate accounted for 56% and 33% of caloric utilization during the FED vs FAST conditions, respectively. At 8 vs 20° C, R represented 54% vs 30% of cabohydrate utilization. Across time, R demonstrated that in both conditions (FED vs FAST) there was a decreased reliance on carbohydrate utilization for energy provision. From these data it appears that while substrate utilization differed between dietary treatment and across time this did not differentially affect O₂or T _re during protracted exposure to 8, 20, and 27° C. The higher R in the 8° C condition for both dietary treatments demonstrates that carbohydrate utilization is increased in shivering cold-exposed humans. However, the reduction in R across time suggests that fat oxidation is also involved in metabolic heat production and core temperature maintenance during shivering in the cold.     

The energetics of middle-distance running

Summary In order to assess the relative contribution of aerobic processes to running velocity (v), 27 male athletes were selected on the basis of their middle-distance performances over 800, 1500, 3000 or 5000 m, during the 1987 track season. To be selected for study, the average running velocity corresponding to their performances had to be superior to 90% of the best French of the season. Maximum O₂ consumption and energy cost of running (C) had been measured within the 2 months preceding the track season, which, together with oxygen consumption at rest allowed us to calculate the maximalv that could be sustained under aerobic conditions: . The treadmill runningv corresponding to a blood lactate of 4 mmol·^–1 (v _la4), was also calculated. In the whole group, C was significantly related to height (r=–0.43;P<0.03). Neither C nor (with, in this case, the exception of the 3000 m athletes) were correlated to . On the other hand,v _{a max} was significantly correlated to over distances longer than 800 m. These were also correlated tov _la4. Howeverv _la4 occurred at 87.5% SD 3.3% ofv _{a max}, this relationship was interpreted as being an expression of the correlation betweenv _{a max} and . Calculation ofv _{a max} provided a useful means of analysing the performances. At the level of achievement studied, sustained over 3000 m corresponded tov _{a max}. The shape of the relationship ofv/v _{a max} as a function of the duration of the event raised the question of a possible change in C as a function of v during middle-distance running competitions.     

The anaerobic recovery of frog muscle

Summary The muscle may undergo a partial recovery of its high energy phosphate stores in the absence of oxygen by the way of glycolysis (anaerobic recovery). This process has been studied in 41 pairs of frog gastrocnemii at different degrees of exhaustion induced by variable trains of supramaximal stimuli. Anaerobic recovery appears to be inadequate to replenish the fraction of muscle high energy phosphate stores (GP=ATP+PC) split as a consequence of the stimulation. The maximal amount of recovery (on the average about 5 Moles of GP per gram of fresh tissue) occurs when the muscle resting stores have been reduced to about 50%. This limitation in the extent of recovery is not a consequence of a reduced availability of glycogen but it is possibly related to the production of some metabolic intermediate, limiting the rate of the glycolytic sequence, likely the accumulation of lactic acid in the fiber. The time course of the anaerobic recovery process is characterized by at1/2 of about 2 min. The efficiency of the process, i.e. the number of the high energy phosphate bonds resynthesized by one Mole of lactic acid, appears to vary between 1.5 and 1.8, being of the same order of magnitude as the GP/L.A. ratio obtained from muscle extracts.     

Thermogenic effect of adrenaline: interaction with insulin

Summary The contribution of insulin (3.6 pmol sd kg body mass^–1·min^–1 to adrenaline-induced (0.164 nmol · kg fat free mass^–1·min^–1) thermogenesis was studied in ten postabsorptive healthy volunteers using two sequential protocols. Variables considered were oxygen consumption as well as carbon dioxide production, heart rate, blood pressure, plasma concentrations of glucose, insulin, glycerol, free fatty acids,-HO-butyrate and lactate. Adrenaline increased plasma concentrations of glucose, glycerol, free fatty acids, and-HO-butyrate, and heart rate and metabolic rate during normo-insulinaemia [61.3 (SEM 6.6) pmol·^–1]. Similar effects were observed during hyperinsulinaemia [167.9 (SEM 18.7) pmol·^–1], but the effect of adrenaline on oxygen consumption was reduced. On average, metabolic rate increased by 12.9% during normo-insulinaemia and by 8.9% during hyperinsulinaemia. We concluded that relative hyperinsulinaemia resulted in decreased adrenaline-induced thermogenesis and therefore increased whole body anabolism.     

Effects of marathon running on running economy and kinematics

The present study was designed to investigate interactions between running economy and mechanics before, during, and after an individually run marathon. Seven experienced triathletes performed a 5-min submaximal running test on a treadmill at an individual constant marathon speed. Heart rate was monitored and the expired respiratory gas was analyzed. Blood samples were drawn to analyze serum creatine kinase activity (S-CK), skeletal troponin I (sTnI), and blood lactate (B-La). A video analysis was performed (200 frames · s⁻¹) to investigate running mechanics. A kinematic arm was used to determine the external work of each subject. The results of the present study demonstrate that after the marathon, a standardized 5-min submaximal running test resulted in an increase in oxygen consumption, ventilation, and heart rate (P < 0.05), with a simultaneous decrease in the oxygen difference (%) between inspired and expired air, and respiratory exchange ratio (P < 0.05). B-La did not change during the marathon, while sTnI and S-CK values increased (P < 0.05), peaking 2 h and 2 days after the marathon, respectively. With regard to the running kinematics, a minor increase in stride frequency and a similar decrease in stride length were observed (P < 0.01). These results demonstrate clearly that weakened running economy cannot be explained by changes in running mechanics. Therefore, it is suggested that the increased physiological loading is due to several mechanisms: increased utilization of fat as an energy substrate, increased demands of body temperature regulation, and possible muscle damage. Accepted: 20 March 2000     

Can energy drinks reduce the depressor effect of ethanol? An experimental study in mice

Although the popularization of the combined use of alcoholic beverages and energy drinks (ED) containing caffeine, taurine and other substances has increased, there are no controlled experimental studies on the effects of ED alone or combined with ethanol. This work aimed at evaluating the effects of different doses of ED combined or not with ethanol, on the locomotor activity of Swiss mice. The administration of 3.57, 10.71 or 17.86 ml/kg of ED alone increased the locomotor activity of the animals in relation to a control group. Low doses of ethanol (0.5, 1.0 and 1.5 g/kg) alone or in combination with 10.71 ml/kg of ED did not affect their locomotor activity. However, the reduction of activity observed after 2.5 g/kg of ethanol was antagonized by 10.71 ml/kg of ED. Further studies on the mechanisms of this interaction are still needed.