Muscle-specific indices to characterise the functional behaviour of human lower-limb muscles during locomotion

The mechanical output of a muscle may be characterised by having distinct functional behaviours, which can shift to satisfy the varying demands of movement, and may vary relative to a proximo-distal gradient in the muscle-tendon architecture (MTU) among lower-limb muscles in humans and other terrestrial vertebrates. We adapted a previous joint-level approach to develop a muscle-specific index-based approach to characterise the functional behaviours of human lower-limb muscles during movement tasks. Using muscle mechanical power and work outputs derived from experimental data and computational simulations of human walking and running, our index-based approach differentiated known distinct functional behaviours with varying mechanical demands, such as greater spring-like function during running compared with walking; with anatomical location, such as greater motor-like function in proximal compared with the distal lower-limb muscles; and with MTU architecture, such as greater strut-like muscles fibre function compared with the MTU in the ankle plantarflexors. The functional indices developed in this study provide distinct quantitative measures of muscle function in the human lower-limb muscles during dynamic movement tasks, which may be beneficial towards tuning the design and control strategies of physiologically-inspired robotic and assistive devices.     

A model of heart ventricles for electrocardiographic evaluation of the state of the myocardium

A mathematical model of heart excitation processes has been developed for describing an electrocardiogram. A verified archive of model electrocardiograms has been created with the use of the model. The model has been used to study how electrocardiograms are affected by individual variability in ventricle shape and heart position in the norm, in myocardial infarction of different localizations, and in ventricular hypertrophy. Correspondence of the specific features of real and model electrocardiograms is discussed.     

Control of the contractility of the myocardium in a feedback system

Experimental evidence strongly suggests that the contractility of the intact heart in situ, in contrast to that of striated muscle elsewhere in the body, is controlled in a close-cycle system. Thus, the variation of intraventricular pressure during systole follows a complex pattern, whose relative form remains quite constant regardless of the duration of ejection. By use of the single-chambered model of the cardiovascular system, a mathematical representation of a feasible feedback mechanism is developed. The requirement that the feedback system must satisfy mathematical principles eliminates relationships apparently reasonable from a physiological viewpoint. A clinical application which the mathematical development suggests is that early arterial hypertension may arise from an abnormal feedback mechanism with excessively large cardiac output in the initial portion of systole.     

Evaluation of the temperature effect on contractility of frog myocardium]

Dynamics of the parameters of mechanical activity of the frog isolated myocardium was studied under conditions of temperature variations. Within the temperature range of 0-35 degrees C the temperature decrease leads to a rise in the strength of contractions. Meanwhile the velocity of contractions reaches its optimum value at 20 degrees C. The velocity of contractions proved to be a more adequate and sensitive criterion for evaluating the cold effect on contractility than the strength of contractions. From the analysis of the evidence obtained it is concluded that muscle contractility of the heart is depressed on cooling.     

Effects of sevoflurane on the contractility of ferret ventricular myocardium.

Isotonic and isometric variables of contractility and relaxation of isolated ferret right ventricular papillary muscles were measured before and during exposure to incremental concentrations of sevoflurane (0-4.9% vol/vol) (30 degrees C) (n = 9). In a second group of muscles (n = 8), effects of sevoflurane were compared with those of low [Ca(2+)](o) (0.45-2.25 mM in steps of 0.45 mM). Sevoflurane caused a reversible concentration-dependent decrease in contractility (ED(50) of developed force 4.6+/-0.9% vol/vol). When compared with twitches of equal amplitude in low extracellular Ca(2+) concentration, sevoflurane accelerated both isometric and isotonic relaxation. The myocardial depressant effect of sevoflurane is less than that of isoflurane and results mainly from a decrease of intracellular Ca(2+) availability. The abbreviated isometric relaxation likely reflects a decrease in Ca(2+) sensitivity and the faster isotonic relaxation may reflect a mild stimulation of Ca(2+) uptake by the sarcoplasmic reticulum.     

An increase in the redox state during reperfusion contributes to the cardioprotective effect of GIK solution

This study aimed at determining whether glucose-insulin-potassium (GIK) solutions modify the NADH/NAD(+) ratio during postischemic reperfusion and whether their cardioprotective effect can be attributed to this change in part through reduction of the mitochondrial reactive oxygen species (ROS) production. The hearts of 72 rats were perfused with a buffer containing glucose (5.5 mM) and hexanoate (0.5 mM). They were maintained in normoxia for 30 min and then subjected to low-flow ischemia (0.5% of the preischemic coronary flow for 20 min) followed by reperfusion (45 min). From the beginning of ischemia, the perfusate was subjected to various changes: enrichment with GIK solution, enrichment with lactate (2 mM), enrichment with pyruvate (2 mM), enrichment with pyruvate (2 mM) plus ethanol (2 mM), or no change for the control group. Left ventricular developed pressure, heart rate, coronary flow, and oxygen consumption were monitored throughout. The lactate/pyruvate ratio of the coronary effluent, known to reflect the cytosolic NADH/NAD(+) ratio and the fructose-6-phosphate/dihydroxyacetone-phosphate (F6P/DHAP) ratio of the reperfused myocardium, were evaluated. Mitochondrial ROS production was also estimated. The GIK solution improved the recovery of mechanical function during reperfusion. This was associated with an enhanced cytosolic NADH/NAD(+) ratio and reduced mitochondrial ROS production. The cardioprotection was also observed when the hearts were perfused with fluids known to increase the cytosolic NADH/NAD(+) ratio (lactate, pyruvate plus ethanol) compared with the other fluids (control and pyruvate groups). The hearts with a high mechanical recovery also displayed a low F6P/DHAP ratio, suggesting that an accelerated glycolysis rate may be responsible for increased cytosolic NADH production. In conclusion, the cardioprotection induced by GIK solutions could occur through an increase in the cytosolic NADH/NAD(+) ratio, leading to a decrease in mitochondrial ROS production.     

Acceleration generated by the myocardium as a criterion of its contractility

In experiments on cat heart-lung preparations cooled from 38 to 25 degrees C, acceleration of the heart muscle (in mm Hg . s-2) was the only criterion of myocardial contractility, which showed identical changes under an equivalent increase (1.5-fold) of heart load with volume or resistance. The latter indicates that acceleration of the myocardium is the most adequate criterion of myocardial contractility.     

High functional activity of mitochondrial creatine kinase of the myocardium before ischemia is tied with the worst restoration of contractility after reperfusion

Isolated Langendorff-perfused guinea pig hearts were arrested with a cardioplegic solution containing 10 mM phosphocreatine + 15 mM glutamate (PG group) or not containing them (control group). Total normothermic ischemia lasted 45 min followed by 30 min reperfusion. Mitochondrial respiration in the absence and presence of different concentrations of ADP and creatine was studied in biopsy samples after saponin treatment. The samples were taken before and after ischemia as well as after the reperfusion period. A slightly better relative recovery of developed pressure (RRDP) in PG group was associated with higher mitochondrial acceptor control ratio after reperfusion. When results in both groups were taken together, marked negative correlations between the preischemic mitochondrial indices (particularly, those related to creatine kinase activity) and RRDP were revealed. Relative changes in these indices after ischemia demonstrated tight positive correlations with RRDP. Thus, the hearts having higher functional activity of mitochondrial creatine kinase are more sensitive to ischemia, other conditions being equal.