Changes in biomechanical properties during drop jumps of incremental height

The purpose of this study was to investigate changing biomechanical properties with increasing drop jump height. Sixteen physically active college students participated in this study and performed drop jumps from heights of 20, 30, 40, 50, and 60 cm (DJ20-DJ60). Kinematic and kinetic data were collected using 11 Eagle cameras and 2 force platforms. Data pertaining to the dominant leg for each of 3 trials for each drop height were recorded and analyzed. Statistical comparisons of vertical ground reaction force (vGRF), impulse, moment, power, work, and stiffness were made between different drop jump heights. The peak vGRF of the dominant leg exceeded 3 times the body weight during DJ50 and DJ60; these values were significantly greater than those for DJ20, DJ30, and DJ40 (all p < 0.004). The height jumped during DJ60 was significantly less than that during DJ20 and DJ30 (both p = 0.010). Both the landing impulse and total impulse during the contact phase were significantly different between each drop height (all p < 0.036) and significantly increased with drop height. There were no significant differences in the takeoff impulse. Peak and mean power absorption and negative work at the knee and ankle joints during DJ40, DJ50, and DJ60 were significantly greater than those during DJ20 and DJ30 (all p < 0.049). Leg, knee, and ankle stiffness during DJ60 were significantly less than during DJ20, DJ30, and DJ40 (all p < 0.037). The results demonstrated that drop jumps from heights >40 cm offered no advantages in terms of mechanical efficiency (SSC power output) and stiffness. Drop jumps from heights in excess of 60 cm are not recommended because of the lack of biomechanical efficiency and the potentially increased risk of injury.     

Decreased defence against free radicals in rat heart during normal reperfusion after hypoxic, ischemic and calcium-free perfusion

Excessive formation of free radicals possibly plays an important role in the origin of irreversible damage of the heart after hypoxic, ischemic or Ca2+-free treatment. The effect of these treatments on the activity of superoxide dismutase and the glutathione system was studied on isolated rat heart. These activities reflect the protective capacity of the heart against reactive substances. In addition the peroxidation of lipids is determined in the treated hearts using malondialdehyde formation as an indicator. All experiments were performed using a Langendorff-apparatus with recirculating perfusion. The observed changes in the components of the glutathione system and superoxide dismutase activity both after hypoxic, ischemic and Ca2+-free perfusion, as measured upon reperfusion, indicate a decrease in cellular defense mechanisms in the heart against free radicals. The effect was most pronounced upon Ca2+-repletion after a period of Ca2+-free perfusion. No malondialdehyde could however be detected either in the tissue of the treated hearts or in the perfusate. Our data give reason to expect beneficial effects of an adequate pharmacological treatment, which replenishes the cellular defence systems.     

Relation between pharmacodynamic and anthropometric parameters during ergometry at rest and after repeated intake of metoprolol in healthy volunteers: results of a pilot study

Metoprolol is a widely used beta1-selective beta-blocker in hypertension and tachycardia. The influence on vital signs at rest and during ergometry (exercise heart rate or blood pressure, effect areas above baseline) was investigated in a pilot study with 18 healthy volunteers (mean age 29.1 years) by means of multiple and pairwise correlation analysis. At rest, the difference between predose and day 5 values were not associated with anthropometric characteristics. During ergometry for weight and height significant negative correlations were found corresponding to marked beta-values in the multiple regression models. Therefore heart rate decreases less markedly in slim persons which should be taken into consideration in exercise tests during metoprolol intake.     

RETRACTED ARTICLE: Changes in parameters of biochemical and oxidative stress in university students during and after examinations

The study aimed to evaluate the parameters of lipid peroxidation and antioxidant protection, biochemical parameters, and cortisol and adrenaline content in the blood of students depending on the effect of exam stress. A total of 135 healthy students (72 female (53.3%) and 63 male (46.7%)) aged from 19 to 21 years (mean age 20.16 ± 0.42 years) of the experimental group underwent detailed medical screening and examination before the inclusion in the study. The control group consisted of 30 healthy students (17 female (56.7%) and 13 male (43.3%)) of corresponding age (mean age 20.23 ± 0.54 years), whose medical examination was performed during breaks in the absence of any stress factors. The blood parameters of the experimental group were investigated 1 h before, 1 h after, and 24 h after the exam. The cortisol content in the blood of experimental group students significantly increased 1.37 times (p < 0.05) an hour before the exam and 1.32 times (p < 0.05) an hour after; adrenalin content in blood increased 1.76 times (p < 0.05) and 1.49 times (p < 0.05), respectively. Compared to the control group, intensification of lipid peroxidation processes with a 1.51-fold (p < 0.05) increase in erythrocyte malonic aldehyde content in blood 1 h before and 1.42-fold (p < 0.05) increase an hour after the exam was observed in students due to the effect of exam stress.. Changes in hormonal homeostasis, activation of lipoperoxidation processes with the development of oxidative stress, and the disintegration of antioxidant protection factors are typical for academic stress in students.     

Prediction of biomechanical parameters in the lumbar spine during static sagittal plane lifting.

A combined approach involving optimization and the finite element technique was used to predict biomechanical parameters in the lumbar spine during static lifting in the sagittal plane. Forces in muscle fascicles of the lumbar region were first predicted using an optimization-based force model including the entire lumbar spine. These muscle forces as well as the distributed upper body weight and the lifted load were then applied to a three-dimensional finite element model of the thoracolumbar spine and rib cage to predict deformation, the intradiskal pressure, strains, stresses, and load transfer paths in the spine. The predicted intradiskal pressures in the L3-4 disk at the most deviated from the in vivo measurements by 8.2 percent for the four lifting cases analyzed. The lumbosacral joint flexed, while the other lumbar joints extended for all of the four lifting cases studied (rotation of a joint is the relative rotation between its two vertebral bodies). High stresses were predicted in the posterolateral regions of the endplates and at the junctions of the pedicles and vertebral bodies. High interlaminar shear stresses were found in the posterolateral regions of the lumbar disks. While the facet joints of the upper two lumbar segments did not transmit any load, the facet joints of the lower two lumbar segments experienced significant loads. The ligaments of all lumbar motion segments except the lumbosacral junction provided only marginal moments. The limitations of the current model and possible improvements are discussed.     

Preconditioning impact on coronary perfusion during ischemia and reperfusion of heart

Recent studies have confirmed that ischemic preconditioning prevents appearance of reperfusion endothelial dysfunction. However, the issue of preconditioning impact on no-reflow phenomenon remains unresolved. The receptor mechanisms involved in the cardioprotective and vasoprotective effects of preconditioning are different. The ability of preconditioning in preventing reperfusion endothelial dysfunction is dependent upon bradykinin B2-receptor activation and not dependent upon adenosine receptor stimulation. The vasoprotective effect of preconditioning is mediated via mechanisms relying in part on activation of protein kinase C, NO-synthase, cyclooxygenase, mitochondrial K(ATP)-channel opening and an enhancement of antioxidative protection of the heart. The delayed preconditioning also exerts endothelium-protective effect. Peroxynitrite, NO* and O2* are the triggers of this effect but a possible end-effector involves endothelial NO-synthase.     

Changes in membrane characteristics of heart muscle during inhibition

Membrane characteristics were studied in isolated muscle strands from auricles of frogs using the "square pulse" technique. Changes in the time course and spatial spread of subthreshold electrotonic potentials were measured. If acetylcholine is applied in concentrations which cause slowing or stoppage of the heart beat, the following changes are produced: (a) the length constant (λ) of the membrane is reduced, (b) the time constant is shortened. The effects are reversible and increase with acetylcholine concentration. The membrane changes caused by acetylcholine dimmish with time. It is concluded that during acetylcholine inhibition, as well as during vagal inhibition, the conductance of the muscle membrane is increased. Appreciable changes in the resting membrane potential need not accompany inhibition.     

Changes in parameters of lipoprotein metabolism during rat hepatic development

Maintenance of whole body cholesterol homeostasis is determined in part by the liver. Thus, changes in expression of hepatic parameters important in the regulation of cholesterol metabolism may play key roles in determining how homeostasis is maintained. The expression of hepatic lipoprotein uptake systems was studied during development using as a ligand very-low density lipoproteins rich in apolipoprotein E that had been obtained from hypercholesterolemic adult rats. These lipoproteins can serve as ligands for cell surface receptors recognizing apolipoproteins B and/or E. Uptake was lowest in freshly isolated fetal rat hepatocytes, increased substantially in hepatocytes from neonates and was intermediate in those from adults. Binding of these lipoproteins to liver membranes prepared from fetal, neonatal, suckling, weaned and adult rats was lowest in fetal preparations, while those from suckling, weaned and adult livers behaved similarly. Numbers of binding sites in neonatal liver membranes were similar to those in adult, but showed a different affinity. On the basis of this data, the ability of hepatocytes to recognize and remove apolipoprotein B/E-containing lipoproteins from the plasma appears to be a function of the differential expression or regulation of lipoprotein-uptake systems during development.     

A biomechanical study of balance recovery during the fall forward

The study deals with the biomechanics of balance recovery of human subjects in a falling forward situation. Eight subjects took part in the experiment. The subject, held in the initial leaning forward position, was released without his knowledge. The instruction was to recover the induced disequilibrium by walking. The biomechanical analysis shows two phases in the balance recovery. The first phase—preparation phase—is characterized by three events at fixed timing whatever the initial inclination. (i) Dynamic reaction time, showing no significant inter-individual variation (mean VALUE = 90.8 ms). (ii) Braking of the forward fall, between 184 ms and 237.2 ms, depending on the subject. (iii) Beginning of the swing phase—i.e. toe-off instant—between 235.9 ms and 328.3 ms, depending on the subject. The second phase—gait execution phase—is characterized by the duration of the swing phase, the duration of the stance phase, the stride length and execution speed. The durations diminish whereas the stride length and the execution speed increase with respect to the initial inclination. For the same execution speed, the stride length is shorter than in normal walking.

It has been concluded that balance recovery following an induced fall forward begins with an invariable preparation process which is followed by an adaptable recovery one.     

Changes in surface EMG parameters during static and dynamic fatiguing contractions.

The effect of contraction types on muscle fiber conduction velocity (MFCV), median frequency (MDF) and mean amplitude (AMP) of surface electromyography was examined in the vastus lateralis of 19 healthy male adults. The subjects performed knee extension both statically and dynamically until they were exhausted. The static contraction was a sustained isometric extension of the knee at a joint angle of 90 degrees with 50% of the maximum voluntary contraction (MVC) load. The dynamic contraction was a repetitive isotonic extension of the knee between the angles of 90 degrees and 180 degrees with the same 50% MVC load at a frequency of 10 times per minute. MFVC during the static contraction significantly decreased during the exercise (p < 0.01). On the other hand, MFVC during the dynamic contraction did not significantly change throughout the exercise. MDF decreased and AMP increased during both types of contractions (p < 0.01). Because the blood flow within the muscle is maintained during the dynamic contraction by enhanced venous return from the contracting muscle, these results suggested that MFVC is affected by the metabolic state in the muscle and the changes in MDF cannot be explained only by that of MFVC.     

The biomechanical function of the heart and its adrenergic regulation after long-term gamma radiation exposure

The study of heart isolated by Langendorf's method has shown that the prolonged gamma irradiation of rats with 1.0 Gy dose (2.8 x 10(-7) Gy/sec) causes the decrease in contraction and relaxation ability, of myocardium, reduces functional response of heart to the stimulation of beta-adrenergic receptors, and increases of myocardium reaction to the stimulation of alpha-adrenergic receptors.     

Oxygen supply to isolated rabbit heart during its perfusion with eryhem and blood

Coronary vessels of the isolated rabbit heart were perfused with eryhem--a solution containing 2.5--3.0 g% of hemoglobin and with the rabbit own blood diluted to the Hb content corresponding to such in eryhem. The oxygen capacity of eryhem as per 1 g of Hb constituted 72% only of the oxygen capacity of erythrocyte Hb. It was shown that eryhem was capable of uptake, transportation and of giving up oxygen. At the same time the myocardium perfused with eryhem was in worse conditions of oxygen supply than the muscle perfused with blood. The required level of oxygen uptake was reached with the increase of coronary blood supply and more complete utilization of blood oxygen reserves.