A Fatigue-Based Model of Disuse Osteoporosis

A theoretical model has been developed for predicting the extent of bone adaptation as a function of exercise level. The model works by estimating the amount of fatigue damage which will occur for a given cyclic strain. This allowed us to predict that there would be a difference in the strengths of bones taken from sedentary individuals compared to those from more active individuals. The predicted difference, expressed as the ratio of the fatigue strengths of the two bones, was 1.53.

Experimental tests were carried out using bones from chickens, comparing those kept in battery cages (who are unable to engage in any strenuous activity) with those kept in normal free-range conditions. A finite element model was used to ensure that the in vitro fatigue tests were conducted at strain levels within the physiological range. Significant differences in both static and fatigue strength were found for the radius and tibia bones. These differences could be accurately predicted by the theoretical model, after a slight difference in weight between the two groups was allowed for. Additional evidence for these predictions can also be found in the literature. This work supports the concept that bone adaptation is driven by the need to optimise fatigue damage accumulation.     

Study of function of fingers by physiological tremor

1. 1. Physiological tremor is invisible mechanical vibration of body parts.

2. 2. It has two peak frequencies in the p ower spectrum. The origin of the peaks has not been well elucidated. Therefore, the mechanism and its application to labor science is studied in the paper.

3. 3. The effect of immersion of fingers in water and of loading weights on fingers are performed to elucidate the mechanism involved.

4. 4. It is found that the power spectrum output has two bands from the lower frequency band (1.5–18 Hz) from the CNS and the higher frequency band (18–50 Hz).

5. 5. These results are applied to the evaluation of fatigue of fingers in tapping and typing work.

6. 6. The total power, which is the sum of power spectrum in frequency range between 1.5 and 50 Hz, is employed as evaluational index. The change of total power explains finger fatigue during tapping and typing loads.

7. 7. Physiological tremor is an effective way of evaluating the function of fingers in human work.

Intracellular effect of β3-adrenoceptor agonist Carazolol on skeletal muscle,a direct interaction with SERCA

Carazolol (CZL) is a known agonist of β3 and antagonist of β1 and β2 adrenoceptors (AR), used in the animal production industry to improve meat quality by reducing animal stress and skeletal muscle (SM) proteolysis. Here we sought to better understand the direct effect CZL has on SM. We study CZL effect on calcium (Ca²⁺) regulation by enzymatic activity kinetics of the Ca²⁺-ATPase (SERCA), in isolated sarcoplasmic reticulum (SR) from SM and on the mechanical properties of isolated muscle. In isolated SR from SM previously incubated with 0.03 mM CZL, but absent during SR isolation and during SERCA activity determination, the activity was reduced by 45%. Thermal analysis of SERCA activity with CZL shifted the transition temperature of inactivation (T_i) from T_i = 47 to 44 °C. When isolated SR from fast and slow SM was exposed to CZL, inhibition of SERCA occurred in a dose dependent manner. Slow and fast SM T_i of SERCA shifted to a lower temperature in the presence of CZL and a second transition appears at temperatures <40 °C. In isolated extensor digitorum longus (EDL) and soleus muscles, CZL reduces the contraction force and increases susceptibility to fatigue. However, recovery force after fatigue in either muscle was higher. Our results suggest that Carazolol penetrates the plasma membrane and interacts with SERCA, thus having an important effect on skeletal muscle function. The inhibition of SERCA may lead to a decrement in SR Ca²⁺-release promoting further failure in muscle contraction.     

Compressive fatigue and endurance of juvenile bovine articular cartilage explants

Articular cartilage is an enduring tissue. For most individuals, articular cartilage facilitates a lifetime of pain-free ambulation, supporting millions of loading cycles from activities of daily living. Although early studies into osteoarthritis focused on the role of mechanical fatigue in articular cartilage degeneration, much is still unknown regarding its strength and endurance characteristics. The compressive strength of juvenile, bovine articular cartilage explants was determined to be loading rate-dependent, reaching a maximum strength of 29.5 ± 4.8 MPa at a strain rate of 0.10 %/sec. The fatigue and endurance properties of articular cartilage were characterized utilizing a material testing system, as well as a custom, validated instrument termed the two degrees-of-freedom endurance meter (endurometer). These instruments characterized fatigue in articular cartilage explants at loading levels ranging from 10 to 80 % strength (%S), up to 100,000 cycles. Cartilage explants displayed characteristics of fatigue – fatigue life increased as the loading magnitude decreased. All explants failed within 14,000 cycles at loading levels between 50 and 80 %S. At 10 and 20 %S, all explants endured 100,000 loading cycles. There was no significant difference in equilibrium compressive modulus between run-out explants and unloaded controls, although the pooled modulus increased in response to testing. Histological staining and biochemical assays revealed no material changes in collagen, sulfated glycosaminoglycan, or hydration content between unloaded controls and explants cyclically loaded to run-out. These results suggest articular cartilage may have a putative endurance limit of 20 %S (5.86 MPa), with implications for articular cartilage biomechanics and mechanobiology.     

Effects of 7-day intake of hydrogen-rich water on physical performance of trained and untrained subjects

Hydrogen-rich water (HRW) is used as a supplement to improve performance and reduce fatigue in athletes. However, the potentially beneficial effects of HRW intake could be mediated by the training status of athletes. The purpose of the study was to analyse the ergogenic effect of intake of HRW for one week on aerobic and anaerobic performance, both in trained and untrained individuals. Thirty-seven volunteers participated in the study and were divided into two experimental groups: trained cyclists and untrained subjects. A double-blind crossover design was performed in which all subjects took a placebo (PW) and nano-bubble HRW (pH: 7.5; hydrogen concentration: 1.9 ppm; oxidation-reduction potential (ORP): -600 mV). At the end of 7-day intake, performance was assessed by an incremental VO₂max test and by a maximum anaerobic test. After HRW intake, only trained cyclists improved their performance in the anaerobic test with an increase in peak power (from 766.2 ± 125.6 to 826.5 ± 143.4 W; d = .51) and mean power (from 350.0 ± 53.5 to 380.2 ± 71.3 W; d = .51), and a decrease in the fatigue index (from 77.6 ± 5.8 to 75.1 ± 5.9%; d = .45). The findings demonstrate that the ergogenic effect of HRW is mediated by the training status, and that 7-day intake of HRW would be an effective strategy for improving anaerobic performance in trained cyclists.     

Damage,repair and regeneration in insect cuticle: The story so far,and possibilities for the future

The exoskeleton of an insect can contain countless specializations across an individual, across developmental stages, and across the class Insecta. Hence, the exoskeleton's building material cuticle must perform a vast variety of functions. Cuticle displays a wide range of material properties which are determined by several known factors: the amount and orientation of the chitin fibres, the constituents and degree of cross-linking and hydration of the protein matrix, the relative amounts of exo- and endocuticle, and the shape of the structures themselves. In comparison to other natural materials such as wood and mammal bone, relatively few investigations into the mechanical properties of insect cuticle have been carried out. Of these, very few have focussed on the need for repair and its effectiveness at restoring mechanical stability to the cuticle. Insect body parts are often subject to prolonged repeated cyclic loads when running and flying, as well as more extreme “emergency” behaviours necessary for survival such as jumping, wedging (squeezing through small holes) and righting (when overturned). What effects have these actions on the cuticle itself? How close to the limits of failure does an insect push its body parts? Can an insect recover from minor or major damage to its exoskeleton “bones”? No current research has answered these questions conclusively.     

Fatigue loading of tendon results in collagen kinking and denaturation but does not change local tissue mechanics

Fatigue loading is a primary cause of tendon degeneration, which is characterized by the disruption of collagen fibers and the appearance of abnormal (e.g., cartilaginous, fatty, calcified) tissue deposits. The formation of such abnormal deposits, which further weakens the tissue, suggests that resident tendon cells acquire an aberrant phenotype in response to fatigue damage and the resulting altered mechanical microenvironment. While fatigue loading produces clear changes in collagen organization and molecular denaturation, no data exist regarding the effect of fatigue on the local tissue mechanical properties. Therefore, the objective of this study was to identify changes in the local tissue stiffness of tendons after fatigue loading. We hypothesized that fatigue damage would reduce local tissue stiffness, particularly in areas with significant structural damage (e.g., collagen denaturation). We tested this hypothesis by identifying regions of local fatigue damage (i.e., collagen fiber kinking and molecular denaturation) via histologic imaging and by measuring the local tissue modulus within these regions via atomic force microscopy (AFM). Counter to our initial hypothesis, we found no change in the local tissue modulus as a consequence of fatigue loading, despite widespread fiber kinking and collagen denaturation. These data suggest that immediate changes in topography and tissue structure – but not local tissue mechanics – initiate the early changes in tendon cell phenotype as a consequence of fatigue loading that ultimately culminate in tendon degeneration.     

A method for assessing muscle fatigue during sprint exercise in humans using a friction-loaded cycle ergometer

This study investigated the mechanical changes induced by muscle fatigue caused by repeated sprints and determined whether a friction-loaded cycle ergometer has any advantages for assessing muscle fatigue. Nine subjects performed 15 sprints, each of 5 s with a 25-s rest, on a friction-loaded cycle ergometer. The averaged force, power and velocity of each push-off were calculated. Maximal power decreased by 17.9%, with a concomittent slowing of muscle contraction, but without any change in the maximal force. These results demonstrated that repeated sprints slow down muscle contraction, leading to a fall in maximal power without any loss of force. This would suggest that fast twitch fibres are selectively fatigued by repeated sprints. However, the ergometer used in the present study made it difficult to evaluate the relative influences of contraction velocity and sprinting time. This was certainly the most important limitation. On the other hand, it showed the advantage of measuring instantaneous power and total work dissipated in the environment simultaneously. It also permitted a force-velocity relationship to be obtained from a single sprint and this relationship is known to be closely related to the muscle fibre composition. Accepted: 5 March 1998     

The effect of caffeine ingestion on physical performance after prolonged exercise

The purpose of this study was to determine the effect of caffeine ingestion on physical performance after prolonged endurance exercise. Twenty three trained male volunteers participated in a 40-km march and were divided into two groups, matched for caffeine clearance rate and aerobic capacity. The experimental group ingested, prior to the march, a caffeinated drink at a dose of 5 mg.kg-1 body mass and at the 3rd and 5th h of marching an additional drink at a dose of 2.5 mg.kg-1 body mass. The control group ingested a drink of equal volume at the same times. Upon termination of the march each subject performed a cycle ergometer test at an intensity of 90% maximal oxygen consumption. Time to exhaustion and rate of perceived exertion (RPE) were recorded. Blood samples were drawn predrink, at the 3rd and 5th h of marching and immediately after the cycle ergometer test, and were analysed for caffeine, free fatty acids (FFA), lactate and glucose levels. Plasma FFA levels increased during the march (p less than 0.05), with no significant difference between groups. Lactate levels increased in the experimental group (p less than 0.05), with no significant change in the control group. Glucose levels did not change significantly in either group. After the cycle ergometer test, lactate levels were significantly higher in the experimental, as compared to the control group (3.77 +/- 0.33 vs 2.52 +/- 0.35 mmol.l-1, respectively). There was no significant difference between treatments in the time to exhaustion on the cycle ergometer, but RPE was different (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of training on the metabolic and respiratory profile of high-intensity cycle ergometer exercise

The tolerable work duration (t) for high-intensity cycling is well described as a hyperbolic function of power (W): W = (W'.t-1) + Wa, where Wa is the upper limit for sustainable power (lying between maximum W and the threshold for sustained blood [lactate] increase, theta lac), and W' is a constant which defines the amount of work which can be performed greater than Wa. As training increases the tolerable duration of high-intensity cycling, we explored whether this reflected an alteration of Wa, W' or both. Before and after a 7-week regimen of intense interval cycle-training by healthy males, we estimated ( ) theta lac and determined maximum O2 uptake (mu VO2); Wa; W'; and the temporal profiles of pulmonary gas exchange, blood gas, acid-base and metabolic response to constant-load cycling at and above Wa. Although training increased theta lac (24%), mu VO2 (15%) and Wa (15%), W' was unaffected. For exercise at Wa, a steady state was attained for VO2, [lactate] and pH both pre- and post-training, despite blood [norepinephrine] and [epinephrine] ([NE], [E]) and rectal temperature continuing to rise. For exercise greater than Wa, there was a progressive increase in VO2 (resulting in mu VO2 at fatigue), [lactate], [NE], [E] and rectal temperature, and a progressive decrease for pH. We conclude that the increased endurance capacity for high-intensity exercise following training reflects an increased W asymptote of the W-t relationship with no effect on its curvature; consequently, there is no appreciable change in the amount of work which can be performed above Wa.(ABSTRACT TRUNCATED AT 250 WORDS)