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In this study we characterize the impact of aging on the spontaneous running performance of the Tgαq*44 mice (transgenic murine model of chronic heart failure) as compared to the wild-type FVB mice. In 166 mice we have recorded the following parameters of their physical activities in the running wheels: the total distance covered during the experiment (Dsum), the maximal distance covered in single-effort (Dmax), mean time spent on running per 24 h (Tmean), mean running speed (vmean), the maximum instantaneous speed of run (vmax) and the number of efforts (i.e. the number of running events undertaken by the mice) during 54 days, in four age groups ~4, ~10, ~12 and ≥12.5 months of age. The level of spontaneous running performance of the FVB mice remained essentially unchanged, but a strong impact of aging in the Tgαq*44 mice on their running performance was found. Namely, the Dsum, Dmax, Tmean and vmean in the Tgαq*44 mice at the age of ≥12.5 months decreased by ~50%, when compared to its level corresponding level at the age of ~4 months, with far lesser effect of aging on their vmax. Surprisingly, the number of attempts to perform running by the Tgαq*44 mice at the age of 4 - 12 months remained essentially unchanged. This suggests that the exercise intolerance of the aging heart failure (HF) mice seems to be more dependent on deterioration of heart and muscles function linked to HF than on a possible ageing-related impairment of the ‘willngness’ to initiate running, generated by the central nervous system. [ABSTRACT FROM AUTHOR]

A child's body is highly sensitive to air quality, especially regarding the concentration of particulate matter (PM). Nevertheless, due to the high cost of precision instruments, measurements of PM concentrations are rarely carried out in school areas where children spend most of their daily time. This paper presents the results of PM measurements made by a validated, low-cost university air pollution measurement system operating in a rural area near schools. An assessment of children's exposure to PM during school hours (8 a.m.-6 p.m.) at different times of the year was carried out. We show that PM 10 concentrations in the air, particularly in winter, often exceeded the alert values of 50 µg m -3 , posing a health risk to children, especially when children exercise outside the school building. We also calculated the rate and total PM 10 deposition in the respiratory tract during various physical activities performed in clean and polluted air. Monitoring actual PM 10 concentrations as presented in this paper, using a low cost sensors, offer school authorities and teachers an opportunity to reduce health risks for children. This can be achieved by adjusting the duration and exercise intensity of children's outdoor physical activities according to the measured air quality., (© 2023. Springer Nature Limited.)

Behavioural indices are recognised as important criteria for assessing animal welfare. One of the basic animal behaviours included in ethograms is their activity. The assessment of fast-moving animals, performed by humans using the visual observation method, is difficult and not very objective. Therefore, the aim of the research was to develop a method of automated analysis of animal activity, particularly useful in the observation of quick and lively individuals, and to prove its suitability for assessing the behaviour of fast-moving animals. A method of automatically assessing animal activity was developed using digital image analysis, with the Python programming language and the OpenCV library being the foundational tools. The research model was Callimico goeldii monkeys housed in a zoological garden. This method has been proved to correlate well (Rs = 0.76) with the visual method of animal behaviour analysis. The developed automatic evaluation of animal behaviour is many times faster than visual analysis, and it enables precise assessment of the daily activity of fast-moving groups of animals. The use of this system makes it possible to obtain an activity index with sub-second resolution, which allows it to be used in online mode as a detector of abnormal animal activity, e.g. early detection of illnesses or sudden events that are manifested by increased or decreased activity in relation to the standard activity pattern., (© 2023. The Author(s).)

Heavy metals are one of the components of smog, which is mainly the product of burning fossil fuels in residential buildings. These elements, introduced into the body of cattle by inhalation, may enter the milk. The goal of this study was to assess the impact of particulate pollution in the atmospheric air on the concentration of particulate matter in the air of a dairy cattle barn and on the content of selected heavy metals in milk from cows present in the building. Measurements were taken between November and April (148 measurement days). The calculations carried out showed a high correlation (R S  = + 0.95) between the concentrations of particulates measured outside and inside the barn, which is indicative of a significant impact of the atmospheric air on the particulate pollution level of the livestock building. The number of days in excess of the daily standard for PM 10 inside was 51. The conducted analysis of the chemical composition of the milk collected under high particulate pollution (February) showed that the permitted lead level had been exceeded-21.93 µg/kg (norm 20.00 µg/kg)., (© 2023. The Author(s).)

Skeletal muscles are an important reservoir of nitric oxide (NO • ) stored in the form of nitrite [NO 2 - ] and nitrate [NO 3 - ] (NO x ). Nitrite, which can be reduced to NO • under hypoxic and acidotic conditions, is considered a physiologically relevant, direct source of bioactive NO • . The aim of the present study was to determine the basal levels of NO x in striated muscles (including rat heart and locomotory muscles) with varied contents of tissue nitrite reductases, such as myoglobin and mitochondrial electron transport chain proteins (ETC-proteins). Muscle NO x was determined using a high-performance liquid chromatography-based method. Muscle proteins were evaluated using western-immunoblotting. We found that oxidative muscles with a higher content of ETC-proteins and myoglobin (such as the heart and slow-twitch locomotory muscles) have lower [NO 2 - ] compared to fast-twitch muscles with a lower content of those proteins. The muscle type had no observed effect on the [NO 3 - ]. Our results demonstrated that fast-twitch muscles possess greater potential to generate NO • via nitrite reduction than slow-twitch muscles and the heart. This property might be of special importance for fast skeletal muscles during strenuous exercise and/or hypoxia since it might support muscle blood flow via additional NO • provision (acidic/hypoxic vasodilation) and delay muscle fatigue.

This paper presents a calibration system for low-cost suspended particulate matter (PM) sensors, consisting of reference instruments, enclosed space in a metal pipe (volume 0.145 m 3 ), a duct fan, a controller and automated control software. The described system is capable of generating stable and repeatable concentrations of suspended PM in the air duct. In this paper, as the final result, we presented the process and effects of calibration of two low-cost air pollution stations-university measuring stations (UMS)-developed and used in the scientific project known as Storm&DustNet, implemented at the Jagiellonian University in Kraków (Poland), for the concentration range of PM from a few up to 240 µg·m -3 . Finally, we postulate that a device of this type should be available for every system composed of a large number of low-cost PM sensors.

Background: In the last decades, marathon running has become a popular form of physical activity among people around the world. It should be noticed that the main marathon races are performed in large cities, where air quality varies considerably. It is well established that breathing polluted air results in a number of harmful effects to the human body. However, there have been no studies to show the impact of marathon run performance on the amount of the deposition of varied fractions of airborne particulate matter (PM) in the respiratory tract of runners. This is why the present study sought to determine the impact of marathon run performance in the air of varying quality on the deposition of the PM 1 , PM 2.5 , PM 10 in the respiratory tract in humans., Methods: The PM 1 , PM 2.5 and PM 10 deposition was determined in an "average runner" (with marathon performance time 4 h: 30 min) and in an "elite marathon runner" (with marathon performance time 2 h: 00 min) at rest, and during a marathon race, based on own measurements of the PM content in the air and the size-resolved DF( d ) profile concept., Results: We have shown that breathing air containing 50 µg m -3 PM 10 (a borderline value according to the 2006 WHO standard - still valid) at minute ventilation (V E ) equal to 8 L min -1 when at rest, resulted in PM 10 deposition rate of approximately 9 µg h -1 , but a marathon run of an average marathon runner with the V E = 62 L min -1 increased the deposition rate up to 45 µg h -1 . In the elite runner, marathon run with the V E = 115 L min -1 increased PM 10 deposition rate to 83 µg h -1 . Interestingly, breathing the air containing 50 µg m -3 of PM 10 at the V E = 115 L min -1 by the elite marathon runner during the race resulted in the same PM 10 deposition rate as the breathing highly polluted air containing as much as 466 µg m -3 of PM 10 when at rest. Furthermore, the total PM 10 deposition in the respiratory tract during a marathon race in average runners is about 22% greater (203 / 166 = 1.22) than in elite runners. According to our calculations, the concentration of PM 10 in the air during a marathon race that would allow one not to exceed the PM 10 deposition rate of 9 µg h -1 should be lower than 10 µg m -3 in the case of an average runner, and it should be lower than 5.5 µg m -3 in the case of an elite runner., Conclusions: We conclude that a marathon run drastically increases the rate of deposition of the airborne PM in the respiratory tract of the runners, as a consequence of the huge V E generated during the race. A decrease of the PM content in the air attenuates this rate. Based on our calculations, we postulate that the PM 10 content in the air during a "clean air marathon run", involving elite marathon runners, should be below 5.5 µg m -3 ., Competing Interests: The authors declare there are no competing interests., (©2021 Zoladz and Nieckarz.)

In this study we characterize the impact of aging on the spontaneous running performance of the Tgα q *44 mice (transgenic murine model of chronic heart failure) as compared to the wild-type FVB mice. In 166 mice we have recorded the following parameters of their physical activities in the running wheels: the total distance covered during the experiment (D sum ), the maximal distance covered in single-effort (D max ), mean time spent on running per 24 h (T mean ), mean running speed (ν mean ), the maximum instantaneous speed of run (ν max ) and the number of efforts (i.e. the number of running events undertaken by the mice) during 54 days, in four age groups ~4, ~10, ~12 and ≥12.5 months of age. The level of spontaneous running performance of the FVB mice remained essentially unchanged, but a strong impact of aging in the Tgα q *44 mice on their running performance was found. Namely, the D sum , D max , T mean and ν mean in the Tgα q *44 mice at the age of ≥12.5 months decreased by ~50%, when compared to its level corresponding level at the age of ~4 months, with far lesser effect of aging on their V max . Surprisingly, the number of attempts to perform running by the Tgαq*44 mice at the age of 4 - 12 months remained essentially unchanged. This suggests that the exercise intolerance of the aging heart failure (HF) mice seems to be more dependent on deterioration of heart and muscles function linked to HF than on a possible ageing-related impairment of the 'willngness' to initiate running, generated by the central nervous system.

The issue of air pollution by particulate matter (PM) concerns many places in the world. At the same time, many residents undertake physical activity (recreation, rehabilitation, sport) in the open air. Generally, the amount of dust concentration depends on both the place (center or periphery of the city) and the time of day. In the present study we describe the outcome of monitoring of the state of air pollution by particle matter (PM 10 ) in the Kraków agglomeration area in order to show that it can provide information concerning air quality in the area where people practice varied kinds of sports in the open air. The measurements of PM 10 have been made by a few stations with identical construction working as one network. The details of the air pollution monitoring system and its data quality verification have been described. The network stations made multipoint observations across the Kraków Metropolitan Area during the year 2017 in eight locations. The locations selected represent a diverse spectrum of terrain conditions in which the Kraków agglomeration community undertakes physical activity. For most months of 2017, the minimum monthly average 4-hour PM 10 concentrations were recorded between 10-14 h, regardless of location, whereas the maximum was between 18-22. We also noticed a huge differences in the average monthly value of PM 10 in some locations within the Kraków agglomeration-ranging between 4.9-339.0 µg m -3 . This indicates that some regions of the city are more suitable for performance of physical activity in the open air than others. In conclusion, we postulate that a low-cost air pollution monitoring system is capable of providing valuable information concerning air quality in a given region, which seems to be of importance also to people who practice varied sports activities in the open air., Competing Interests: The authors declare there are no competing interests., (©2020 Nieckarz and Zoladz.)

Persistent negligence in the field of environmental protection in Poland as well as strong dependance of the energy sector on the fossil fuels have led to serious pollution of the air with particulate matter, which at high concentrations is capable of penetrating into the buildings. The aim of this study is to assess the impact of particulate airborne pollution on the concentration of particulate matter inside the antelope house in the Silesian Zoological Garden located within the Upper Silesia in Poland. The research was conducted from February to May in 2018. The records taken in the research period show that the concentration of PM10 exceeded the level of 50 µg/m 3 outside the building during 26 days and 11 days when it comes to the concentration of particulate matter inside the antelope house. The quantity of particulate matter in the antelope house is strongly correlated with the concentration of the particles in the air. Despite fitting existing ventilation system with a filter that reduces the dust level by 60% during the highest level of smog, particulate matter concentration in the antelope house exceeded acceptable limit for PM10 more than twofold. Particle size-fraction analysis revealed that as much as 85% of the particles detected in the studied compartment constitute PM2.5., Competing Interests: The authors declare that they have no competing interests., (© 2020 Pawlak and Nieckarz.)

Physical activity is emerging as an alternative nonpharmaceutical strategy to prevent and treat a variety of cardiovascular diseases due to its cardiac and skeletal muscle beneficial effects. Oxidative stress occurs in skeletal muscle of chronic heart failure (CHF) patients with possible impact on muscle function decline. We determined the effect of voluntary-free wheel running (VFWR) in preventing protein damage in Tgαq*44 transgenic mice (Tg) characterized by a delayed CHF progression. In the early (6 months) and transition (12 months) phase of CHF, VFWR increased the daily mean distance covered by Tg mice eliminating the difference between Tg and WT present before exercise at 12 months of age (WT Pre-EX 3.62 ± 1.66 vs. Tg Pre-EX 1.51 ± 1.09 km, P < 0.005; WT Post-EX 5.72 ± 3.42 vs. Tg Post-EX 4.17 ± 1.8 km, P > 0.005). This effect was concomitant with an improvement of in vivo cardiac performance [(Cardiac Index (mL/min/cm 2 ): 6 months, untrained-Tg 0.167 ± 0.005 vs. trained-Tg 0.21 ± 0.003, P < 0.005; 12 months, untrained-Tg 0.1 ± 0.009 vs. trained-Tg 0.133 ± 0.005, P < 0.005]. Such effects were associated with a skeletal muscle antioxidant response effective in preventing oxidative damage induced by CHF at the transition phase (untrained-Tg 0.438 ± 0.25 vs. trained-Tg 0.114 ± 0.010, P < 0.05) and with an increased expression of protein control markers (MuRF-1, untrained-Tg 1.12 ± 0.29 vs. trained-Tg 14.14 ± 3.04, P < 0.0001; Atrogin-1, untrained-Tg 0.9 ± 0.38 vs. trained-Tg 7.79 ± 2.03, P < 0.01; Cathepsin L, untrained-Tg 0.91 ± 0.27 vs. trained-Tg 2.14 ± 0.55, P < 0.01). At the end-stage of CHF (14 months), trained-Tg mice showed a worsening of physical performance (decrease in daily activity and weekly distance and time of activity) compared to trained age-matched WT in association with oxidative protein damage of a similar level to that of untrained-Tg mice (untrained-Tg 0.62 ± 0.24 vs. trained-Tg 0.64 ± 0.13, P > 0.05). Prolonged voluntary physical activity performed before the onset of CHF end-stage, appears to be a useful tool to increase cardiac function and to reduce skeletal muscle oxidative damage counteracting physical activity decline., (© 2019 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of The Physiological Society and the American Physiological Society.)

It has been repeatedly shown that regular aerobic exercise exerts beneficial effects on incidence and progression of cancer. However, the data regarding effects of exercise on metastatic dissemination remain conflicting. Therefore, in the present study the possible preventive effects of voluntary wheel running on primary tumor growth and metastases formation in the model of spontaneous pulmonary metastasis were analyzed after orthotopic injection of 4T1 breast cancer cells into mammary fat pads of female Balb/C mice. This study identified that in the mice injected with 4T1 breast cancer cells and running on the wheels (4T1 ex) the volume and size of the primary tumor were not affected, but the number of secondary nodules formed in the lungs was significantly increased compared to their sedentary counterparts (4T1 sed). This effect was associated with decreased NO production in the isolated aorta of exercising mice (4T1 ex), suggesting deterioration of endothelial function that was associated with lower platelet count without their overactivation. This was evidenced by comparable selectin P, active GPIIb/IIIa expression, fibrinogen and vWF binding on the platelet surface. In conclusion, voluntary wheel running appeared to impair, rather than improve endothelial function, and to promote, but not decrease metastasis in the murine orthotopic model of metastatic breast cancer. These results call for revising the notion of the persistent beneficial effects of voluntary exercise on breast cancer progression, though further studies are needed to elucidate mechanisms involved in pro-metastatic effects of voluntary exercise., Competing Interests: None.

Cardiac function, skeletal (soleus) muscle oxidative metabolism, and the effects of exercise training were evaluated in a transgenic murine model (Tgα q *44) of chronic heart failure during the critical period between the occurrence of an impairment of cardiac function and the stage at which overt cardiac failure ensues (i.e., from 10 to 12 mo of age). Forty-eight Tgα q *44 mice and 43 wild-type FVB controls were randomly assigned to control groups and to groups undergoing 2 mo of intense exercise training (spontaneous running on an instrumented wheel). In mice evaluated at the beginning and at the end of training we determined: exercise performance (mean distance covered daily on the wheel); cardiac function in vivo (by magnetic resonance imaging); soleus mitochondrial respiration ex vivo (by high-resolution respirometry); muscle phenotype [myosin heavy chain (MHC) isoform content; citrate synthase (CS) activity]; and variables related to the energy status of muscle fibers [ratio of phosphorylated 5'-AMP-activated protein kinase (AMPK) to unphosphorylated AMPK] and mitochondrial biogenesis and function [peroxisome proliferative-activated receptor-γ coactivator-α (PGC-1α)]. In the untrained Tgα q *44 mice functional impairments of exercise performance, cardiac function, and soleus muscle mitochondrial respiration were observed. The impairment of mitochondrial respiration was related to the function of complex I of the respiratory chain, and it was not associated with differences in CS activity, MHC isoforms, p-AMPK/AMPK, and PGC-1α levels. Exercise training improved exercise performance and cardiac function, but it did not affect mitochondrial respiration, even in the presence of an increased percentage of type 1 MHC isoforms. Factors "upstream" of mitochondria were likely mainly responsible for the improved exercise performance. NEW & NOTEWORTHY Functional impairments in exercise performance, cardiac function, and soleus muscle mitochondrial respiration were observed in transgenic chronic heart failure mice, evaluated in the critical period between the occurrence of an impairment of cardiac function and the terminal stage of the disease. Exercise training improved exercise performance and cardiac function, but it did not affect the impaired mitochondrial respiration. Factors "upstream" of mitochondria, including an enhanced cardiovascular O 2 delivery, were mainly responsible for the functional improvement., (Copyright © 2017 the American Physiological Society.)

In this study we examined the relationship between fast myosin heavy chain (MyHC2) content in the vastus lateralis and the rate of oxygen uptake (VO2) and heart rate (HR) increase during an incremental exercise in 38, young, healthy men. Prior to the exercise test, muscle biopsies were taken in order to evaluate the MyHC composition. It was found that during cycling performed below the lactate threshold (LT), a positive relationship between MyHC2 and the intercept of the oxygen uptake and power output (VO2-PO) relationship existed (r=0.49, P=0.002), despite no correlation between MyHC2 and the slope value of the VO2-PO relationship (r= -0.18, P=0.29). During cycling performed above the LT, MyHC2 correlated positively with the magnitude of the nonlinearity in the VO2-PO relationship; i.e. with the accumulated VO2'excess' (r=0.44, P=0.006) and peak VO2'excess' (r=0.44, P=0.006), as well as with the slope of the HR-PO relationship (r=0.49, P=0.002). We have concluded that a greater MyHC2 content in the vastus lateralis is accompanied by a higher oxygen cost of cycling during exercise performed below the LT. This seems to be related to the higher energy cost of the non-cross-bridge activities in the muscles possessing a greater proportion of MyHC2 content. In the case of heavy-intensity exercise, a higher MyHC2 content in the vastus lateralis is accompanied by greater non-linearity in the VO2-PO relationship, as well as a steeper increase in HR in the function of an increase of PO. This relationship can be explained by greater disturbances in metabolic stability in type II muscle fibres during exercise, resulting in a decrease of muscle mechanical efficiency and greater increase of heart rate at a given power output. Therefore, MyHC composition has an impact on the oxygen cost of cycling both below and above the LT.

Presented studies were conducted to assess the influence of alternating (AC) and/or static (DC) magnetic fields (MF) or combined with puromycin (PMC) on U937 cell line viability. The magnitude of DC MF was enclosed in the range (2 ÷ 6) mT. In case of AC MF, four frequencies were set: 12 Hz, 25 Hz, 35 Hz, 50 Hz and magnetic induction value was adjusted to 6.5(rms) mT. The reaction of samples exposed to MF and/or PMC was presented as cell viability coefficient S defined as a ratio of viable cells in the sample to viable cells in the relevant control sample, analyzed by flow cytometry. For PMC treated sample the percentage of viable cells decreased about factor 3. MF alone did not influence cell viability regardless of the type of exposure while simultaneous action of ACDC MF mode and PMC significantly influenced viability of U937 cells. The viability coefficient S was in the range of (0.13 ∓ 0.07 ÷ 0.64 ∓ 0.20) and exerted a non-linear relation with frequency of the AC MF component. The maximal lethal influence (S = 0.13 ∓ 0.07) was observed for DC MF = 6 mT and frequency of AC MF equal to 35 Hz. Observed bio-effects were confronted with the physical models assuming the Ca2+ ions transport disturbance and/or protein complexes dysfunction in cells due to MF action.

Introduction: This study attempted to determine the effect of a 1800 MHz electromagnetic field (EMF) (only carrier frequency) on thyroxine (T4), triiodothyronine (T3) and corticosterone (CORT) concentrations in the blood plasma of chick embryos, and to investigate the effect of electromagnetic field (EMF) exposure during embryogenesis on the level of these hormones in birds that are ready for slaughter., Material and Methods: Throughout the incubation period, embryos from the experimental group were exposed to a 1800 MHz EMF with power density of 0.1 W/m(2), 10 times during 24 h for 4 min. Blood samples were collected to determine T4, T3 and CORT concentrations on the 12th (E12) and 18th (E18) day of incubation, from newly hatched chicks (D1) and from birds ready for slaughter (D42)., Results: The experiment showed that T4 and T3 concentrations decreased markedly and CORT levels increased in the embryos and in the newly hatched chicks exposed to EMF during embryogenesis. However, no changes were found in the level of the analyzed hormones in the birds ready for slaughter. Differences in T4 and T3 plasma concentrations between the EMF-exposed group and the embryos incubated without additional EMF were the highest in the newly hatched chicks, which may be indicative of the cumulative effect of electromagnetic field on the hypothalamo-pituitary-thyroid axis (HPT)., Discussion: The obtained results suggest that additional 1800 MHz radio frequency electromagnetic field inhibits function of HPT axis, however, it stimulates hypothalamo-pituitary-adrenal axis by inducing adrenal steroidogenic cells to synthesize corticosterone. Further investigations are needed to elucidate the mechanisms by which radio EMFs affect HPT and HPA axis function in the chicken embryos.

In this study, we hypothesized that 5 weeks of cycling endurance training can decrease the magnitude of the non-proportional increase in oxygen uptake (V(O(2))) to power output relationship (V(O(2)) 'excess') at exercise intensities exceeding the lactate threshold (LT). Ten untrained, physically active men performed a bout of incremental cycling exercise until exhaustion before and after training. The mitochondrial DNA copy number, myosin heavy chain composition and content of uncoupling protein 3 and sarcoplasmic reticulum Ca(2+)-ATPases (SERCAs) were analysed in muscle biopsies taken from vastus lateralis before and after training. The training resulted in an enhancement of the power-generating capabilities at maximal oxygen uptake (V(O(2)max)) by ∼7% (P = 0.002) despite there being no changes in V(O(2)max) (P = 0.49). This effect was due to a considerable reduction in the magnitude of the V(O(2)) 'excess' (P < 0.05) above the LT. A decrease in plasma ammonia concentration was found during exercise after training (P < 0.05). A downregulation of SERCA2 in vastus lateralis (P = 0.006) was observed after training. No changes in myosin heavy chain composition, selected electron transport chain proteins, uncoupling protein 3 or the mitochondrial DNA copy number (P > 0.05) were found after training. We conclude that the training-induced increase in power-generating capabilities at V(O(2)max) was due to attenuation of the V(O(2)) 'excess' above the LT. This adaptive response seems to be related to the improvement of muscle metabolic stability, as judged by a lowering of plasma ammonia concentration. The enhancement of muscle metabolic stability after training could be caused by a decrease in ATP usage at a given power output owing to downregulation of SERCA2 pumps.

The "slow component" of O2 uptake (VO2) kinetics during constant-load heavy-intensity exercise is traditionally thought to derive from a progressive recruitment of muscle fibers. In this study, which represents a reanalysis of data taken from a previous study by our group (Grassi B, Hogan MC, Greenhaff PL, Hamann JJ, Kelley KM, Aschenbach WG, Constantin-Teodosiu D, Gladden LB. J Physiol 538: 195-207, 2002) we evaluated the presence of a slow component-like response in the isolated dog gastrocnemius in situ (n=6) during 4 min of contractions at approximately 60-70% of peak VO2. In this preparation all muscle fibers are maximally activated by electrical stimulation from the beginning of the contraction period, and no progressive recruitment of fibers is possible. Muscle VO2 was calculated as blood flow multiplied by arteriovenous O2 content difference. The muscle fatigued (force decreased by approximately 20-25%) during contractions. Kinetics of adjustment were evaluated for 1) VO2, uncorrected for force development; 2) VO2 normalized for peak force; 3) VO2 normalized for force-time integral. A slow component-like response, described in only one muscle out of six when uncorrected VO2 was considered, was observed in all muscles when VO2/peak force and VO2/force-time were considered. The amplitude of the slow component-like response, expressed as a fraction of the total response, was higher for VO2/peak force (0.18+/-0.06, means+/-SE) and for VO2/force-time (0.22+/-0.05) compared with uncorrected VO2 (0.04+/-0.04). A progressive recruitment of muscle fibers may not be necessary for the development of the slow component of VO2 kinetics, which may be caused by the metabolic factors that induce muscle fatigue and, as a consequence, reduce the efficiency of muscle contractions.