Izšla je deveta številka revije Maxx Fitness & Bodybuilding

Maxx Fitness & Bodybuilding

 
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Moderatorji: UrosS, Metka, Stasa, Grom

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Pridružen: Ne feb 28, 2010 17:17

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Forefoot strikers exhibit lower running-induced knee loading than rearfoot strikers

Abstract
Purpose. Knee pain and Achilles tendinopathies are the most common complaints among runners. The differences in the running mechanics may play an important role in the pathogenesis of lower limb overuse injuries. However, the effect of a runner's foot strike pattern on the ankle and especially on the knee loading is poorly understood. The purpose of this study was to examine whether runners using a forefoot strike pattern exhibit a different lower limb loading profile than runners who use rearfoot strike pattern. Methods. Nineteen female athletes with a natural forefoot strike pattern and pair-matched females with rearfoot strike pattern (n = 19) underwent 3D running analysis at 4 m/s. Joint angles and moments, patellofemoral contact forces and stresses, and Achilles tendon forces were analyzed and compared between groups. Results. Forefoot strikers demonstrated 1ower patellofemoral contact force and stress compared to heel strikers (4.3 +/- 1.2 vs. 5.1 +/- 1.1 body weight, BW; P = 0.029 and 11.1 +/- 2.9 vs. 13.0 +/- 2.8 Mpa; P = 0.04). In addition, knee frontal plane moment was lower in the forefoot strikers compared heel strikers (1.49 +/- 0.51 vs. 1.97 +/- 0.66 Nm/kg; P = 0.015). At the ankle level, forefoot strikers showed higher plantarflexor moment (3.12 +/- 0.40 vs. 2.54 +/- 0.37 Nm/kg; P = 0.001) and Achilles tendon force (6.3 +/- 0.8 vs. 5.1 +/- 1.3 BW; P = 0.002) compared to rearfoot strikers. Conclusions. To our knowledge, this is the first study to show differences in patellofemoral loading and knee frontal plane moment between forefoot and rearfoot strikers. Forefoot strikers exhibit both lower patellofemoral stress and knee frontal plane moment than rearfoot strikers which may reduce the risk of running-related knee injuries. On the other hand, parallel increase in ankle plantarflexor and Achilles tendon loading may increase risk for ankle and foot injuries.

(C)2013The American College of Sports Medicine
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The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation.

Phinney SD, Bistrian BR, Evans WJ, Gervino E, Blackburn GL.

Abstract
To study the effect of chronic ketosis on exercise performance in endurance-trained humans, five well-trained cyclists were fed a eucaloric balanced diet (EBD) for one week providing 35-50 kcal/kg/d, 1.75 g protein/kg/d and the remainder of kilocalories as two-thirds carbohydrate (CHO) and one-third fat. This was followed by four weeks of a eucaloric ketogenic diet (EKD), isocaloric and isonitrogenous with the EBD but providing less than 20 g CHO daily. Both diets were appropriately supplemented to meet the recommended daily allowances for vitamins and minerals. Pedal ergometer testing of maximal oxygen uptake (VO2max) was unchanged between the control week (EBD-1) and week 3 of the ketogenic diet (EKD-3). The mean ergometer endurance time for continuous exercise to exhaustion (ENDUR) at 62%-64% of VO2max was 147 minutes at EBD-1 and 151 minutes at EKD-4. The ENDUR steady-state RQ dropped from 0.83 to 0.72 (P less than 0.01) from EBD-1 to EKD-4. In agreement with this were a three-fold drop in glucose oxidation (from 15.1 to 5.1 mg/kg/min, P less than 0.05) and a four-fold reduction in muscle glycogen use (0.61 to 0.13 mmol/kg/min, P less than 0.01). Neither clinical nor biochemical evidence of hypoglycemia was observed during ENDUR at EKD-4. These results indicate that aerobic endurance exercise by well-trained cyclists was not compromised by four weeks of ketosis. This was accomplished by a dramatic physiologic adaptation that conserved limited carbohydrate stores (both glucose and muscle glycogen) and made fat the predominant muscle substrate at this submaximal power level.
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First Human Tests of New Biosensor That Warns When Athletes Are About to 'Hit the Wall'

July 24, 2013 — A new biosensor, applied to the human skin like a temporary tattoo, can alert marathoners, competitive bikers and other "extreme" athletes that they're about to "bonk," or "hit the wall," scientists are reporting. The study, in ACS' journal Analytical Chemistry, describes the first human tests of the sensor, which also could help soldiers and others who engage in intense exercise -- and their trainers -- monitor stamina and fitness.

Joseph Wang and colleagues explain that the sensor monitors lactate, a form of lactic acid released in sweat. Lactate forms when the muscles need more energy than the body can supply from the "aerobic" respiration that suffices during mild exercise. The body shifts to "anaerobic" metabolism, producing lactic acid and lactate. That helps for a while, but lactate builds up in the body, causing extreme fatigue and the infamous "bonking out," where an athlete just cannot continue. Current methods of measuring lactate are cumbersome, require blood samples or do not give instant results. Wang's team sought to develop a better approach.

They describe the first human tests of a lactate sensor applied to the skin like a temporary tattoo that stays on and flexes with body movements. Tests on 10 human volunteers showed that the sensor accurately measured lactate levels in sweat during exercise. "Such skin-worn metabolite biosensors could lead to useful insights into physical performance and overall physiological status, hence offering considerable promise for diverse sport, military, and biomedical applications," say the scientists. Future research will further correlate sweat lactate levels with fitness, performance and blood lactate levels, Wang added.

The authors acknowledge funding from the National Science Foundation, the National Institutes of Health IMSD program, the UCSD von Liebig Entrepreneurism Center under the U.S. Department of Energy-sponsored Southern California Clean Energy Technology Acceleration Program and the National Natural Science Foundation of China.

Slika

VIR
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Pridružen: Ne feb 28, 2010 17:17

Odgovor Po jul 29, 2013 10:24

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Ni več tako enostavno :)

Zadnje ugotovitve kažejo, da laktat ni tako slab. Tu imaš malo razloženo KLIK

Ali pa tule KLIK
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The human metabolic response to chronic ketosis without caloric restriction: Preservation of submaximal exercise capability with reduced carbohydrate oxidation

S.D. PhinneyCorresponding author contact information, a, B.R. Bistrianb, W.J. Evansc, E. Gervinoc, G.L. Blackburnb
a Department of Nutrition and Food Science, Massachusetts Institute of Technology, Cambridge, MA, USA
b the Cancer Research Institute, New England Deaconess Hospital, Harvard Medical School, Boston, USA
c the Exercise Biochemistry Laboratory, Sargent College of Allied Health Sciences, Boston University, Boston, MA, USA
http://dx.doi.org/10.1016/0026-0495(83)90106-3, How to Cite or Link Using DOI

Abstract
To study the effect of chronic ketosis on exercise performance in endurance-trained humans, five well-trained cyclists were fed a eucaloric balanced diet (EBD) for one week providing 35–50 kcal/kg/d, 1.75 g protein/kg/d and the remainder of kilocalories as two-thirds carbohydrate (CHO) and one-third fat. This was followed by four weeks of a eucaloric ketogenic diet (EKD), isocaloric and isonitrogenous with the EBD but providing less than 20 g CHO daily. Both diets were appropriately supplemented to meet the recommended daily allowances for vitamins and minerals. Pedal ergometer testing of maximal oxygen uptake (V̇O2max) was unchanged between the control week (EBD-1) and week 3 of the ketogenic diet (EKD-3). The mean ergometer endurance time for continuous exercise to exhaustion (ENDUR) at 62%–64% of V̇O2max was 147 minutes at EBD-1 and 151 minutes at EKD-4. The ENDUR steady-state RQ dropped from 0.83 to 0.72 (P < 0.01) from EBD-1 to EKD-4. In agreement with this were a three-fold drop in glucose oxidation (from 15.1 to 5.1 mg/kg/min, P < 0.05) and a four-fold reduction in muscle glycogen use (0.61 to 0.13 mmol/kg/min, P < 0.01). Neither clinical nor biochemical evidence of hypoglycemia was observed during ENDUR at EKD-4. These results indicate that aerobic endurance exercise by well-trained cyclists was not compromised by four weeks of ketosis. This was accomplished by a dramatic physiologic adaptation that conserved limited carbohydrate stores (both glucose and muscle glycogen) and made fat the predominant muscle substrate at this submaximal power level.
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Metabolic adaptations to a high-fat diet in endurance cyclists

Julia H. GoedeckeCorresponding author contact information, a, b, Candice Christiea, b, Gary Wilsona, b, Steven C. Dennisa, b, Timothy D. Noakesa, b, William G. Hopkinsa, b, Estelle V. Lamberta, b
a Medical Research Council/University of Capetown Bioenergetics of Exercise Research Unit, University of Cape Town Medical School, Cape Town, South Africa
b Department of Physiology, School of Medical Science, University of Otago, Dunedin, New Zealand
http://dx.doi.org/10.1016/S0026-0495(99)90238-X, How to Cite or Link Using DOI

Abstract
We examined the time course of metabolic adaptations to 15 days of a high-fat diet (HFD). Sixteen endurance-trained cyclists were assigned randomly to a control (CON) group, who consumed their habitual diet (30% ± 8% mJ fat), or a HFD group, who consumed a high-fat isocaloric diet (69% ± 1% mJ fat). At 5-day intervals, the subjects underwent an oral glucose tolerance test (OGTT); on the next day, they performed a 2.5-hour constant-load ride at 70% peak oxygen consumption (VO2peak), followed by a simulated 40-km cycling time-trial while ingesting a 10% 14C-glucose + 3.44% medium-chain triglyceride (MCT) emulsion at a rate of 600 mL/h. In the OGTT, plasma glucose concentrations at 30 minutes increased significantly after 5 days of the HFD and remained elevated at days 10 and 15 versus the levels measured prior to the HFD (P < .05). The activity of carnitine acyltransferase (CAT) in biopsies of the vastus lateralis muscle also increased from 0.45 to 0.54 μmol/g/min over days 0 to 10 of the HFD (P < .01) without any change in citrate synthase (CS) or 3-hydroxyacyl-coenzyme A dehydrogenase (3-HAD) activities. Changes in glucose tolerance and CAT activity were associated with a shift from carbohydrate (CHO) to fat oxidation during exercise (P < .001), which occurred within 5 to 10 days of the HFD. During the constant-load ride, the calculated oxidation of muscle glycogen was reduced from 1.5 to 1.0 g/min (P < .001) after 15 days of the HFD. Ingestion of a HFD for as little as 5 to 10 days significantly altered substrate utilization during submaximal exercise but did not attenuate the 40-km time-trial performance.
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Changes in the Energy Cost of Running during a 24-h Treadmill Exercise
Gimenez, Philippe1; Kerhervé, Hugo2; Messonnier, Laurent A.3; Féasson, Léonard1,4; Millet, Guillaume Y.1

Collapse BoxAbstract
Purpose: Although fatigue generally increases the energy cost of running (Cr), the changes of Cr and associated variables during an ultramarathon are not known. This study aimed to determine the changes of metabolic and cardiovascular adjustments during an ultraendurance exercise.

Methods: Twelve healthy males ran 24 h on a motorized treadmill (24TR). Overall oxygen consumption (V˙O2 mL·min−1·kg−1), net energy cost (Cr J·kg−1·m−1), and respiratory exchange ratio (RER) were determined before, every 2 h, and after the 24TR at 8 km·h−1. Running speed and heart rate (HR) were continuously measured during the 24TR.

Results: V˙O2 increased (+7.6%, P < 0.001) during the 24TR, principally in the first 8 h of exercise. The RER mirrored changes in V˙O2, that is, decreased significantly until the eighth hour and remained constant thereafter. As a consequence of RER decrease, the increased Cr was markedly attenuated but was still significantly higher at the 8th and 12th hour compared with pre-24TR. Speed was constant over the first 6 h then significantly decreased during the 24TR. HR increased until the sixth hour (i.e., HR drift), then decreased until post-24TR. Furthermore, a significant positive correlation (R = 0.75, P < 0.01) was observed between the velocity sustained during the 24TR (expressed in percentage of the velocity attained at V˙O2max: %VV˙O2max) and the pre- to postchanges in Cr.

Conclusions: The present study characterized accurately the changes of energy cost and substrate use during an extreme run, showing a plateau after 8 h of exercise. It is also concluded that the participants who maintained the highest %VV˙O2max were also those having most deteriorated their Cr over the 24TR, supporting the notion of a trade-off between running speed (relative to VV˙O2max) and Cr.

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