Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Why exercising muscles tire when needed most

09.11.2006
Researchers at Rice and Harvard link metabolism to muscle fatigue in the body

The cause of muscle fatigue during intense exercise is linked directly to the muscle’s reliance on anaerobic metabolism for force production, according to a new study by researchers at Rice and Harvard universities.

Published in the November issue of the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology, the study implicates the reliance on anaerobic energy release as a key factor in the onset of muscle fatigue and impaired exercise performance. While the mechanism of how anaerobic pathways might impair force production remains under active investigation, the new results suggest that the mechanisms of muscular fatigue in the body are probably similar to the mechanisms being discovered in laboratory research on cell and tissue samples.

The researchers had six males perform 15 all-out sprints on a stationary cycle at varying pedal forces, which meant varying muscle-force requirements. Besides conventional cycling, the researchers also had the study participants perform similar all-out sprints with only one leg while the unused leg rested on an adjacent stool. Although this approach may seem unorthodox, the Rice-Harvard group knew from previous work that the metabolic pathways providing the chemical energy necessary for contraction would differ appreciably during the one- and two-legged conditions, said principal investigator Peter Weyand, assistant professor in kinesiology at Rice.

During exercise, muscles continuously break down and resynthesize the chemical ATP (adenosine triphosphate), which serves as the immediate source of energy for muscle contractions. During less vigorous muscular activity, essentially all of the ATP needed for muscular contraction can be provided via aerobic pathways that utilize oxygen delivered via the bloodstream. The aerobic pathways allow moderate levels of force to be generated without fatigue for prolonged periods, but can only support modest levels of muscular activity, due to the upper limits on how rapidly blood and oxygen can be supplied to the working muscles by the heart. Consequently, during more vigorous exercise, such as sprinting or lifting heavy loads or weights, the aerobic provision of ATP is supplemented by anaerobic pathways that do not rely on oxygen delivery. While the anaerobic pathways provide ATP very rapidly, their capacity is finite and must be replenished after each bout.

The researchers knew that the rates of oxygen delivery, aerobic metabolism and the amount of “aerobic” muscle force generated would be much greater in the active leg under the one-legged condition simply because the heart and circulation can provide relatively more blood and oxygen when only one limb is active. Thus, the researchers were confident that a much greater fraction of the muscle force required would be provided via chemical energy that came from aerobic pathways for all of the one-legged versus the two-legged sprint trials.

The cyclists were asked to pedal stationary cycles for a series of sprints at the rate of 100 revolutions per minute, continuing an all-out effort until they could no longer maintain this speed for at least five seconds. The researchers simultaneously measured the forces the subjects applied to the pedals, the amount of oxygen they inhaled and the electrical activity of the thigh muscles used to apply pedal force. Electrodes were attached to the skin of the thigh to measure electrical activity in the leg muscles.

Weyand and colleagues found that the electrical activity of the leg muscles increased throughout each workout. Such increases are common during fatiguing contractions as individual muscle fibers develop less force over time. “Under these conditions, the exercise can be continued only if the individual activates new, unfatigued muscle to augment the impaired force from the muscle fibers originally activated,” Weyand said. “The increase in electrical signals from the active muscles can be used to indirectly assess the amount of fatigue the muscles are experiencing.”

As the researchers had hypothesized, the subjects had much higher peak rates of aerobic metabolism and pedal forces per leg when they used just one leg. During both the one- and two-legged sprints performed at pedal forces greater than those that could be supported via the aerobic pathways, the researchers observed progressive increases in electrical activity in the thigh muscles. “This indicates that new muscle fibers were being recruited throughout each sprint trial to provide the muscle force necessary to maintain a constant pedal force required by the sprint,” Weyand said.

Due to the lesser pedal forces supported via the aerobic pathways during two-legged cycling, the onset of compensatory muscle recruitment occurred at lower thresholds of pedal and muscle force in this mode. Similarly, at equivalent pedal forces, the rates of increase in compensatory electrical activity in the muscles were greater during two-legged than one-legged sprint cycling. “We attribute these between-mode differences in the rates at which muscles become fatigued and additional muscle is recruited to the greater reliance on anaerobic pathways of ATP resynthesis for force production during two-legged cycling versus one-legged cycling,” Weyand said.

“Although scientists have observed similar fatiguing patterns of electrical activity in people holding heavy objects, performing calisthenics and fine-motor tasks, muscular force decrements had not been shown previously to be so closely linked to the anaerobic pathways of ATP resynthesis,” he said.

Weyand suggested that the study raises the possibility that relying on the anaerobic pathways for chemical energy might be intrinsically fatiguing. “Experts focusing on locomotion and whole-body activities have attributed performance limitations during running, cycling, swimming and other athletic activities that involve many muscles simultaneously to the maximum rates at which ATP can be resynthesized from all pathways and not to an impaired ability of skeletal muscles to produce force during contraction,” he said. “Although bicep curls might not induce huffing, puffing and the same level of discomfort incurred by an all-out sprint, your muscles might not know the difference.”

Weyand’s coauthors on the paper are Matthew Bundle, formerly a Rice research fellow in the Department of Kinesiology and now an assistant professor at the University of Wyoming; and Carrie Ernst, Matthew Bellizzi and Seth Wright, all at Harvard.

The study was funded by the U.S. Army Medical and Materiel Command, the National Institutes of Health and the National Research Council.

B.J. Almond | EurekAlert!
Further information:
http://www.rice.edu

More articles from Studies and Analyses:

nachricht Smart Data Transformation – Surfing the Big Wave
02.12.2016 | Fraunhofer-Institut für Angewandte Informationstechnik FIT

nachricht Climate change could outpace EPA Lake Champlain protections
18.11.2016 | University of Vermont

All articles from Studies and Analyses >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Novel silicon etching technique crafts 3-D gradient refractive index micro-optics

A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.

Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...

Im Focus: Quantum Particles Form Droplets

In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.

“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...

Im Focus: MADMAX: Max Planck Institute for Physics takes up axion research

The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.

The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...

Im Focus: Molecules change shape when wet

Broadband rotational spectroscopy unravels structural reshaping of isolated molecules in the gas phase to accommodate water

In two recent publications in the Journal of Chemical Physics and in the Journal of Physical Chemistry Letters, researchers around Melanie Schnell from the Max...

Im Focus: Fraunhofer ISE Develops Highly Compact, High Frequency DC/DC Converter for Aviation

The efficiency of power electronic systems is not solely dependent on electrical efficiency but also on weight, for example, in mobile systems. When the weight of relevant components and devices in airplanes, for instance, is reduced, fuel savings can be achieved and correspondingly greenhouse gas emissions decreased. New materials and components based on gallium nitride (GaN) can help to reduce weight and increase the efficiency. With these new materials, power electronic switches can be operated at higher switching frequency, resulting in higher power density and lower material costs.

Researchers at the Fraunhofer Institute for Solar Energy Systems ISE together with partners have investigated how these materials can be used to make power...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

ICTM Conference 2017: Production technology for turbomachine manufacturing of the future

16.11.2016 | Event News

Innovation Day Laser Technology – Laser Additive Manufacturing

01.11.2016 | Event News

#IC2S2: When Social Science meets Computer Science - GESIS will host the IC2S2 conference 2017

14.10.2016 | Event News

 
Latest News

UTSA study describes new minimally invasive device to treat cancer and other illnesses

02.12.2016 | Medical Engineering

Plasma-zapping process could yield trans fat-free soybean oil product

02.12.2016 | Agricultural and Forestry Science

What do Netflix, Google and planetary systems have in common?

02.12.2016 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>