Why Exercising Muscles Tire When Needed Most

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Why Exercising Muscles Tire When Needed Most

http://www.sciencedaily.com/releases/2006/11/061108102523.htm

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 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 Bundle, formerly a Rice

research fellow in the Department of Kinesiology and now an

assistant professor at the University of Wyoming; and Ernst,

Bellizzi and Seth , 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.