Source: Rice University Date: November 8, 2006

Why Exercising Muscles Tire When

Needed Most

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.