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Force decline during fatigue is due to both a decrease in the force per individual cross-bridge and the number of
Marta Nocella1, Barbara Colombini, Giulia Benelli
1Department of Physiological Sciences, Universit`a degli Studi di Firenze, Viale G.B. Morgagni 63, 50134 Florence, Italy.
The Journal of Physiology
|May 5, 2011
Summary
Exercise fatigue involves reduced force output. Initially, individual cross-bridge force decreases, then the number of active cross-bridges declines, impacting muscle performance during prolonged activity.
Area of Science:
- Muscle physiology
- Exercise science
- Biophysics
Background:
- Exercise fatigue is characterized by the inability to maintain force or power output.
- Fatigue mechanisms involve either reduced force per cross-bridge or fewer active cross-bridges.
- Understanding these mechanisms is crucial for exercise performance and rehabilitation.
Purpose of the Study:
- To investigate the contributions of cross-bridge force and number to force loss during fatigue.
- To differentiate the mechanisms causing force decline in early versus late stages of fatigue.
Main Methods:
- Experiments conducted on single muscle fibers or small bundles from mouse flexor digitorum brevis (FDB) muscle.
- Tetanic contractions (105) were induced, with force and stiffness measured via high-frequency sinusoidal length oscillations (2.5 or 4 kHz).
- Stiffness data were corrected for series compliance to isolate cross-bridge behavior.
Main Results:
- Early fatigue (first ~20 contractions) showed force decline primarily due to reduced individual cross-bridge force.
- As fatigue progressed, a decrease in the number of force-generating cross-bridges became the dominant factor.
- Interestingly, early fatigue paradoxically increased the rate of tetanic force development and relaxation, which slowed in later stages.
Conclusions:
- Muscle fatigue onset is characterized by decreased individual cross-bridge force.
- With more severe fatigue, the number of active cross-bridges diminishes, contributing significantly to force loss.
- These findings provide insights into the complex, stage-dependent mechanisms underlying skeletal muscle fatigue.
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