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1 Department of Kinesiology, The College of William & Mary, Williamsburg, Virginia 23187-8795; and 2 The Human Performance Laboratory, Department of Kinesiology, The University of Connecticut, Storrs, Connecticut 06269-1110
Strength decrements observed after
extended (4-6 wk) periods of muscle unloading are associated with
significant atrophy. Because early (up to 2 wk) strength gains from
resistance exercise are related to improved neural recruitment, we
hypothesized that the loss of strength resulting from 2 wk of muscle
unloading [unilateral lower limb suspension (ULLS)] was due to
impaired neural activation of the affected muscle. Blood samples,
muscle biopsy specimens, muscle function data, and electromyography
(EMG) recordings were analyzed before and after 14 days of muscle
unloading. Pre- to postunloading data showed significant
(P 
0.05) decrements in peak torque and total work
performed by knee extensors and flexors. This was coupled with
decreased EMG activity, but no change in neuromuscular efficiency
(total torque/EMG). Resistance to muscle fatigue was enhanced after
ULLS. The 14-day intervention failed to alter the size or fiber type
distribution of muscle samples. However, resting plasma cortisol levels
were significantly increased after muscle unloading, suggesting an
endocrine environment favorable to muscle atrophy. Our data confirm
that the diminution in muscle function displayed after 2 wk of
unloading is mainly due to neural, rather than contractile, disturbances.
unweighting; neuromuscular; cortisol; adrenocorticotropic hormone; electromyography
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