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1 Department of Geriatrics, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205, USA; Department of Physiology and Pharmacology, Karolinska Institutet, S-171 77 Stockholm, Sweden; Department of Clinical Physiology, Huddinge University Hospital, Karolinska Institutet, S-171 77 Stockholm, Sweden
2 Department of Geriatrics, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205, USA
3 Department of Clinical Physiology, Huddinge University Hospital, Karolinska Institutet, S-171 77 Stockholm, Sweden
* To whom correspondence should be addressed. E-mail: Per.Tesch{at}fyfa.ki.se.
In an effort to simulate the compromised function and atrophy of lower limb muscles experienced by astronauts after spaceflight, twenty-one men and women age 30-56 yrs, were subjected to unilateral lower limb unloading for five wk. Whereas ten of these subjects performed unilateral knee extensor resistance exercise (ULRE) two or three times weekly, eleven subjects (UL) refrained from training. The exercise regimen consisted of four sets of seven maximal actions, using an apparatus that offers concentric and eccentric resistance by utilizing the inertia of rotating flywheel(s). Knee extensor muscle strength was measured before and after UL and ULRE and, volume of knee extensor and ankle plantar flexor muscles was determined by means of magnetic resonance imaging. Surface EMG activity measured following UL inferred increased muscle use to perform a given motor task. UL induced an 8.8% decrease (p<0.05) in knee extensor muscle volume. Following ULRE and, as a result of only about 16 min of maximal contractile activity over the five wk course, muscle volume increased (p<0.05) 7.7%. Muscle strength decreased 24-32% (p<0.05) in response to UL. Grp ULRE showed maintained (p>0.05) strength. Ankle plantar flexor muscle volume of the unloaded limb decreased (p<0.05) in both groups (UL 10.5%; ULRE 11.1%). In neither group did the right weight-bearing limb show any change (p>0.05) in muscle volume or strength. The results of this study provide evidence that resistance exercise may not only offset muscle atrophy, but is in fact capable of promoting marked hypertrophy of chronically unloaded muscle.
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