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1 Institute of Sports Medicine Copenhagen/Team Danmark Testcenter, Bispebjerg Hospital, 2400-Copenhagen NV, Denmark; Copenhagen Muscle Research Center, Rigshospitalet, 2100-Copenhagen, Denmark
2 Copenhagen Muscle Research Center, Rigshospitalet, 2100-Copenhagen, Denmark
3 Institute of Sports Medicine Copenhagen/Team Danmark Testcenter, Bispebjerg Hospital, 2400-Copenhagen NV, Denmark
* To whom correspondence should be addressed. E-mail: ll_andersen{at}yahoo.dk.
Previous studies show that cessation of resistance training, commonly known as "detraining", is
associated with strength loss, decreased neural drive and muscular atrophy. Detraining may also increase the expression of fast muscle myosin heavy chain (MHC) isoforms. The present study examined the effect of detraining subsequent to resistance training on contractile performance during slow-to-medium velocity isokinetic muscle contraction vs. performance of maximal velocity "unloaded" limb movement (i.e. no external loading of the limb). Maximal knee extensor strength was measured in an isokinetic dynamometer at 30 and 240 °s-1, and performance of maximal velocity limb movement was measured with a goniometer during maximal
unloaded knee extension. Muscle cross-sectional area (CSA) was determined with MRI. EMG signals were measured in the quadriceps and hamstring muscles. Twitch contractions were evoked in the passive vastus lateralis muscle. MHC isoform composition was determined with SDS-PAGE. Isokinetic muscle strength increased 18 % (P<0.01) and 10 % (P<0.05) at slow and medium velocities, respectively, along with gains in muscle CSA and increased EMG in response to 3
months of resistance training. After 3 months of detraining these gains were lost, while in contrast
maximal unloaded knee extension velocity and power increased 14% (P<0.05) and 44 % (P<0.05), respectively. Additionally, faster muscle twitch contractile properties along with an increased and decreased amount of MHC type II and MHC type I isoforms, respectively, were observed.
In conclusion, detraining subsequent to resistance training increases maximal unloaded movement
speed and power in previously untrained subjects. A phenotypic shift towards faster muscle MHC
isoforms (I 
IIA IIX) and faster electrically evoked muscle contractile properties in response to detraining may explain the present results.
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