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We investigated changes in the extensor digitorum longus (EDL) muscle of Daurian ground squirrels (Spermophilus dauricus) in different seasons in normal (unrestrained) and tail-suspended (a disuse atrophy model) animals to evaluate mechanisms to overcome fast twitch muscle atrophy.Squirrels were randomly assigned to 7 groups of six animals: Summer-control,Autumn-control,Winter-control (Hibernation) ,Winter-active,Summer-TS (Tail suspension) ,Autumn-TS and Winter-TS.Muscle weight,muscle-to-body mass ratio,fiber cross sectional area (CSA,video analysis) and fiber type distribution were determined for all animals.Muscle-to-body mass ratios and fiber CSA in EDL were similar in all control groups while percentage of type I fibers was lower in the Autumn-control animals (p<0.01) .Compared to controls,body weight was 9.6%lower only in Winter-TS (p<0.01) ,while muscle wet mass was lower in Autumn-TS and Winter-TS (p<0.05) .Neither muscle-to-body mass ratio nor fiber type distribution was different between control and TS groups.In Summer-TS,Autumn-TS and Winter-TS type I fiber CSA was lower (range 19-27%) than in controls.We conclude that hibernating for 2 months did not affect the properties of ground squirrels EDL,including muscle-to-body mass ratios,CSA and fiber type distribution; 14-day tail suspension in different seasons significantly decreased EDL fiber CSA of ground squirrels.Taken together,ground squirrels have a protective mechanism against muscle atrophy during hibernation; this mechanism cannot be trigged by one of the factors alone,including muscle disuse,fat storage and short photoperiod,which are closely associated with hibernation,but only by the combination of these factors.