It has been reported that gravitational unloading by actual spaceflight [1][2][3] or simulation models, such as hind limb suspension of rats [3] or human bed rest [4,5] causes atrophy in antigravity muscles. These responses are more profound in slow-twitch than in fasttwitch fibers of ankle plantarflexor muscles [2,3]. It is clear that changes of the muscular properties following gravitational unloading are caused by the decreased normal activity. However, it is not necessarily known what the key factor is for such muscle-specific effects of gravitational unloading. Therefore the effects of various kinds of activities-such as electrical activity estimated by an electromyogram (EMG) recording, contractile activity induced by electrical stimulation at low or high frequencies, and length-related loading on the muscles-on the properties of hind limb muscles were studied in the present investigation.Leonard et al. [6] reported that the mean decrease of lean body mass in nine astronauts was 1.5 kg after 28-84 d of spaceflight, and 1.1 kg was due to water loss. In some studies, the absolute protein content, not Japanese Journal of Physiology, 52, 235-245, 2002 Key words: hind limb suspension, atrophy and growth failure, protein and water contents, electromyogram, muscle length. Abstract:The mechanisms responsible for the morphological and metabolic adaptation of skeletal muscles to the removal of antigravity activity were investigated in rats. Significant atrophy relative to the levels before suspension was induced in ankle plantarflexsors, may be due to a reduced tension production caused by decreased muscle length and electromyogram activity. Growth failure was significant in ankle dorsiflexors, although these muscles did not atrophy. Forced muscle contraction through electrical stimulation at 1 or 100 Hz during hind limb suspension generally had detrimental effects. The percent contribution of water loss to the suspension-related change in weight was 85, 88, and 93% in soleus, plantaris, and extensor digitorum longus, respectively. The total levels of both ␤-hydroxyacyl CoA dehydrogenase (HAD) and lactate dehydrogenase (LDH) were less in the suspended muscles than in the controls, having high positive correlations with the total protein content. The specific activity of HAD, but not of LDH, of the suspended muscles was lower than in the controls (25-61%). These data suggest that the cause of muscle atrophy and changes in metabolic properties may be a decreased tension development, not necessarily the reduction of electrical or contractile activity. Further, it is clearly suggested that electrical stimulation of a muscle group with different composition of fiber phenotype at a certain pattern or frequency is not suitable for the countermeasure. It is also suggested that the major cause of the decreased muscle weight was loss of water, even though protein content was also lowered after suspension. Moreover, the data suggest that the HAD level was affected more than the total protein content and LDH.