Considerable information has accumulated covering a large number of spaceflight and HS experiments that span a time frame of ~4 to 22 days for spaceflight and from 1 to 56 days for HS. These experiments have primarily focused on extensor muscles used extensively for postural support and locomotor activity. The review by Roy, Baldwin, and Edgerton provides one of the most comprehensive reviews on rodents in the space environment. Additional reviews on this topic have been published. The collective observations clearly show that these types of muscle undergo significant reductions in muscle mass (muscle weight) along with a concomitant loss in total protein and myofibrillar (the fraction that is composed of the contractile machinery of structural proteins) protein content of the targeted muscles. In some experiments, it has been reported that the myofibrillar fraction can be degraded to a greater extent than other muscle fractions. The general pattern demonstrates that a rapid loss in muscle weight and net total and myofibrillar protein content (concentration (mg/g X muscle weight) occurs during the first 7–10 days of unloading and this is followed by a more gradual loss in these constituents. The net result is that between 25 and 46% of the muscle mass can be lost in antigravity muscles of the lower extremity such as the soleus (Sol; a calf muscle) and vastus intermedius (VI; a deep layered quadriceps muscle), which are composed mostly of the slow Type I myofibers containing the slow myosin heavy chain (MHC) protein. MHC is the most abundant protein expressed in striated muscle; and this structural / regulatory protein serves as the motor protein that regulates, in synergy with its companion protein actin, the contraction process that derives the force, work, and power generation that is necessary for the muscle groups to bring about both movement and stabilizing types of activity (posture). It is also important to point out that fast-twitch synergistic muscles (expressing fast isoforms of MHC) are also targeted, but these muscles and their fibers are apparently not as sensitive to the unloading stimulus as the slower types of muscle are. Compared to both the slow and fast types of muscle, atrophy of the corresponding joint flexors, for example the tibialis anterior and extensor digitorum longus muscles in the leg, is markedly less.
