[Dynamics of human muscle fibers atrophy during long-term head-down hypokinesia]

The dynamics of atrophy in the muscles-extensors of the leg and thigh during long-term hypokinesia was studied with the model of head-down tilt (HDT -6 degrees) in a series of two experiments (120 and 60 days in duration) with participation of men and women. Dynamics of the cross-sectional area of human muscular fibers was characterized by similar atrophy of the fast and slow fibers on the first stage of HDT. Later on, the size of fast fibers in m. soleus and m. gastrocnemius lateralis reached plateau whereas the size of slow fibers continued to diminish. These were only fast fibers that underwent further atrophy in locomotor m. vastus lateralis. A supposition can be made that the controversy between literature data on the character of atrophy of muscular fibers in experiments with human subjects and rodents is primarily linked with different rates of atrophy developed by muscles in various species; besides, investigators describe different stages of one process. Analysis of the benefit from preventive exercises for the muscular atrophy control during HDT indicated that locomotor (with loading restraint belts) and resistive exercises can preclude slow fibers atrophy under the conditions of hypokinesia.