IN-VIVO IIX AND IIB FIBER RECRUITMENT IN GASTROCNEMIUS-MUSCLE OF THE RAT IS COMPARTMENT RELATED

The purpose of the present study was to investigate to what extent fas t-twitch IIX and IIB fiber recruitment was related to the natural exis ting muscle compartments (subvolumes of muscle innervated by different primary nerve branches) in rat medial gastrocnemius. Three groups (n= 6) of rats trotted on a motor-driven treadmill (20 degrees incline) at different speeds. A fourth group served as controls, and a fifth grou p received in situ electrical stimulation of all medial gastrocnemius muscle fibers. Postexercise glycogen levels (periodic acid-Schiff stai ning intensities) were made. Running caused more and in situ stimulati on caused less glycogen breakdown in the proximal IIX and IIB fibers c ompared with the fibers of the same type in the most distal compartmen t. Furthermore, the boundaries of the most distal compartment could of ten be recog nized in the periodic acid-Schiff-stained cross sections. It was concluded that during running the proximal IIX and IIB fibers were recruited to a greater extent (and at lower treadmill speeds) com pared with the distal IIX and IIB fibers, respectively.