PHYSIOLOGICAL-CHARACTERISTICS OF 2 EXTREME MUSCLE COMPARTMENTS IN GASTROCNEMIUS-MEDIALIS OF THE ANESTHETIZED RAT

Rat medial gastrocnemius (GM) muscle is a compartmentalized muscle. Th e functional properties and fibre type composition of the most proxima l and most distal compartment were studied in in situ preparations. Th e proximal compartment contained predominantly fast twitch oxidative f ibres. The distal compartment was mainly composed of fast twitch glyco lytic fibres. With the use of two small electrodes placed around the p rimary nerve branches, both compartments could be separately stimulate d within the same muscle. The length-force relationship was less broad and maximal twitch and tetanic forces were obtained at lower muscle l engths for the proximal compartment. The differences (mm) were 0.9 +/- 0.2 and 1.2 +/- 0.2 for maximal twitch and tetanic force (120 Hz) pro duction, respectively (P < 0.001). The shortening velocity for maximal power production was lower (P < 0.001) for the proximal compartment ( proximal: 57.1 +/- 2.7 mm s(-1), distal: 73.1 +/- 3.0 mm s(-1)). Durin g a standard fatigue test the fatiguability was significantly lower fo r the proximal compared with the distal fibres. Our findings suggest t hat the proximal compartment is likely to be activated in vivo during activities requiring relatively low power outputs for longer time peri ods. In contrast the distal compartment is probably recruited only dur ing high power demanding short lasting activities. The presented model makes it possible to study fatigue related changes in power productio n of the 'red' and 'white' areas of the GM separately in a way that is probably meaningful with respect to in vivo function.