The proximal and distal compartments of rat medial gastrocnemius muscle are dominated, respectively by, fast-twitch oxidative and fast twitch glycolytic fibres. In the present study it was hypothesized that repetitive in situ activation with an intact blood supply would cause greater declines in maximal tetanic force, compound action potential (CAP) amplitude and CAP area in the distal compared to the proximal compartment. Furthermore, it was hypothesized that these differences would be eliminated after occlusion of the blood supply to the muscle. A twitch followed by a tetanus (120 Hz, 200 ms duration) was given every 3 s for 2 min. This exercise protocol was applied once with, and once without, blood supply. During the first minute of the exercise, as expected, occlusion enhanced the decline of proximal force to 77.4 ± 0.8%, a level comparable (P > 0.05) to the decline of distal force (79.7 ± 1.2%). In contrast with the hypothesis, CAP amplitude was not significantly affected by occlusion and it changed significantly less in the proximal (to 102.9 ± 4.5%) compared to the distal (to 69.0 ± 6.7%) compartment. During the second minute of activation without blood flow, sudden declines of distal CAP amplitude (to 18.4 ± 3.4%) coupled with parallel declines in force (to 17.6 ± 2.8%) were observed to occur in the distal but not in the proximal compartment. Proximal final force and CAP amplitude were 54.2 ± 2.6% and 80.6 ± 5.9% respectively. Thus, in contrast with the hypothesis, occlusion enhanced the differences between compartments. These results are discussed in relation to fibre type composition and metabolic changes. It is suggested that a loss of force caused by a decreased muscle fibre excitability upon repetitive activation depends not only on fibre type, but also on the intramuscular location of the fibres.
- Action potential
- Muscle fibres