BLOOD-FLOW OCCLUSION, MAXIMAL FORCE PRODUCTION AND EMG IN 2 RAT GASTROCNEMIUS-MUSCLE COMPARTMENTS

The proximal and distal compartments of rat medial gastrocnemius muscl e are dominated, respectively by, fast-twitch oxidative and fast twitc h glycolytic fibres. In the present study it was hypothesized that rep etitive in situ activation with an intact blood supply would cause gre ater declines in maximal tetanic force, compound action potential (CAP ) amplitude and CAP area in the distal compared to the proximal compar tment. Furthermore, it was hypothesized that these differences would b e eliminated after occlusion of the blood supply to the muscle. A twit ch 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 wit hout, blood supply. During the first minute of the exercise, as expect ed, 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 signific antly affected by occlusion and it changed significantly less in the p roximal (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 wi th parallel declines in force (to 17.6 +/- 2.8%) were observed to occu r in the distal but not in the proximal compartment. Proximal final fo rce and CAP amplitude were 54.2 +/- 2.6% and 80.6 +/- 5.9% respectivel y. Thus, in contrast with the hypothesis, occlusion enhanced the diffe rences between compartments. These results are discussed in relation t o fibre type composition and metabolic changes. It is suggested that a loss of force caused by a decreased muscle fibre excitability upon re petitive activation depends not only on fibre type, but also on the in tramuscular location of the fibres.