Regional intramuscular pressure development and fatigue in the canine gastrocnemius muscle in situ

Intramuscular pressure (PIM) was measured simultaneously in zones of the medial head of the gastrocnemius-plantaris muscle group (zone I, popliteal origin; zone II, central; zone III, near calcaneus tendon) to determine regional muscle mechanics during isometric tetanic contractions. Peak PIM averages were 586, 1,676, and 993 mmHg deep in zones I, II, and III and 170, 371, and 351 mmHg superficially in zones I, II, and III, respectively. During fatigue, loss of PIM across zones was greatest in zone III (-81%) and least in zone I (-60%) when whole muscle tension loss was -49%. Recovery of PIM was greatest in zone III and least in zone II, achieving 86% and 67% of initial PIM, respectively, when tension recovered to 89%. These data demonstrate that 1) regional mechanical performance can be measured as PIM within a whole muscle, 2) PIM is nonuniform within the canine gastrocnemius-plantaris muscle, being greatest in the deep central zone, and 3) fatigue and recovery of PIM are dissimilar across regions. These differences suggest distinct local effects that integrate to determine whole muscle mechanical capacity during and after intense exercise.