Heart Muscle Mechanics

Selected experimental studies are reviewed that deal directly with force generation, shortening velocity, and compliance in heart muscle. The first detailed description of force, velocity, and compliance of heart muscle was presented by Abbott & Mommaerts (1) about 20 years ago. They used papillary muscle and trabeculae carneae preparations comparable to those developed by Cattell & Gold (16) and Ullrich & Whitehorn (69). By apply­ ing the methods that had been used for discovering the mechanical proper­ ties of skeletal muscle (31, 32, 53, 70), they described the following: (a) the staircase phenomenon; (b) the length-tension diagram; (c) the series elastic component; (d) the duration of the active state; (e) the force-velocity rela­ tion; and (j) the effect of quick stretch. Later work refined the methods and the data and fit them into the evolving concepts of muscle contraction. The general approach was based on the rationale worked out for skeletal muscle. That trend has continued, and thus data from skeletal muscle work will be discussed in this review where they are important for understanding heart muscle experiments. Working with heart muscle involves certain difficulties. Compliance is large, which means considerable internal movement occurs. Furthermore, during a normal contraction-relaxation cycle, internal Ca2+ levels continu­ ously change. Two new techniques, which were developed using skeletal muscle, help deal with these problems. The first is the important advance of visualizing the sarcomeres during activity and describing or controlling their length. The second concerns making the sarcolemma of intact or fragmented heart muscle freely permeable to ions and small molecules.