TASK-SPECIFIC DESIGN OF SKELETAL-MUSCLE - BALANCING MUSCLE STRUCTURALCOMPOSITION

Skeletal muscle fibers are composed of three structural elements, each contributing a unique aspect of muscle function, yet each 'competing' in a sense for space inside the-cell. The volume occupied by myofibri ls determines the force of contraction, the volume of sarcoplasmic ret iculum sets the rate of onset and relaxation of a fiber's contraction and hence contraction frequency, and the volume of mitochondria sets t he level of sustained performance. The entirety of functional outcomes in muscle, from sustained isometric to high frequency contractions, a nd from high power output to high endurance, are all primarily attribu table to shifts in the proportions (and relationships) of those three structures. This paper examines and reviews these components of muscle first to identify and summarize structure-function 'rules', and secon d to examine the balance between sometimes competing demands. In parti cular, we focus on those muscles in which power, endurance and frequen cy are all simultaneously high (flight muscles), and examine how muscl e has 'solved' problems of space and energy demand. From these results and observations it would appear that for flight to have evolved in s mall animals, the double packing of inner mitochondrial membranes may be expected in animals under 50-80 g in mass, and asynchronous muscle is structurally essential for flight in small insects with wing beat f requencies above about 100 Hz. (C) 1998 Elsevier Science Inc. All righ ts reserved.