Tension development in highly stretched vertebrate muscle fibres.

1. An apparatus is described by which the length of a selected part of an isolated muscle fibre can be held constant, giving isometric conditions, or alternatively its length can be measured while tension is held constant (isotonic). Control can be switched between length and tension so as to carry out afterloaded contractions with a shortening stop.2. When a part of a fibre with uniform striation spacing is stretched so far that there is presumably no overlap of filaments, the tension developed during an isometric tetanus with this apparatus is very small (not more than 3-5% of the tension developed at optimum length).3. If the tendon ends are held stationary, a fibre with the same initial length develops a large amount of tension (order of 30-40% of tension at optimum length) with a slow time course. This additional tension is due to shortening of the end parts of the fibre, where the striation spacing is smaller and overlap of filaments still exists.4. The resistance to elongation of a part of a fibre where there is no overlap is only slightly increased on stimulation.5. To a first approximation, the results are in good agreement with expectations based on the sliding filament theory. The development of detectable amounts of tension, and of a slight increase of stiffness, on stimulation, are however not expected on the simplest form of this theory; possible explanations are discussed.