EIGHT FIGURES
I n isometrically recorded contractions of skeletal muscle, the initial, total, and developed tensions vary as the length of the muscle is changed.
The initial tension, produced as the muscle at rest is stretched, does not follow Hooke's law of elastic bodies. The relation of the initial tension to length follows a hyperbola-like curve (Weber, 1846; Wertheim, 1847; Dresser, 1890) concave toward the tension axis.2 Later investigators (Brodie, 1895, and Haycroft, '04) describe the curve as following Hooke's law at small tensions (up to 10 to 15 gm. for the frog gastrocnemius according to Haycroft) but giving a hyperbola-like curve a t higher tensions. Sichel ( '34) found that the individual muscle fibers obey Hooke's law. He attempted to explain the fact that the whole muscle did not follow it by assuming muscle fibers of different length, Le., at the beginning not all fibers develop tension. A similar suggestion was made by Dresser (1890).
The total isometric tension curve in relation to the length of the muscle was described as S-shaped by Blix (1895). The total tension produced by the muscle increases rapidly at first, as the muscle is stretched, decreases at greater lengths, and increases again more rapidly at still greater lengths. S-shaped total isometric tension curves similar to Blix's have been described by Evans and Hill ('14), Beck ( '22), Debler ( '36). Colle ( '29) observed in the frog's sartorius, that the first part of the total tension curve was a linear function of the length up to a stretch of about four-tenths of the muscle length, but he observed a drop in the, total tension at higher lengths and then a quick rise as did Blix. Most of these observations were made on isolated muscles.
'Aided by a grant from the Charlton Research Fund, Tufts College Medical School. a This refers, of course, to the tension remaining when the extra tension due to the viscouselastic properties of the muscle (the after-lengthening effect of Blix, 1893) has disappeared.
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' A silk stocking was likewise found to give an exponential tension curve for the mine reasons.