The muscle work test (Ingle, D.J. Proc. Soc. Exper. Biol. & Med., 31: 163, 1933) has been useful as a quantitative index of vigor for more than twenty-five years. When the gastrocnemius muscle of an anesthetized normal rat is made to contract by intermittent faradic stimulation, it can work vigorously for several days. Fed rats can sustain a high energy output for ten days and longer, and individual animals may work continuously for as long as 17 days. The rat loses its ability to work within a few hours after removal of either the adrenal glands or the pituitary gland, but it is responsive to replacement therapy with the hormones of these organs. The ability of the experimental animal to work over long periods is a criterion of vigor, for it is a stressful procedure which will quickly reveal physiologic deficiencies not demonstrable under resting conditions.

The standard procedure involved the use of male rats weighing 200 gm. They were anesthetized with phenobarbital sodium and cyclopal sodium. The gastrocnemius muscle of the left hind leg was weighted with 100 gm. and stimulated to contract five times per second. The distance which the weight was lifted was recorded on automatic motion accumulators. The animals were inclosed in a constant temperature cabinet at 280 + 0.50. Test substances in aqueous solution were injected into the jugular vein by a constant infusion machine. The apparatus permitted the study of twelve rats simultaneously.

Some years ago it became possible to normalize the work performance of adrenalectomized rats by the continuous intravenous injection of adrenal cortex extract. The work performance of hypophysectomized rats could be greatly increased, but not to normal, by the administration of adrenal cortex extract. In 1953 it was discovered that commercial ACTH would normalize the work performance of hypophysectomized rats, and, when given with adrenal cortex extract, it would normalize the work performance of hypophysectomized-adrenalectomized rats. Here was evidence for an extra-adrenal effect of ACTH, possibly caused by a separate principle contaminating ACTH.