Cortisol

Cortisol is a catabolic hormone that promotes protein breakdown and inhibits protein synthesis.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 98.} It stimulates the conversion of amino acids to carbohydrate, raises protein-degrading enzyme levels and suppresses immune cell function.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 98.} Rippetoe describes cortisol as a counter to anabolic hormones, released in response to hard training and psychological stress such as sleep loss or relationship problems.^{2Rippetoe et al., Practical Programming for Strength Training, 3rd ed., p. 96.}

Acute increases help clear damaged proteins and maintain blood glucose as muscle remodels and grows; chronically high levels are the concern.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 99.}

In practice

• Rippetoe describes a post-workout rise as an expected response to overload: training hard enough to drive adaptation raises cortisol, which returns to baseline within hours.^{4Rippetoe et al., Practical Programming for Strength Training, 3rd ed., p. 97.}
• Sessions with high training volume, large muscle groups and short rest intervals raise serum cortisol.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 99.}
• Rippetoe warns about sessions overlapping so closely that cortisol never returns to baseline.^{4Rippetoe et al., Practical Programming for Strength Training, 3rd ed., p. 97.} Combined with psychological stress, chronic elevation may contribute to overtraining.^{4Rippetoe et al., Practical Programming for Strength Training, 3rd ed., p. 97.}
• Serum cortisol and the testosterone-to-cortisol ratio have had limited success in predicting strength and power changes; the rise in cortisol may need to exceed 800 nmol/L before signaling possible overtraining.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 99.} The ratio can help track training stress, but it can’t diagnose overtraining alone.^{5Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 196.}