Creatine kinase

Creatine kinase is the enzyme that makes ATP from creatine phosphate and ADP.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 61.} This reaction supplies energy at a high rate in the phosphagen system.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 61.} Muscle holds four to six times more creatine phosphate than ATP, so the reaction acts as a reserve for quickly replacing ATP.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 62.} Higher-volume resistance training likely increases creatine kinase in muscle.^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 94.}

When muscle is damaged, proteins such as creatine kinase leak into the blood.^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.} After downhill running, blood levels peak about 12 to 24 hours later.^{5Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 57.} After resistance training, the rise may not show for about 48 hours and can peak 4 to 6 days after the workout.^{5Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 57.} A repeat of the same eccentric workout two weeks later produces a much smaller rise.^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.}

In practice

• Unusually high training volume can raise creatine kinase, a sign of muscle damage.^{6Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 145.}
• Increased creatine kinase is one of many proposed markers of overtraining, but no single marker predicts it.^{7Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 147.},^{8Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 125.}
• Read it with caution: creatine kinase levels do not necessarily reflect how much damage occurred or when.^{9Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 58.}