Blood lactate

Blood lactate is the concentration of lactate circulating in the blood, reported in mmol/L; resting levels normally range from 0.5 to 2.2 mmol/L.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 64.} It reflects the balance between production and clearance, with production rising as exercise intensity increases and running higher in type II muscle fibers.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 64.}

Lactate isn’t the cause of muscle fatigue: the reaction produces lactate rather than lactic acid, while the buildup of hydrogen ions interferes with contraction.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 62.} Lactate also serves as fuel for muscle fibers, especially type I fibers and heart muscle, and the liver can convert it back to glucose.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 64.} The NSCA’s personal training text describes clearance rate as an indicator of recovery ability.^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 45.}

In practice

• Blood lactate peaks about 5 minutes after exercise stops and usually returns to preexercise levels within an hour; light active recovery speeds clearance.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 64.}
• Resistance training and sprints produce higher levels than lower-intensity continuous exercise, while trained clients have lower levels at the same absolute workload.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 64.}
• The steep rise associated with lactate threshold begins around 50 to 60% of maximal oxygen uptake in untrained people and 70 to 80% in trained people.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 46.} Training near or above that point is thought to shift it to a higher intensity.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 46.}

Alex Viada describes lactate as an easy-to-measure proxy for fatigue-causing metabolites and a marker of relative intensity, rather than a firm cutoff.^{5Viada, The Ultimate Hybrid Athlete, p. 143.}